28 Dec
2003
28 Dec
'03
3:01 p.m.
Tony,
I pulled the power supply from the laserdisc player. I couldn't see it
adiquately in the machine.
Tony Duell wrote:
Can you tell us a little about the PSU design? Is it a
'linear' PSU, with
an iron-cored mains-frequecy transformer? Or a switcher, with HV
capacitors (look for a couple rated at 200V each) and a chopper transistor?
I find one cylindrical cap that is 200v and 220uf and one 250v .1uf cap
that looks like a chicklet (kind of chewing gum). I don't have any idea
how to identify a chopper transistor..... could that possibly be labeled
"Q1" and have a heat sink on it? I also see a couple transformers and a
bunch of resistors an IC and assume most of the D#'s are diodes. There
are quire a few "Q#" labeled items too.
This power supply does smell a little like something let the magic smoke
out.
If the former, can you disconnect the secondary windings of the
transformer from the PCB? If you can, then try a new fuse with the
windings disconnected. If that blows, then you have shorted turns in the
transformer, or a short in some component on the mains input side (RF
filter capacitor?).
If it's a switcher, then you have fun (for suitable values of 'fun'. You
need to check forwards from the mains input and at least find the mains
rectifier diodes and the chopper transistor. Check these for shorts. If
the transistor is OK, and one or more of the diodes is shorted, then
replace the diodes (use something rated at _at least_ 400V 2A) and try
again. If the chopper is shorted, well, you've probably got other
problems too, and _I_ wouldn't want to do much more without a schematic...
How would I check the rectifier diodes and the chopper transistor for
shorts? Check for continuity with ground on each pin?
Chad Fernandez
Michigan, USA