On 3/30/2019 2:25 PM, Charles via cctalk wrote: > I have a PDP-11/23+ and the power supply (H786) "last ran when parked" a > year or so ago. But there's no DC output at all today, and the fans are > running so there is AC power... > I also have the original H7861 that came with it, which had a blown > chopper transistor. I couldn't find anything else bad, so I replaced the > transistor and within a few seconds of running, it blew again. :( > > So I need some help - I've never been good at fixing switching supplies, > not to mention the high-side hazards. > The simplest solution would be just to replace it with a working unit. > Anyone got one to sell, hopefully cheap? :) > If not, can anyone fix one or both of mine? > > thanks! > Charles >
Sorry, no spares (and certainly no tested spares. ;) ). My playbook for these sorts of things goes like this. Others will undoubtedly chime in with probably better advice. 1. Is anything obviously burned / overheated / fuses blow? Note those, and what feeds them. 2. Check the semiconductors - often I will lift one leg of a diode or two legs of a transistor to make sure I get a good test. It's a pain, but usually worth it. Sometimes, depending on what the schematic looks like, I'll test without lifting as many / any legs. 3. Specialized regulators / op-amps, etc., require checking their voltage while running. Not usually easy to test them. 4. Check the capacitors, especially input and output for excessive leakage. I often will reform input and output caps just because. Check their ESR, at least with one of the cheap component testers out there. I use one of these for testing, as well as an ohmmeter, etc. https://www.ebay.com/itm/All-in-1-LCR-Component-Tester-Transistor-Diode-CapacitanceESR-Meter-InductanceBH/183706764001 I also use a "Blue ESR Meter" of late, as well: https://anatekinstruments.com/products/anatek-blue-esr-meter-full-kit-for-self-assembly-besr_kit But before I had these, I just used an ohmmeter to test each semiconductor junction, and monitored voltage while reforming capacitors to get an idea of their ESR.