OK, time to pick the EEV brains again. I've got yet another scope to repair - a Hitachi V-665. The PSU squeals loudly at about 10kHz. I've checked all the capacitors and diodes, and as far as I can tell, they're good. None of the rails are shorted, and unplugging either one of the two power output cables doesn't make it happy (so I'm assuming the fault isn't on one of the other boards). Nothing overheats, even if I allow it to run like this for a minute or so, but nothing works correctly either.
+110V, +55V, -12V rails are reasonably close to regulation (I don't have the exact numbers with me, but about 120V, 58V, -12V). The user manual doesn't specify what the acceptable tolerance is, and I haven't been able to find a service manual. However, the +12V and +5V rails are down to 9V and 4V, and as mentioned above, oscillating angrily. Tweaking the "+12V adjust" pot just makes it louder.
Attached are scope captures of the +5V rail before and after the output LC filter, and the relevant schematic page. The other rails look exactly the same on the scope, so I didn't bother capturing them.
Unfortunately, all the interesting functions are mains-side (and also it's a massive pain in the ass to disassemble), so before I start dicking around aimlessly with an isolation transformer and my safety, I thought I'd ask if anyone has any suggestions of exactly what to suspect. I'm not 100% sure I understand at a glance how the controller chip works, though I'll dig into the schematic a bit more before doing any real work. I haven't been able to find a datasheet that isn't in Japanese.
A few thoughts and comments:
- I'd assume the STK7308 controller/switch chip is good, since it is running, if poorly.
- Since only two of the rails are out of regulation, even though the only thing linking them is the turns ratio of the transformer, I'd normally suspect the transformer. However, the actual scope circuitry doesn't appear to be running properly, so I think it's safe to assume that the loads on the rails aren't distributed the way they're supposed to be.
- Complete shot in the dark: Maybe the soft start bypass TRIAC is fried and isn't closing? Would an SMPS stuck in soft-start behave anything like this? And do they often fail open?
- Would it be risking damage to disconnect the power supply and externally supply power to just the +/-12V and 5V rails (not the 55V and 110V), to see how they behave when supplied with a known good PSU?