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| Ohm meter will not zero |
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| clay1905:
Hello folks, Of late I have been working towards the completion of a restoration project, an early V.T.V.M. Part of the reason behind this restoration is to get a better understanding of the workings of radio valves. OK, the meter seems to be pretty good on all functions except for Ohms. In this function the diode bridge is out of balance. Obtaining a movement zero with the probes shorted isn't possible. However, zeroing is quite possible in all other functions. From what I presently think I know, this imbalance boils down to the voltage difference on the grids of V3. In particular, the negative voltage on G1. At about 7.6 volts negative relative to its cathode, that section should be near cutoff. In any case, almost all of the components test very well. Marked resistors are all well within 10%. The capacitor is mica and isn't leaking. The wire wound resistors are the only ones I can't be certain of, but the two of 21K test very well. So the question is, what to do? There are a few places where I could make change, but I'm wary of them all. The meter must have worked at some time in the past with these components. Now it doesn't and I'm not too sure why. Increasing resistor value for R10 may get the drop required, but I"m concerned about pushing up the voltages on the grids of V2 as a consequence. It might make no difference, but I'm still a bit too green to make that call. Shorting R's 9 to the chassis gets a good response from the movement, but taking them out of circuit seems the wrong thing to do. I would like a little guidance here on determining the best course of action to get the bridge back into balance on the Ohms range. I don't want to be doctoring things, but rather finish a proper restoration where everything is as it was originally intended. And if possible, I'd like to be a little the wiser too. Clay. |
| trevatxtal:
Sorry if this is a silly question! Have you checked the 3 volt battery ? If it is down you will never zero the scale! |
| clay1905:
Hi trevatxtal, Yes, the battery has charge. Worth revisiting to be sure though. Thanks for the prompting. In the mean time I've hit upon a snag. I disassembled the movement and chassis from the front panel in preparation for a repaint. No components were harmed in this process, or so I would have thought. Something has developed a big weird short. Now, there is no subtlety about the instrument. In two functions, DC+ and Ohms the needle swings hard downscale, and in the other two functions is a hard upscale swing. Hmm, seems I've provoked something that is simple, and consequently, damnably hard to diagnose. I'll be back soon (I hope). |
| Prehistoricman:
If you've not made a short, then you may have cold solder joints that got disturbed. |
| trevatxtal:
hello again I note QUOTE the battery has charge END Quote That was not what I said if it is a rechargeable battery it will not be 3 Volt I suspect you may have the wrong batteries. if so it will never zero. Trevor |
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