Tel-Instrument oscilloscope repair

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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Sun Apr 05, 2015 10:37 pm

Thanks Steve the pot seems fine adjusting feel wise but it will not center the scan so your advice sounds good to me ,i will have a look tomorrow now i have more time .

i will try a tap test as well , i am happy its going but if it does still have problems yes better find out now.

Its a nice mid sized scope cheapest scope i ever bought.
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 » Mon Apr 06, 2015 10:44 am

Steve Anderson wrote: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.


OK i had a look at this its 7.44 volts in my case either side again must just be my power supply voltages every thing seems a bit lower .

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.


This is the case around the middle correct position but with a little fiddling it does center and stay there your thoughts of the pot being worn perhaps being handled ruff sound like the case .

I think i will leave it due to its a hard one to replace and live with it i suppose i could just replace it but its a lot of work to get it out and i can still adjust it now knowing what up .

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.


The tap test only effects the horizontal pot makes the trace jump but i will do the full day test and warm it back into service :wink:


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

Steve A.


I am very happy with it as is, i mean 5 dollar dual trace scope with z axis only my 60s one can do that i don't like using it much in case a valve gos .

I just have to replace that plug at the back as it got bent from what ever knock that started this all.

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


Yep i will live with it i bolted the little transformer back on tight so keep on power supplying what ever i am happy.

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.


Thats something i didn't think of but with my interests being slow scan and nbtv mainly it really only needs to be on the lower side from what the scope can do ,but i will look into how close both channel time bases are .

Thank you again Steve for you help here it made fixing it much easier !
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IMG_0655.JPG
bit if tight adjusting horizontal position can be got
IMG_0655.JPG (137.1 KiB) Viewed 4694 times
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Re: Tel-Instrument oscilloscope repair

Postby Steve Anderson » Mon Apr 06, 2015 3:18 pm

You're welcome Harry. I quite enjoy doing things like this by 'remote control'. If anything else comes up in the future don't hesitate to ask, if I can help I will - and that applies to everyone.

If you're happy to live with that faulty H.POS pot, OK. I did suspect it was going to be tricky to replace, as you say it may be worn out rather than broken as a result of the impact.

The +8/-8 supplies are a bit low for the output of 7808/7908s but unless this causes problems, again I would leave it alone. You're not going to use it as a laboratory instrument so it should be fine.

I had a quick look at the Z-axis modulation arrangement the other day, you may be in luck, it appears to be DC-coupled, but I need to study it further.

Steve A.

A little later...nope, the Z-modulation isn't DC-coupled, but it is DC-restored with the timebase acting as an internal sync pulse (what they call 'unblanking'). Thankfully the horizontal timebase frequencies for both NBTV and SSTV are very similar, 12.5Hz (NBTV frame) and 15Hz or 16.7Hz (SSTV line). So with a following wind it shouldn't need any modification. However, the Z-input voltage is going to be a bit strange, it needs to be negative, but that's something to look at later...not impossible, just odd.

Unfortunately for NBTV the internal horizontal timebase goes from left-to-right resulting in a left-right reversed image. But it does appear to have an X-Y mode on the main timebase control, fully clockwise. CH2 then becomes the input for an external timebase. But in doing this you lose those DC-restoration pulses, so they would need to be mixed in the the Z-mod signal. Or for NBTV just view the picture in a mirror!
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Mon Apr 06, 2015 4:50 pm

Steve Anderson wrote:You're welcome Harry. I quite enjoy doing things like this by 'remote control'. If anything else comes up in the future don't hesitate to ask, if I can help I will - and that applies to everyone.


I hate bothering any one with my problems but it sure is reassuring taking on a repair that i am taking the right path fault finding ! So knowing i had you looking over my shoulder this was good i am happy i have a new scope infact testing it next to the dick smith one i have i think i like this one better.

If you're happy to live with that faulty H.POS pot, OK. I did suspect it was going to be tricky to replace, as you say it may be worn out rather than broken as a result of the impact.


If i had to i would replace it but tossing up the work the position control doesn't bother me that much .

The +8/-8 supplies are a bit low for the output of 7808/7908s but unless this causes problems, again I would leave it alone. You're not going to use it as a laboratory instrument so it should be fine.


I see the transformer can be switched over to wide range of world mains voltages but it is set to ours ,could be they just didn't get it right for our country may be the reason all the voltages are a bit low.

I had a quick look at the Z-axis modulation arrangement the other day, you may be in luck, it appears to be DC-coupled, but I need to study it further.

Steve A.


I would like to use it so thats promising !

A little later...nope, the Z-modulation isn't DC-coupled, but it is DC-restored with the timebase acting as an internal sync pulse (what they call 'unblanking'). Thankfully the horizontal timebase frequencies for both NBTV and SSTV are very similar, 12.5Hz (NBTV frame) and 15Hz or 16.7Hz (SSTV line). So with a following wind it shouldn't need any modification. However, the Z-input voltage is going to be a bit strange, it needs to be negative, but that's something to look at later...not impossible, just odd.


So i would have to invert what ever into this ,i still have my old 60s scope that has the z axis as well if all else fails ,i wondered why it was call blanking input and not Z

Unfortunately for NBTV the internal horizontal timebase goes from left-to-right resulting in a left-right reversed image. But it does appear to have an X-Y mode on the main timebase control, fully clockwise. CH2 then becomes the input for an external timebase. But in doing this you lose those DC-restoration pulses, so they would need to be mixed in the the Z-mod signal. Or for NBTV just view the picture in a mirror!


Pity its not mechanical just reverse the motors , your information i will get back to when i get around to it as i would not have known this ...Thanks for this Steve and you get to knock off work early today for this ! :wink:

Now time to put the top and bottoms case lids back on ....
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 08, 2015 6:25 pm

Harry Dalek wrote:...and you get to knock off work early today for this ! :wink:....

"Job and knock" as they say in the UK...

Steve A.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Sun Jun 28, 2020 9:51 pm

Well this was 5 years ago and i find my self with 2 scopes to fix ...i will be getting if i can a new digital scope in a few weeks but will look into fixing my CRT ones while i wait or at least this one ...put the Project on hold till i have some thing to test it with .
Cluttering up the project build page so may as well add to this one and slowly look into it .
Renamed the files for a start to what parts of the circuit they were the Tel-Instrument ti-5252 looks to be from 1992 ..
recap I found it 5 yeas ago at a reuse shop at the tip 5 dollar scope original problem was it's inverter didn't work that's been fine since the last posting .

Problems..and so it should it looked like it was used as a football when i found it /
The scope has a ...long term intermittent problem second deflection amplifier trace ,both vertical position controls are intermittent same both signal inputs
Trace horizontal position seems warn
The Time Div seems out or failing on the lower end whats expected
Time delay switch control i am not sure is operating

When the scope works you tend to forgive its faults .

I have tapped the circuit boards to try and locate any thing poorly soldered or loose plugs all seems ok which is a bugger as it would be an easier fix .

Think next i will get the meter out and check the voltages are going where shown ,in the mean time i am studying the circuits .
Attachments
CRT.jpg
delay.jpg
Horizontalamp.jpg
inputs.jpg
powersupply.jpg
sweepgenerator.jpg
Triggenerator.jpg
VET.jpg
VETAMP.jpg
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 » Mon Jun 29, 2020 3:07 pm

It's seems to be much the same as I mentioned in another thread recently - it's usually the electro-mechanical things that fail. Switches, pots, connectors, relays and so on...anything with moving parts. The problem is getting replacements. Often they're 'specials' made to the manufacturers mechanical specification, electrically they're generally the same as any other 'industry standard' part.

The good news with getting a digital scope is that the front-panel switches and pots are often opto-sensors (rotary encoders) with no sliding contacts (pots/switches). The gain/time settings are all done in the software which controls the the gain of the channel and the sampling rate of the A-Ds. The scope I bought over 10 years ago (new) has never had any issue with this problem, I hope it never will.

The other advantage, no CRT. So the whole thing is much more compact - mine is the size of a ladies handbag and takes up little room on the work-bench. No high-voltage power and a lot less power consumption, runs cool, improved reliability... plus many have colour screens which makes each trace easier to identify....also a USB port for saving the whole screen to a flash drive as a GIF/JPG/TIFF, so easy to publish here.

Some also have a network connection (Ethernet) so it can be controlled/automated via a PC. Though I do have a network here I've not found a need to connect the 'scope to it...yet.

It is a significant investment, but you won't want to go back to a CRT scope after you've used a digital one for just a short while.

Steve A.

...and that's me saying that! Someone who has a keen interest in CRTs! Would you want to go back to an analogue multi-meter?

Added later...for all the plus points of digital 'scopes one thing I was a little disappointed with was the the screen resolution. My Tek is 320x240 which is OK (just) when using it in the normal manner, but the saved screen-shots are the same resolution. As it is 10 years old I would hope screen resolutions have got better these days. Usually when posting here I double the size of the TIFF file from the 'scope and convert it to a GIF, example follows...
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Mon Jun 29, 2020 5:12 pm

Steve Anderson wrote:It's seems to be much the same as I mentioned in another thread recently - it's usually the electro-mechanical things that fail.


Switches plugs trimmers Yes i have been taking on this advice Steve so far i have not found a culprit..i am going to see if i can take off the front panel there are parts of it i can't get at with out do this and i do have to replace a position control seem to control poorly .


Switches, pots, connectors, relays and so on...anything with moving parts. The problem is getting replacements. Often they're 'specials' made to the manufacturers mechanical specification, electrically they're generally the same as any other 'industry standard' part.


Looking at every thing apart from the crt and transformer i think its all replaceable ...knowing it works at times the vertical signal and deflection side much be ok i would expect as you think a mechanical thing i just haven't tracked down yet ...i did find a broken wire yesterday but must be double grounds to the little board from some where else find out when i take the from panel off .
Yes replacements exact same may be a problem but doing the exact same thing is best i can do as say a pot size shape .

The good news with getting a digital scope is that the front-panel switches and pots are often opto-sensors (rotary encoders) with no sliding contacts (pots/switches). The gain/time settings are all done in the software which controls the the gain of the channel and the sampling rate of the A-Ds. The scope I bought over 10 years ago (new) has never had any issue with this problem, I hope it never will.


After all these problems i think its time to update even the wife thinks i need a new one so that birthday coming up i pointed and now i have to wait !... ihope this one i am getting lasts too , i would think it would be beyond me to ever fix a 2020 digital scope looks to be a 100 mhz bandwidth ..so i am happy to hear yours has lasted

youtu.be/p_Bb-IWqnKM

The other advantage, no CRT. So the whole thing is much more compact - mine is the size of a ladies handbag and takes up little room on the work-bench. No high-voltage power and a lot less power consumption, runs cool, improved reliability... plus many have colour screens which makes each trace easier to identify....also a USB port for saving the whole screen to a flash drive as a GIF/JPG/TIFF, so easy to publish here.


Yes they look very small less bench space indeed ! hope it shows frequency that comes in very handy and seeing your posts on screen grabs this is something i have never had i will have to put the camera down now .

Some also have a network connection (Ethernet) so it can be controlled/automated via a PC. Though I do have a network here I've not found a need to connect the 'scope to it...yet.


I think it can but never even thought about this

It is a significant investment, but you won't want to go back to a CRT scope after you've used a digital one for just a short while.

Steve A.

...and that's me saying that! Someone who has a keen interest in CRTs! Would you want to go back to an analogue multi-meter?


MMM i can understand that like the fellow in the video i would think it's like if you never had one you would never miss it but when you have used one there's no going back ,i never thought i would get one as the CRT main scope worked well all these years but we all get old and sick ...Like you i will always still like them ...i will keep the new one in the house and if the old scopes are fixable they have a spot in my shed shop .
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 » Mon Jun 29, 2020 5:23 pm

Hmm...I'm not so keen on the wide screen display, OK here it's only 2 traces, but when you have four (or even five with trigger view) a more conventional aspect ratio is to be preferred. But I guess you'd get used to it. Horses for courses...interesting to note that Tek haven't done a wide-screen version, but I haven't checked in a few years...so again, I'm probably wrong.

Steve A.

Here's a promising contender, around US$500, up to 4 channels, 100MHz, 8", yes, eight inch colour display, battery portable, guess where its made? It's almost disposable, if it lasts two years, throw it away and buy a new one...60 cents a day....when your income depends on it, 60 cents a day is a bargain!
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Mon Jun 29, 2020 10:02 pm

Steve Anderson wrote:Hmm...I'm not so keen on the wide screen display, OK here it's only 2 traces, but when you have four (or even five with trigger view) a more conventional aspect ratio is to be preferred. But I guess you'd get used to it. Horses for courses...interesting to note that Tek haven't done a wide-screen version, but I haven't checked in a few years...so again, I'm probably wrong.

Steve A.

Here's a promising contender, around US$500, up to 4 channels, 100MHz, 8", yes, eight inch colour display, battery portable, guess where its made? It's almost disposable, if it lasts two years, throw it away and buy a new one...60 cents a day....when your income depends on it, 60 cents a day is a bargain!



Hell there all Nice Steve there all not to bad price wise go bonkers looking at whats about I can't really push my luck here and be to fussy this was a very economical option down to $297 family will be pleased and Harry will be more than happy .The 4 input scope looks nice but i am getting others to get this so i am pretty much going with what i would be happy with ....As the rolling stones song you can't always get what you want love to own a porsche but alas A vw for me .
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 Jul 01, 2020 11:45 am

Yep, I'm still bombing around in my 30-year old Nissan with 550,000km on the clock, original diesel engine. As much as I'd love a Jag, this will have to do...

Steve A.
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Wed Jul 01, 2020 1:31 pm

Steve Anderson wrote:Yep, I'm still bombing around in my 30-year old Nissan with 550,000km on the clock, original diesel engine. As much as I'd love a Jag, this will have to do...

Steve A.


Yes you have to put up with your lot really ..As with the scopes as long as they go it tends not to bother you i didn't care the best one was 30 years old ...lucky to have a tv that long! well i do have a 1962 job in a box still working hoarder i am ...well got many years out of the scope before any problems..

Came home and noticed a ebay purchase just the right size if i didn't know any better it is what i hope ...better not mention it to the wife :wink: darn have to wait till the 18th...oh well i will review it in the mean time and keep hush about it .

Yesterday i pulled the crt scopes front panel off they didn't make these easy to repair ! a lot of the pots i have found also are pull switch types the horizontal position is i think the only one causing problem it says the switch is a 10 mag when pulled to on .

i will leave that be for now i need to see whats happening with no vertical deflection side of it going to check the voltage around the deflection amplifier lucky i have a schematic its a nightmare to work on as is .
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 Jul 01, 2020 3:53 pm

I looked into the vertical Deflection...i haven't been able to find R 942 and 943 220k on the board tried to test the resistance between 470 ohm on the base of the 2 transistors to the +120v but got nothing close there ...

The 120v is fine showing 122v testing at the power supply i would expect a higher voltage than 9v..here so i think the 8v you see feeding in around the emitter area and 5v volts also must be 10 v i have to check this at there test points but the 120v is non existent open ?


i am point at the 2 470 ohm resistors on the base of the 2 transistors other end of those resistors i was expecting a 220k but no where to be seen yet

The problem area seems around where there was board damage but missing bit was still there track wise to what go's where .

My thoughts here are get another 2 220k resistors and wire the 120v to the Q973 and Q966..but not knowing here the board resistors are worries me if they connect again a higher voltage will pass to this point ...i can't see them any where near by i will keep on looking i haven't followed the circuit tracks yet that should help follow the trail .
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DSCN6742.JPG
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DSCN6744.JPG
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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 » Thu Jul 02, 2020 3:18 pm

Can you move the trace(s) up and down with the position pot(s)? If you can that means at least the vertical amplifier is working, just not getting a signal. Can you get two traces?

Steve A.

Quite an unusual vertical amp, not that it matters right now...
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Re: Tel-Instrument oscilloscope repair

Postby Harry Dalek » Thu Jul 02, 2020 5:40 pm

Steve Anderson wrote:Can you move the trace(s) up and down with the position pot(s)? If you can that means at least the vertical amplifier is working, just not getting a signal. Can you get two traces?

Steve A.

Quite an unusual vertical amp, not that it matters right now...


No vertical or vertical deflection positioning control its all out... that's why i have been looking at the deflection amplifier for now ...it looks to me like its not getting the 120v positive still trying to track down both 220k resistors that should be there even where the 120v comes in to that board ...sort of a catch 22 the board needs to be connected to find the problem ..i am trying to take it apart just enough to get at the problems.
I will try and find R998 and 999 the 120v is also feeding in there might help .Edit ...i found one of them R998 no conductivity to the 120 v supply either
So have no positioning control for vertical and second trace is out also .
At the power supply the 120v is there ..i will keep poking around .
Edit ..........i have found R942 and both 220ks r942 and r943 well both getting 120v fine so its not that move onto the lower voltages....feeding the deflection
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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