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Old 1st Mar 2021, 5:28 pm   #1
cakerack
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Default Telequipment D75 Scope

Hi All

I could do with a bit of guidance with what I think is a Telequipment D75 with a S2A timebase module. When I got it, the green light would illuminate but nothing else. I have given it a strip down and cleaned all of the switches and potentiometers. Its in excellent condition for its age and would love to have it fully working. I can now get it powered up, however there are two remaining issues:

1). The trace never syncs no matter what I try. I have attached a photo. The top trace is flat from Ch1, the bottom trace is from Ch2 and I think is supposed to be a square wave but like I say it will never sync. I get the same results from both Ch1 and 2. The timebase knob never really seems to have a great effect. For instance, slowing down the timebase to 1s never shows an observable trace - it just remains as a 'double' trace with an unsync'd square wave.

2). The other issue I could probably live with but would be nice to fix. After it has been on for 10 minutes or so the trace gets steadily dimmer. I can adjust the intensity to still see the screen clearly so like I say its not that important, but if anyone had an insight to what that might be (or even confirmation that its a symptom of an old scope!), that would be great.

Thanks for any help you can provide!

Cheers

Sean
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Old 1st Mar 2021, 7:14 pm   #2
ajgriff
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Default Re: Telequipment D75 Scope

Have you got a copy of the manuals? I think you may need to study the operating instructions. At the moment things don't look right. For example it looks like you are feeding a square wave into CH2 with the scope set to trigger on CH1.

Alan
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Old 1st Mar 2021, 9:27 pm   #3
WME_bill
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Default Re: Telequipment D75 Scope

D75 scope.
There are so many options with these modern scopes. so while you are discovering how to use it, I suggest you should keep it as simple as possible.
Agriff is quite right and has noticed already that some of the control settings seem peculiar.
Try to get a single trace so set the Y Plug in to ch2,display ch2, trigger ch2. Input to ch2 only.
Then set the A Timebase to Sweep Repetitive, Auto trigger. A display only, polarity +,Source Int, Sweep speed by the A red knob. Check the large TB sweep knob is not at the ExtnX position. You will have to play with the Trigger Level and the A timebase speed. You may have to switch from Auto Trigger Mode to AC.
And as Agriff say, get the manual as that has very helpful instructions on setting it up and how it works. I think the manual is available readily on the web. Either the D75 or the D83 is virtually the same, just a bigger screen.

Your picture shows input to ch2 only, yet you talk about an input to ch1 as well as ch2. This confuses us somewhat.
They are nice scopes, and well worth getting going.
wme_bill.
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Old 1st Mar 2021, 9:41 pm   #4
Pinörkel
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Default Re: Telequipment D75 Scope

Hi Sean,
congratulations on your scope. If you not already have it, grab the service Manual of the D75 here. My scanned manual of the D755 there will also work(much better scan quality), but has some minor differences.

1) In you photo, there are a couple of things wrong: You input a signal into channel 2, but trigger on channel 1 which will of course not work. Set TRIG SELECT to CH2, CHANNEL SELECT to CH2 and the trigger mode to A ONLY. Then work the TRIG LEVEL knob to make it trigger.
With the timebase on 1s you should see a slowly moving vertical line with two bright ending spots. With the timebase at 1mS you should be able to see a rectangle signal.

2) My guess for this issue would be that one or more of the 10M resistors in the brightness chain in high voltage section are bad(have gone high). Try to measure the resistors R317, R318, R319, R321 and R322 when the scope is unplugged. Be careful there, since there are dangerous voltages in this section. Usually the best option is to replace the resistors with Vishay VR37 high voltage resistors. You can find additional information on this in my thread around this post.

Denis

Last edited by Pinörkel; 1st Mar 2021 at 9:49 pm.
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Old 2nd Mar 2021, 3:20 pm   #5
cakerack
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Default Re: Telequipment D75 Scope

Hi Alan/Bill/Denis

Firstly thanks so much for your quick reply! Also I wanted to apologise for the 'bum steer' with my original photo - I can see why it caused confusion. Saying that, your suggestions were really helpful as it helped me confirm that there is an issue somewhere. I am finding that the trigger pot seems to have absolutely no effect at all and (possibly as a result) I still do not get a sync. I have attached 2 more photos showing the behaviour - one at an angle so you can hopefully see what I have switched on. In the meantime I have now downloaded the manual and will start to go through the operation side and see what I can find. I'm slightly worried it has been tampered with previously as the metal shields are both missing off the S2A module where they are both present on th V4 module. More info as I get it!

Cheers

Sean
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Old 2nd Mar 2021, 5:30 pm   #6
WME_bill
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Default Re: Telequipment D75 Scope

D75.
What do you get when trigger set to AUTO instead of AC. All the rest look fine from your photos. wme_bill
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Old 2nd Mar 2021, 7:00 pm   #7
Pinörkel
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Default Re: Telequipment D75 Scope

The settings on you photos look much better now. I think, your photos show that the sweep works, but the trigger does not lock on to anything which results in a constant sweep.

Just to milk some more information out of the front panel:
Can you please test what happens with the trace and the orange trigger lamp if no input is connected, the trigger is set to auto, the timebase is set to 0.1 second, the sweep is set to single and then you shortly press the reset button in the sweep section (rest of the setting should remain as in your last photo)? If this works correctly you should have no trace and no orange light at the beginning, and after you press the button the trace should move as a dot from left to right within one second. Only while the trace moves, the orange light should be on.

To do a rudimentary check of the trigger level A potentiometer you could measure the resistance between its middle leg and one of the two outer ones while turning the knob with a multimeter. The contacts can be reached easily by just removing the plugin from the unit. The measured resistance value should vary between approximately 0 Ohms and 7K5 Ohms as you rotate the knob. This must work for both outer contacts and the central one.

Last edited by Pinörkel; 2nd Mar 2021 at 7:07 pm.
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Old 2nd Mar 2021, 7:37 pm   #8
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Default Re: Telequipment D75 Scope

An addition to the previous post:
If the manual triggering tested ok, you can try the following. Reconnect the input of the calibration signal, put the timebase back to 1 ms, set the sweep to REP, the trigger to AUTO and triggering to LINE. Now, if you rotate the small VARIABLE TIME DIV A knob right above the timebase knob slowly, can you get the trace to stopp rolling and show a steady square wave?
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Old 2nd Mar 2021, 9:36 pm   #9
cakerack
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Default Re: Telequipment D75 Scope

Thanks again for your responses! I'll answer in order:

1). Setting the trigger to Auto makes no difference at all

2). With no input and the trigger set to AUTO and timebase set to 0.1s, pressing the RESET button shows a momentary flash of a dot at the LHS of the screen and the orange light stays on for 1 second then goes off. The dot doesn't move from left to right - just flashes on for a fraction of a second and then off.

3). I have checked the potentiometer and it does indeed sweep from zero to 7.5k when measured between the centre and both external legs in turn.

Bizarre!

Cheers

Sean
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Old 2nd Mar 2021, 9:55 pm   #10
cakerack
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Default Re: Telequipment D75 Scope

I have also uploaded a video of the behaviour when I set the timebase to .1ms and then play with the variable Time Div control - I can get the square wave to stabilise but its certainly not based on any trigger as it slowly moves. A Trig LEVEL makes no difference whatsoever. You will also notice that the trace fades towards the right hand side of the screen.

Have a look at https://drive.google.com/file/d/1njT...ew?usp=sharing

I apologise for the background sounds - that is the sound of 40 pints of beer fermenting!
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Old 3rd Mar 2021, 12:00 am   #11
Pinörkel
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Default Re: Telequipment D75 Scope

Quote:
Originally Posted by cakerack View Post
1). Setting the trigger to Auto makes no difference at all
Quote:
Originally Posted by cakerack View Post
2). With no input and the trigger set to AUTO and timebase set to 0.1s, pressing the RESET button shows a momentary flash of a dot at the LHS of the screen and the orange light stays on for 1 second then goes off. The dot doesn't move from left to right - just flashes on for a fraction of a second and then off.
That is very strange. Your pictures and video show that your scope can generate a continuous sweep. The orange light being lit for 1 second indicates that the sweep runs when pushing RESET in SINGLE mode and also ends, but the dot not sweeping from left to right indicates that the sweep does not work in that situation.

If pushing the TRIG MODE buttons makes no difference at all, and it is continuously sweeping in all modes, then something seems to auto-fire the trigger regardless of input.

To see, if the scope can at least trigger on the line voltage, you can try the following:
Set everything like in the first picture in your post here. Then set the TRIG MODE to AUTO, the SOURCE to LINE, the VOLTS/DIV for CH2 to 0.5 V and the timebase to 5 ms. Then take the probe, disconnect the ground lead and touch the probe tip with your finger. Your body should then act as an antenna and couple a very ugly 50Hz sine wave into the scope. If the scope can trigger on the line voltage, this ugly sine wave is steady, otherwise you might see a scrolling or verticaly double-mirrored sine wave.

One additional thing you could try is, to reseat all transistors on the two PCBs, PC132 and PC133, of the S2A module. Carefully take each transistor that is in a socket and wiggle it back and forth without bending the legs. That could eliminate potential contact issues.

Last edited by Pinörkel; 3rd Mar 2021 at 12:14 am.
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Old 3rd Mar 2021, 9:51 am   #12
cakerack
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Default Re: Telequipment D75 Scope

Hi Denis

OK so I have followed your instructions and I get some results - I get the start of the ugly sine wave you describe but only for about half a cm at the left of the display. If I change the timebase, the waveform fills more of the screen the faster the timebase is set to. It seems to be connected in part to the fact that the waveform gets dimmer towards the right hand side of the screen possibly?

I have attached another video to demonstrate this - have a look at https://photos.app.goo.gl/6YkwyGbQvk6Nv79A6 and you will see what I mean.

During this test the orange trigger light stays on permanently.

I hadn't realised the transistors were pluggable! I have re-seated all of the transistors and there is no change in the symptoms.

I also checked the +24v, -24v and +105v power supply voltages last night and they are all correct.

Cheers

Sean
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Old 3rd Mar 2021, 4:30 pm   #13
cakerack
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Default Re: Telequipment D75 Scope

OK I have another update! I was running the scope on its side so I could meter some signals and I left it running on its side (left side down). After about half an hour I noticed the square wave signal had sync'd perfectly (although it was still dim at the right hand side and I am beginning to think that is a different problem)! So - I turned it back the right way around and it continued to work. 10 mins later it lost its sync so I was wondering if the heat rising from the rest of the scope was warming up the S2A module. Long story short - hair dryer on the S2A PCB - I can now reproduce proper syncing at will, whenever I warm up the S2A board. Next chapter - I have a can of freezer on order and I will do some more digging.

I still have the dim side of the display when running any timebase slower than 50uS though...

Cheers

Sean
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Old 3rd Mar 2021, 4:59 pm   #14
WME_bill
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Default Re: Telequipment D75 Scope

D75 no trigger.
Cakerak: Your posting #10 shows that the scope appears to be working fine except that there is no trigger.
You need to follow the Y signal through from the channel switching in the Y amplifier unit across the main board to the TB-X Amplifier. At this stage, don't start worrying about the trace becoming dimmer.That is probably something in the Bright-up circuit. Quite different.
I am glad you have checked all supply rail voltages. First thing to do always.

Easiest way is I'm afraid to follow the signal with another oscilloscope, a very simple one will do.
Alternatively, with a multimeter,and input and trigger set to DC, you can confirm the clear signal path by each stages. Input say 1V from a battery and confirm a change all along the circuit chain.

To see if the failure is in the Y Amp module or the TB module, try feeding your test signal into both Y input and the Ext-trigger set to DC.

I suggest you clean the contacts on all the switches in the trigger signal path.
The Trigger route switching is quite complex and you will need to sit down quietly and work out the route.
There is a DC path through the Y Amp to trgger out on the plug board to the main board, SK602, pin 7 or 10.
Then across the main board PC147.
Then to the TB module SK601, pin11 or 12.

And then check the Trigger monostable transistors TR3, 4, 5. The voltage changes around there are given in the manual.
Otherwise the usual voltage test around each transistor (with the circuits all non-operating by setting the TB to EXT X). Rule of thumb for NPN: emitter 0.6v negative of base. Collector much more positive of base. Reverse polarity of PNP.
I am working on a D75 at present where the gremlins have altered the wiring onto the main board. Very jolly.
wme_bill

Last edited by WME_bill; 3rd Mar 2021 at 5:04 pm.
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Old 4th Mar 2021, 12:36 am   #15
Pinörkel
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Default Re: Telequipment D75 Scope

Quote:
Originally Posted by cakerack View Post
Hi Denis

OK so I have followed your instructions and I get some results - I get the start of the ugly sine wave you describe but only for about half a cm at the left of the display. If I change the timebase, the waveform fills more of the screen the faster the timebase is set to. It seems to be connected in part to the fact that the waveform gets dimmer towards the right hand side of the screen possibly?

I have attached another video to demonstrate this - have a look at https://photos.app.goo.gl/6YkwyGbQvk6Nv79A6 and you will see what I mean.

During this test the orange trigger light stays on permanently.

I hadn't realised the transistors were pluggable! I have re-seated all of the transistors and there is no change in the symptoms.

I also checked the +24v, -24v and +105v power supply voltages last night and they are all correct.

Cheers

Sean
I agree with WME_bill on the source of the trace dimming issue. However, try to handle one issue at a time. To me, it looks like the scope does not trigger on the LINE source either. If my observation is correct, then the issue should be located after the A TRIG SOURCE switch in the A TRIGGER AMPLIFIER on PC 132. So, the first thing I would do is, to check all transistors and diodes on the right hand side of the circuit diagram of PC 132. If you have no second scope, maybe you can use setting the trigger source to LINE to some extend. In this configuration, you should be able to measure a 50 Hz frequency at the base of TR3, if the A TRIG SOURCE switch S1 routes the signal correctly. Then you could try to trace that signal.

However it is kind of hard to get in there while the plugin is installed and running. To get better access, you can remove the aluminum bar at the side by removing the scope handle via the nut at the inside of the case. A more sophisticated option would be to make an extension cable, like in my posts here and here using a Cinch 81 731 AA 16 edge connector (search for "Cinch 81/731/16/EG" on google to find some).
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Old Yesterday, 11:21 am   #16
cakerack
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Default Re: Telequipment D75 Scope

Hi All

Sorry for the long delay in replying - I have been trying to find where this sync loss issue was being caused. So I managed to track it down to TR63 on the S2A module. Basically a hair dryer and a can of freezer was my friend there. I changes TR63 and TR64 for good measure and now the sync appears to be spot on.

So now to the other issue of the screen fading to the right hand side of the display (photo attached). If I have the scope on a fast timebase, its fine - which suggests to me (correct me if I'm wrong) that the fault ISN'T in the HT side of things. If I have it on a slow timebase, for instance 1mS, the trace barely gets 2 divisions across the screen. Set it to 100uS and the trace gets all the way across to the right of the display. Where do I start?!

Cheers

Sean
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Old Yesterday, 5:25 pm   #17
Pinörkel
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Default Re: Telequipment D75 Scope

Nice work on locating the fault. I did not expect it to be this far left of the trigger source switch. Is the trace dimming from left to right also visible when pressing the trace locate button? Based on the current situation, i would still suspect the resistors, I mentioned in my first post above. I found a thread with a D83 with exactly the same issue here and the resistors were the source of the fault. I have put some information on how to loosen screws on the respective board here.

Last edited by Pinörkel; Yesterday at 5:48 pm.
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Old Yesterday, 7:15 pm   #18
cakerack
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Default Re: Telequipment D75 Scope

Hi - yes, the trace locator display also fades to the right. I'll give your suggestion a try and see how I get on - thanks so much for your assistance with this!

Cheers

Sean
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