Heathkit IM-28 VTVM Restoration

I recently restored a Heathkit IM-28 VTVM.

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I had always wanted Heathkit’s IM-13 bench model VTVM. The IM-28 is the “new look” version of the IM-13, and I couldn’t pass it by when I saw it at the MIT Flea.

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Unlike the RCA WV-97A VTVM that I recently restored, most of the precision range resistors were still within tolerance. 

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I replace all that were badly out of tolerance, but left in a few that were close, such as R17, the 6.838M Ω resistor in the DC voltage divider,

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and R2 and R4 in the AC voltage divider. 

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The film capacitors were all modern plastic dielectric, not paper, so I left them in place.

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Of course, I replaced the electrolytic filter capacitor. The rectifier was a silicon device, so I left it in place.

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I replaced the battery holder with a full-wave bridge rectifier feeding an LM317 regulator set to 1.55 volts.

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I cut the wire that grounded one side of the filament circuit, since the negative side of the full wave bridge has to be grounded. If the ground on the AC side isn’t lifted, it will short out the full wave bridge.

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Here’s another view of the battery eliminator circuit.

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All back together.

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I calibrated it using my HP 6920B meter calibrator. The DC and Ohms ranges are pretty good, usually better than the specified 3% accuracy, but a few of the spots on the AC ranges are out of the 5% spec. That’s probably due to the slightly out of tolerance resistors in the AC voltage divider that I didn’t replace.

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The unit came without the mounting bracket, so I drilled out a pair of angle irons to fit the mounting thumbscrews on the ends of the IM-28 cabinet.

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They are a workable substitute.

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The unit came without a probe, so I built a homebrew switchable DC/AC-Ohms probe for it.

© Steve Byan 2011-2016