Heathkit bench supply mini-restoration

[james_s]

I was poking around in my mom's garage and came across the Heathkit IP-2718 I had when I was a kid. I managed to blow something up in it decades ago and then somehow the lid got misplaced. It also has been missing the dual knob for the B supply for as long as I've had it. I ended up snagging a parts unit on ebay for the opening bid with local pickup so I thought I'd restore mine. Well after some cleaning and polishing and a few squirts of control cleaner I soon realized my "parts" unit was in better shape than the one I had planned to fix with the parts. It had an issue though, when shutting it off the A supply would spike up to around 8V briefly, obviously not a good situation. Digging in a bit I found a very cooked looking diode which had been desoldered at one end with a larger Schottkey diode soldered across it on the back of the PCB. I removed that mod and checked the original diode, finding it was quite leaky. I replaced that with a new 1N4007 and found the voltage spike still occurred. More poking around located another leaky diode, replacing that solved the spike. I was about to button things up and call it done when I noticed the ripple on the A supply was quite a bit higher at some settings than the B supply. I then decided to carefully test the short circuit current limiting and found that while the B supply would limit at around 650mA as designed, the A supply would top out at under 200mA. Another half hour of poking around found a tiny solder bridge hidden under a blob of flux between two adjacent joints. I removed the bridge and everything works perfect. I strongly suspect that whoever built this thing in 1977 simply put up with the degraded performance and never had it working quite right, that must have been frustrating as it was otherwise quite well constructed. It's not a spectacular power supply, lacking adjustable constant current mode but it cleaned up nicely IMO and Heathkit brings a certain degree of nostalgia.

So now I guess the problem is I still need a knob and lid for the other one! I can make a new lid using the one I have as a pattern but the knob may be trickier to find.

[bitseeker]

Nice restoration, James. Sounds like you're in the market for another parts unit. And so it goes...