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Heathkit V-7A Valve Voltmeter

The Heathkit V-7A is a basic Vacuum Tube Voltmeter (VTVM), sold as a kit or completely assembled. After some work, I was able to restore my two V-7A's, one in original state and one slightly patched.

	  My second V7A.

My second V7A.

The V-7A is a basic valve voltmeter (VTVM), using a double triode in a bridge configuration. This type of circuit, shown in the diagram below, has been used in many basic VTVMs that served hobbyists and repairmen.

	  Principle of operation: two-triode bridge.

Principle of operation: two-triode bridge.

Two triodes are used as cathode followers, with large cathode resistors, in a bridge configuration. The cathode follower circuit has the advantage of a high input impedance while using two triodes in a bridge results in a symmetrical circuit with relatively (for a circuit using valves) low drift. The power supply is floating with respect to the input. A voltage divider is used to set the anode current ('bias'). In a real circuit the cathode resistors can be adjusted to correct the balance and make the zero adjustment of the meter. If you want to read more on this type of VTVM circuit, take a look at the Idiot's guide to VTVMs on the Tone Lizzard website.

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.

	  Compare the V-7A on the left to the IM-11D on the right.

Compare the V-7A on the left to the IM-11D on the right.

It is a bit hard to date these instruments. In the Netherlands, the V-7A kits have been on sale from 1955 into the early sixties. The earliest reference to the V-7A I found was in an advertisement in the Radio Electronica magazine related to the 1955 Firato exhibition. According to the IM-11 page on the Heathkit Virtual Museum, the IM11 has been introduced in 1962 as an improvement of the V-7A. Interestingly, while the V-7A's unequivocally have a US origin, both my IM-11/D VTVMs have been made in Germany.
inside the V7-A

Inside the V7-A after repair. Click to go to the IM-11D and compare.

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.

V7-A circuitry with a replacement transformer

Inside the V7-A that had a replacement transformer.

The other V-7A was a bit rusty, and its phone jack had been replaced by a BNC receptacle. And someone had drilled a hole in the front to mount a pilot light at the location where the text “V-7A” should be.

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.

Both dismantled

Both V7-A VTVMs partly dismantled.

Then I unscrewed the nuts of the switches and potmeters and I unmounted the circuit board of both VTVMs. I cleaned the front and the knobs. I dug up a phone jack from the junk box and replaced the banana plug receptacle. Then I mounted the circuitry with the original power transformer on the unaltered front plate with the rebalanced meter movement.

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.


Fixing the second V7A

Two power transformers

Two power transformers, one from the IM-11/D with mounting bracket.

After this, I had a number of V7A components having non-original details, but that could still be made into a fine working valve voltmeter. I had a front plate with a green pilot light, and a circuit board with a power transformer that wasn't fitting. I decided to use the power transformer of my second IM11. The second IM-11/D was looking bad, and I only had 3 complete meter movements, because two of them had a missing front cover.

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.

The patched V-7A inside

The patched V-7A has an IM-11 power transformer.

So here I had a problem. But hey: I could use the meter movement from the other IM-11/D, that was almost identical to the V-7A's meter.
The patched V-7A after completion

Except for the pilot light, nobody can see the difference.

That one had a different problem: beside the missing front cover, it also had a broken case. So I took the actual meter movement out of its clear plastic case and transplanted it into the other meter movement case. Everything fitted just fine. I rebalanced the meter using some nail polish. Then I tested it. It stuck halfway so I had to remove some dust. Then it was OK and I could add the front cover.

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