| 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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