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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bit if tight adjusting horizontal position can be got
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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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Harry Dalek
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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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Harry Dalek
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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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