Power Designs TP340a Knob Upgrade

My Power Designs TP340a bench power supply has a look and feel of quality stemming from its industrial design roots in the 1960s. Unfortunately, my supply was built in the mid-1990s and the design was compromised by the use of plastic knobs for the voltage controls on each of the three independent power sources.

TP340a with "original" plastic knobs.

TP340a with “original” plastic knobs.

Since getting the supply up and running, I’ve been on the look out for some suitable replacement knobs. Other/earlier models of these Power Designs power supplies appear to have used knurled aluminum knobs supplied by Kilo International. Kilo sells similar knobs to this day, but they apparently long-ago discontinued the model Power Designs used (which is probably the reason for the substitution in the first place).

In the current lineup, Kilo has two lines that might serve as suitable substitutes. The JD series has the right overall profile, with a flat top rounding into a straight side, but it lacks the flared skirt of the originals. The DDS series has the skirt, but the rest of the profile isn’t right, with a flat top dropping to a flat side that then flares slightly for the knurling.

I suspect that the original knobs were a skirted variant of the JD series, probably called JDS. I scoured eBay for old stock or used knobs, without luck, but I did find a seller with used knobs of similar design. In fact, they look like knobs that Power Designs used on a number of supplies made in the late 60’s or early 70’s. The price wasn’t bad either, 5 knobs for about $17, shipped.

They still weren’t quite right though. The fact that the design wasn’t an exact match wasn’t the issue. Rather, I was concerned that they were a little wider than the original knobs, and  that their proportions would seem off (I lack the skill to create a nice, well proportioned design, but I can tell when things are off). Because of this, I dithered on ordering them, but after seeing his stock decline, I finally pulled the trigger.

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The knobs were in pretty good condition, other than being a bit gunked up. I gave them a washing and scrubbed them gently with some Barkeepers Friend (a mildly abrasive cleaner for glassware and stainless steel) before replacing the existing knobs.

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I think my concern about proportions were warranted. The I think the knobs give the supply a bit of a big-nosed look. Still, I think they look better than the plastic knobs that it had before, and the definitely feel better to use.

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The seller also offered some knobs with the same design but a smaller diameter. I would have purchased them instead, but for the fact that they were drilled for small diameter potentiometer shaft. I considered getting them and drilling them out, but I don’t really have the right tools and so I figured it would be a frustrating project that I could do without right now.

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