Heathkit IP-5220 Variable Isolated AC Power Supply

I'm restoring a Heathkit IP-5220 Variable Isolated AC Power Supply. 

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It's a nice combination of  a 3 Ampere isolation transformer, variac, and an AC voltmeter and a range-switched ammeter with 1 amp and 3 amp ranges. I found a nice scan of the manual on the 'net.

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Aside from dust, a glitter star sticker on the front panel, and a few dead bugs inside, it was in good shape. 

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Except that some cretin took off the caps from the panel-mount fuse holders.

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The 3AG panel-mount fuse holders are an older style that uses a screw-lock cap. All of the modern fuse holders that I have in my parts stock have bayonet-lock caps except for some cheap surplus ones I bought from All Electronics. But they use a much finer thread than the fuse holders on the Heathkit.

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I searched Mouser and the Littelfuse and Bussmann web sites, but all I found were bayonet-lock style fuse holders. They show up on some of my older equipment such as an old 1970's era DEC rack-mount power supply and my HP 400C meters, so they were common from the 1950's at least through the late 1970's or early 1980's.

Does anyone know who made these screw-lock fuse holders? Are they still in production? I'd rather not replace the fuse holders in the IP-5220, so I'm hoping I can find a couple of these old-style fuse holders or  a couple of replacement caps.

In the mean-time, I cleaned it up and stole a couple of fuse holder caps off of other equipment. The voltmeter calibration is pretty good, especially around 110 V to 120 V. I haven't checked the ammeter calibration yet. The three amp range uses a shunt that the builder was supposed to adjust to calibrate that range. The one amp range just depends on the intrinsic meter accuracy.


I replaced the fuse holder caps with screw caps from a Littelfuse 8AG fuse holder. The 8AG fuses are the same diameter as a 3AG fuse, but are shorter. It turns out that the cap is interchangeable between the two fuse holders. A fellow on the Antique Radio Forum pointed me to Talon Electronics, who had the 8AG fuse holders for only $1.75 each. I noticed that the Littelfuse 372001A 8AG fuse holder caps have a blue-filled legend, while the 342001A 3AG fuse holder caps have a white-filled legend. I suppose the color coding is to indicate the correct type of fuse to use for replacement.

I discovered that the isolated AC outlet was so worn that contact to a plug was intermittent. The unit must have had extensive use in a repair shop. I ordered a couple varieties of AC sockets from All Electronics. I tried both of them and found that the Leviton# 1374-001 socket fit the panel cut-out perfectly. 

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Rather than the solder lugs of the original socket (on the right in the photo), the replacement had wire leads. 

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I dismounted the Variac to make it easier to access the socket wiring. I added a Euro-style terminal strip to connect to the new socket. I mounted it using the chassis hole previously used for the ground lead from the socket, replacing the original 6-32 x 3/8 binder-head screw with a 6-32 x 5/8 binder-head screw. I relocated the socket ground to share the screw for the non-isolated socket.

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I re-mounted the Variac.

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Here's a full view of the internal layout.

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The unit is working fine now, back in the power supply stack at the end of the bench.

© Steve Byan 2011-2019