TDS5/6/7xx PSU behavior in standby?

[siggi]

Hi y'all,

I'm trying to repair a TDS684A, which came to me with a broken standby power supply. I've fixed that, but when I power the scope, it hiccups, e.g. the front panel lights and fan turn on and off. I've tried disconnecting the CRT and acq boards, but this doesn't change anything, so I'm looking at the PSU again in more detail.

One thing I noticed is that my PFC regulates fairly high at 417V when I feed it 60V DC for input. When I feed it mains, it runs from 424V to 437V where it bumps into the over voltage lockout, at a frequency of about 1Hz. This is one of the older PSUs that has a PFC riser board full of discretes, btw. I've checked the 2.5V reference, which is spot on, and everything in the PFC otherwise looks as described in the TDS520 SM.

I wonder if this is behaving as expected, as I can then strike it off my list of things to further look into.
Anyone?

Also, the 684A SM has a table with expected output voltages from an unloaded supply. Mine's reading low on the 24V rail in particular (it's ramping from 8-9V with a period of ~180ms).
Since these PSUs were outsourced and frequently revised, I wonder how much stock I can put in these troubleshooting guidelines in the SM?

Siggi

[nctnico]

In standby these PSUs are in hick-up mode. If you listen closely to the scope in standby you'll hear it ticking in a 1 HZ ritme.

[siggi]

In standby these PSUs are in hick-up mode. If you listen closely to the scope in standby you'll hear it ticking in a 1 HZ ritme.

Thanks - this is indeed what I observed. I've bolted the PSU back in the scope, essentially just with a fixed standby supply and a new bulk cap.
It was now able to power the processor and CRT without trouble, but when I added the acquisition board it went into hiccups. After a while it "cleared up", guess the PSU burned through shorted bypass caps.
In any case, I have suspicious-looking residue around some of the SMT caps on the acquisition board, so likely I suffer from capacitor plague.
At least that's something I believe I can fix.