Heathkit IM-13 VTVM Restoration

After warming up on my IM-28 restoration, I went on to do my IM-13.

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I don’t have a “before” picture, but the cosmetics were pretty much the same as this “after” photo. 

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One difference is that I swapped meters with the IM-28, as the red scale on the IM-13 had faded. 

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I want to keep the IM-13 as part of my Heathkit Classic II style collection. I planned to sell off the IM-28, and did so at the fall NEARC meet in Brookline.

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I came pretty close to completely re-kitting it. 

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I didn’t pull the tube sockets or the calibration pots, and I left a few of the old components that were good.

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Compared to the IM-28, more of the range resistors on the IM-13 had drifted out of tolerance. That’s not surprising, as the IM-13 could be a decade older than the IM-28. 

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I replaced the electrolytic, of course, but the original silicon diode was fine.

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Some of the five 22 MΩ resistors had drifted out of tolerance, but only two of them, R6 and R7, are actually critical to the calibration of the meter. I replaced those with 1% metal film resistors, and re-used the best of the remaining carbon composition resistors. 

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The high-ohmage 1% metal films are somewhat expensive, so I didn’t want to deplete my stock unnecessarily.

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I replaced a fair number of the range resistors.

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I also re-soldered most of the joints. The original builder was a bit stingy with the solder.

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While the range switch was out and easily accessible, I applied a bit of DeOxit D100 to the contacts and worked the switch to clean any corrosion from the contact fingers.

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I cleaned the old grease from the ball-bearing detent mechanism and re-greased it. 

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I finished by putting a drop of Mobile One 5W-20 oil on the shaft bearing.

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I removed all the wiring from the function switch, ...

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… and gave the contacts and mechanism the same treatment as for the range switch.

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Here I’ve started the re-assembly and re-wiring. The Aerovox 0.047 µF 1600V capacitor tested at 100 GΩ insulation resistance at 500 volts on my GR 1864-B, so I’m re-using it.

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In some places I re-used the old wire, but it had tended to work-harden between its initial installation and my removal and re-installation. I had trouble with the wire breaking when I tried to re-wrap it on the terminals.

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So, as I moved along with the reconstruction, I started using new wire. Fortunately I have a high quality scan of the full assembly manual for the IM-13, obtained before DataPro chased most of the full manual scans off the net.

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The manual specifies the length for each wire in the kit. This saves some time when re-assembling and helps keep the lead dress neat.

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A few more steps further along the way.

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I should have followed the instructions and installed all the wires on the tube chassis before bolting it to the front panel. It was a bit challenging to make some of the connections with everything in the way.

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I removed the D cell battery holder, as I plan to replace it with a line-powered regulated power supply. I ran the wire for the power feed to the resistance range section, but left the battery end unconnected, until the time that I install the power supply.

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I haven’t populated the boards yet, but  you can see that the power supply is about the same size as the original D cell battery holder in the IM-13. 


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The battery eliminator PCB mounts on three 1/4 inch long 6-32 threaded standoffs. Two of the standoffs take the place of the mounting screws for the original battery holder. I had to drill a hole for the third.

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It would have been a lot easier to drill and mount the board if I had removed the chassis from the front panel.

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Next time I’ll have the boards on-hand to install the battery eliminator earlier in the restoration.

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The VTVM battery eliminator uses its own power transformer, so it doesn’t load the original Heathkit power transformer at all.

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The transformer is a Triad FS12-200-C2. I bought it from Mouser.

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It’s just a simple LM317 circuit.

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I wired the two secondaries in series as a 6.3-0-6.3 volt center-tapped transformer, and used two 1N5822 Schottky diode rectifiers to make a full-wave rectifier feeding about 6 volts to the filter capacitor.

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The filter capacitor is grossly oversized. I bought a batch of 4700 µF 16 volt electrolytics some time ago for this project. I must have slipped a decimal point, 470 µF would have been more like it. If I had to do it over again, I’d use a 680 µF to give a little more margin, as the minimum capacitance needed to keep the LM317 in regulation is 466 µF.

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Here’s the schematic. I used KiCad on my Mac to draw the schematic and lay out the board. The schematic drawing tool is much clunkier than the defunct but sorely-missed DesignWorks package from Capilano Computing. The PC layout editor is also a bit clunky, but the package is usable. KiCad does have an excellent set of libraries.

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Here I’ve mounted the completed board to the standoffs.

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The left standoff connects the battery eliminator to the chassis ground, so I used internal star lockwashers on all three mounting screws.

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There’s plenty of room for the battery eliminator.

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The positive output connects to the resistance section of the range switch.

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The battery eliminator picks up the unswitched side of the line from the terminal strip for the line cord.

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It connects to the switched side at the same terminal as the VTVM power transformer.

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I also completed another switchable VTVM probe for the IM-13.

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I use a 1/4 inch stereo (tip-ring-sleeve) phone plug with the ring unconnected to give a little extra insulation between the probe tip and the instrument ground.

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Fitting both the RG-58A/U and the Belden test lead wire through the shell of the 1/4 inch phone plug is a tight squeeze. This time, I tried filing a recess to pass the ground lead. It worked out well.


Calibration is a lot of boring knob-twisting. Setting the calibration points is quick and easy, given a meter calibrator and an accurate and precise DMM. But verifying each range is tedious. Here’s how my IM-13 came out, after the restoration.

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Everything is well within Heathkit’s specified tolerance.

© Steve Byan 2011-2019