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Old 16th Jun 2019, 12:20 pm   #21
Anode_to_Joy
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Default 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
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Old 16th Jun 2019, 6:15 pm   #22
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Default Re: Telequipment D75 scope.

Quote:
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.
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

Last edited by Pinörkel; 16th Jun 2019 at 6:24 pm.
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Old 17th Jun 2019, 12:12 pm   #23
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Default 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
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Old 17th Jun 2019, 12:57 pm   #24
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Default 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
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Old 18th Jun 2019, 12:23 am   #25
Pinörkel
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by micheal View Post
Have you removed the two chrome slotted nuts behind the variable A & B T/B knobs? again use pointed noise pliers to remove nuts.
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.

Quote:
Originally Posted by Alistair D View Post
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.
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.
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Old 23rd Jun 2019, 7:41 pm   #26
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Default 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.
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Old 29th Jun 2019, 10:26 am   #27
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Default 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?
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Old 30th Jun 2019, 6:39 pm   #28
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Default 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.
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Old 13th Jul 2019, 9:12 pm   #29
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Default 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.
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Old 14th Jul 2019, 8:50 am   #30
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Default 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.
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Old 16th Jul 2019, 12:43 am   #31
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Default 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
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Old 16th Jul 2019, 10:01 pm   #32
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Default 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.
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Old 17th Jul 2019, 9:43 am   #33
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Default 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
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Old 19th Jul 2019, 12:19 am   #34
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Default 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
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Old 21st Jul 2019, 3:15 pm   #35
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Default 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?
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Old 21st Jul 2019, 4:38 pm   #36
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Default 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 ....
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Old 22nd Jul 2019, 12:09 am   #37
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Default 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
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Old 25th Jul 2019, 7:52 pm   #38
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Default 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.
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Old 25th Jul 2019, 8:25 pm   #39
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Default Re: Telequipment D75 scope.

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

Al
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Old 26th Jul 2019, 9:46 am   #40
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Default 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

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