Some friends were crashing at my place after MAGFest for the local Awesome Games Done Quick (AGDQ) event. At the time I had a lamp in one of the spare bedrooms but the 2nd spare bedroom had no light source. There is a switched outlet tied to a light switch, but I couldn’t think of any lamp fixtures sitting around that aren’t coherent light or require a DMX-512 protocol. Womp womp, just remembered one in the garage come to think of it. Any how, as a quick fix to solve the issue what better thing to use for a lamp than a 15′ strip of LEDs and a bench power supply? Both were handy, so I grabbed them and cobbled together a 1 minute solution to solve the temporary problem. All is good, except the strip curled back up on itself and shorted. The power supply, a nice BK Precision 1672 with current limiting went into protect mode, but something strange happened. The channel was killed.
Upon investigation the unit would always be stuck in constant current (CC) mode and there was no output. My first suspect was easy, the TO3 transistor might be shorted. No, not it. There were 4 or 5 fuses on each of the power supply boards inside, all were fine. I shot off an email to their support. They kindly replied fairly quickly saying they couldn’t help – but here is the schematics. How is that for service? So awesome.
Looking at the schematics and halfway guessing, I notice a lot of 741 opamps. They are all socketed, so the first easy thing to try is just replace them all. Cheap component, easy work (sort of.) My co-worker Will was placing a Mouser order and offered to let me jump in on it, so some replacement DIP 741’s were had.
Removing the PCB was actually a pretty huge pain. The way the unit is put together, it’s pretty tough to service (although with the bottom of the PCB exposed, I bet shops highly familiar can make easy work of taking measurements.) After freeing the board which requires moving the front knobs, the pot retaining nuts, 4 screws that hold the board in, the metal bracket above the board, and loosening the front plastic and disconnecting a bunch of cables that have little slack I was able to quickly and easily replace all op-amps.
Upon first test it didn’t seem to work right., the voltage adjustment worked like a champ but the current adjustment didn’t — no output. I wanted to keep the unit all together, and figured I’ll just use the right channel and +5vdc.
Upon putting it all back together, I tested it again and it worked fine. I’m thinking one of the connectors wasn’t seated fully, at least that is what I’m telling myself. So the lovely power supply is back in action 100%, thanks for the schematics BK and thanks Will for the 741’s!
And yea, I got a simple lamp for the room.