The other day I bought in an auction a Sanwa Vacuum Tube Voltage Meter, perhaps from the 1960s, for $20.
There are several reasons why you should have a VTVM. I was looking for one, because they are excellent when dealing with high voltages. For example, I once fried my digital tester, while measuring anode voltage with a high voltage probe, because the input opamps of the digital tester were too sensitive to the initial voltage spike. The 1000:1 high voltage probe is basically a very high resistor divider (100Mohm), in which you measure 1V; but when you make contact with high voltage, there is a delay for the electrons to flow and establish the current which causes the 1000V to drop to 1V. In my case, I was measuring 30KV, and the spike was surely well above my 1600V, and that killed my digital tester.
Back to the Sanwa, it doesn't mention any model and I haven't found its schematic, but they are almost all of them based on the same circuit, and here you have an excellent guide on these instruments:
Basically the instrument has a 6AL5 (double diode) for AC signal rectification, and a 12AU7 (double triode) for amplification and buffer for the instrument.
The restoration was simple, about 1 hour:
- I replaced the selenium rectifier with a 1N4007 diode, the selenium diode was open, but they are very unreliable, so you should replace it even if it is good.
- I replaced 3 electrolytic capacitors
- The instrument was open, so I gave it a better look, and found that one of the springs was cut (they actually behave like fuses). I was lucky, because it was broken very near the outer end, so I simply shifted the contact, and soldered the spring again. And it worked at once!
- The 3V battery that is used for the ohmeter was missing, somebody took it out, but too late.... a big part of the chasis was corroded by the leak. I didn't remove the rust, I simply left it as is, and I didn't add a new battery because I really won't use it as ohmeter.
It is a nice bench instrument to have around, and it was really easy to restore back to a working state.
See the faulty selenium rectifier
See the rust caused by the leaky battery
You may see the small 1N4007 replacing the selenium rectifier at the center top inside a cloth spaghetti
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