Quote:
Originally Posted by WaveyDipole
Colin, that sounds like a sound plan. I will try 120v first, but unfortunately I don't have a variac, but I can certainly use a 40w bulb to drop the current. There is a back round cable retainer of the very common type, but this flat rigid 3-core cable looks rather awkward in it. I will be replacing this with a piece of round flex. If it runs off the UK mains then I will use a UK standard plug. If not, then I have a step down transformer that I can use with it.
What you say about C1 makes sense, but can I clarify. You say that V-7A VTVM uses a 0.01 uF, 1600 V capacitor? This IM-28 uses a 0.047uF (47nF), my proposed replacement is 0.1uF (100nF). Can I just confirm that you did mean 0.01uF and not 0.1uF? If its 0.1uF, then it seems worth a try, but if its 0.01uF then I'm not so sure?
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Ok, so if you don't have a Variac but
do have a step-down transformer, why not try powering it up with the transformer first? if the heaters look to be glowing right and the heater voltage is 6.3 VAC, then it is a 120 V model and no harm has been done.
Yes, the equivalent capacitor in the V-7A is 0.01uF at 1600 V. If you just want to see if the VTVM works, and you aren't going to put a signal above a few volts into it, then I don't think you need to bother about the capacitance value too much and that a cap with a couple of hundred volts rating will do. I was hoping that I might find a suitable replacement to give you in my collection, but regrettably not. You might be able to find a suitable capacitor from a UK seller. Bitsbox has 10nF, 3kV for £0.28 (
https://www.bitsbox.co.uk/index.php?...oducts_id=3262), but Dave Cantelon at Just Radios has tubular axial metallised polyproylene caps 0.01uF at 1600 v for $1.49 (
https://www.justradios.com/cart.html). His minimum order is $20 plus shipping, but if you take a look at his site, he has lots of really interesting stuff - regrettably he is located in Canada, so shipping-times are a little long.
Colin.