V.T.V.M. woes

[clay1905]

Hi Folks,
I have an old V.T.V.M., made by Palec of Sydney. The meter appears to be of 1947 vintage, based on the date on the movement scale. The meter is a model is VTM V.T.- (probe) Multimeter.
Considering it was such a popular meter in it's day, there's a serious lack of info on this one out there. I believe it was world famous all over Mascot.

Anyway, the instrument seems to have a few issues. The one that's driving me nuts is a small piece of tinned wire that forms a link between two of the function selector switches. This little bit of wire effectively bypasses the range resistor tree, passing whatever input is on the D.C. in socket directly onto one grid of the first amplifier stage. I don't think that +1000V on the grid of any twin-triode would do any good.
So, the question is "what on earth were they thinking?" This piece of wire looks just like it's been there since 1947. It follows that I must be missing something else.

I have attached a schematic-cum-layout diagram. Because I wanted to show the various wires in their colours, white is represented as a simple dotted line, and the mystery wire as dots and dashes. The actual mystery wire links between terminals 5 & 6 on switch 1 of the function selector, then passes across to switch 2, where it links terminals 4, 5 & 6.
I believe the schematic is correct now. If any anomalies are spotted, I would very much like to know.  Two possibilities. 1. I have made an error. 2. The meter has been modified, altered etc, and I'm not spotting it.
Disconnecting the mystery wire makes the DC function work, but I'm not at all happy with that arrangement as I've made a change I don't fully understand.
At the moment, I think that this bypassing link might be related to switching in/out a few small resistors associated with small AC measurements. However, I have been poring over the drawing for too long.
Any assistance with this would be very much appreciated.

Clay.

[trobbins]

Hi Clay, does the circuitry generally follow the typical VTVM circuit configuration described in:
http://www.angelfire.com/electronic/funwithtubes/VTVM.html
and used by Heathkit:
https://www.rsp-italy.it/Electronics/Kits/_contents/Heathkit/Kits/Heathkit%20IM-11%20VTVM%20manual.pdf

My B&K 375 is pretty similar to those.

[clay1905]

Hi trobbins,

In very general terms, yes. The circuit I'm working on is a full 4 triode bridge. The basic operating principle is the same, but with a second duo-triode valve that amplifies the output of the first.
The most similar circuit design I can find is an American instrument, one that seems equally obscure,  "Clippard Laboratories Model 406". (P 293 of Vacuum Tube Voltmeters, by John F. Rider), available here:
http://www.tubebooks.org/Books/rider_VTVM.pdf

I have been studying the above text, along with another book, "The V.T.V.M., How it Works and How to Use It." By Rhys Samuel, and available here:
https://www.jumpjet.info/Pioneering-Wireless/eBooks/useful/1956a.pdf
Both of which have been quite useful.

From what I can tell, the meter is almost as it should be, and some silly little thing is wrong but I'm blind to it.

[clay1905]

I should add that I'm presently trying to disentangle the overall schematic to make a simplified DC circuit drawing. Early versions of these instruments were a lot more complex than was ultimately needed. I'll post as soon as I'm satisfied that it actually represents more than a collection of assumptions and errors.

[trobbins]

Yes Paton were a pain with their schematics - easy to track wire connections, but ugly to comprehend what was in circuit and what was switched out of circuit (I restored a Palec VCT2 valve tester).

[clay1905]

Oh. Um. I have one of those in the cupboard, waiting. The filament winding in the transformer needs replacing. It's potted in tarry stuff (if they're the same model) and it's waiting until I feel game enough to tackle it.
We may talk again on this subject.
I haven't been able to source an "official" schematic for the V.T.V.M., so I only have the one I've traced out.
And you've given me a little idea that just *might* help with finding what that link wire is really about.

[duak]

Clay, it seems to me that the black wire connected to the terminal on the left is not original - it looks like it's just laid on.  Both the black wire and the yellow wire look like a modern PVC insulation rather than the fabric-ish stuff on the others.

I was going to suggest the Rider book - I see you've already found it.

Keep us posted - all the best!

[clay1905]

Hello Duak,
Thanks for your observation. I now realise that photo could be very misleading.
Indeed, many of the bridging wires on the switches are cotton covered, but all the rest of the wiring is P.V.C insulated. How I wish....

[clay1905]

Looking at this meter, there are so many reasons I see for the wire bridge being incorrect. All the other wiring fits where it belongs like a hand in a glove. The circuit diagram shows that the bridge shorts out the constant impedance of the meter. As I mentioned, it also puts whatever is on the positive DC terminal straight onto the grid of V2.
The wire is too long to be just a tail left by the assembler, but it can't have gone to where it was connected last either. For the time being, I think I'll leave it disconnected. As I look at the schematic, I think it might be bound up with the capacitor associated with the A.C. input. I think that capacitor is a coupling capacitor, and is out of place.

[trobbins]

Is there any plausible use for the wire for testing some circuitry or part, or for fault finding ?

[clay1905]

trobbins
Hey, I hadn't thought of something like a test point. This might be worth following up too.

I need to study some more, particularly the schematic, but I'm becoming more convinced that there were originally two bridging wires, and the one on switch 1 is the end of a capacitor leg. And also that they never originally connected.

Thus far I haven't paid to much mind to A.C. input. One thing and a time and all that. But the schematic isn't making a whole lot of sense around that area at the moment. As things are, an A.C. signal would go straight onto G1 of V3. This provides no means of attenuation or removing any D.C. component. I don't think that's right. Actually, just glancing at the schematic now, A.C. in looks more like a very tidy bowl of spaghetti than a a map of a circuit.

It's a funny rainy smokey stormy day. I think I'll stay indoors and try to figure out some more of this meter. I'm not too sure what it was, but some little thing you said twigged me to look at this capacitor and the A.C. input in general. Today's the day.

[h4x0r]

Hi Clay,

If you have no success, perhaps I'd suggest getting in touch with Kevin Chant (via his website) as he may have a schematic for this. His site helped me a few years ago restoring an old AWA vtvm, and master instruments who made the meter scale had not long moved from Marrickville, unfortunately it meant the specific parts I needed were not available, nor repairable. I did learn quite a lot in the process though.  https://www.kevinchant.com/palec.html

[clay1905]

After spending some time today, I've found a few things to follow up on. It looks like the capacitor should be connected where the tail of wire is.
The A.C. input is connected to the D.C. terminal on the function selector switch.
There's a connection to one of the triode heaters from the A.C. socket, and that might be about keeping the heaters and cathodes at a similar potential to stop a flashover, but nothing further. Each pole of the A.C. being measured should ultimately be applied to the grid of one of the valves, but via different resistance trees. I've just got to work out which tree is which, and the rest should be not too hard.
In this old girl, the resistance trees are more spread around the chassis than in later designs. And they're not all geographically contiguous. Anyway, I think I'm getting closer to a solution.

[clay1905]

In order to get a clearer idea of how the V.T.V.M. deviates from how the A.C. circuit ought to be, I enlarged the schematic of the Clippard 406, and traced out the A.C. circuit on it using different coloured texta pens.
Aha! The deviation is profound and quite as clear as mud. An entire resistance tree has been bypassed, and one pole of the A.C. input connected to the nearest convenient terminal. In this case the heater of one of the twin-triodes.
This Palec instruments' resistor trees are built up in odd ways. Parts of different trees share common real estate, but no electrical connections. Determining where one stops and another starts is next. It's on the schematic, just gotta transfer to the chassis. That way I'll know where to reconnect half the A.C. input, and all being well, Bob's your aunty.

[clay1905]

This meter had an active A.C. probe. The penny has just dropped. I'm looking at the heater supply for the probe as if it was part of the A.C. in circuit. No wonder it makes no sense.
There's no rectifier for an A.C. signal on the main chassis, so it must have been elsewhere. Early practice was a tiny diode valve in the probe. Now it's getting interesting.

[clay1905]

Once I saw the need for an active probe, the rest was simple.
Attached is the final schematic. (I hope) I've changed some colours in this version to make the relevant paths clear.

[clay1905]

That solution was too easy. No bias voltage on the probe anode. I can't see where that might have come from yet. I don't think it was a "crystal" probe, as that wouldn't stand the high voltage range.
It seems that such a voltage could come from one of the cathodes of a triode.
Back to the drawing board.
Actually, I'm looking at some other schematics of probes with diodes in them, and I'm not understanding how bias is applied. I would have liked to see a B+ line, and dropping resistors to anode and screen voltages, but this ain't no A.M. radio.
Help?!?

[trobbins]

R&H has a VTM article in Sept 1949, and an extension article in Oct 1949 on an RF probe.  Likely inspired by commercial products like the Paton VCT and its RF probe.

[clay1905]

Thanks, I'll see what I can find.
To date my primary experience is with battery/electric receivers. (And not a huge amount there.) This instrument has a lot of differences. One is that there are voltages both above and below nominal zero of the chassis. I have some vague suspicion the probe diode is maintained just at cut off by this difference when there is no voltage being measured. All diodes have voltage drop, and I'd have thought that would have to be taken up, so as to speak, before the instrument could measure voltages smaller than the drop of the diode.
Now I'm curious. How does this thing work?

[clay1905]

Right. It seems there are a few different format diode probes, not including solid state.
They are all remarkably sensitive. Able to discern down to about 0.1V. There were a number of different valves, but basically two families, single and dual diodes. The dual diodes have separate cathodes and are configured as voltage doublers.
Which just about addresses my concerns. These little diodes can pass enough current to tickle the fancy of a triode.
This was from "The V.T.V.M. How It's Made, and How To Use It." By Rhys Samuel. He has a very thorough chapter on probes and cables.
(Must read page 177 to see how to measure voltages lower than 0.1.)
Now all I need do is work out which probe format would best suit this meter. There seems to be some info on later Palec stuff, and Paton Electrical in general, but this meter seems to have slipped through the cracks. The later ones used half a dual diode to rectify A.C. signals, and a simple direct probe.
At the moment I'm leaning towards a 6AL5, as this is what is in the Clippard 406's probe. This meter seems to be virtually identical to the Palec, with the one exception being the resistance tree values are different to reflect different range values on the cabinet front.
And here's a picture.

[clay1905]

AAArgh! It's in the name, isn't it?

Vacuum Tube (Probe) Multimeter. I'll crawl back under my rock now.

[clay1905]

Final of the schematic.
Includes an A.C. probe circuit, the most likely in my opinion.