Electronics > Repair

Old VTVM (Precise Model 909)

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Analog Kid:
Oops: R7 = 15MΩ. Dunno where I got 1M from.
Also not sure why they show 2 resistors there.
Another weirdness: looking for the battery connection, I looked up R8 in the manual to locate it.
It listed it as "5M or 6M Res.". Ackshooly it's 5.6M. Typist got a little wonky there? or maybe it really doesn't matter, and depends on what they had on hand? In any case, it's listed as a non-precision resistor.

floobydust:
I found what might be a HV probe (x100 max. 60kV) 2,490MEG sort of implies 10MEG? for the 909.
Precision Test Equipment 1953 Catalog

Sometimes in old gear like this I will add a couple 5W zeners in the B+ filter to regulate it. Hate it when VTVM needs moves around due to mains fluctuations.

TimFox:

--- Quote from: Analog Kid on November 11, 2024, 09:09:50 pm ---Oops: R7 = 15MΩ. Dunno where I got 1M from.
Also not sure why they show 2 resistors there.
Another weirdness: looking for the battery connection, I looked up R8 in the manual to locate it.
It listed it as "5M or 6M Res.". Ackshooly it's 5.6M. Typist got a little wonky there? or maybe it really doesn't matter, and depends on what they had on hand? In any case, it's listed as a non-precision resistor.

--- End quote ---

To isolate the system under test from the cable capacitance, the resistance should be in the DCV probe before the cable.
Did they split that into two resistors?

Analog Kid:
No idea, as it didn't come with a probe. So you're saying the resistance should be right at the attachment point (at the microphone connector) before the cable? I'll do that when I make a probe for it.

TimFox:

--- Quote from: Analog Kid on November 11, 2024, 09:45:53 pm ---No idea, as it didn't come with a probe. So you're saying the resistance should be right at the attachment point (at the microphone connector) before the cable? I'll do that when I make a probe for it.

--- End quote ---

No, the large resistor should go inside the probe handle, as close as possible to the tip, before the voltage goes through the shielded cable to the microphone connector.

[Tip] - [resistor] - [coax cable] - [cable connector] - [panel connector] - [internal switched voltage divider] - etc.
----(probe)----

Any safe probe-tip and handle can be used. so long as the resistor is close to the pointy end.

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