Tel-Instrument oscilloscope repair

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Tel-Instrument oscilloscope repair

Postby Harry Dalek » Tue Mar 31, 2015 3:49 pm

I have swapped over here from Steves SSTV thread as i think it may be of interest to others and may go on for a while and take over Steve's thread ! ,any help to get this going would be great free advice is gold .

This scope was given up on if it were not for our reuse shops it would be in land fill !

So far nothing out of that inverter circuit .
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The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Tel-Instrument oscilloscope repair

Postby Steve Anderson » Tue Mar 31, 2015 4:43 pm

OK Harry, no problem with either cluttering up the previous thread or starting this new one...

Those voltages are in the right 'ball-park', it depends on your actual wall outlet AC voltage and the load which will vary a bit. So they're nominal voltages, anything within 10% should be fine.

If you've already checked the inverter transformer and it appears OK, it's time to check the semis, Q401, Q402 and Q405. Should any be defective they should be easy to replace with an equivalent, I downloaded the datasheets for them yesterday, none of them are particularly special.

If they appear OK try lifting one end of R421 and see if you get anything across C404, it could be anything up to 2kV so be careful. Most multimeters won't handle 2kV so you'll have to knock up a simple resistive voltage divider, say 10M in series with 1M, measure across the 1M. Don't worry about the actual reading, all we want to know is - is the inverter inverting? Remember the output voltage will (should) be negative.

Steve A.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Tue Mar 31, 2015 7:10 pm

Steve Anderson wrote:OK Harry, no problem with either cluttering up the previous thread or starting this new one...

Those voltages are in the right 'ball-park', it depends on your actual wall outlet AC voltage and the load which will vary a bit. So they're nominal voltages, anything within 10% should be fine.

If you've already checked the inverter transformer and it appears OK, it's time to check the semis, Q401, Q402 and Q405. Should any be defective they should be easy to replace with an equivalent, I downloaded the datasheets for them yesterday, none of them are particularly special.

If they appear OK try lifting one end of R421 and see if you get anything across C404, it could be anything up to 2kV so be careful. Most multimeters won't handle 2kV so you'll have to knock up a simple resistive voltage divider, say 10M in series with 1M, measure across the 1M. Don't worry about the actual reading, all we want to know is - is the inverter inverting? Remember the output voltage will (should) be negative.

Steve A.


Steve i just went to my shed to do the test and got no results 0 volts but ! ................ i went to test the +117 again and i pressed down a little bit harder on the plug connection and i got a fright as the inverter started to work it was sparking across the high voltage diode ! i had R421 still half disconnected .


So this is promising ! i will leave another look till tomorrow afternoon when i look into this plug i think it sounds like a bad soldering connection ,sounds easy but to get to the bottom of that board is not easy with the panel pots and such in the way.

Any case interesting !

i forgot to mention that if you release the pressure it stops so it seems like a soldering or plug problem ..
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The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Tel-Instrument oscilloscope repair

Postby Steve Anderson » Tue Mar 31, 2015 8:49 pm

To test I would simply chop off the plug and solder the wires directly to the PCB. If you can re-terminate the wires into the plug later, fine. If not leave them hard-wired. But before that...

Re-connect R421 before you go any further. Seriously. Do not install the CRT yet, let's verify other voltages are correct first. Most other components can be replaced or substituted, the CRT cannot.

Before you take the side-cutters to the wires, try some contact-cleaner in the plug/socket and insert and remove the plug several times. This may provide the wiping and cleaning action required. DO NOT use WD40!! Also before chopping wires just run over the socket-pcb joints with an iron, just in case one is dry - it happens. Before you power-up check continuity through the connector with a meter, PSU-to-inverter, just to make sure there's no open circuits.

Most electrical/electronic faults are of a electro-mechanical nature. Switches, pots, connectors, especially if it has moving parts (e.g. relays).

In fact it was a bit remiss of me not to mention unplugging and re-plugging connectors a few times. Something I do as a matter of course...also gently tug on the wires into the connector just in case one is loose.

There appears to be hope after all!

Steve A.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Tue Mar 31, 2015 10:10 pm

Yes Steve looks hopeful now .

This might be a easy fix after all ,and i agree i need to fix the plug or connection problem first since when i release the pressure of my multimeter probe off the 117 volts it stops so some thing is up .

Would not have picked this to be the problem live and learn ,i will take your advice reconnect the resistor and try your easy fix tests on the plug first ,worst case i get my soldering iron out if its still playing up.

Let you know how it gos tomorrow afternoon ,thanks always for the help Steve !
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Wed Apr 01, 2015 5:48 pm

I tried looking at the problem if there is one on the +117 -13 +12 volt plug after checking if it was loose or not connecting to me the plug seems fine , but the pressure on this plug gets the inverter working release it stops ,playing around a bit it can be got to stay on but a bump to the case and off it gos .

So i really needed to see whats happening on the soldered side i traced the connections for conductivity from this plug to every where its connected to and again its fine ...

So all i did today was look i have not re soldered the connections yet .....But what i have noticed is the ground to the board should be grounded to the case via those brass legs screwed to the case that holds it in place but they were not screwed in so the board was just sitting there ,some one has been here before !.

I did do a ground test as it is wire grounded to another board so i am not sure thats a problem yes or no as the pressure on the board could be grounding it to the case via a ground in the circuit thats not grounded to the other side of the board ,it sort of looks like that either side of the boards ground looking at the solder side .

I know some times different models use the same circuit boards and and have more or less components i found this on the board as well i hope there has been no parts removed.

I will double check tomorrow either sides of the board are circuit board ground connected ...i hope not as that would be the problem .
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under side of the inverter board
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top side of the inverter board
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missing parts
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IMG_0588.JPG
missing parts bottom solder side
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The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Tel-Instrument oscilloscope repair

Postby Steve Anderson » Wed Apr 01, 2015 6:30 pm

Yes, I prefer a 'proper' grounding wire and not rely on the PCB support pillars for it. One place where I worked the brass board spacers/pillars were replaced with plastic ones 'cos the bean-counters (accountants) said they were cheaper. Yep, you guessed it, the whole production batch of 500 radar transceivers refused to work. It cost far more than the bean-counters saved, 1) to rectify the problem, 2) to update the service manuals with the additional grounding wires being shown.

If anyone here is an accountant, I apologise.

Harry, the missing parts look like they were missing as it left the factory, not removed later, but I could be wrong. Remember it did work once in the past without whatever those components might have been. Components being deleted as a production batch goes along is not uncommon, or it's a provision for an option or an extra. A different CRT phosphor may require slightly different bias voltages for example. Unless it's really obvious that these components are essential for the thing to operate I wouldn't fret about it.

It's not easy to see from the photos what parts are actually missing, can you identify them? Rxxx, Cxxx etc..

Steve A.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Thu Apr 02, 2015 9:30 am

Steve Anderson wrote:Yes, I prefer a 'proper' grounding wire and not rely on the PCB support pillars for it. One place where I worked the brass board spacers/pillars were replaced with plastic ones 'cos the bean-counters (accountants) said they were cheaper. Yep, you guessed it, the whole production batch of 500 radar transceivers refused to work. It cost far more than the bean-counters saved, 1) to rectify the problem, 2) to update the service manuals with the additional grounding wires being shown.

If anyone here is an accountant, I apologise.


Yes Steve some one stuffed up and i bet cost them more trouble than its worth ,it shows you who should not be in electrical decision making process ,i can see a standard procedure drawn up when it gets to the accountants stage disregard go back to the electronics technician !

Harry, the missing parts look like they were missing as it left the factory, not removed later, but I could be wrong. Remember it did work once in the past without whatever those components might have been. Components being deleted as a production batch goes along is not uncommon, or it's a provision for an option or an extra. A different CRT phosphor may require slightly different bias voltages for example. Unless it's really obvious that these components are essential for the thing to operate I wouldn't fret about it.

It's not easy to see from the photos what parts are actually missing, can you identify them? Rxxx, Cxxx etc..

Steve A.


There are a few parts
R433
R432
R426 which is near the inverter transformer
Vr440 trimmer
C413

I was thinking perhaps its more what you are thinking parts for another model scope not needed for this one but i was looking to see if it looked like there was any desodering.

I am going to double check all the parts circuit board conductivity but just in case i will re heat the solder resolder the parts in .

I checked the board ground as is all grounding on the circuit board seems ok

Not much i can do here more apart from that put it back together and test again .

There must be a reason for the problem fault the board is not mechanically case grounded correctly so i fix this as well and report back here with the results.
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Thu Apr 02, 2015 12:27 pm

Ok i had productive morning and found what looks like the fault on the high voltage side of things ,when i was re soldering the connections to the inverter output side of the transformer i found one the one in the middle see picture was poorly soldered i did end up fixing the other 2 right side as well a bit better .

I reconnected every thing with the inverter board and its grounded nicely now .

I then did get my visual stimulation with those neons on turn off they flash .

I then put the crt back plugged it to its socket reconnected the rotation coil plug to its circuit ,that was a pain no room for my hand !

When now turning it on a very good visual stimulation of the green trace ! so things are good there as far as lighting up the crt .

What is fine is focus brightness control no problems here

But !!!!!! ...from what i can see not knowing what all the switches do and what i am seeing from a quick test ...No vertical control of the trace and horizontal either swings from one side or to the other on its adjustment .

ON touching the input its more like z axis thing where the brightness of the trace changes, i could very well have settings wrong but any case it looks to me it still needs looking at for some other problems.
Happy today one problem is fixed at least .
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before fixing inverter transformer
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getting ready to resolder
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fixed
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The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Tel-Instrument oscilloscope repair

Postby Steve Anderson » Fri Apr 03, 2015 7:52 pm

OK, you're getting there... I have very limited time today, but once back in Bangkok I'll study this and add my thoughts...tomorrow...

Steve A.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Fri Apr 03, 2015 9:06 pm

Steve Anderson wrote:OK, you're getting there... I have very limited time today, but once back in Bangkok I'll study this and add my thoughts...tomorrow...

Steve A.



NO problems Steve i think i do need advice as to what up now ,it looks to me like any vertical deflection is not working as i have no vertical beam movement or signal in this direction.

Now this is a problem for a scope !
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Tel-Instrument oscilloscope repair

Postby Steve Anderson » Sat Apr 04, 2015 1:09 pm

Harry, some voltage checks are in order, but first I suggest trying this...

Try CH2, in the photos only CH1 appears to be selected. Try the ALT/CHOP/ADD buttons (the centre row of five). Put the timebase into 'calibrated' so the UNCAL LED goes off - but I doubt it will make any difference. Note any change in the trace position vertically as you try different modes. Put the 'DISPLAY' switch to 'NORM'.

Voltages, check the +5V, +8V, -8V and +120V are present - C903/C902/C904/C901 respectively - use the circuit diagram called 'VETAMP', Document Number TI-2009. Check the voltages on the CRT deflection plates, that is either end of R985 & R986. It's hard to guess from the diagram what the voltage might be, but anything from 10V-100V means that something is 'alive'. Make sure the vertical POSITION controls (both) are approximately centred.

Power off, unplug and re-plug CO801/CO901 several times. You may as well do the same with any others too. Let us know what you find.

Note that I'm mainly aiming at mechanical issues, connectors etc.. As this has had quite a hard bang I suspect what faults are to be fixed will be of an electro-mechanical nature.

Steve A.

Later...I forgot these...'MODE' switch -> 'AUTO', 'COUPLING' -> 'AC', 'SOURCE' -> 'INT'.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Sat Apr 04, 2015 5:29 pm

Steve Anderson wrote:Harry, some voltage checks are in order, but first I suggest trying this...


Thanks Steve i have very little time today or tomorrow due to the easter bunny ! :D but i will see what i can find out with your help here, with the time i do have to nick out to the shed and test i will report back .

I agree the the knock its had is a likely cause for most of its problems ,i still don't understand how that CRT survived it got a nasty wack from the back .

I did hope it was just a high voltage problem my luck its not , i will look into your tests and check for any thing mechanically caused ...thanks also for the setting i wasn't sure whats correct and whats not on this front ...its a little different to my other scopes.
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Sat Apr 04, 2015 7:46 pm

Between sorting easter eggs for tomorrow morning i did have a few minutes to go to the shed and try some of the settings .

The vertical does work ! it was the settings ! thank you Steve and i didn't even have to do much ...now again i am not sure its something that just works because moved it ,i suppose i will have to do a tap light bump test or am i pushing my luck !

The dual trace works yippy...

The horizontal still moves from side and jumps to one side does not like being centered so its either one side or the other if again this is a setting problem or not i am not sure i have not done all your settings or voltage tested yet .

Another step forward as it is now a working oscilloscope ! Now this is better than a few days ago ! :wink:

Heres a few pictures of simple touching of the probe test to check the vertical .
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anigif.gif
anigif.gif (306.55 KiB) Viewed 6244 times
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Harry Dalek
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Re: Tel-Instrument oscilloscope repair

Postby Steve Anderson » Sat Apr 04, 2015 8:02 pm

Harry, as you now have a trace/traces, forget the voltage measurements I mentioned earlier. Instead carefully measure the voltage on the wiper of the H.POS pot as you vary it from one end to the other, it should go smoothly from +8V to -8V (or the other way around). If the pot isn't easy to reach measure at C317/R353. Circuit is 'HOR AMP', Document Number TI2003, bottom left.

If it jumps suddenly from one voltage to another, (which may not be +8/-8) either the internal pot track is broken and/or the wiper isn't in good contact with the track. If this is so, the pot will need replacing. But check for dry joints or cracked tracks on the board if the pot pins are PCB mounted. The problem wll be finding a replacement as it also has the push-pull 'x10' switch, but you could opt to do without it by wiring it in the 'x1' position permanently. I find I very rarely use the 'x10' function, if ever.

There's no harm in going around the whole instrument tapping (gently) with the insulated handle of a screwdriver to see if there are any more gremlins that might otherwise surface later. It's better to do this now while you still have the covers off. When you can soak-test it for a full day with the covers on so it gets to normal temperatures.

With any luck you have a 25MHz dual-trace scope for five bucks and a bit of time and effort.

Steve A.

We still don't know what that extra transformer does, but if the thing's working - leave it!

Later on you may want to check the calibration, both the timebase (time/cm) and CH1/2 (V/cm). But that can be left for now.
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