Electronics > Repair

Old VTVM (Precise Model 909)

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Analog Kid:
Sorry to bug you, but you didn't really answer my question: is the little schematic snippet I gave above correct?
Hell, I'd use an OA2 if I had one, but I'm fresh out.

TimFox:
With correct values, that should work.
1.  High-voltage Zeners have large temperature co-efficient, so it is better to use several lower-voltage units in series.  With high voltage and reasonable current, self-heating is important.
2.  Typically, the current through the Zener string would be roughly equal to the expected current through the tubes in the voltmeter circuit.  You can play with that current, but you don't want too much current, or for the Zener current to drop to zero, under the range of operation.
3.  The existing two resistors in your circuit are probably too large for the extra Zener current, and need to be calculated accordingly.  Note that the voltage across the filter capacitor is considerably less than in a circuit where the rectifier connects directly to the capacitor.
4.  Looks like you need to do some math. 

Analog Kid:
OK, help a guy out here if you would:
To do the math, I'd need to know what the two triodes supplied by the HV draw, correct?
How can I determine that? Would I want to break that circuit and insert an ammeter?

Not saying I'm even going to do this; I'm fine leaving the VTVM stock ATM. But a guy would like to know anyhow.

floobydust:
You can see what the VTVM circuit is drawing for current by reading the voltage drop across R27 or R28.
What do you have for their values? The 9071 is 10k, 4.7k and another 4.7k, they'd use the same transformer. So I would be lazy and copy that.
Or for the math... you know min/max VTVM current draw and then give extra for teh zener at low AC line. The zener is a bit oversized so higher current through it is not a problem.

But first in the restoration: C4 is surely dead, dried out and needs to be replaced. Note it has no bleeder resistor - don't let it bite. I'd add a mains fuse too.
Measure the carbon resistors to see if they are on value.

I couldn't remember what the cigar-tube probes usually have inside for a resistor for DCV.

Analog Kid:

--- Quote from: floobydust on November 12, 2024, 07:00:59 pm ---But first in the restoration: C4 is surely dead, dried out and needs to be replaced. Note it has no bleeder resistor - don't let it bite.

--- End quote ---

Maybe not; saw it yesterday and it says "dry electrolytic" on it, so maybe it's OK.
Probly better to replace it, though.
Going to build a capacitor tester one of these days ...

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