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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Mon 05, 2018 1:10 am 
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Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
Thank you RRM. I appreciate your time spent!

I had tried a larger trimpot for R32, hoping to reduce the current through the meter for the first tube I had purchased. But this alone did not help.

I understand what you are saying and will try this, or at least something similar, depending on the values I have in my parts drawer.

As pointed out by Fred Scoles on Feb 10 in this same thread, Triplett may have specified GE tubes. But after trying three additional GE tubes, I think it goes even further then specifying a tube supplier. In my previous life I used a lot of photomultiplier tubes. We arranged with our supplier to sort the tubes on their production line and pick out ones with extra sensitivity to a particular wavelength range. Of course we paid a premium on top of the regular price, but it made our own testing possible.

This may take me a few days.


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Mon 05, 2018 5:01 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3374
Location: Florida
R5 is the one that is critical to balance. The voltage drop across it must be equal to the battery voltage (22.5) to have zero current through the meter.

RRM


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Tue 06, 2018 1:10 am 
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Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
Hello RRM,

Success. Meter is zeroed for even the worst performing tube.

Following the spirit of your recomendation, a 50K trim pot was added in series with R5. The calibration pot (originaly a 3.5K trim pot but now replaced with a 2.5K trim pot because it was frozen in the 1.7K position thanks to a failed D cell during long term storage) was left alone.

For the selected worst performing tube (still a GE 1R5) and the front panel zero adjust set to minimum meter deflection, the 50K trim pot needed to be set at ~40K to achieve zero meter deflection, current through R5 was ~0.29mA. A 9V battery (reads 9.4V on my Simpson 260) read 9.2V with the calibration pot position untouched (it was calibrated with the original GE tube that could zero the meter without any klooging).

For the same tube and the front panel zero adjust set to maximum meter deflection, the 50K trim pot needed to be set at ~2.5K to achieve zero meter deflection, current through R5 was ~0.52mA. The 9V battery read 9.5V with the calibration pot position still untouched. The meter reading could easily be recalibrated with the existing 2.5K calibration trim pot.

As you said, the voltage drop across the "new" R5 is 23.2V and 22.1V for the two conditions above when the meter was zeroed, respctively, nearly the same as the battery (I am sure there is considerable error in both my resistance and current readings as it was just a quick glance to get an idea).

Thank you again for your time and help!


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Wed 07, 2018 3:14 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3374
Location: Florida
Good show. Making things work is fun.

I almost goofed and forgot to document how this thing works. Now the info is safely in my file in case I wind up with one of these sometime.

RRM


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