2 Oct
2013
2 Oct
'13
12:37 p.m.
It seems, that I have todo some reverse engeneering on
the PSU of the TSZ07
to find out what's wrong. It isn't the main switching part of the PSU that
gets hot, more something like Parts of the Standby Power regulation.
The Power Resistor that was originally in there (smaller as that what I've
soldered in for replacement) is made to get hot, but unfortunately it gets
hotter than is good for the surrounding parts, inkluding the PCB itself.
It would be interesting to know what this resistor is for.
If there's a capacitor-resistor-capacitor smoothing circuit (that is a
capacitor acorss the supply (maybe rectified mains), then a series
resistor, then another capacitor, then if the first capacitor goes high
impedance (either losing capacitance or increased ESR) then more ripple
current willflow through the resistor and second capacitor. Ripple
current heats up resistors just as well as any other current :-). So a
failed capacitor can cause this.
Another thign that DEC certainly did was to put a power resisotr in
series with the mains input to limit the inrush current. This resistor
is bypassed by a triac which is triggered once the PSU gets going. If the
triace of its control circuit has failed, the resistor carries the full
PSU input current and overheats. I think DEC even once used a relay to
short out the resistor, but anyway.
-tony