Electronics > Repair

Oscilloscope SMPS repair - where to look?

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c4757p:
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?

SeanB:
C1507, C1508 and C1512 would be good bets to change in the first place. The higher voltage loads probably have enough capacitance not to drop when the supply is oscillating, but the low voltage rails have a larger load. The triac will be working, as otherwise the smoke would be coming out of R1501. With these Sanken supplies I am always suspicious of small electrolytics on the primary if they are not working well and the outputs are not dead shorted.

Paul Price:
I'd put my money on C1561 and C1551 and possibly C1552 but I can't tell because you've not presented a scope trace here.

You should scope the capacitors with a ground reference on the left side of the small L1510, L15aa chokes that connect all the filter caps to ground, but that would mean you would isolate the ground lead of your scope and you wont do that..I would.

At telltale sign of a diseased cap is any bulging of the vent area on top of the filter caps, a visual check can be quicker than a scope to find the problem.

You can make a good check of the inrush triac circuit by simply unplugging the power supply and checking (oops don't burn yourself) the temp of the resistor it shunts. The resistor should be reasonably cool if the triac is doing its job.

I would use an AC multimeter to check the AC ripple across the caps in question, it is a safe and isolated way to spot an open or opening capacitor.

I think measuring the DC voltage and AC ripple just post input bridge filter cap point,  this is also an important place to look  at. The voltage should be in the 350V range and ripple should be <10V RMS.

c4757p:
Thanks for responding quickly as always!


--- Quote from: Paul Price on July 07, 2013, 04:47:29 pm ---I'd put my money on C1561

--- End quote ---

I'll check it more carefully. It didn't stand out with a visual check and in-circuit ESR probe (admittedly probably inaccurate because of the filter chokes), and it's a top quality Nippon, so I didn't suspect it, but I will. I've got an appropriate replacement on hand, so I can just test by replacing it.

Paul Price:
Measuring ripple with a MTM and/or shunting a new capacitor across a suspected cap is much faster, easier trouble-shooting technique that results in quickly finding problems due to capacitors.

There are just two reasons for the +5 and +12V supplies to sag, they are either not supplying enough power because of power supply problems or the circuits they connect too are greedily asking for too much. Disconnecting the load, if practically possible, and shunting the gap with a .1 or .01 ohm resistor will reveal the current drawn by the respective supplies with ohms law and a low-voltage reading across the resistor. I sometimes do this using an razor to cut a tiny paper-thin break in the PCB copper (that can be easily repaired with small wire bridge after tests) and insert the current-sense resistor across the gap.

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