Telequipment D75 scope.
 

@Anode_to_Joy: Congratulations on your successful repair!

https://www.vintage-radio.net/forum/...d.php?t=155339

I stumbled upon a D75 in form of a dumpster-find a few weeks ago (picture attached). Unfortunately the unit was in very poor condition: dirty, missing and fractured knobs, missing labeling on the amplifier unit, non-functional or loose push buttons, and missing screws. Since I have been planning to buy an oscilloscope for years, I could not resist trying to bring the unit back to life. I have no specific experience with repairing oscilloscopes but have done a lot of successful repair work on defective CRT and LCD displays and some tube radios. So repairing the D75 sounds feasible for me, if I get some kind hints from more experienced tinkerers. I already tried to find solutions for my current issues online, but the descriptions for most D75 and D83 related issues are PSU or EHT related.

So far, I performed an overall cleaning of the unit, acquired the service manual and made a basic checkup of the unit for potential short circuits, especially regarding the PSU section. After fixing a few things, I powered the unit on and did a calibration of the output voltages, which are now spot on. Unfortunately, I was not able to check on the cathode voltage due to insufficient measuring equipment. The trace looks OK to me and reacts like I would expect it, when doing CRT related adjustments. After fixing the push buttons, I tried to conduct a basic function check of the unit. Most functions seem to work to some degree including the delayed timebase, with the most obvious problems being inconsistent Y-offsets, wonky triggering and the missing knobs.

In principle, the Y-offsets for channel 1 and 2 can be adjusted properly including an acceptable match with the 5x magnification. Aside from the overly sensitive 5x trimmer, the potentiometers are a little bit jumpy and need some inside cleaning. The traces do not show any continuous drift. However, if the position knobs have been moved significantly or some channel switching has been performed, the match with the 5x magnification is lost and needs to be trimmed again to avoid jumping of the trace when switching the magnification on and off. Also, the zero position of the trace in GND mode is off by a few ticks sometimes for no obvious reason. So far, I have no idea what is causing this. In the same way, the horizontal position adjustment seems to change sometimes by 2 or 3mm, when working with the unit. An in depth cleaning of the CH2 position and trim potentiometers resulted in a more stable operation of the knob, but did not resolve the offset problem. Are there some known usual culprits for the D75 that could be related to this problem?

The triggering seems to mostly work. If a periodic 1kHz sine or rectangle signal is applied, the scope triggers and shows no drift. Unfortunately I get unexpected behavior when moving the knob for trigger level A. As far as I remember, putting the scope in AC or DC trigger mode should result in a smooth change of the trigger start position of a sine wave, as long as I am in a valid triggering interval. Also, the lamp of the trigger should illuminate when a valid trigger point has been found and be off otherwise. On my D75, rotating the trigger level knob does not consistently change the trigger level. The triggering stays mostly at the same position and erratically jumps somewhat up and down, but only while the knob is being moved. This movement has a clear correlation with the rotation direction of the knob but is very jumpy. As expected, leaving the valid triggering interval in any direction causes the triggering to be lost. Regardless of the triggering being successful or not, the trigger status lamp is always on. My first try to solve the problem was to try cleaning the trigger level potentiometer. However, I failed to fully open its case which is hold together by four screws but has a kind of sealed inner part.

For the knobs, the time base section is not only missing the plastic knobs, but also the metal rings, which secure the buttons on the shafts. I think, they are called shaft collars in English (picture attached). However, I was not able to find any information on where to get the utilized variety with a serrated exterior side and the specific dimensions required to fit the shafts. Especially the lower one for the Time/Div-knob seems to be impossible to find, since it needs two grub screws in an 120° angle. In addition to that, machine parts that use imperial units are very hard to find in Germany. Is a special name for this type of component that I just do not know but which is required to find spares on the net? Unlike the missing shaft collars, replacements for the plastic parts of the knobs are no problem since I can simply reverse engineer the knob geometry in digital 3D and create new knobs with a 3D printer. This even works for the transparent delay time knob. Nonetheless, original spare knobs would be very nice and also solve the problem of the missing shaft collars.

Re: Telequipment D75 scope.
 

I think I may have a square volts/div knob and am pretty sure someone was offering D75 spares a while back but can't find the thread. I'll try and dig it out.

Andy.

Re: Telequipment D75 scope.
 

Hello Andy, thank you for taking the time to read my post and for the kind reply. Your offer of an original D75 Volts/Div square knob sounds very interesting and could solve at least 1.5 of my current part acquisition issues. I will also try to find the user, you mentioned, which offered D75 spares in earlier posts. My previous attempts to get some spare parts like knobs and screws online always resulted in accumulated costs that would be more than sufficient to buy a defective scope on ebay for parts, which would unfortunately waste a lot of parts that could be useful elsewhere. If said user maybe has a non-repairable D75, I could try to buy several parts from him. Especially since I discovered some severe cracks on other knobs in the meantime.

That aside my reverse engineering attempts for the plastic parts of the Time/Div, Volts/Div and horizontal position knobs went really well, especially the transparent Time/Div knob. However the silver-gray color of my printing material does not really fit the original color of the other knobs. I will try to take some pictures and post them here. If anybody wants to try printing their own knobs I can also post the 3D model files on request.

Greetings
Denis

Re: Telequipment D75 scope.
 

Well done on the work you have done so far on saving another Telequipment scope from the scrap heap.

It was me who was offering D75/D83 parts for sale. The only knob I have left is the smaller A timebase knob. The cost is £2.00+p&p. I have no idea what the postage cost will be but I can find out for you. If you buy it I will include the remains of a B timebase knob as seen here. https://www.vintage-radio.net/forum/...d.php?t=135544 Perhaps you can fabricate something from it.

The collar in your second picture is actually an integral part of a Telequipment knob. The plastic parts have broken off it.

Al

Re: Telequipment D75 scope.
 

Hello Al, thanks for your reply. No way I can let something like this just sit in a dumpster, waiting to die. ;)

An original D75 A-timebase knob sounds perfect, I'm in! Until now, I used that shaft collar core in the second picture, which is the remains of the CH2 Volts/Div knob to occasionally operate the A Time/Div which happens to have the same shaft diameter. Having that knob would be extremely helpful. Also a shaft collar core of the B-timebase knob (if it is the one you showed here) would enable me to build a near perfect replica of that knob using an SLA-Printer and transparent resin. Maybe you can determine postage costs and drop me a message. I could pay via PayPal in advance.

Denis

Re: Telequipment D75 scope.
 

Found the knobs I have, see pics. Some may or may not be Telequipment. Drop us a PM if any are of any use to you.

Andy.

Re: Telequipment D75 scope.
 

Top row knobs 1 & 2 look like V/div knobs. TQ used 2 identical looking but different diameter knobs on some of their modules. I would be worth double checking the diameter before sending them off.

The V/div knobs on a V4 module are 22mm dia.

The other size knob is 28mm

Al

Re: Telequipment D75 scope.
 

Hello Andy,
thank you for digging the knobs out. Unfortunately, I can tell from the pictures that none of the knobs is from a D75. The square ones with four indices are a lot bigger than the ones required for the Volts/Div on the D75 and also have slightly different proportions.

I attached a render of my attempt to reconstruct the Volts/Div knob in 3D and a photo showing three printed knobs that I plan to use as substitutes until I get my hands on original knobs. The geometry is very accurate. However, the 3D printing resolution is of course no match for injection molding and I need to find a better matching PLA color.

Denis

Re: Telequipment D75 scope.
 

I reckon six of Andy's knobs are "modern" TQ, most of which are for 63,75 and 83 series. Most of the rest are "old type" TQ, with a couple of strugs thrown in for good measure.
Les.

Re: Telequipment D75 scope.
 

Oh well, that's a shame, your copy looks well though.

Thanks Les.

Andy

Re: Telequipment D75 scope.
 

Today I had some time to clean the front panels and do a test fitting of my temporary replacement knobs for CH2 Volts/Div, Horizontal Position, Time/Div A and Time/Div B, as can be seen

here

(sorry for the link, but the forum does not accept higher resolution images for upload). The first two knobs already work but the Time/Div knobs are still non functional due to the missing shaft collars. The Time/Div B knob is still a prototype, which I did only for experimenting with different ways of material post-processing to get the desired transparent look. I'm still unsure whether I will try to reproduce the original knob as closely as possible or go with the evenly toothed exterior shown here, which has better grip.

The glossy stuff on the V4 amplifier is a piece of OHP transparent film on which I printed a reconstruction of the front panel labels. This is a test for checking the accuracy of a label reconstruction I created because many parts of the original labels are damaged or missing. I am planning to print the final labels on a waterslide decal paper that can be used to transfer highly detailed labels onto a surface by placing a thin foil with the help of water.

Unfortunately, a closer inspection of the knobs on the V4 revealed that most of them have cracked. At the moment, I am still unsure if this can be fixed. Trying to glue the cracks back together would require the internal shaft collars to be removed in beforehand to get rid of the mechanical stress. However the knobs will surely break again upon re-insertion of the shaft collars.

Re: Telequipment D75 scope.
 

Thanks to Alistair D, I got some Time/Div knob components today. :) The special shaft collar from the resin remains of the Time/Div B knob seems to perfectly fit my prototype knob. When I tried to put the Time/Div A knob on the respective shaft I noticed that the internal shaft collar is not firmly connected to the plastic exterior. This causes the knob to wiggle. Upon decomposing the knob into its three parts (shaft collar, cap and shell) I discovered that the issue had its origin in what looks like kind of a construction error. The shaft collar, which oddly also has a 120° angle between the set screws instead of 90°, does not have a linearly grooved exterior side but a diamond pattern instead (see image). This does not allow the shaft collar to be pressed into the plastic shell without damaging it, finally resulting in a wiggly connection. At the moment I am trying to find a solution for this, without having to glue the shaft collar into the plastic shell.

With the new knob components I noticed that the shafts of the Time/Div knobs require a very high amount of torque to be rotated and the position snapping produces very hard clicks. In contrast, the Volts/Div knobs can be rotated easily. Does anybody know if this is supposed to to be like this on a D75? I suspect that the internal mechanism in which a spring-loaded brass roller rotates around a gear wheel is either set too hard or the gear wheel geometry is worn out. Unfortunately, I couldn't find a way to reach this mechanism, as the two outer circuit boards in the S2A unit do not seem to be removable without desoldering tons of wires. Does anybody know a way to get to the Time/Div switching mechanisms without desoldering half of the unit?

Re: Telequipment D75 scope.
 

Sorry about that Denis. I did not know that the knob was loose on it's collar. I am more than happy to give a you a full refund on the cost. Perhaps a little epoxy adhesive will sort the problem.

I have owned 3 scopes that used the S2A module. On every one of them the timebase knob is much stiffer to operate than the V/div control. I think that the extra stiffness is due to the timebase switch having more wafers.

Al

Re: Telequipment D75 scope.
 

No need for a refund, everything is fine. The parts are as perfect as can be expected from used 1975 items. And the really important parts, the special purpose shaft collars are intact. Nothing there that can not be fixed. :)

Thanks for this information. I do not think the extra wafers are causing this, but I will try to clean and lubricate them once I find a way to safely access this part of the switch mechanism.

Re: Telequipment D75 scope.
 

Hi, you can access the switch roller mechanisms if you remove the top and bottom centre aluminium panels which run from front to back, the only draw back is you have to take front panel of to access screws that hold panels in place, (same as back panel) and you will need to set time ten control up afterwards.

Regards Mike.

Re: Telequipment D75 scope.
 

Ahh, that was also my first guess but I failed to find a secure way to remove the Delay/Time Mult knob for getting the front panel off. Removing the knobs front part was simple but the back part with the printed on coarse scale would not come off. I removed the larger copper ring(see image) which did not result in any progress in removing the knob. Maybe the inner most silver ring with the two 180° notches needs to be rotated? Are there any known traps in setting up the time ten control again? I would just set the knob to 5, remove the front part and not rotate the shaft afterwards. Then I would screw on the front part in the position that is determined by the set screw marks on the shaft.

Re: Telequipment D75 scope.
 

I have looked at a functionally similar 'Duodial' turns counter in my spares box and on that one, the stationary part of the counter mechanism is just pegged to the panel by a stand-off washer with a broad peg on the counter side and a smaller round peg on the panel side. The rotating knob is fixed to the mechanism by a circlip.

If you have removed the knob, then it seems to me that there is no need to remove the counter body to remove the panel, as the connection is merely via the ten turn pot shaft, which you have disconnected.

I'll post some photos of the Duodial turns counter underside shortly.

Ron

Re: Telequipment D75 scope.
 

The inner collar does unscrew. I normally use a small pair of circlip pliers with right angled tips to loosen them.

No need to worry about damaging anything as I have an almost complete S2A that you can have any parts from.

Al

Re: Telequipment D75 scope.
 

You have to undo slotted recessed nut i use a pair of pointed nose pliers and two hands one on handles of pliers and the other on the jaws.
As for recalibrating try not to turn the thin shaft, and when putting back together again align knob screw with indentation on shaft , there are calibration pots which allow you to fine fine tune the delay time multiplier control in alignment and set up instructions in s2a manual .

Mike

Re: Telequipment D75 scope.
 

Thanks to your hints, I managed to remove the Delay/Time Mult knob. Now, there is something stuck near the labeled plastic cylinder of the Time/Div knob that is preventing me from taking the front panel off. Seems like I have to figure this out another time.

Today, I thoroughly cleaned the vertical position double potis including the trimmers. Most of their hick ups are gone now, however, the trimmers are still very fiddly.

Upon pressing the shaft collar into the modified Time/Div knob I got from Alistair D, I noticed a small crack on it that started to widen. This was easily fixed with some epoxy and two cable ties pressing the knob together.

For calibration of the scope I acquired two Cinch 81 731 AA 16 edge connectors (see images). These are just like the build in Cinch 81 774 AN 16 module connectors, except they have gold plated contacts on both sides and do not have the reverse polarity protection gap. I am planning to use these and some custom cut PCBs to build extension cables for both modules for live measurements and calibration.

This week I was allowed to play with an equally old Hameg 412-5 scope. I was amazed how responsive and stable it was compared to my D75. No trace drifting over time, no 5x trace offset, a consistently and stable working trigger control, virtually no warmup time and absolutely no trace hick ups on using any switches. At the moment I can't judge how a D75 should behave in perfect condition. Do you have any experiences on that? This would make it much easier for me to identify aspects that should be fixed by, for example, locating and replacing bad and heat sensitive components.

Re: Telequipment D75 scope.
 

Thanks @Pinörkel, my D75 continues to work well.
Sorry for the slow reply, far too much going on at the moment!
Thank to the great generosity of @Mr Bungle, I now have some spare parts for these 'scopes. Attached should be a picture of the spare knobs. If they are of use to you, please let me know and I'll post them to you for the cost of the postage. Let me know if you want a more detailed picture.
Regarding the performance of a D75, the trace on mine doesn't drift noticeably over time although the trigger may need adjusting occasionally. I haven't tried the 5x magnification, I'll try it later.
John

Re: Telequipment D75 scope.
 

Hello John,
thanks for the offer. These look very tempting. If they are not cracked I am definitely interested in the two square ones for Volts/Div and the three round knobs with the 1/8" holes in the top for vertical positions 1/2 and horizontal position. These are the ones that are currently missing on my scope or already have cracks that will go fatal in the near future. The former users of my D75 did not handle that unit well. I did not expect an opportunity for getting original knobs that soon, however, some detailed pictures via PN would be nice.

Greetings
Denis

Re: Telequipment D75 scope.
 

Have you removed the two chrome slotted nuts behind the variable A & B T/B knobs? again use pointed noise pliers to remove nuts.

Mike

Re: Telequipment D75 scope.
 

Re your drift problem. Read the DC balance mod paragraph from this site https://www.markhennessy.co.uk/dm63/scope2.htm

I can provide photographs of factory modified V4s if it would help.

Al

Re: Telequipment D75 scope.
 

Jep, I removed those, but with no effect. On the occasion, I also disassembled the 7.5K Allen Bradley Mod Pot for trigger A control, fed a tinzy winzy drop of non aggressive contact spray in there(center part can not be disassembled non-destructively) and fixed one of its solder joints. Now the trigger finally behaves much more like I would expect it. However, the pot value is more like 8.5K which is why the trigger center point is not at the half rotated position of the pot.

This article looks interesting. I will have a thorough look at it. I thought about measuring the power supply quality before, but at the moment I do not have access to a suitable second oscilloscope and suitable accessories for that. I assume the noise should be measured with a differential probe setup.

Since the time drift of the vertical position is way more pronounced on channel 2 than on channel 1, I assume, that there is a heat sensitive component in the CH2 vertical amplifier. This particularly affects the 5x magnified mode, where the trace is initially off by about 1.5 full divisions. After 10 minutes of warming up, the non magnified traces are mostly within three ticks of the center position. If I recenter the traces then, they stay comparatively stable. For channel 2, this readjustment is required every time the scope is switched on, channel 1 often does not need readjustment after the 10 minute warmup.

Re: Telequipment D75 scope.
 

Today I applied the DC balance mod like apparently applied by Telequipment in later units (see image). The result were slightly less fiddly vertical position controls, especially for the 5x calibration. However, the drift problem of CH2 was not eliminated. Hopefully, my noobtastic self-taught electronic skills will suffice for localizing the temperature sensitive component that causes it. ;) CH1 is pretty good now.

As an additional 3D printing mod, I designed some clip-on covers for the scope handlebar. These seem to get lost or to get broken on most D75s I have seen on the net. Possibly because fastening the screws beneath requires removing the covers which then tend to break. I did not follow the original design exactly, but omitted the raised ring on the outer side of the covers because it simplifies 3D printing a lot. The covers require quite some force to press them on, but then they fit perfectly. If anybody is interested in the 3D model, do not hesitate to drop me a note.

Re: Telequipment D75 scope.
 

Great work once again with the 3D printing.
On the advice I got on this forum, I bought a can of good quality freezer spray and it certainly helped me locate the temperature sensitive components. I think it may help you here if you have not tried it yet?

Re: Telequipment D75 scope.
 

@Anode_to_Joy: I have planned to get some ice spray once I created my custom extension adapters to be able to operate the modules outside the case and do some live measurements.

Today I decided to try creating a Time/Div knob true to the original design. I am unsure if I captured the top rim correctly, since I found no good sideways images of this part on the net. Due to technical limitations of the SLA printing process, the knob is printed in two parts (see first image) and assembled after time-consuming post-processing of the individual prints. This includes a lot of sanding and still does not produce perfectly clear parts. Also, I am having difficulties with the print quality since the used printer is old and technically nearly dead. Therefore, only images of imperfect prototype parts at the moment. In the second image, I tried to clear coat an assembled knob. Although the knob has some unpleasant print defects and my spraying capabilities are quite lousy, the knob turned out not too bad. Since I noticed some issues with my production procedure, I will create a better quality knob later.
Fun fact: Since the knob is printed using uv-active clear resin, the resulting knob is uv-light sensitive and will become just as brittle (aka shitty :-) ) as the original one over time. Maybe the clearcoat can delay this process.

Re: Telequipment D75 scope.
 

Over the last few evenings I completed the Time/Div knob. Since I found some images on the net, which showed that my previous design did not correspond to the original form, I reworked the design. With the help of a custom made toroidal sanding tool I managed to realize a single part design that is very close to the original. The knob's transparency was again achieved by spraying the knob with a clear coat. I can post the 3D model and additional processing information on request.

Apart from the button, I cleaned and lubricated most switches and potentiometers. However, I have to do something about the stiffness of the two Time/Div controls. Maybe I can loosen the spring which presses the brass cylinder onto the gear wheel which is responsible for the snap movement of the knobs. Otherwise it is very likely that the buttons will be destroyed again by the mechanical stress.

I also started with a little electrical debugging. My plan is to fix all electrical issues before trying to calibrate the scope according to the manual. The first problem I want to tackle is an initial drift in the vertical position of channel 2. At the moment, when I switch on the scope, the zero position of the trace is off by 3 ticks and off by 1.5 divisions in 5x mode. After 10 minutes the trace has slowly moved to the zero position I set it to and matches with the trace in 5x mode. The same effect can be observed for channel 1, but much less pronounced. There, the 5x trace is only off by 1 tick at the beginning. In addition to that, the channel 1 trace settles at the right position after approximately 1 minute, which seems OK to me. A similar small drift can be observed for the horizontal position.

Seeing that channel 1 behaves comparatively well, I am suspecting a thermally sensitive of off-valued component near the CH2 DC balance potentiometer. Initial measurements of the resistors in that region did not reveal an obvious culprit. However, seeing that all three potentiometers act as a voltage divider between the +24V and the -24V rail of the scope, I had a look on the voltage stability of the power supply. I adjusted the voltages after a 30 minute warm up time of the scope according to the manual and then measured the voltages after a cold start of the scope with a DMM.

For the +24V rail i get:
24.052V at startup
24.006V after 5 minutes
24.000V after 10 minutes

For the -24V rail i get:
-24.001V at startup
-23.986V after 5 minutes
-23.981V after 10 minutes

The maximum output variation measured at the output of the vertical DC balance potentiometers was no more than 0.04V.

Does anyone know if the observed voltage drifts of the power supply are within the specifications? The manual does not mention acceptable tolerances for the power supply rails.

Re: Telequipment D75 scope.
 

Nice work on that knob, brilliant.

Both rails look fine. A recap in the affected area might help but it could be leaky trannies. As you say it could be a thermal issue, try some freezer spray.

Andy.

Re: Telequipment D75 scope.
 

Thank you for the assessment, Andy. Regarding the freezer spray, I am still unsure how to use this correctly without damaging the components. Upon freezing I suspect some water to be drawn from the air humidity. Couldn't this cause a short? Also, couldn't the quick temperature change cause cracks in resistors, solder joints or similar components?

I am currently fiddling together a module extension cable to be able to do some live measurements. Still unsure how long I should make the cable to prevent screwing up the connected signals. Maybe I will use beefy 50 cm long cables from an ATX power supply. To be able to use the extension cable for both, vertical and horizontal plugins, I will create a plug without a notch. Since the notch is there to prevent wrong insertion of plugins and is realized by a small plastic piece in the edge connector (as can be seen in my post here), I can simply temporarily remove the plastic piece from the scopes connector and use the cable for both plugin types. Of course then I have to pay attention to use the right cable orientation myself.

Denis

Re: Telequipment D75 scope.
 

Freezer spray. You suspect a transistor of intermittency, spray it. Instant change is likely diagnosis. Beware small changes which are thermal drift which will go back to normal as it warms back up. You can check a suspect voltage, see if immediate big change. You can spray diodes, but normally we only spray transistors.
I think I made extender cables for D75/83/63 for both V and T/B. I definitely made an extender for the vertical based on a pair of plug/sockets and a length of strip board. Don't remember the pitch. Just long enough to give good access to all presets during V plug in calibration.
Les.

Re: Telequipment D75 scope.
 

Thank you for your answer, Les. If only transistors are sprayed for diagnosis, I maybe do not need to use freezer spray at all. Nearly all transistors on the D75 are socketet. So in the vertical amplifier, I could simply swap transistors between channels and observe the change.

As for the cable I took a similar approach and created a plug from a piece of copper layered circuit board by custom cutting the strips. The pitch is 0.15" with 0.1" wide contacts and 0.5" gaps. However, the corresponding cinch edge connectors are listed on the net to have a 0.156" pitch, which is definitely wrong. Dimensioning the length of the cable just to reveal the trimmer holes on the module side is not sufficient in my case, since I also want to take live measurements in the unit with removed aluminum side covers. Maybe I just have to try if a length of 40 or 50 cm works.

Denis

Re: Telequipment D75 scope.
 

Meanwhile I completed my extension cable (see images). It works like a charm and lets me debug the v4 amplifier lying on it's side outside the case. I did some investigations and identified at least two issues, which seem to be part of the vertical drift problem of channel 2. Seeing the error type, I specifically searched for an off-valued component and a drifting voltage. My measurements are shown in the small circuit diagram cutout of that area. Sorry for the rotated image, but the forum would make the image unreadable by scaling it down if uploaded upright. Please download the image and rotate it. I marked seemingly good values in green, suspicious ones in orange, and off-valued stuff in red. A photo of the respective area on the board can be seen in my previous post.

• The voltage behind the TR603 dual FET MD2369B in CH2 is off. It is supposed to be 0.8V but is at 1.11V for both FET sides. Resistors R706, R615 and R616 seem to be within tolerance(or at least match the values in CH1 very close) and the voltage of 22.33V between them is spot on like in CH1. I have not found the cause of this voltage deviation yet.
• Swapping TR605 with TR607 does not swap the trace offset for channel 1 and 2, so I suspect those to be OK.
• The voltage behind the D604 diode (10V Zener) continuously drifts in the first 5 to 10 minutes after startup from 13.5V to 13.3V. The 24V reference on the other side of the diode only changes marginally. Blowing the area around the diode leads to the voltage rising again and the trace drifting in the opposite direction. This also impacts the voltage on the left hand side of R619 and has a bigger effect on CH2, since R618 is kind of off-valued as opposed to R617. This one is crying for a spritz of freezer spray.
• Since I want to avoid desoldering components, I did not try swapping the dual FETs TR603 with TR601. One leg is soldered to a 100 ohm resistor bypassing the socket. Since the voltage is off symmetrically for both individual FETs in TR603, I think its unlikely to be defective.

Additional thoughts on my measurements and conclusions are very welcome as I am not very experienced in debugging electronics in the way a real electronics engineer would do it.

Denis

Re: Telequipment D75 scope.
 

Since it required no backplate soldering, I removed the dual FETs TR601 and TR603 and swapped them, which also swapped the off-voltage from channel 2 to channel 1. Now the 0.8V measuring point on channel 2 was spot on and the one in channel 1 at approximately 1.22V. So, seemingly TR603 is defective, which unfortunately happens to be a difficult to source Tektronix custom dual FET 151-1036-00 and not a Motorola MD2369b like I thought first. The removed FETs also allowed some better measurements of R617(222 ohm) and R618(215 ohm) which seem to be within spec. The now measurable R615 and R616 with 4k72 ohms and 4K82 ohms are also close enough to the intended values of 4K7 ohms. Does anybody know a reasonably priced source for those 151-1036-00 FETs? The only source I found called a price high enough to get a whole D75 scope on ebay.

I also had a look at the drifting zener diode. According to literature, some temperature drift seems to be normal, especially for diodes deviating significantly from 6.2V. Maybe the diode is not defective but only exhibits a normal temperature drift? However, if it is supposed to be a 10V zener, it drops about 0.7V too much voltage when warmed up and drifts about 0.2V. Some docs about the temperature drift of zener diodes suggest it can be expected to be in the order of magnitude of 10^-4 for a 10V zener. Could it be that the effect is much larger on vintage zener diodes?

Re: Telequipment D75 scope.
 

I have some 151-1036-00 FETs. PM me your address and I’ll send you a couple of them FOC. They were from a D83. Might be a week or so before I can send though due to a family emergency.

Edit: my other half got stuck on phone before we had to bail out of the door so I’ve extracted them ....

Re: Telequipment D75 scope.
 

Hello, thank you very much for your offer. Your helpfulness is just great. I'll contact you via PM tomorrow. And no need to hurry, take your time. I have several other repair issues to take care of in the meantime. For example, one of the vertical position double potentiometers is still very jumpy although I disassembled it and cleaned it thoroughly(maybe end of life). And the trigger A control does not behave as it should although I can get stable triggering(maybe some off-valued components, maybe just needs recalibration).

Best wishes for a good outcome to your family emergency.

Denis

Re: Telequipment D75 scope.
 

Today, I had some time to test the FETs which were generously send to me by MrBungle. The first one lead to a voltage of 0.63V at the nearby probing points which are supposed to have 0.8V. This totally threw off the vertical position control to a point that could be barely compensated by turning the DC-balance to its limit. However, the second FET was a nearly perfect match to my existing one in channel 1 and lead to 0.78V at the probing point. The vertical position control and its DC-balance of channel 2 are now centered much better. The drifting problem, on the other hand, was barely affected by the change. After trying to do a basic calibration of DC-offsets according to the manual, I realized that I probably need to tackle the drift problem first, or the unit can not be calibrated properly. Every time one setting was adjusted properly, It was off again after some time or after switching channels. In addition to that, the calibration trimmers for the variable gain DC-balances (R703, R645) are kind of mushy.

Regarding the drifting voltage behind the D604 10V zener diode: Has anyone of you ever measured the usual voltage drift at this point on a working V4 amplifier? I am quite unsure what to do about this issue, especially because I do not know what behavior should be expected. I could try to swap the zener diode, which is very risky, since I would have to dismantle most of the unit to get to the backside of the board. Some sources suggest to replace a zener with an combination of a zener and a normal diode to get rid of potential temperature drifts. Anyhow, the documents I found about temperature drifts on zener diodes suggest that the range should be much lower than what I observed here.

Re: Telequipment D75 scope.
 

I will try and set up my DM63 and make the measurement you require tomorrow.

Al

Re: Telequipment D75 scope.
 

Thank you Al, that sounds perfect. While you are at it, do you think you could maybe also have a short peek at some of the other measuring points in the mid section of PC152? I measured the ones on mine and had the impression that all values are a little bit too low. Here are my values, measured from top to bottom in the respective circuit diagram.
0.8V -> (0.77, 0.77, 0.77, 0.77)
6V -> (5.87, 5.85, 5.97, 5.97)
10.5V -> (10.17, 10.15, 10.36, 10.25)
12V -> (11.81, 11.79, 11.89, 11.87)
11.3V -> (11.10, 11.10, 11.17, 11.18)

Denis

Re: Telequipment D75 scope.
 

I have 3xV4s here. These are my results for the D604 readings.

V4/1 V4/2 V4/3
0 mins 10.473 10.429 10.430
5 mins 10.657 10.581 10.589
10 mins 10.678 10.600 10.612

On all 3 units the voltage rose quite rapidly during the first 30 seconds or so.

Regarding the other measurements you want, can you give me the exact circuit references just to be sure I am measuring the exact point you require. I assume that these readings are to be taken after 10 minutes.

Al

Re: Telequipment D75 scope.
 

With regards to the 10V zener, it is expected to have a positive tempco, meaning the voltage across the diode will increase with heating. That could be expected to slightly alter the balance but most of the drift on that power rail should be largely ignored , but it could be worth trying a new one and as suggested adding a series diode to help cancel some of that drift.

It looks to me though that the designers were struggling with temperature drifts which is why they added the thermistors in x5 mode. Also as I recall those dual fets do run warm. Looking at the board layout, there is a different physical spacing between the thermistors and the dual fets for the two channels. Try bending the CH2 thermistor (TH602) away from the body of the fet(TR607) so the the physical spacing of it from the fet is more like the channel 1 situation.

Re: Telequipment D75 scope.
 

Hello Al,
thank you for the measurements. As I can see from your measurement values, you measured the voltage across the diode instead of the voltage to ground. I just did the same on my unit and got the following values:
10.32V 0 min
10.57V 1 min
10.63V 3 min
10.646V 5 min
10.648V 10 min
This looks a lot like the diodes in your V4s, so I think, its normal.

Regarding the additional measurements, I will prepare a marked circuit diagram and a PC152 photo for you later, if I run out of other debugging options. I do not want to steal more of your valuable time than necessary. ;)

Hello Argus25,
thank you for your expertise regarding the 10V zener. I did not expect the drift at this point to be kind of unimportant. Your analysis of the role of the thermistors sounds very interesting. I just did what you suggested with TH602 and the result was a much more consistent convergence of the CH2 drift. It now settles after about 5 minutes. However, it still has a kind of inconsistent convergence point which I attribute to the highly wiggle sensitive CH2 vertical position potentiometer. CH1 is perfectly usable after only 1 minute of warm-up and does not have any wiggle problems at the corresponding potentiometer.

Since the initial deviations from previously optimized vertical positions on cold startup of CH2 are 5 times more off than for CH1, I still have to find the component causing this. I have also the impression, that CH2 reacts more sensitive to the potentiometer than CH1. Maybe I should also try a second cleanup of the double vertical position potentiometer of CH2, although I already did that very thoroughly on both double potentiometers.

Denis

Re: Telequipment D75 scope.
 

The voltages I measured were across D604 as you said. When I am next working on the scope I will do one set ground referenced.

No need to mark up a diagram, a list of component references will be fine.

Al

Re: Telequipment D75 scope.
 

Just a heads up here and I hope this doesn't violate the ebay policy on here but there's a load of telequipment plugins appeared on ebay here: https://www.ebay.co.uk/itm/362703426707

Re: Telequipment D75 scope.
 

I did some additional measurements to further pin down the location of the fault, but did not succeed. From the fact that channel 1 works comparatively well, I conclude that the voltage drift at D604 can only be a small part of the problem. A second thorough cleaning of the corresponding vertical position double potentiometer did not solve the problem. At least It killed some of the pots wiggle sensitivity. Maybe I will get an optical interface adapter for my DMM to log the voltage drifts on my PC.

Thank you very much for the hint. I'll keep an eye on that. Some of these seem to have a fair bit of cosmetic damage and all serial number plates have been removed. So maybe they could be good for spare parts. They seem to only ship to UK though.

Apart from that, I could not stop myself from acquiring a D755 Opt 66 with a V4 and an S2C amplifier which will arrive this week (X-Y mode. Yay!). Unfortunately the units case was painted deep blue by the former owners and modified with a custom power cord (drilled a hole in the case for that). That makes it unsuitable for a restauration. The modules seem to be sort of OK, but the mainframe has some serious corrosion issues due to moisture. As I could see on some supplied images, nearly all the trim pots on PC147 and the PCBs around the tube have to be replaced. Hopefully the unit does not get damaged during transport. IMHO the most interesting aspect of this unit is, that the modules use much newer electric and mechanical parts than the ones on my D75. The push switches and black plastic button rods resemble those, I once saw in a Tek 465b. Most electronic components like resistors are much newer models than in my D75. However I will first concentrate to fix my D75. Maybe the modules from the D755 can help with that.

Re: Telequipment D75 scope.
 

I tried to do some comparative measurements on my newly acquired partially defective D755 to further pin down the drifting problem in the vertical amplifier of my D75. To my great surprise, the D755 had a different dual FET type installed at positions TR601 and TR603, which reads as TFD1551 instead of Tek 151-1036-00. Even more surprised, I found the voltages behind the FETs being at 0.42V for channel 1 and 0.27V for channel 2, which are supposed to be 0.8V. Nonetheless, both channels on this V4 amplifier do not show any drift like the one in my D75. The vertical positions on the V4 of the D755 are spot on right from the start, as well as the DC-balance, and do not need any recalibration after powering the unit on. In addition to that, the D604 zener has a similar, but somewhat faster voltage drift like the one in my D75 that does not cause the traces to drift vertically. Maybe I searched for the error at the wrong place. In addition to that, the voltages at the test points shown in the circuit diagram of the vertical amplifier are roughly as inaccurate in the D755 as in the D75. So they also do not seem to be the cause of the drifting issue. A confusing but challenging issue.

That aside, I tried to analyze the D75 power supply by self probing the +24V and -24V power rails with the scope. Both rails exhibit a small amount of ripple that can be seen at 5 V/div. In addition to that, the +24V rail has kind of a superimposed sawtooth wave at double the line frequency. Since I could not observe this sawtooth wave on the D755, this is probably something I should fix in the power supply. The first image shows the ripple on the +24V (top) and the -24V (bottom) rails. The second image shows the sawtooth ripple of the +24V rail with the overlay of an inductively coupled sine wave from the scopes power cord. Looks like a capacitor induced rectifier issue to me, maybe C405.

Re: Telequipment D75 scope.
 

Just noticed a small but important typo in my last post: The ripple that can not be seen at 5 V/div but only at 5 mV/div. Would be catastrophic otherwise.

Re: Telequipment D75 scope.
 

Today, I had some time to work on the D75 again. Regarding the sawtooth on the -24V rail, I removed and checked the two 2200μF Erie electrolytic capacitors in the power supply, but the capacitance of C405 and C406 looks OK, being at 2600μF and 2800μF. Since the ripple does not have a noticeable effect on the scope's operation, I left it that way for now.

Regarding the trace drifting issue, I tried to identify temperature sensitive components in the suspected area of the vertical amplifier. So far, I noticed no hot and overheating components, which means that the drifting issues probably happen in a temperature range below 60 degree Celsius. This made freezer spray seem a little overpowered. Thus, I build a small device from a straw and tape that allowed me to selectively cool down components by blowing into the straw without affecting other nearby components. The results were pretty interesting. I discovered that the carbon film resistors R615 and R616 in the channel 2 circuitry(see attached image) and the respective R612 and R613 in the channel 1 circuitry are extremely temperature sensitive. By blowing onto R616, you can effectively move the trace in 5x mode down by a full division in a matter of seconds. Blowing onto R615 moves the trace in the opposite direction. Even touching the legs of the resistors with a room temperature DMM probe moves the trace in 1x and 5x mode. After that, the trace slowly returns to its former position. IMHO my issues with channel 2 could be easily explained if R615 and R616 have a different temperature coefficient. I am somewhat puzzled that Telequipment used highly temperature sensitive components at a seemingly critical location like this. Assuming that those resistors behave like common 5% tolerance carbon film resistors, their temperature coefficient should be around 350 to 500 ppm/K (according to some resistor datasheets). Now, to tackle the issue, I could try to replace these resistors on both channels with 0.1% tolerance metal film resistors with 25 ppm/K temperature coefficients. In addition to that, I could try to temperature compensate the D604 zener diode (would have to read some lectures on that first). However, having to do that would be a pity since the components would no longer look original. Besides, I am still a little bit unsure if my above analysis sounds reasonable.

On a side note, the respective resistors on the V4 amplifier in my defective D755 behave in the same way when cooled. However, in the D755 there is no observable vertical trace drift. In addition the V4 amplifier of the D75 shows the temperature drift even when put in the D755. This should rule out the power supply being the source of the drift.

Re: Telequipment D75 scope.
 

I was thinking about how isolate which section of the amplifier is drifting. The idea I have come up with is to use two meters. Connect one to the 2 gates of TR601, connect the other meter likewise to TR603. Monitor the voltages as the scope warms up. Both meters should drift by the same amount. Repeat this process with TR605/607 etc until you find where the two meters disagree. Hope it helps.

Al