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Need help with surface part ID and finding a schematic

S

SteveB

Jan 1, 1970
0
I fix surface mount power supplies for a living and still I can't recognise
some of these parts. Even having a rough idea doesn't help a lot, for
instance the pink non-surface mount 15/16 items are diodes but could be any
type and rating, but they look a bit like the bidirectional avalanche ones
as used in the gates of triac ac dimmer circuits as I can't see a cathode
marking. Measure them on a DVM diode range to see if you get an approx 0.7v
drop across them, if so then they are ordinary diodes or perhaps zeners, if
not they may be the bidirectional ones I mentioned.
Item 17 is a capacitor, probably about 100nF 50v, but again that's pure
guesswork as it could be 10nF 500v.
Other guesses:-
1,2,3,4 are power diodes, very likely all the same type.
5,19 (presuming there are 3 connections) are power transistors, FETs ,
triacs or thyristors. My guess is triacs because of those pink things). I
would expect them to both be of the same type.
11 is a small value capacitor, perhaps 1nF 50v
12,13 are probably transistors or less likely small FETs but there are
diodes and zeners the same shape with 3 connections where one goes nowhere.
7,8,9,10 no idea but as there are 4 of them and seem to be only two
connections I suspect medium power diodes.

I wish you luck as it is almost impossible to identify unmarked surface
mount components accurately. I would measure everything with a DVM, if
anything measures short then remove it and check it's still short off the
pcb for starters. Check all identical looking components actually measure
the same on a DVM diode range both ways round. These sort of semiconductors
tend to go short or very low resistance (<100 ohm) in both directions when
they blow up. Unfortunately they also sometimes go short first then blow
open circuit if lots of current then flows as a result of the short. If you
get 0.7v readings across 2 or 3 contacts of the 2 or 3 available then that
component is likely OK (this is NOT foolproof!). The 5,19 components are
the most likely to have failed and if they have will be difficult to
identify. If only one has gone short then use the other to find out if it's
a triac or whatever. If you don't know how to identify basic semiconductor
types with a meter ask here.
 
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