Many thanks to Pelule for these useful information! I just got a HP6624A which showed "HW ERROR CH4", and trying to repair it. I have downloaded the service manuals, and found it is quite difficult to follow. So far, I have a couple of questions and hope someone may help.
1. The AC cooling fan is very loud. I noticed that when the fan was first powered on, it was quite quite and the rotation speed looked reasonable. Then, maybe a 30-40 seconds later, the fan speed increased and gave loud noise. Is the fan bad (bearing)? Or, is this loud noise has anything to do with the capacity near it? Any suggestion how to reduce the noise?
2. When I turned on HP6624A, the screen directly displayed "HW ERROR CH4". According to the manual, it first should display "ADDRESS 5". Is my GPIB card also bad?
3. When I turned on HP6624A, the screen directly displayed "HW ERROR CH4". I tried each key on the front panel, but there is no any response from the screen. Is my keypad bad?
4. If I don't want to repair output board 4, can I use my 6624A with only 3 output boards?
Thanks in advance!