Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Nov Tue 13, 2018 5:17 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 24 posts ]  Go to page 1, 2  Next
Author Message
 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: 686
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?


Top
 Profile  
 
 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: 280
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


Top
 Profile  
 
 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: 686
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


Top
 Profile  
 
 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: 280
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.

_________________
--73


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Feb Fri 09, 2018 10:10 pm 
New Member

Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
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 1340 times ]


Top
 Profile  
 
 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: 2009
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


Top
 Profile  
 
 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: 11
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.


Top
 Profile  
 
 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: 686
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.


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Feb Mon 12, 2018 4:15 pm 
New Member

Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
Thank you Sparksalot.

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


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Feb Wed 28, 2018 2:59 pm 
New Member

Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
I wanted to follow up on a few remarks in this thread. It may be of interest to others repairing Triplett Model 631 VTVMs, especially those with problems zeroing their VTVM meter who come upon this thread in their search for help.

As mentioned above, I repaired a Triplett Model 631 (bad calibration pot), but then had trouble to zero the meter in VTVM mode with a replacement 1R5 tube. (On the other hand, the GE tube originally in the meter allowed the meter to be zeroed just fine.) From the posts here, and from other dialog, it seemed a possible (but not probable) solution might be to "pre-age" the tube using a recipe like that offered by Sparksalot. In addition, per Fred Scoles, it can be the case that Triplett specified GE brand tubes for the Model 631 for just this reason (i.e. not all 1R5 tubes are created equal).

Here is what I did...
I procured five 1R5 tubes from four different sources. Included were a Siemens DK41 (equivilant to the 1R5), a RadTel 1R5, and three additional GE 1R5 tubes of different vintages (based on variations in the construction details including the shape of the getter). Amazingly, none of the tubes allowed the meter to be zeroed: after five minutes, the mimimum achieved meter settings on the VTVM 30V scale (using VTVM probes with internal 1M Ohm resistor shorted out) were as follows:
Siemens 13.0V
RadTel 12.5V
GE1 3.5V
GE2 1.5V
GE3 27.5V

Four of the tubes were then treated as follows with 1.4V across the filament:
Siemens No treatment
RadTel 48 hours at 90V plate voltage followed by 48 hours at 125V plate voltage
GE1 48 hours at 90V plate voltage
GE2 48 hours at 125V plate voltage
GE3 48 hours at 90V plate voltage

Anode and filament voltages were monitored with separate multimeters as well as the filament current (50mA). Voltages were from power supplies rather than batteries.

The reason to use 90V is this is the maximum voltage stated on the GE data sheet for the 1R5 tube whereas the 125V was taken from the offered recipe. The connection details are attached.

All five tubes were then checked for functionality along with the original tube (the latter still could zero the meter just fine). Listed below are the minimum achieved meter readings after five minutes on the VTVM 30V scale with the probes shorted and when connected to a 9V battery.

Special note: The RadTel tube was also checked after the 48 hours at 90V plate voltage with the same result as indicated below - there was no change in its behavior with the additional 48 hours at 125V plate voltage.
min with 9V battery
Siemens 13.0V 23.0V (no treatment and no change)
RadTel 2.0V 11.5V (much better, even after the 48 hour first treatment)
GE1 5.0V 14.7V (a little worse)
GE2 3.0V 12.6V (a little worse)
GE3 meter is off scale (worse)

The 9V battery readings are all for the same position of the calibration pot, but working with the GE2 tube and setting the calibration pot to give a total meter deflection to match the 9.4V reading on my Simpson Model 260 made no significant difference in zeroing of the meter.

So, in conclusion, pre-age treated tubes gave mixed performance and was certainly not the cure all for the "zeroing the meter" problem. The Siemens tube is still in its original state, and the other tubes are all here, so if someone would like me to run any additional tests please let me know. Thanks to Sparksalot and Fred Scoles for their valuable help.


Attachments:
1R5PreAgeCircuit.jpg
1R5PreAgeCircuit.jpg [ 255.94 KiB | Viewed 1143 times ]
Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Thu 01, 2018 1:48 am 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3434
Location: Florida
Has anyone puzzled out the equivalent circuit Triplett used for the VTVM circuit? Tracing things out through the switches is a pain and so far the circuit doesn't seem to be the sort of half tube/half resistor bridge used in older single-triode VTVM's.

I don't have a 631, just curious as to the design. Once sorted out it may be possible to substitute a FET and a single 9V battery.

RRM


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Thu 01, 2018 2:27 pm 
New Member

Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
Here is what I have. Please excuse the poor quality. The pot near the meter is to calibrate the meter with a known voltage source connected to the TTT631. The pot closer to the tube is to zero the meter with the TTT631 connections shorted. This is for models with serial number 25000 or larger.

Attachment:
TTT631VTVM.jpg
TTT631VTVM.jpg [ 48.06 KiB | Viewed 1078 times ]


Edited by PFH2QVO on March 3, 2018:
The diagram above has an error. There should be a connection across the top from the VTVM jack to the + side of the meter. See additional posts below.


Last edited by PFH2QVO on Mar Sun 04, 2018 2:09 am, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Thu 01, 2018 8:16 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3434
Location: Florida
delete


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Sat 03, 2018 12:10 am 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3434
Location: Florida
Using the schematic on the web I came up with something similar but that kind of circuit has a big problem in that the meter will never see zero current. I expected some kind of bridge similar to the usual VTVM circuit but with a resistor in place of one of the triodes. I have an EMC 106 VTVM that uses this kind of circuit for its ac ranges.

I suspect the only way to sort this out is to trace out the circuit or find a service manual. Notice that they didn't show the pins of the 1R5, just depicted it as a triode. I suspect that's not the only omission or error.

RRM


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Sat 03, 2018 3:22 pm 
New Member

Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
Hello RRM,

I am so sorry. There is a problem with my drawing above. The VTVM input is also connected to the + side of the meter.

I checked the tube connections on my TTT631.
Attachment:
1R5fromGE.jpg
1R5fromGE.jpg [ 9.42 KiB | Viewed 1022 times ]


The drawing comes from the GE data sheet for the 1R5.

Of course pins 1 and 7 are to the 1.5V battery, positive side to pin 7. Pins 2, 3 and 6 are connected together and go to +22.5V. Pin 5 is internally connected to pin 1 (checked my tube with meter to confirm). Pin 4 connects to the switch to sample the voltage to be measured.

I am by no means an expert, but I think it works like this....
There is a loop formed by the 1R5, the two 22.5 volt batteries, the 40K resistor and part of the 6K resistor. The meter, along with the 3K resistor, the 3.5K resistor that calibrates the meter, and the rest of the 6K resistor not in the loop is sensitive to the voltage difference on two points in this loop. If the effective resistance of the tube is the same as the 40K resistor plus the part of the 6K resistor in the loop, the meter should read zero. We adjust the 6K resistor for this when we zero the meter with the input shorted. With a voltage applied to the input jack, the loop is not balanced on these two points, and the meter now measures a current. We should adjust the 3.5K resistor to make the reading correct using a known applied voltage.

I am interested in your comments. Feel free to shoot it down.


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Sun 04, 2018 12:10 am 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3434
Location: Florida
Ok I took another look. The key to this circuit is the use of two batteries in one loop and one in the other. Current from both loops flow through a common resistor (the 40K connected to the pot slider). If the voltage drop across this resistor exactly matches the voltage driving the single battery loop there will be no current flow in that loop (meter movement). If it exceeds it the meter will be driven backwards; if it's less the meter goes upscale. The zero pot balances things out. This is a clever circuit but is highly dependent on the tube characteristics.

Now tell me what that 10 meg resistor (R34) is doing.

RRM


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Sun 04, 2018 2:06 am 
New Member

Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
Hello RRM,

R34 is not active when the TTT631 is in VTVM mode.

For all non-VTVM dial settings, R34 and C2 are in parallel with the meter. It has a time constant of 1sec and, I guess, damps the meter movement.

This feature is one of the corrections made in the circuit for the TTT631 a few posts above. The circuit diagram in the manuals and circulating on the internet has switch S2-2B and switch S2-3B wrong. This was checked with a continuity meter on my unit, and it makes sense. S2-2B breaks the connection between R34 and the wire attached at the X100K post on the switch (ultimately leads to the + side of the meter) when any VTVM setting is selected. In a similar way, S2-3B breaks the connection between R18 and switch S1-1F for any VTVM scale selection (I have traced the circuit for each dial setting, the design is remarkable and I can see no possible reduction in the number of components or possible further simplification).

But in VTVM mode, C2 is now in parallel with R6, R7, R8 and R9. When connected to a voltage source at the VTVM input jack (and using the VTVM probes that have an external 1M resistor in series with the input jack within the lead package), C2 charges through the resistor in the probe set (time constant 0.1sec) and then discharges through the combination of R6, R7, R8, and R9 (total of 10M, thus a time constant of 1 sec).

You are right about the VTVM circuit being highly dependent on the tube characteristics. As posted above, five purchased tubes failed to allow the meter to be zeroed with the VTVM input shorted... but the tube in the meter as received works just fine.


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Sun 04, 2018 2:23 am 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3434
Location: Florida
Which direction was the zeroing problem? Was the meter always reading above or below zero?

RRM


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Sun 04, 2018 3:46 am 
New Member

Joined: Feb Fri 09, 2018 9:26 pm
Posts: 11
Hello RRM,

Always above.

Thanks in advance for any ideas you might have.


Top
 Profile  
 
 Post subject: Re: Troubleshooting a Triplett 631 VTVM
PostPosted: Mar Mon 05, 2018 12:40 am 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3434
Location: Florida
Ok. The meter always reading upscale means the tube isn't pulling enough current.

I found a 1R5 in my tube stash, set up a breadboard of the 631 circuit, and verified the functional description I posted earlier. In a nutshell, higher tube current will drive the meter downscale, lower upscale. The tube I had (Amperex) pulled too much current. My meter always went downscale and could not be zeroed. The fix in my klooge was to lower the value of R5. This reduced the voltage drop, allowing it to equal the battery voltage instead of exceeding it. Raising the value of R5 will compensate for low current draw. How much depends on the tube's idle current.

As might be expected, things interact in this circuit so more than one change may be needed to make things happy with a particular tube. If I owned a 631 I'd be tempted to change R5 to 33k or so with a 25K trimpot in series. If this allowed zeroing but not calibration, then also replacing R36 with a 10K trimpot. These are only guesses for starting values. The idea is to get things set up so that the zero and cal pots are about mid scale.

Due to the circuit being very dependent on tube characteristics I wondered about linearity. It turned out to be very good in my klooge . I calibrated for full scale at 1V in and the meter then read about 0.52 or so for 0.5 in. That is 2% of full scale which is not bad for an analog meter.

Now if I had a 631 of my own I could have some more fun.

Happy klooging.

RRM

p.s. I'm sure Triplett knew all of this but adding trimpots would have meant added cost. More parts to buy and install and longer to calibrate. Much easier to specify a particular tube brand, probably the one they designed around.


Top
 Profile  
 
Post New Topic Post Reply  [ 24 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot], Lou deGonzague and 8 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB