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Old 31st Mar 2019, 6:55 pm   #1
Anode_to_Joy
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Default Telequipment D75 Oscilloscope restoration.

Hello everyone, this is my first post here.

I've been restoring a Telequipment D75 oscilloscope which was had no trace when I got it but it's in better health now.

I replaced all the high value carbon resistors with good quality high voltage types along with many of the transistors, diodes and capacitors. This has resulted in a nice bright, focused trace and all controls appear to work but sadly, only for 5 minutes or so! I'm now getting stuck and can't figure out what else it could be. Any pointers would be very gratefully received.

After about 5 minutes, the accurate sine wave trace suddenly starts to expand and altering the Intensity control makes the trace even bigger as it's increased. (When warming up it only increases the brightness, as it should). The HT on the cathode starts to fall according to my AVO 8 from -2.5Kv down to -1.8Kv before I turn the scope off. All the other voltages from the power supply remain constant.

The voltage from the bright up board seems to be working. The intensity control varies the voltage at pin 148/18 of the EHT form 6 volts to 60 volts with a regular pulse down to 6 volts. This stays constant when the scope is working properly or not.

Replaced components on PC148:

All 4 transistors
2 x HT diodes, 2 signal diodes and 130v zener diode
All high voltage resistors over 1M
3 x 10nF HT capacitors

I've had a good look at previous posts on these Telequipment scopes and tried to follow all the advice in them, but they all appear closed now. Hope someone is able to advise me as where to look next. I'll try to get a few pictures of traces probing the primary of the transformer if that helps? Cheers, J.
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Old 1st Apr 2019, 12:55 pm   #2
micheal
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Default Re: Telequipment D75 Oscilloscope restoration.

This could be due to leakage of EHT from the pin on transformer to the clamp/fixing screw through the paxolin due to damp
The pin in question has the quadrupler connected to it.
Try warming the paxolin up with hairdryer for about ten minutes or so and see if this gives a longer run time, but be careful the wire is very fine.
Could also be due to quadrupler leaky ie diode or capacitor.

Regards Mike
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Old 1st Apr 2019, 1:31 pm   #3
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Default Re: Telequipment D75 Oscilloscope restoration.

Agreed. Had similar issues on my D83.

I've got another D83 with a knackered focus pot and limping power supply here. If you need any parts, drop me a PM. I have a good EHT transformer and multiplier that should fit in the D75. The D83 is the upright version of the D75.
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Old 1st Apr 2019, 8:09 pm   #4
WME_bill
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Default Re: Telequipment D75 Oscilloscope restoration.

D75.
Sounds like EHT transformer is failing. Have you tried looking at it in a darkened room. Be careful your nose does not get too close. I suggest have a torch handy so you can see what you are doing.
If there is leakage across the paxolin terminal board or in the transformer windings, it will often show up in the dark.
There was some discussion about this paxolin board leakage a couple of years ago. Have you searched. Try under D75 or D83, which is identical electrically. D63/DM63 is similar.
Have you checked to see if the EHT still drops with the EHT multiplier disconnected. That would clarify if transformer or the multiplier.
Tube will not come to harm if operated with no PDA for a short time. I believe the Tektronix tube uses has a domed mesh giving scan magnification with the PDA working, so any trace will be dim anyway and probably smaller.
Sounds as though you have done good work in testing the Bright-up circuit, which usually foxes people.
Mr Bungle's offer sounds the ideal solution.
wme_bill
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Old 1st Apr 2019, 8:48 pm   #5
MotorBikeLes
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Default Re: Telequipment D75 Oscilloscope restoration.

Those symptoms show low EHT as already advised. Leaky paxolin on EHT transformer, possibly leakage on surface of CRT near EHT connector. Some (not all?) are boded so should be OK.
I have had problems with one of the short lengths of co-ax on the EHT area. The covering or just the end sleeving can become conductive.
Bill's darkened room test should yield an answer.
Les.
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Old 3rd Apr 2019, 7:21 pm   #6
Anode_to_Joy
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Default Re: Telequipment D75 Oscilloscope restoration.

Thank you all for your very helpful advice and offers of spares.
I found the thread showing the problem with the EHT transformer and it seems like it could well be the same issue.
I took a look at the EHT transformer in the dark last night but couldn't see any arcing - although I did hear a very faint sizzling, but it was impossible to hear or see where it came from. I then unsoldered the lead to the quintupler and the trace was indeed dimmer and shorter, (and no sizzling). It must have taken over 10 minutes but the fault eventually came back. I'll probably have to leave it until the weekend but I think I'll remove the transformer and take a closer look. Hopefully I'll get it apart without damaging it and rebuild it with some fibreglass instead of the paxolin. I'll also check the co-ax.
There are a couple of black axial 20nF 5kv capacitors. I don't have spares or any way to test them at working voltage but could these be part of the problem? I'm not sure if they could become leaky after the unit warms up?
Cheers, John.
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Old 3rd Apr 2019, 9:44 pm   #7
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Default Re: Telequipment D75 Oscilloscope restoration.

If they are the black ones it's probably those. Every telequipment scope I've had has knackered ones in it. I replaced all mine with ones from hvstuff.com: http://hvstuff.com/high-voltage-capacitors/ceramic/disc

I wired 2 in parallel. After much digging I found the pictures I took:

Before (note the black capacitors)

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During (hvstuff ones installed)

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After (transformer reinstalled)

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Watch the little black diodes and the coax soldered to the board - they are incredibly fragile! I found this out the hard way
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Old 4th Apr 2019, 11:39 am   #8
Anode_to_Joy
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Default Re: Telequipment D75 Oscilloscope restoration.

Yep, they're the ones.
Thanks for the link - that's a great price on the hv caps, I'll stock up on those.
I noticed the trimmer pots for the -2.5kv set and grid cut-off are different on your pcb. I think I'll upgrade them when I take the board out as well. I've made it a bit easier to remove the pcb but it's still a pain!
I installed 2 new high voltage diodes so I shouldn't have any trouble there but I did have to re-solder a piece of co-ax as you mentioned.
John.
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Old 4th Apr 2019, 8:32 pm   #9
MotorBikeLes
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Default Re: Telequipment D75 Oscilloscope restoration.

That EHT trimpot in the middle should be replaced by an upright cermet type.
Les.
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Old 4th Apr 2019, 10:41 pm   #10
MrBungle
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Default Re: Telequipment D75 Oscilloscope restoration.

I must admit it's a total pain to get that board in and out. Really I'm not sure how they assembled most of the telequipment kit. It wasn't designed with maintenance in mind at all! I had a problem when I had the D83 arrive and that was there were two of them. One had a duff power supply and the other one had a duff everything else. I ended up breaking both scopes into two assemblies (top and bottom) and doing an entire "bum transplant".

A couple of motivational photos... mid transplant.

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Had to wait until Mrs Bungle was at her sister's for the weekend with the kids to do the work which took up half the living room!

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Honestly, and this isn't a positive comment, I regret buying the things after this experience and I can't face even getting the good one out now and I actually cannibalised the iffy unit for project parts so many times that it's just a shell of a scope! I also did my back in moving them around!
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Old 4th Apr 2019, 11:03 pm   #11
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Default Re: Telequipment D75 Oscilloscope restoration.

I don't suppose either of your scopes had a V5 vertical amplifier. If yes I would be interested in purchasing one.

Al
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Old 4th Apr 2019, 11:35 pm   #12
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Default Re: Telequipment D75 Oscilloscope restoration.

No - just boring old V4s. And both hideously broken.
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Old 5th Apr 2019, 8:25 am   #13
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Default Re: Telequipment D75 Oscilloscope restoration.

Thanks anyway.

Al
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Old 5th May 2019, 10:59 am   #14
Anode_to_Joy
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Default Re: Telequipment D75 Oscilloscope restoration.

I've finally got some time (and hv caps) to work on the old Telequipment D75 scope again. I feel your pain Mr Bungle! I've been suffering with my back the past few weeks and moving this lump around takes a lot of care (the scope that is!).. I've been more like Mr Zippy if truth be known.

I replaced the two black 20nF high voltage caps and a 150v zener diode. Put it back together and it worked - for 40 minutes. The HT started to collapse again and the trace started to stretch in all directions. Part of me thought, I can live with 40 minutes operation but I can't stop now!

Looking at some old threads, I decided to make a probe with 2 x 10 Meg high voltage resistors in series so I could use a DMM. My Avo 8 needle tends to stick and I didn't trust the readings. I calibrated it with an accurate voltage source and it turned out the readings on the DMM were exactly 1/3 of the source as expected, so it's easy to scale up.

When I power up, the HT is -2.37kv and the sinewave trace seems to lock after a few seconds as if the HT supply is regulating, but it actually drops at a rate of about 3v per second for a minute then slows to a volt per second after 10 minutes. Twenty minutes in and the HT had dropped to -1.86kv and then the trace began to expand and the HT dropped quickly. Increasing the Intensity control expanded the trace and decreased the HT. The 105 volt DC rail was steady, and the voltage on the wiper of of R301 (Set -2.5kv) was steady at 76v. Indecently, the HT can't be adjusted to more than -2.37kv.

I decided to take another look at the waveforms on T301, the oscillator / high voltage generator transformer. Hopefully, there is an attachment I copied from an earlier post of a circuit diagram with illustrations of the correct waveforms along with a picture of what I am seeing. Oscillating steadily at 26.6KHz, points 1 & 2 are as expected, but point 3 looks very different. Instead of a sort of clipped sawtooth I am seeing more of a sawtooth. The voltages look wrong. Instead of 1v p/p there is 3.7v p/p. Instead of it being centred around -3v, it doesn't go lower than -2.44v. Seeing as the other waveforms appear correct, I'm not sure if this is important?

As has been pointed out earlier, I think there may be a problem with the transformer although I took it out and had a good look at it and couldn't see any signs of damage to the paxolin. Disconnecting the quintupler reduced the size of the trace as expected but the HT collapsed just the same.

Thanks for the advice so far. I have restored a Telequipment CT71 curve tracer and it would be great to have the old scope working on the bench alongside it and I won't have to keep lugging it around any more.
Cheers, John.


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Old 5th May 2019, 7:40 pm   #15
MotorBikeLes
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Default Re: Telequipment D75 Oscilloscope restoration.

I think this is time for a drop of freezer. On the incoming side, you have 4 transistors and two diodes, as well as the HV rectifying diodes (I replace those with BY182 or very similar). When the screen expands, a tiny squirt of freezer spay on one of those 6 semis will likely find a temp sensitive culprit.
Les.

Last edited by MotorBikeLes; 5th May 2019 at 7:42 pm. Reason: 2 off sp, plus I can't count.
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Old 5th May 2019, 8:27 pm   #16
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Default Re: Telequipment D75 Oscilloscope restoration.

Agreed. Also check the trimmers. I had one of them go temperature sensitive as well.

Let me know if you need a spares D83 - yours FOC. Has a mostly good HT and good HT and isolation transformer in it at least. I'm not far from you in Twickenham and need to clear out my last D83 chassis before SWMBO shoots me.
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Old 1st Jun 2019, 12:42 pm   #17
Anode_to_Joy
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Thumbs up Re: Telequipment D75 Oscilloscope restoration.

Success! The Telequipment D75 has been running perfectly for a couple of days now. I took MBLes's advice and bought some good quality freezer spray which was far more effective than the compressed air stuff I was using and found the fault disappeared when I sprayed around the 3 x 3.3M resistors in parallel (marked as R333 1M on the circuit diagram). So I replaced them. I had already replaced the trimmer for the -2.5kv set with a cermet type. I also cleaned out the sockets for the small transistors with some switch cleaner, so something there seems to have fix the fault. To be honest, there aren't many components left I haven't replaced! I'll take a look at the rest of the power supplies capacitors later but I'm very happy with it now. I often prefer using this scope over the modern digital one as things like modulated RF waveforms seem much easier to understand.
Thanks for all the advice and offers - very kind and reassuring as I hadn't worked on the EHT stuff before.
Cheers,
John
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