So I’ve managed to avoid blogging for a full year. It’s not that I had a reason to stop blogging, nor is there a reason I’m posting almost exactly one year later. I just think it’s a bit neat. I’ve also noticed my last post was about CNCing — I can’t believe I’ve been working on that project for this long. A lot has happened, but I’ll save that for a future post. I just wanted to recount a debugging episode as I think it might be useful to someone.
I bought an HP 6236B triple output benchtop powersupply off ebay some time ago, and a few months back, the output voltage dropped accoring to it’s analog display, about 10-20%. I thought it a strange failure mode, but I didn’t have much need for it and put off trying to fix it – which I did over this past weekend. One thing I love about older electronics is that they actually expected people to fix them, rather than designing things so complex that they have to be replace or entire circuitboards replaced. I found the manual online (adding copy here for posterity), and not only did it contain the schematic but a debugging guide on what to check for what set of conditions.
Manual for HP 6236A, 6236B, 6237A, 6237B
So the instructions tell you, for a lower output voltage, to first start off by checking the reference voltages. I did, and found all the voltages were lower than they should have been – this seemed quite odd to me. Here’s the schematic of the reference voltage stage:
The debugging instructions said that if the +7.5V supply (first one it had me check) was too low, to check for a short in VR3, a zener diode. So I removed it from the PCB and tested it — worked fine. It gave me a finite list of other possible failure modes: U1, U2, U3, and U4 — all the op amps in the device — but I was reluctant to try to desolder them, and I didn’t expect the problem to be there. I poked around for a few hours but couldn’t think of the cause of the problem so I retired for the night. The next day I started to poke around some more, and wanted to look at the power off the transformer which would have been AC across CR56, CR55 using my oscilloscope. It looked reasonable, and having that out, I decided to use it to poke around a bit more – and noticed that it wasn’t that the reference voltages were lower, rather, they had dips in them corresponding to the lines 60Hz voltage. Sure enough, the multimeter was averaging the value and giving me a stable readout even though the votlage was not constant – and that had led me down the wrong path. The instruction manual even said to use a multimeter for debugging – and no where did it suggest I check out what is, in electronics, the most common point of failure – the capacitors. A bit of testing later, and I had narrowed the problem down to C33 – the capacitor that’s supposed to generate a DC voltage from the AC power line input. It had a strange configuration – 4 prongs (3 GND on the can and one positive terminal). It was also very very light and large – I’m still planning on taking it apart, but don’t think it’s electrolytic. I replaced the 85V 490uF cap with 2 50V 1000uF in series that I had in my workbench, and sure enough, it works like new again.
I did learn two lessons I would like to share. The instructions told me where to look, and that was great, but it also led me down the wrong path. Had I had the schematic and no debugging instructions I would have solved the problem more quickly. It’s important to rely on your instincts as well. In addition, I learned that older PCBs can’t take the heat. I tried to use my hot air reflow station to desoler the cap, but only managed to delaminate and burn part of the back of the PCB. I’ve not had that problem with newer PCBs — so don’t do that.