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