Hi everyone, I've been trying to figure out what is wrong with the 12V rail on my H7861 (BA11-S) power supply. It's showing about 4.2V. The 5V rail is spot on.
Page 39 of the following schematics is the main part board of the PSU. http://bitsavers.org/pdf/dec/qbus/MP01233_BA11-S_schem_Mar81.pdf Going into the collector of Q3 is about 80V coming straight from T2 (I think I measured it at about 100Hz), but the emitter is putting out the 4.2V, which is the same as the base voltage and output voltage. I tried replacing this transistor because the hFE was about 80 and a good one was about 120. Unfortunately it didn't do anything. None of the capacitors look swollen and I don't see any leakage. There is a smaller board which I think goes into J4. The 12V side seems to have a 555 timer and adjusting the pot doesn't change the voltage at all. My understanding of switchmode power supplies is very poor. Does anyone have some pointers on what to check or what might be the possible cause? Hopefully I can get my PDP up and running again... Only got about 20 minutes use out of it. Thanks! Aaron.
