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Old 23rd Dec 2019, 11:08 am   #101
MrBungle
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Default Re: Telequipment D75 scope.

That sounds like telequipment for sure on the cost optimisation.

I used to love the challenge. Alas no more. I’ve done 43 scopes now, mostly Tek. I’m done. I seem to be collecting frequency counters now instead though
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Old 23rd Dec 2019, 11:25 am   #102
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Default Re: Telequipment D75 scope.

I'm 100% confident that my examples came from the factory with no covers.

Mine came from work. Knowing the people who have worked here over the years, there's absolutely no way the covers of both fixtures would have been removed and lost.

For what it's worth, the serial number of my vertical unit is E543428 and the date codes I can see on the transistors are 1974 for both 2N2369s and 1977 for the TFD1551. Of course, I've no way of knowing if the JFET is original or a replacement. I can look up the details for the X unit when I next go up in the attic. But having said that, I doubt we'll be able to gather any meaningful data about how these units evolved, given the small numbers out there today.
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Old 23rd Dec 2019, 11:44 am   #103
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Pinörkel View Post
Quote:
Originally Posted by mhennessy View Post
Your pictures on the Tekwiki are excellent. I'm comparing your vertical unit to my one, as it is right in front of me now, and they look very similar indeed. I suspect that R1015 might be a replacement, given that it's a different type to R1012. Otherwise, all looks good to me.
Thanks for the compliment. I tried to make the images as good as possible. Yes, R1015 might be a replacement one. However, to me, TQ seems to have had a habit to not always use the same components, but the stuff the currently had a supply of. You can see this when you compare my images of circuit boards of the S2A and S2C plugins (http://w140.com/tekwiki/wiki/Telequipment_S2).
You might be right - does the soldering on that resistor give any clues?

That said, looking at the resistors on the timebase module, I'm seeing the usual types that were used by Telequipment - nothing unusual there. In the case of the vertical calibrator, that top resistor is a type that I've not seen before in a Telequipment unit. Given the balanced nature of the circuit, I'm sure they would have made the effort to put two identical ones in.

But having said that, Les has just sent me a photo of his unit. It looks like he has a very early one - perhaps even a prototype, as he says it came a TQ site. The PCB in his unit does not have a silkscreen, so no component designators, etc. There can't be too many like that about!

Anyway, on Les's unit, I can see that the 910R resistors do look like your top one. And they measure high (around 1k1). Which suggests that they are carbon-composition rather than film.

Given that the value of these resistors will affect the calibration - both at LF and HF - I suggest that we can't derive a calibration procedure from Les's example. But overnight, I have been thinking about this, and have a couple of ideas that might work - bear with me for a while, as this holiday is going to be a very busy one for me, but in those moments when I need a distraction, I'll put some effort into this.

Spoiler: it will need a pair of bench power supplies, a high bandwidth oscilloscope and a function generator that goes fairly high. And an edge connector. I'm hoping that the calibrator is just tuned for maximum flatness within the passband - that would be most logical, but I'm not making any assumptions at this time.
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Old 23rd Dec 2019, 1:27 pm   #104
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by mhennessy View Post
For what it's worth, the serial number of my vertical unit is E543428 and the date codes I can see on the transistors are 1974 for both 2N2369s and 1977 for the TFD1551. Of course, I've no way of knowing if the JFET is original or a replacement. I can look up the details for the X unit when I next go up in the attic. But having said that, I doubt we'll be able to gather any meaningful data about how these units evolved, given the small numbers out there today.
The serial numbers of my units ar similar to yours, 543091 for the 067-672-00 and 541073 for the 067-673-00.
Quote:
Originally Posted by mhennessy View Post
You might be right - does the soldering on that resistor give any clues?
The soldering looks unsuspicious. However, the cutting angle of the R1015 resistor is the only one that is not parallel to the board, which could hint at it being installed after the board was put into the frame.
Quote:
Anyway, on Les's unit, I can see that the 910R resistors do look like your top one. And they measure high (around 1k1). Which suggests that they are carbon-composition rather than film.
Just measured my resistors with a somewhat decent LCR-meter at 100hz. The R1015 measures 0.9261 kΩ and the R1012 0.9106 kΩ. Unfortunately there is no way knowing if swapping R1015 for a better matching one is a good idea or not, because the unit might or might not have been calibrated against the currently installed resistor.
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Old 23rd Dec 2019, 10:27 pm   #105
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Default Re: Telequipment D75 scope.

I just went to look at the numbers on mine. V unit is E543052, so earlier than both yours (unsurprisingly). The "H" unit however: No "normal" TQ label. Instead a paper label with "Serial No. TG1172". Does TG mean testing group, or somebody's signiature? -- I have a few internal circulation documents, if i get time I will look for those initials. The 1172 obviously suggests Nov '72. The TEKWIKI photo (Your P I now see) has PC155 as the H board. My board is totally different. It is a convenient board (I don't recognise it) that has been used as a placeholder for the few components. It looks like the board for a differential Y amp (preamp), but I may show a photo of it later.
I MAY complete both the circuits, with some errors spotted by Mark corrected, and my "made up" component numbers replaced by those on your amp.
Mark, I have thought about calibration. Since these were intended for D(M)63, D75 and D83, they need only to be "OK" for something above 50MHz.
I have a Tek 475, so maybe could monitor the output (at 50 ohms??) whilst powering the rails from a D63 mainframe. Then it could perhaps be optimised for a good spare input pulse. Maybe my C3 calibrator would be good enough.
Just thinking aloud.
les.
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Old 21st Mar 2020, 12:54 pm   #106
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Default Re: Telequipment D75 scope.

That virus got me working on my scopes again. This time I tried to repair the stands of my D755 which are broken because of impact damage from a drop. In addition to that, the former owner tried to repair them with glue and also painted them blue. So I tried to 3D-model the stand geometry which consists of two parts, the stand itself and a screwed-on cable holder. Then, I printed the parts on an Ultimaker 3 and got quite satisfying results. The images show a comparison of the 3D-printed replacement and an original stand of my D75. Unfortunately, I didn't have any brass inserts for machine screws at hand to mold into the 3D-print but instead had to use two deep thread wood screws to join the stand and the cable holder.
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Old 21st Mar 2020, 12:58 pm   #107
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Default Re: Telequipment D75 scope.

The attached images show the new stand attached to the scope. Imho, it fits quite nice. The geometry was constructed to fit on an undamaged D75/D755 frame. If the frame got bent from an impact, the parts with the brass insert for the holding screws have to be bent back to their original position first for the screw holes of the stand to fit the frame(see last image). If you want the 3D-files to print a replacement stand of your own, just let me know.
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Old 21st Mar 2020, 1:29 pm   #108
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Default Re: Telequipment D75 scope.

Brilliant
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Old 21st Mar 2020, 9:36 pm   #109
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Default Re: Telequipment D75 scope.

TQ did produce a mod kit to fit over the voltage selector panel as it was then considered unsafe. -- well, if you will poke a wire in to see what happens.
It comprised, from memory, two extended threaded posts to replace the existing fixing screws, and an upside down plastic box to keep the probing wire out.
Les.
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Old 21st Mar 2020, 11:15 pm   #110
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Default Re: Telequipment D75 scope.

Hello Les,
you probably mean parts number 200-2208-01 (Cover) and 129-0706-00 (6BA spacer) as shown in the attached image of the back of my horribly blue painted D755. I think this part was standard for the D755. At least it is shown in the exploded view in the manual.

Some time ago, I purchased a D755 manual in bad shape and did a very time consuming digital restoration on it. I tried to fix all the printing issues, time imposed damage, misshaped letters and diagram parts, even with color separated layers on the PCB layouts. The result was a 102 page 1200dpi digital version of the manual with much better quality than the original. I am planning to release a more screen-reading friendly version of this soon for free to Tekwiki. The PDF will also show the original white, canary and pink paper color (used by TQ to mark the parts for the main frame, the V4 and the S2C) on screen, but not when printing. The manual is complete and even comes with exploded views for ann components and manual amendment lists for error corrections. If you like to see a first version of the manual, just drop me a PM.
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Old 22nd Mar 2020, 8:12 pm   #111
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Default Re: Telequipment D75 scope.

Yes, that is the kit. The early versions were TQ D75. Later they became Tektronix. Various ones, could have been D751 thro to D755.The D83 was sold as a chassis plus vertical plug in plus sweep plug in. For D75 models, you bought a complete unit. Obviously the plug ins were removable and could be changed, but the factory only supplied as a complete scope to a specification which could be specified by customer. (my understanding anyway)
The "kit" was not present on any earlier models (D65 thro D67, D(M)63, D75, D83 and many others. Later they became standard, with retro fit kits available.
That type of voltage selector panel was common to all (most anyway) of TQ scope production from the early S31 onwards.
From the D75 series, the D755 is the most common because they were a forces specification, and were "manufactured and supported" after general production finished as I understand it. Often two different phosphors, yellow and blue predominant
Les.
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Old 22nd Mar 2020, 8:20 pm   #112
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Default Re: Telequipment D75 scope.

Yes, I updated the Tekwiki with regard to plugin variations and options with the information from my manual. Option 4 was P7 phosphor and amber filter instead of P31 and blue filter.
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Old 11th May 2020, 7:30 pm   #113
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Default Re: Telequipment D75 scope.

Today I wanted to continue analysing the stability issues of my D755 power supply. However, the scope did not turn on properly as it used to. The screen showed only a mushy glow with a very blurry hint of a trace at the middle left. So I tried milking the front panel a bit:
- Brightness control does influence the brightness of the glow.
- The blurry leftover of the trace cannot be moved at all (see first image).
- The diffuse glow gets very intense when I rotate the transparent Delay/Time knob to the EXT-X position (see second image, same exposure as the first).
- The S2C works in my D75.
- The focus beam button shows a beam.

Then, while I tried to take a photo of the beam with the focus button pressed, something single arced in the HV-section of the scope and the scope went dark. Also, no light at the green power indicator lamp. However, neither the scopes fuse nor the mains fuse has blown. I immediately unplugged the scope and searched for obvious shorts, but could not find anything. So I ran a second test and the scope turned on again in mushy glow mode. At the moment I am suspecting something in the power supply or the HV-section has fallen victim to age or corrosion. I had to do quite some corrosion cleanup when I got the scope since it had been stored in humid conditions. Also the previous owner did some reworking in the power supply like replacing the filter caps. Unfortunately I do not have an HV-probe, so i cannot measure all parts of the power section at the moment.
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Old 12th May 2020, 6:00 am   #114
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Default Re: Telequipment D75 scope.

"Unfortunately I do not have an HV-probe, so i cannot measure all parts of the power section at the moment." You can though measure across parts of the HV PSU , like resistors, diodes etc to get a rough idea of what's what. Most DMM's are 600v +, your unlikely (though possible) to get more than a 600v drop across R's. You can also check it cold, resistances etc.

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Old 12th May 2020, 7:11 pm   #115
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Default Re: Telequipment D75 scope.

Thank you for your input. I have a Voltcraft VC870 DMM rated 750V for both AC and DC. For more accurate component testing, I have a DE-5000 LCR meter. Given the unclear state of the power supply modifications by the previous owner, my current plan is to check the power supply on PC149 first and make sure the +24V, -24V, +30V and +105V produce a clean output. As can be seen in this post, the +24 had some suspicious ripple on it, the last time I checked. Then, when all power rails are OK, I will advance to the HV section.

One further thing I noticed while examining the scope today was that the power lamp is lighting up again. I find this strange, since the power lamp has its own separate 6.5V transformer winding. The only other thing connected there is the 50Hz line trigger input of the horizontal section. So how could it stop working in connection with this fault and then lit up again? It should be always on as soon as the transformer gets power. It does not seem to be a case of randomly connecting dangling filament in the lamp, since it does not react to the orientation I hold the scope.
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Old 12th May 2020, 11:32 pm   #116
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Default Re: Telequipment D75 scope.

I just did some measurement on PC149 and discovered that the +105, +30V and -24V rails gave a spot-on output, but the +24 rail only delivered about -1.6V garbage. I assumed that, since the +30V rail seemed OK, the error must be somewhere within the red rectangle in the attached circuit diagram. So I checked all transistors (in continuity and diode mode), capacitors and resistors with my DMM but I could not identify any component that had gone open circuit or was significantly off-valued or shorted. At first I suspected TR405 being bad, but since the 25.2V at the collector of TR408 were not there, I think the fault could be TR408 or something connected to its collector or base. However all components I tested there seemed to be OK.
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Old 14th May 2020, 12:09 am   #117
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Default Re: Telequipment D75 scope.

Just measured all components again and de-soldered some components where necessary. Everything was near perfect or at least well within tolerance, except for C414. This was a 33 μF capacitor at 32 μF, that hat been put in place of the required 22 μF at that point. Since this seems to be a filter capacitor, I think this should be OK. So I pulled the PC149 out of the case and did some live measurements on the voltages going in and followed their path. Now, I finally got 25.3 V at the collector side of TR408 and 26.1V at the emitter side of TR405 but still no output at the connector. Since all I had done was reseating some components and resoldering several connections, I suspected some cold solder joints. So I checked the traces from the emitter of TR405 to the PCB connector and both of the beefy vias on the path were dead cold solder joints. After fixing this, I got my +24V. Then I did a rudimentary PSU calibration and the scope started running again. I should probably touch up all the solder joints on PC149 just to be on the safe side before continuing with the rest of the scope and then recalibrate the PSU.
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Old 21st May 2020, 3:39 pm   #118
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Default Re: Telequipment D75 scope.

Further analysis of the D755 PSU revealed additional problems. The +24V line was not as stable as I expected it to be and suffered occasional breakdowns with characteristic alternating 50 Hz positive spikes. Since I suspected a rectifier issue, I had a look at the waveforms at the filter capacitor C405. This revealed a non uniform triangle waveform with alternating 100 Hz high and low triangles that occasionally fused into a single uniform 50 Hz triangle waveform.

So the diodes had to be replaced. Since the triangle waveform at the -24 V line filter capacitor also showed signs of alternating triangle wave heights, I replaced the diodes on all three rectifiers. The original diodes were rated with 500 mA and 450 V, according to the manual. The closest match, I could get my hands on were some 1N4005 standard speed rectifier diodes rated 1 A and 600V. After the replacement all rectifiers finally produced clean 100 Hz triangle waveforms. However, the 30 V rail, which I measured at 30.5 V half a year ago before the PSU issues appeared, now is only at 28.8 V and I am not sure about why this is different. Maybe I chose the wrong diodes?

All PSU output lines now seem to be kind of OK. The +24 V and -24 V rails have about 2.5 mV ripple VPP, the +105 V rail has about 15 mV and the +30 V rail 1.7 V. As a comparison, on my D75, the -24 V rail has about 2.5 mV ripple VPP, the +24 V rail 6.25 mV, the +105 V rail has about 12.5 mV and the +30 V rail 1.25 V. So I think the optimal values are ~2.5 mV on the +24 V and -24 V rails, ~15 mV on the +105 V rail and ~1.5 V on the +30 V rail. This means the +24 V line on my D75 has some optimization potential. The higher value in comparison to my D755 manifests itself in form of a superimposed 100 Hz triangle wave. I am still trying to find out where this is coming from, but I can already say that it is not related to TR405 and TR406 (tested by swapping).

In general, the PSU ripple on my D75 is way cleaner than on my D755, although the D75 still has its original filter capacitors and rectifiers. On my D755, there is a lot of seemingly random stuff on the ripple and some small 100 Hz periodic spikes. So I am still not really satisfied with the output. In addition to that, R413 and the capacitors which were swapped by the previous owner look kind of ugly. I wish I could get my hands on a defective set of original filter capacitors in optically nice condition. Then I could hide some new capacitors inside and preserve the original look. See attached images for a comparison of PC149 of my D7 and D755. Also, I am still searching for a source of metal oxide resistors that look similar to the grey-colored ones in my D75.

An additional thing that bugs me with the D755 is the look of the trace when switching the scope on. When that trace appears, it looks like a dashed line at first and then transitions to a continuous line in about 8 seconds. My D75 does not have this issue. So, maybe this is a first indicator of a problem with the CRT. On the other hand, the CRT of my D755 seems to be in much better condition than the one of my D75, when judging by the achievable intensity.
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Old 3rd Jun 2020, 10:03 pm   #119
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Default Re: Telequipment D75 scope.

After I did some additional optimization on the D755 PSU in terms of reworking solder joints, I finally got acceptable outputs. However, after a while of testing, the issues withe trace issue got worse. The uneven trace illumination after switching on turned into a continuous horizontal twitching of the left hand side of the trace. This got so bad that the trace got compressed all the way to the right. After ruling out a fault in the S2C horizontal module by swapping, I suspected something on the horizontal amplifier on PC 150 to be faulty. An initial unpowered DMM check of the transistors did not reveal anything suspicious. So I just went for the easiest thing first and reseated all transistors in their sockets. To my great surprise, the issues completely vanished after that. The trace is now perfectly stable right from the beginning and everything else seems to work, aside from some stuff that should be fixable with a calibration. So, if you encounter an bad looking issue with continuous horizontal twitching of the left hand side of the trace, try to reseat all transistors on PC 150 and maybe do some contact cleaning.
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Old 5th Sep 2020, 1:14 pm   #120
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Default Re: Telequipment D75 scope.

Yesterday, my D755 did not switch on anymore. Turned out, the fuse had blown silently. Maybe this was caused by the recent defects in the PSU that I repaired. I found a Eaton & BUSSMANN TDC11 1A time delay fuse in it. Unfortunately I did not find a usable source for TDC11 1A fuses on the net, where one does not have to buy 1000 fuses minimum. Searching for good a replacement I noticed that all locally available time delay fuses had a very different arcing behavior in low over-current situations. While the TDC11 1A takes only 1 to 4 seconds to blow at 2A, the replacements I found can last from 100 seconds up to 10 minutes at that amperage. Now I am afraid that a fuse like this will not protect the scope reliably. However, I am not sure if the TDC11 was the original fuse to begin with. Maybe not and it is already too sensitive in the low amperage area to reliably survive the powering on phase of the scope. Has anyone some useful experiences to share on this?
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