Telequipment D61 scope repair

[TransistorMan]

Hi!

I was given 2 used, semi-functional D61 scopes.
Currently I'm trying to repair them, and I ran at 2 pretty weird problems with one of them.
I went through standard calibration procedure, but still, I got some issues.

First problem:
When I turn-on oscilloscope, some timebases (for example, 5ms) don't work for 5-10 minutes , and then they decide to magically work.
I suspect cold joints on range switch, but I really don't want to mess with re-soldering all joints on range switch,unless necessary.
Could it be something else?

Second problem - [FIXED - bad input resistor R900]  :
Channel 2 has got significantly lower bandwidth than channel 1. Square wave gets deformed much quicker on channel 2 (difference is noticable allready at arround 50kHz) than on ch1. I tried switching transistors (TR702,TR703,TR704 with TR602, TR603 and TR604 respectevly) between channels inputs, but it does not make any difference. Is there any (passive?) component that I sould check?

Oscilloscope manual link: http://www.r-type.org/pdfs/d61.pdf

Thanks in advance,
Nikola

[tggzzz]

Unless you can see faulty solder joints, they are not very likely to be the problem (but sometimes they are!).

Often dirty switches can be "repaired" by repeatedly switching them. That clears oxidisation and general crud off the contacts. Beyond that, a deoxidising agent can help; Caig DeOxit is often mentioned, but others can be used.

If you accurately define which ranges do and don't exhibit the problem, then the schematic will enable you to narrow down the problematic paths and components.

Expect there to be subtle (and not so subtle) problems with electrolytic capacitors. They "dry out" with age. I had a Tek 485 which would take 1 minute to turn on, but if turned off and on again it started instantly. Difficult to fault-find a problem that only occurs twice a day, but I traced it to an electrolytic that took in the SMPS startup circuit.

There's lots of help available on the Yahoo TekScopes group.

[IanMacdonald]

Oxidised wipers on presets are another favourite on vintage gear, though if you've gone through cal you would probably have noticed any like that.

[Armadillo]

Do all the contacts cleaning with deoxit or contact cleaner first.

[TransistorMan]

Thanks for advices, I will get contact cleaner soon, and take care of those switches.

I fixed channel 2 issue - it was just that bloody R900 that burned(input resistor, 10 ohms, it was between BNC and AC/DC/GND switch).
It's resistance was arround 40k, so just not enought to make obvious difference at DC or lower freuquencies.
I still got no idea how someone could burn that little guy.

Timebases that don't work are:
500ms, 5ms and 50us.
Those timebases correspond to positions 3,9, and 15 on time/div switch.
At first I suspected that 2M37 resistor (R1,  fig. 4 in manual), but then, 20us should not work either, but that one seems to work fine.

Still, I will have to measure it, becuase some timebases, including previously noted timebases seem to be...non linear, with signal periods geting shorter towards right end of the screen. Not an huge issue,since i don't plan to use this scope for any kind of precise measurements, but still pretty annoying.

Expect there to be subtle (and not so subtle) problems with electrolytic capacitors. They "dry out" with age. I had a Tek 485 which would take 1 minute to turn on, but if turned off and on again it started instantly. Difficult to fault-find a problem that only occurs twice a day, but I traced it to an electrolytic that took in the SMPS startup circuit.

There's lots of help available on the Yahoo TekScopes group.

Similar case here - if I turn off scope, and then turn it on few minutes after that (obviously, I'm avoiding doing that), all timebases work instanteniously. I guess that I will have to check capacitors too, but still, bad supply capacitors would probbaly cause all timebases to be dead.
I think that somehow most electrolytes are still just fine.

Another issue I ran at is somewhat bad triggering of squarewaves at frequencys higher than 30-ish kHz.
I added picture in attachments. Traces seem to have this "afterglow". Not shure if this is normal or not.
I'm using Arduino uno as squarewave generator, since I don't have propre func. gen 
I will have to hack together some sine oscillator to see if similar issues appear with other waveforms.

[tggzzz]

Expect there to be subtle (and not so subtle) problems with electrolytic capacitors. They "dry out" with age. I had a Tek 485 which would take 1 minute to turn on, but if turned off and on again it started instantly. Difficult to fault-find a problem that only occurs twice a day, but I traced it to an electrolytic that took in the SMPS startup circuit.

There's lots of help available on the Yahoo TekScopes group.

Similar case here - if I turn off scope, and then turn it on few minutes after that (obviously, I'm avoiding doing that), all timebases work instanteniously. I guess that I will have to check capacitors too, but still, bad supply capacitors would probbaly cause all timebases to be dead.
I think that somehow most electrolytes are still just fine.

Another issue I ran at is somewhat bad triggering of squarewaves at frequencys higher than 30-ish kHz.
I added picture in attachments. Traces seem to have this "afterglow". Not shure if this is normal or not.
I'm using Arduino uno as squarewave generator, since I don't have propre func. gen 
I will have to hack together some sine oscillator to see if similar issues appear with other waveforms.

Before looking at anything like that, make sure all the power supplies have the correct DC voltage and the ripple is within specification. A bad PSU makes everything bad, in subtle ways.

A quick-and-dirty trick to see if a suspect PSU capacitor has too high an ESR is to temporarily solder another capacitor across its terminals. If it improves proportionately, then the suspect capacitor is almost certainly faulty.

[sarahMCML]

Hi,

I suspect that you are correct in thinking R1 the 2M37 resistor as being defective. I think you have misread the S1 3R diagram slightly. The contact shown as 1 actually marks the switch positions for the timebase settings 500ms, 5ms & 50us. The position marked 16 is wrong, and is actually on the next contact shown as 2, and will select 20us!

Hope this helps,

Sarah.

[TransistorMan]

I'm not shure if it's okay to lift this post,since it's quite old, but I'm trying to fix "second" scope now.

First scope seems to work fine now - even faulty timebases now work perfect. No idea why.

So I decided that it's time to fix second scope.
I measured voltage rails, and they are all fine.
I cleaned switches, and I ran into some pretty weird issues.


First, for some reason, trigger level was causing traces to move horizontally. Most likely, it was leaky C23 capacitor. I replaced it with a new one, and this issue seems solved for now.
Also, trigger seems to work more or less fine. I think that I just have to go thrugh calibration procedure to polish trigger response.

Second issue is much more serious. For some reason, horisontal preamplifier sometimes starts to mix ramp and square wave (I'm feeding scope calibration signal to both inputs, just like service manual asks to do) after certain /point(voltage?)/ when moving trace verticaly. So, if I'm feeding CH2 in trigger, when moving CH2 position, square wave from CH2 gets mixed with ramp, and it basically totaly ruins screen picture on scope. Moving CH1 vertically does not cause any issues when CH2 is selected as trigger input.

When CH1 is trig. input, moving CH1 causes ramp to be mixed with square wave, and moving CH2 does not.

When I put trigger source to external, neither CH1 or CH2 vertical position affects ramp.

Also, +/- switch affects this. If this switch is at +, moving trace upwards ruins ramp, and if it is at -, movign trace down will ruin ramp signal.

So I'm pretty shure that problem is somewhere in trig. input and horizontal amplifier section(Schematic, fig3 in service manual).
I probbaly tested every single diode in this circuit(maaybe I missed some, hopefully not the faulty one/s,hmm), I tested transistors and everything seems just fine.

No visual damage is apperent (burnt resistors or something like that), so I got no idea what is wrong.

Any suggestions on what to check? Could it be even faulty channel amplifiers? I think it's pretty unlikety that they are faulty :/

Thanks,
Nikola

[sarahMCML]

Hi Nicola,

Would it be possible to show some screen shots of your second problem, as I'm having some difficulty in determining what you are describing?

Regards, Sarah.

[TransistorMan]

Hi Nicola,

Would it be possible to show some screen shots of your second problem, as I'm having some difficulty in determining what you are describing?

Regards, Sarah.

Here are some images of scope screen

Ramp signals are measured with my working scope, and other 2 pics are screen of scope that I'm trying to repair.
Pictures named normal and ramp are taken "at same time", while scope is working sort of properly.
Other 2 are taken when signal gets ruined. Pic dragged.png contains only one signal,because I had to move other one a bit off screen to mess up signal.

One signal is not perfect square, because I haven't adjusted attenuator compensation pots yet.

I inspected ramp at point 1596/9C as well, it's basically perfect ramp.

Also, traces moves  quite a bit horizontally  when I switch trig input from ch1/ch2 to EXT :/

Thanks

[TransistorMan]

This pic may show what's happening a bit cleaner.
Ramp signal is chopped by square wave, and that causes parts of wave on screen to go off screen.
Also, it seems like that only negative halfwave gets infused in ramp.

[TransistorMan]

UPDATE:

I just realised that input switch and amplifiers, on both channels, behave...weirdly.
Outputs between 0,01V/div and 0,1V/div are more or less same.
Also, channel amplifiers don't have enought gain - their gain is like 10x less than it should be 

So I guess that trigger and horizontal amp/preamp might work more or less just fine, and that problems start because they get overdriven, because input attenuators are pushing out too much signal.

Also, I can't balance horizontal preamp using R49. Diode D29 seemd broken, so I replaced it with 1N4148. Sadly, old one fell somewhere so I can't re-test it.
So looks like that whole scope might be fryed all over the place, but not enought to stop it from working 

[TransistorMan]

Okay, for some reason, just connecting any metal part of input switch to ground restores amplitude(I had input switches pulled out. During normal operation, switch is connected to chassy ground anyway).   

I guess that coax or some wire is rotten, I don't think that switch is intentionaly made in such a way that it operates properly only when screwed in place.

Right now...everything seems fine. All previous issues fixed with 2 aligator clips...   

I'm pretty tired now, so tommorow I will go throught calibration procedure and hopefully there won't be any more nasty surprises 

[sarahMCML]

Hello once again,

Has it really cured ALL the previous funnies? If so, it wouldn't be the first time that a conveniently placed switch or socket metal body has been used as an earthing point for some other component/s. Such usage will certainly lead to strange results if disconnected during servicing.

Although I worked on quite a few of the Telequipment D65 and D66 models (and also the earlier D43 valve based true dual beam units with various plug-in amplifier modules), I never got to see the insides of a D61 like yours.

Regards, Sarah.

[TransistorMan]

Hello once again,

Has it really cured ALL the previous funnies? If so, it wouldn't be the first time that a conveniently placed switch or socket metal body has been used as an earthing point for some other component/s. Such usage will certainly lead to strange results if disconnected during servicing.

Although I worked on quite a few of the Telequipment D65 and D66 models (and also the earlier D43 valve based true dual beam units with various plug-in amplifier modules), I never got to see the insides of a D61 like yours.

Regards, Sarah.

Thanks four Yours interest in this repair

Yes, I would say that, for now, EVERYTHING works just fine
I went trought calibration procedure, and scope works normally now.
I might have to re-adjust trigger and horizontal preamp pulse responses to polish triggering(on both scopes), but apart from that, I see no issues 

Since resistors R905-R909  are mounted on V/div switch, improper grounding of those leaves trigger input switch "floating",without low resistance path to ground, and I guess that causes TR23 (schematic fig. 3) base voltage to go a bit too high, ruining horizontal preamplifier's balance.

I really hope that this is it, and that scopes will keep running well for at least a few years 

[sarahMCML]

Hello again,

Great news that that this seems to have sorted out all your problems, and hopefully you will now be able to get plenty of use of the instruments.

Also, I apologise for incorrectly calling you Nicola instead of Nikola in an earlier post.

Regards, Sarah.

[TransistorMan]

Also, I apologise for incorrectly calling you Nicola instead of Nikola in an earlier post.

No problem 

Out of curiosity, I took a look at D61a manual.
Nice follow-up might be upgrating scope to something more like D61a. Most of the schematic and main board seem identical,apart from small add-on board. Also, D61a manual offers us single shot circuit. That might be interesting to try out as well. 

Regards,
Nikola