I bought a Heathkit VTVM in reasonable condition on the quarterly radio swapmeet of the NVHR. At the same occasion, I found a wreck of the same VTVM. A few months later, I bought two more Heathkit VTVM's of the same type. Or so I thought. I put all four of them in a box to fix them later on. But in 2006 when I opened the box, I found out I had two slightly different but very similar types of VTVM: twice V-7A and twice IM 11D. They are the same size, they have the same controls and front lay-out. There are some differences in styling, such as the type of knobs. The IM-11's have a thinner circuit board and sightly more modern components.
One of my V-7A's has a grey plastic handle, the other one has a cast aluminium handle. The first V-7A was in good condition, but somebody had replaced the input phone jack by a banana plug receptacle. Its power transformer did not quite fit as its core was touching the case. It was mounted on two long screws and did not look quite original. It looked like a replacement transformer of German origin.
After some thought, I decided to use the front panel of the first one and use the circuit board with the original power transformer of the second one. The first thing I had to do was to fix an unbalance in the meter movement of the first V-7A. I made a small ring of copper wire and glued it to the tail of the pointer using some nail polish. Then I put the movement back into place and tried the meter. It was working fine and its sensitivity was ok.
The time had come to try my rehabilitated V-7A. I did have one DC probe assembly (phone jack on one end, test prod with 1 MΩ resistor on the other end). After switching on, the supply voltage was well within range. The DC range hardly needed callibration. The AC ranges were considerably off. I suspect a marginal 6AL5 rectifier. After this, I was the proud owner of a V-7A with original parts including a metal handle with text “Heathkit Precision” and working well.
First, I tried to rebalance the meter of my V-7A. At that point, I had some bad luck. I noticed the pointer was slightly corroded. I figured that the added oxygen might account for the extra weight to the pointer, so I tried to scratch away some of the aluminium oxyde. That was a bad idea. The metal had become brittle and a piece just broke off.
After this, I mounted the meter movement in the case. I took the power transformer from the cracked IM-11/D circuit board and mounted it on the V-7A board. I also added a fuse, which the IM-11 had, but the V-7A had not. Then I placed the circuit board at the back of the meter movement and mounted the potmeters, range switches and input jack to the front. I soldered the wires from the pilot light to the circuit board and I also soldered a new power cord. Then I checked everything and switched the meter on.
It seemed to work. I did the callibration procedure, and then I had two working V-7A valve voltmeters, one really original and the other patched but working excellent.
Copyright © 2006 - 2009 by Onno's E-page published 2006-06-05, last updated 2009-05-05