Calibrating Weston 981 type 3 tube tester

[Dekatron]

I can't say if it is ok or not since I have never dealt with these testers, only looked at the schematics and found similarities with a blocing oscillator design as I have built a few of those in experimenting with Trochotrons.

However I found this document: http://www.kw.igs.net/~knickerson/img/GmAnalysis.pdf at this webpage: http://www.kw.igs.net/~knickerson/heathkit.html which includes a scope picture of the grid voltage and explanation for the gm measurements. It looks like your scope picture show a positive peak whereas it should be negative (perhaps you swapped electrodes on the scope probe?). You might have to adjust your trigger to get a stable trace on your scope.

Here's another page on problems with the Heathkit TT-1: https://paulcarbone.com/blog/heathkit-tt-1-troubles/ which also references the other webpage but it shows a few capacitors being out of tolerance, however I couldn't see those in the Weston tester after a quick look on the schematic diagram. Checking capacitors is good anyways as they age and change their values. If any of them would change in the oscillator it would probably change frequency and possibly the amplitude of the grid signal and that might affect the measurements.

Checking all components to make sure that they are within the stated tolerances is always good.

[nzoomed]

Originally Posted by Dekatron

I can't say if it is ok or not since I have never dealt with these testers, only looked at the schematics and found similarities with a blocing oscillator design as I have built a few of those in experimenting with Trochotrons.

However I found this document: http://www.kw.igs.net/~knickerson/img/GmAnalysis.pdf at this webpage: http://www.kw.igs.net/~knickerson/heathkit.html which includes a scope picture of the grid voltage and explanation for the gm measurements. It looks like your scope picture show a positive peak whereas it should be negative (perhaps you swapped electrodes on the scope probe?). You might have to adjust your trigger to get a stable trace on your scope.

Here's another page on problems with the Heathkit TT-1: https://paulcarbone.com/blog/heathkit-tt-1-troubles/ which also references the other webpage but it shows a few capacitors being out of tolerance, however I couldn't see those in the Weston tester after a quick look on the schematic diagram. Checking capacitors is good anyways as they age and change their values. If any of them would change in the oscillator it would probably change frequency and possibly the amplitude of the grid signal and that might affect the measurements.

Checking all components to make sure that they are within the stated tolerances is always good.

I just stuck the scope probe across the pins on the socket where the instructions said to measure. Perhaps the scope is out of phase, and reversing them may show it different, what I can say is your probably right about it showing a positive peak, as when I connected the scope, the wave shifted up on the CRT and I had to move the trace line lower down, in other words, its mostly reading above zero volts and not much swinging below zero, which would suggest its pulsing?

Anyway, regarding capacitors, I did replace all the old electrolytics.
There are no paper caps in this unit from what I can tell.

[Dekatron]

Just be careful when you hook up a scope as yopu might need an isolation transformer for either the scope or the tester as you might otherwise form a ground loop shorting them!

It looks like there are at least a few non-electrolytic caps from the component list: C1, C4, C5 & C8 - I couldn't find C8 in the schematic though.

C6 seems to be a non-polarised electrolytic, as it doesn't have any polarity shown - but in the Heathkit TT-1 it is drawn like a normal polarized electrolytic.

[nzoomed]

Originally Posted by Dekatron

I can't say if it is ok or not since I have never dealt with these testers, only looked at the schematics and found similarities with a blocing oscillator design as I have built a few of those in experimenting with Trochotrons.

However I found this document: http://www.kw.igs.net/~knickerson/img/GmAnalysis.pdf at this webpage: http://www.kw.igs.net/~knickerson/heathkit.html which includes a scope picture of the grid voltage and explanation for the gm measurements. It looks like your scope picture show a positive peak whereas it should be negative (perhaps you swapped electrodes on the scope probe?). You might have to adjust your trigger to get a stable trace on your scope.

Here's another page on problems with the Heathkit TT-1: https://paulcarbone.com/blog/heathkit-tt-1-troubles/ which also references the other webpage but it shows a few capacitors being out of tolerance, however I couldn't see those in the Weston tester after a quick look on the schematic diagram. Checking capacitors is good anyways as they age and change their values. If any of them would change in the oscillator it would probably change frequency and possibly the amplitude of the grid signal and that might affect the measurements.

Checking all components to make sure that they are within the stated tolerances is always good.

Originally Posted by Dekatron

Just be careful when you hook up a scope as yopu might need an isolation transformer for either the scope or the tester as you might otherwise form a ground loop shorting them!

It looks like there are at least a few non-electrolytic caps from the component list: C1, C4, C5 & C8 - I couldn't find C8 in the schematic though.

C6 seems to be a non-polarised electrolytic, as it doesn't have any polarity shown - but in the Heathkit TT-1 it is drawn like a normal polarized electrolytic.

All the electrolytics I found were polarised, there were 3 large ones and one small one iirc.
The rest appear to be a polyester type capacitor, and everyone who has recapped these things appear to have left these alone

My Tektronix scope doesnt have any earth, so its floating with no ground reference, same for the tester for that matter, so im assuming thats OK?

[Dekatron]

If all capacitors are within their tolerances it must be your scope that wasn't triggering properly, if not you have a very unstable signal from the oscillator.

As long as the measurements are ok you won't need to replace any of the polyester capacitors, but they too age and sometimes degrade over time.

[nzoomed]

Originally Posted by Dekatron

If all capacitors are within their tolerances it must be your scope that wasn't triggering properly, if not you have a very unstable signal from the oscillator.

As long as the measurements are ok you won't need to replace any of the polyester capacitors, but they too age and sometimes degrade over time.

I think I will test it out on my mate's digital scope, that way, it can settle my mind!