Javier Albinarrate - LU8AJA

  • Increase font size
  • Default font size
  • Decrease font size

Sanwa Cony VTVM

Print

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.

Restoration

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.

Photos

 Front

Front

Back

Back

See the faulty selenium rectifier

 Top

Top

See the rust caused by the leaky battery

Bottom

Bottom

You may see the small 1N4007 replacing the selenium rectifier at the center top inside a cloth spaghetti

Last Updated on Friday, 26 June 2009 19:44  

Google Translate

English French German Italian Portuguese Spanish