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 Post subject: Troubleshooting a Triplett 631 VTVM
PostPosted: Oct Thu 27, 2016 3:02 pm 
Member

Joined: Dec Sat 22, 2012 3:20 pm
Posts: 659
Location: Marenisco, MI
Been trying to get the VTVM portion of my Triplett 631 VOM/VTVM to work. It indicates
nothing on any range in AC, DC or resistance. I've verified that the probes are good and that
the probe connector and wiring to the switches is good. I have carefully cleaned all the switch
contacts. The voltage divider resistors are all within tolerance. The rectifier was replaced by its previous owner but the two glass diodes cleck good. Their associated resistors I "think" are good (previous thread about the resistors and my resistors measure approximately what others measured on their 631s).
The 1R5 tube is new and has been aged as per the instructions (150 VDC on plate etc etc). The meter movement is good and works perfectly on the VOM. The VOM/VTVM switch is good and has been cleaned.
The batteries are fresh (two 22.5 volt and one D-sized 1.5 volt ).

So why doesn't this thing indicate anything when I input a voltage?

So I have been examing the 1R5 wiring and not sure it is correct. It has all of the grids connected together and these grids (pins 3, 4 and 6) are jumpered to pin 2 (the plate) and I measure 45 volts on all grids and the plate. H-m-m-m. This didn't seem right to me so I diconnected the jumper to the plate but there was no change in the meter's not indicating anything when I input an AC or a DC voltage.

Ideas?


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Oct Thu 27, 2016 6:40 pm 
Member

Joined: Nov Mon 09, 2015 2:55 am
Posts: 229
Location: St. Louis, MO
I assume you have the manual with the schematic:

http://bama.edebris.com/manuals/triplett/631/

It sounds like you checked the VTVM probe with another meter and it is good.

Do you have the range switch set to the VTVM ranges, not the regular VOM DC ranges? I know that threw me for a loop when I first got my 631. The VTVM ranges are down at the bottom of the dial.

Obviously you have to put the probes in the VTVM specific jacks.

The B+ tests OK with the test button (and a meter)?

As far as the wiring goes compare it to the schematic in the above link.

_________________
--73


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Oct Thu 27, 2016 10:31 pm 
Member

Joined: Dec Sat 22, 2012 3:20 pm
Posts: 659
Location: Marenisco, MI
FlyingHacker, thank you. Boy am I embarrassed...and stupid! I did not realize the 631
only measured DC on the VTVM. Sure 'nuff, when I selected those bottom ranges my 631
measured accurately. I have used the VOM portions for years and never attempted
to use the VTVM portion until my "regular" VTVM was giving me some screwy readings and I
wanted to verify them with another VTVM. As it turns out, both of my VTVMs are correct and
the screwy readings are evidently due to a circuit problem in the gear I'm troubleshooting.

Guess I'd better read the manual which I obviously have never done.

Thanks again for turning on the light bulb over my head!

Sparkie


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Oct Fri 28, 2016 5:15 am 
Member

Joined: Nov Mon 09, 2015 2:55 am
Posts: 229
Location: St. Louis, MO
Yeah, would be nice if it would read AC as well, but it is compact and battery operated.

It must have taken me five or ten minutes to figure out that I needed to use those bottom ranges for VTVM DC. It is always a good thing to read the friendly manual.

_________________
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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Feb Fri 09, 2018 10:10 pm 
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Joined: Feb Fri 09, 2018 9:26 pm
Posts: 3
Just repaired a Triplett Model 631. The VTVM calibration pot (R31) had a break in the winding. A new pot, plus two 412A batteries and all works fine. I had also purchased a new 1R5 tube as well, but can not adjust the meter to zero with the VTVM probes shorted. With the old tube, no problem. So, perhaps I need to "age the tube" as mentioned above? Can any person give me instructions?

As an aside, I noted a few minor errors in the circuit diagram widely distributed on the internet. You find these in switch S2-2B and S2-3B. In addition, since my SN is 3315, and the circulating diagram is for SN above 25000, I would add that the values of R31 and R36 are 3.7K and 3.4K in my Model 631, and there is an additional 2.5K resistor between R31 and B3 when the unit is set for VTVM operation.

Attachment:
EditedTriplettModel631Schematic.jpg
EditedTriplettModel631Schematic.jpg [ 33.89 KiB | Viewed 156 times ]


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Feb Sat 10, 2018 12:07 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1952
Location: Oswego, NY, USA
You need the Apr '54 thru 1958 version of the manual for your early Triplett 631. I have some archive versions of them, but no extra copies, so I could email to you a multi-page pdf we scanned a few years ago which documents all of the documented parts changes made to the 631 from Apr '54 (Run #1 for model 631, designed by Norman Triplett). The text pages never changed, so all you'd need are the correct ckt dwg & parts lists to make a complete manual for your older 631. PM to me your email. The older 631 manauls list R31 Variable as 2K; R26 as 3.4K, and the additional resistor you mentioned is R37 2.5K. Your sn is probably not newer than 1958. I could also email a couple relevant files. fwiw, factory sheets usually specified a GE brand tube, because some (even at that time) Sylvania and certain Euro versions (DK91) tubes would not zero properly in a 631, even after ageing. Realize that the ckt is running the multi-element 1R5 (hexode...?) tube simply as a triode...only because at that time they couldn't find a dependable triode physically small enough and requiring only 1.5V filament (from the D cell) and 22V for the other elements....so they compromised by using the 1R5.

Fred


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Feb Sat 10, 2018 4:11 pm 
New Member

Joined: Feb Fri 09, 2018 9:26 pm
Posts: 3
Thank you Fred.

That is a big help. Yes, the original tube is from GE.

The replacement I purchased is by Rad Tel Tube Co. Hoping to improve my chances to zero the meter I already ordered a Siemens DK91 and expect it middle of next week. I will do what I can but I will not be too optimistic. Your comments at least remove some of the mystery for me.


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Feb Mon 12, 2018 4:09 pm 
Member

Joined: Dec Sat 22, 2012 3:20 pm
Posts: 659
Location: Marenisco, MI
Hi Joe,

I have not found "pre-aging" a 1R5 (or any other tube used in test equipment for that matter) to be
of significance toward calibration or zeroing problems. For the most part I have only done it
just so I can say to myself I did it and get a feeling that I have done all I can to solve a problem.
It is recommended for tubes that are driven hard to minimize premature failure.

But here goes the "recipe":

- Connect 100-125 volts to the plate
- Connect the grids and cathode together
- Apply the rated DC voltage to the filament
- Let it "cook" for 40 to 50 hours

This is not an old family recipe. It came out of Joseph Risse's book, VOM VTVM Handbook.

Hope this helps.


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 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Feb Mon 12, 2018 4:15 pm 
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Joined: Feb Fri 09, 2018 9:26 pm
Posts: 3
Thank you Sparksalot.

I have a second tube on the way and will probably try that first.


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