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
OK, that sounds like the mains smoothing cap of a switcher...
that looks like a chicklet (kind of chewing gum). I
don't have any idea
Or a tkind of keyboard :-)
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
Yes. Q = transistor on most PCBs. The chopper with be on the mains side
of the PSU (and thus will probably have a fairly low number, like Q1) and
will almost certainly be on a heatsink.
bunch of resistors an IC and assume most of the
D#'s are diodes. There
Are there any recognisable markings on the IC?
are quire a few "Q#" labeled items too.
This power supply does smell a little like something let the magic smoke
How would I check the rectifier diodes and the chopper
shorts? Check for continuity with ground on each pin?
Does your meter have a 'diode test' range? Most do.
If the rectifier is made of individual diodes, then desolder at least one
end of each one, and test them with the diode test range of the meter.
They should conduct (forward voltage -- what the meter displays -- about
0.7V) in one direction, and be open-circuit (meter shows overrange in
some manner) in the other.
If the rectifier is a 4-terminal bridge 'block' then the 4 leands will be
+ve, -ve, and 2 AC. You'll have to work out which is which.
Check (with the rectifier out-of-circuit)
AC to AC : open both ways
Each AC to +ve (2 separate tests) : acts as a diode
Each AC to -ve : acts as a diode
+ve to -ve : looks like a diode with twice the forward drop (about 1.4V)
For the chopper transistor, again take it out of circuit. You need to
identify the connections -- look up the type number in a databook (or get
me to see if I can find it :-)). If it's a flat plastic package with a
metal tab and 3 pins out of one end, then, with the tab downwards and the
leads towards you, the order is likely to be b,c,e from the left. The
collector (c) is also likely to be connected to the tab.
Anyway, once you've done that, check (using the diode test range) :
c to e : open circuit both ways
b to c : acts as a diode
b to e : acts as a diode