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My new VTVM (needs repair)

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carbonzit

Active Member
Coincident with several recent threads here about these ancient instruments, I very recently acquired my first VTVM. It's a working Simpson 311, a bit battered on the outside but apparently working well on the inside. Got it for $5 at a local recycled-goods store! The plastic meter cover has been broken on one side, and rather crudely, yet cleverly repaired by cutting a piece of a plastic box to fit over it. Ugly, but it works.

It appears it might be a military version: the case looks regular, but both tubes inside are JAN (Joint Army-Navy) types. Plus the D cell is an olive-drab issue! (Plus I picked up another meter with this one at the same place, an even more ancient (1939) Weston "Volt-Ohmmeter" in a Bakelite case, a Signal Corps issue according to the nameplate on top, both with hand-written tags by the same person. This one, unfortunately, seems to be non-working, with a cracked glass meter face, but it looks so cool!)

It's a way cool piece of test equipment, in my view: I've always wanted one, ever since I was a kid. Only had to wait ... lessee ... many decades, anyhow, to get it.

But of course it's not working perfectly. I've only tested low DC voltages so far. A 9-volt battery reads 11-something volts. So obviously it needs some adjustment. I see several adjustment pots inside (which I am not going to touch until I know what I'm doing).

Questions (and I'm especially hoping Dean Huster might take an interest and reply here):

1. Anyone know where I can get a manual and schematic for this, hopefully for free? I see them being sold all over the place, so this might not be possible. At least I need a service manual for it.

2. It came with part of a "probe". Does the probe contain any resistors or anything else other than just a straight-through conductor? Can I make one myself? I can certainly pick up a BNC connector locally. The ground connector just has an alligator clip on it; this too is easily made with just a banana plug.

3. If it seems to work OK, is it likely that it actually is working OK? I don't have any good references to test it against, other than zeners or 78xx regulators.
 

carbonzit

Active Member
You, sir, are a gentleman and a scholar, as they used to say.

Thanks a million!

I'm surprised how simple a circuit my "new" VTVM is.
 

carbonzit

Active Member
According to the schematic, the probe has a switchable 7M resistor in it, for use as an isolating resistor for DC measurements, according to the manual. Should I just get a 7M resistor for a DC probe?
 

carbonzit

Active Member
it has some some uuF (pF) capacitor, perhaps. Resistor is not for DC blocking !![/url]
No, it only shows a 7M resistor, as I wrote. Concerning the probe, the manual (the one you so kindly provided) says

A switch on the probe handle sets up a direct connection for either AC or ohms readings, or connects an isolating resistor into the circuit for DC readings.
and the schematic (page 21) shows a 7M resistor.
 
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unclejed613

Well-Known Member
Most Helpful Member
the handwritten tags are most likely calibration stickers. you might also try doing a search for the Army calibration manuals for these meters. they should begin with "TM-" and end with a "-5", "-15", "-25", "-35", or "-45".
 

mvs sarma

Well-Known Member
that is a series resistor at input for range extension and to prevent too high a voltage,sparking inside the probe across wires. They are called High voltage probes.
 

carbonzit

Active Member
that is a series resistor at input for range extension and to prevent too high a voltage,sparking inside the probe across wires. They are called High voltage probes.
I don't think that's correct.

As I stated directly from the manual, the resistor is an isolating resistor for DC readings (meaning all DC readings, not just high-voltage ones). This seems to make sense, since even after calibration, the meter consistently reads too high on DC values. (I went through the calibration procedure in the manual.)

I'm going to rig up a 7M resistor setup and give it a try.

==================================

Update: Sure 'nuff, I rigged up a 7M resistance (2-10M in parallel with 2-1M in series), and whaddya know: a 9-volt battery measured ... 9 volts, dead-on (confirmed with another meter). So I'm gonna build me a probe with the resistors and a switch, and I'll be in business.
 
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unclejed613

Well-Known Member
Most Helpful Member
for future reference, i found the mirror for the Boat Anchor Manual Archive, which has manuals for a lot of old electronic equipment.

http://bama.edebris.com/manuals/
 
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