VTVM AC function out of whack

[clay1905]

Hi folks,
I have a Palec VTVM, model TV-M. This meter has been repaired in the distant past a number of times. Many of the carbon resistors had since wandered way out of range, and so I have replaced all the capacitors and resistors.
Now everything appears to be working just fine, with the exception of the AC ranges which are reading about 50% low. I have checked the rotary switches for resistance in the contacts, the valve bases, anything at all that might be a cause. I have even gone so far as to test and check that I have put all the components in their right places. It seems I have.
Now oddly enough, the meter seems to use the same probe (with isolating resistor) for both AC and DC measurements. I have made up a direct probe for AC use, but the meter needle barely flickers and then returns to zero if I use it. The isolated probe (1meg) gives me the 50% low reading.
Adjusting the trimmer VR2 to the minimum resistance brings the needle up a little closer to the correct value, but nowhere near enough.
Both the 12AU7 and the 12AL5 test as good on an emission type tester. (Palec ET-4a)
Attached is my attempt at a schematic diagram. I think it's complete and correct.
So, if you can see it, please let me know where I'm going off the rails.

Thanks,
Clay.

[bob91343]

Difficult to read any values or other captions on the diagram.  It looks as though the 12AL5 rectifies the signal and presents it to the VTVM.  The diagram is so involved that it's hard to trace what goes where.  Seems to be a peak detector so the dc part has to be set up to account for calibration difference.

How are you testing it?

[clay1905]

Firstly, sorry about the cluttered schematic. It wasn't ever meant to be published. It was intended as much as a layout guide as a schematic, so I could more easily reassemble the switch clusters after cleaning them. I have attached a somewhat cleaned up version. I simplified the function switch wiring a bit to make things clearer there, and increased font size.

Yes, your assessment  of the meter is correct. It's a peak meter and the 12AL5 is the AC input rectifier.

Testing is primitive and basic. I have another VTVM that's functional. Both meters should present similar impedance. I measure the no load output of a transformer, and see how well the meter under test replicates the correct measurement. The transformer I'm using presently outputs about 19.5VAC, and the DUT is showing almost 11VAC.
The same error is showing in both the 15 and 50 volt ranges. However, messing about with the meter again this morning, I'm seeing some rather erratic behaviour. It appears to be centred around the range switch. Switching to the 150V range has the needle swinging as hard down scale as is possible. This is in spite of the meter being warmed up, and the zero already set on lower ranges. No amount of turning the zero set knob will move the needle off the stop in this condition.

As I'm not the most knowledgeable, you can guess that I'm pretty bamboozled by this behaviour. Any suggestions on how to narrow down the search for fault(s) would be greatly appreciated.

Clay.

[bob91343]

It's probably either a dirty switch or a bad ground connection.  The intermittent nature of the problem indicates such.

Take a look at the 12AL5 socket as well.  Since that's only used on ac it could be a source of trouble.

[clay1905]

Yes, I've more or less come to the same conclusion that there's some problem with the output from the 12AL5, or the path to the 12AU7.
Seeing the DC measurements are pretty good, I think its safe to assume that the problem is between the input socket and the grid on the triode. I'll try giving the rectifier socket a spritz of Deoxit to see what comes of it.
I doubt the switches are a problem. I have stripped them down and cleaned up all the oil, dirt and oxide. As I'm seeing exactly the same voltage on two different ranges, it's unlikely to be there. If nothing else brings any joy, I'll revisit them then.

Clay.

[clay1905]

I'm afraid none of the above did the trick.
Resistance through the AC resistor tree is spot on. The path between the 6AL5 and 12AU7, via the DC resistor tree measures as exactly what it should too.
I've swapped the 6AL5 over for another that also tests as above 95% of original emission for both halves. Same with the 12 AU7, even though it can't be the problem.
So to sum up:
Easy version-
Nothing is wrong except the meter reading.
Not so Easy Version-
The conducting path from the input socket, all the way to the 6AL5 is free from excess resistance.
The conducting path from the 6AL5 to 12AU7 is good.
And finally, the path from the 12AU7 to the movement is also testing as good.
The meter measures DC well, so the amplifier and movement are working correctly
So far everything is testing and measuring as good. Yet the meter is failing to measure AC voltages correctly.

I think what I will do now is arrange for a supply of 19.5VDC. and measure the 12AU7's response to it in at least two ranges. Probably 50 and 150 volts.
Compare this with when 19.5 R.M.S. VAC is applied, looking particularly at the voltages on the grids of the triodes. (The scales are calibrated in P to P and RMS AC Voltages.) 19.5 RMS VAC *should* be pushing an equivalent current through the movement coil I think, and so should also have similar voltages and currents to 19.5VDC in the meter circuit proper.
Any thoughts on this strategy?

[bob91343]

Are you using a 6AL5 when you should be using a 12AL5?

[TimFox]

Note that most VTVMs with those tubes measure the peak-to-peak AC voltage (with separate actual pk-pk and equivalent sine wave RMS scales).  However, the DC probe includes a 1 megohm series resistor that is not used for the AC measurement.  The 12AU7 heater can be wired for 6.3 or 12.6 V.

[clay1905]

Thanks for pointing this out, but actually the reverse is true. I had labeled the rectifier as 12AL5, but it's actually a 6AL5, and the 12AU7 has the filaments in parallel.

[clay1905]

Yes, this probe thing is a bit of a mystery. Indeed the DC probe has a 1 megohm resistor inside. But as I pointed out at the start of this discussion, the meter won't measure AC without this resistor, which I think is odd and might be a clue that I'm not recognising. The other possibility that I haven't yet considered is that the AC resistor tree is "lacking" by about 1 megohm, which might explain the one probe for two jobs.
This evening I hope to get around to setting up the voltages AC and DC so I can compare performance.

[TimFox]

Ordinarily, you do not want the series 1 megohm resistor on AC, since it forms a low-pass filter with the substantial cable and input capacitance of the meter.  My VTVMs all have a separate input connection for AC, but I know that some have a switch in the probe to short out the resistor.  The resistor is needed when measuring DC voltages in the middle of high-frequency amplifiers (such as the plate of an IF amplifier tube), since the cable capacitance could upset the amplifier operation.

[bob91343]

I suspect the switch in the probe that may be leaving the resistor in the circuit on ac.  Or perhaps just poor contact.

Apply a known ac voltage to the circuit after the resistor.

[clay1905]

Thanks all,
I'm quite aware that in most cases VTVM's don't have an isolating resistor in the probe for AC measurements. Where did this talk about a switch in the probe come from? I don't have one, and haven't mentioned one either.
As I wrote earlier, the meter does not respond to a "straight through" probe. No needle movement whatsoever. Very strange. Adding a 1megohm resistor to the straight through probe immediately gets the needle moving.
Also, as written above, all the switches have all been carefully overhauled and measured for contact resistance. There isn't any.
This morning I measured the voltage on g2 of the 12AU7, with 20VDC & 19.5VAC at the probe to see the difference.
I measure about -0.4VDC when I apply 20VDC to the probe. I see about -0.1VDC on the grid when I apply 19.5VAC. This is all with the range switch set to 50V.
Both the rectifier valves I've tried in the socket test to very good, so I doubt that's where the problem lies, and this is very frustrating, I can tell you.
I'm off to the electronics store to buy a 1megohm variable resistor to experiment with probe resistance. It's about all I can think of to try for the minute. Maybe about 500K will work for AC, seeing the readings are close to 50% of expected.

[clay1905]

OK,
Just for reference, some VTVM's have separate input sockets for AC and DC measurements. Others have two probes, one for each current type, sharing one socket. Still others have a probe with internal switching. And then there are those with a diode, often a valve, built into the probe. Maybe there are other designs too but I haven't encountered them yet. I am presently looking at the separate AC & DC probes with one socket variety.
A quick experiment shows that between 390 and 470 Kohms at the probe gets the needle so close that I *should* be able to get to correct readings by tweakingVR2.
I don't quite understand what the designer was thinking if this is the correct solution. However, seeing the needle settle on a value this close is something of a relief for the frustration.
This is what was making me wonder about whether the designer intentionally left the AC resistor tree "lacking" some amount of resistance, making it possible to use the same probe for both functions.