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

ColinTheAmpMan1 · 11th Oct 2019, 4:34 pm

For the sake of completeness, the double-diode originally in my V-7AU was a Brimar EB91 6AL5, made in England. The EB91s that I have are Mullard M8212 "made in Great Britain" and also CV4025, so presumable military-grade? I have another nine of them.
It seems like you have a US-specific IM-28. I think if I were you, I would power it up using a Variac (if you have one, of course!) set initially to 120 VAC and check that the heaters are glowing. If all seems good and the heater-voltage is 6.3 VAC, then it is probably set up for US mains. If the heaters are dim, then slowly crank up the voltage on the Variac and if the heaters start to glow like bulbs, take the voltage back down. Otherwise, carry on to 240 VAC and (hopefully) the instrument will work fine. Once that has been ascertained, change the US plug and mains lead for a decent UK one and an appropriate mains fuse. Heathkit were notorious for their bad mains wiring, just using a knot in the mains-lead to retain it from being accidentally pulled out and not having an earth connection at all. It looks like you have an earth-connection, but if there is a knot, replace it with a proper cable retainer. Much safer!
Colin.

11th Oct 2019, 4:34 pm	#6
ColinTheAmpMan1 Octode Join Date: Jan 2012 Location: Wimbledon, London, UK. Posts: 1,465	Re: Heathkit IM-28 VTVM For the sake of completeness, the double-diode originally in my V-7AU was a Brimar EB91 6AL5, made in England. The EB91s that I have are Mullard M8212 "made in Great Britain" and also CV4025, so presumable military-grade? I have another nine of them. It seems like you have a US-specific IM-28. I think if I were you, I would power it up using a Variac (if you have one, of course!) set initially to 120 VAC and check that the heaters are glowing. If all seems good and the heater-voltage is 6.3 VAC, then it is probably set up for US mains. If the heaters are dim, then slowly crank up the voltage on the Variac and if the heaters start to glow like bulbs, take the voltage back down. Otherwise, carry on to 240 VAC and (hopefully) the instrument will work fine. Once that has been ascertained, change the US plug and mains lead for a decent UK one and an appropriate mains fuse. Heathkit were notorious for their bad mains wiring, just using a knot in the mains-lead to retain it from being accidentally pulled out and not having an earth connection at all. It looks like you have an earth-connection, but if there is a knot, replace it with a proper cable retainer. Much safer! Colin.