UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Heathkit IM-28 VTVM

ColinTheAmpMan1 · 11th Oct 2019, 2:39 pm

Check the front panel of the VTVM. If the model-number ends in a "U", then it is a UK model, but having said that, it looks to me as if the mains transformer can be connected for 120 or 240 VAC by means of a selector, which looks, from the schematic, to incorporate a mains fuse-holder. Another way of telling if Heathkit items were produced in the UK will be that the meter will either show "Pullin" at the very top for UK models and also show "DAYSTROM Ltd - GLOUCESTER ENGLAND". If I remember correctly, Heathkit used Simpson meters on US-produced kit and the address on the meter was "BENTON HARBOR MICH."

Unfortunately, I do not have any other documentation on the IM-28 VTVM and I know it is difficult to find free documentation on the web, but many of these Heathkit VTVMs had similar circuitry and only really differed in cosmetics. The V-7A VTVM uses a 0.01 uF, 1600 V capacitor in the equivalent position to your C1, so I assume that the voltage is probably more important than the capacitance value. Incidentally, be careful of putting too high a voltage on the double-diode V1 (6AL5). It is a bit delicate, but if you do kill it, I have a load of EB91 equivalents that I bought on eBay when I blew mine.

Incidentally, can't the member who sold you this VTVM provide you with any other info, like why it is wired for 250 VAC, but still has a US plug?

Good Luck, Colin.

11th Oct 2019, 2:39 pm	#4
ColinTheAmpMan1 Octode Join Date: Jan 2012 Location: Wimbledon, London, UK. Posts: 1,465	Re: Heathkit IM-28 VTVM Check the front panel of the VTVM. If the model-number ends in a "U", then it is a UK model, but having said that, it looks to me as if the mains transformer can be connected for 120 or 240 VAC by means of a selector, which looks, from the schematic, to incorporate a mains fuse-holder. Another way of telling if Heathkit items were produced in the UK will be that the meter will either show "Pullin" at the very top for UK models and also show "DAYSTROM Ltd - GLOUCESTER ENGLAND". If I remember correctly, Heathkit used Simpson meters on US-produced kit and the address on the meter was "BENTON HARBOR MICH." Unfortunately, I do not have any other documentation on the IM-28 VTVM and I know it is difficult to find free documentation on the web, but many of these Heathkit VTVMs had similar circuitry and only really differed in cosmetics. The V-7A VTVM uses a 0.01 uF, 1600 V capacitor in the equivalent position to your C1, so I assume that the voltage is probably more important than the capacitance value. Incidentally, be careful of putting too high a voltage on the double-diode V1 (6AL5). It is a bit delicate, but if you do kill it, I have a load of EB91 equivalents that I bought on eBay when I blew mine. Incidentally, can't the member who sold you this VTVM provide you with any other info, like why it is wired for 250 VAC, but still has a US plug? Good Luck, Colin.