6 Aug
2012
6 Aug
'12
11:09 a.m.
So true. I can't tell the number of times that I
repaired my parent's TV
set. It regularily killed the HOT about once every two years. I've tried
many things in order to prevent this (assumed a poor circuit design, after
all, the poor BD508 has to provide enough power for a 90cm CRT). I
replaced all suspect components (HOT driver IC, flyback, cascade), but
still, it blew (i.e. shorted C-E) after some time. One day, I had the idea
to suspect not an electronic component, but think it might be something
different. I had a deeper look at the isolating mica piece between the HOT
and the chassis. There was nothing visible, but nonetheless I replaced it
ALthought I hate ;shotgun debugging' as you well know, I gernally change
the mica insulator when I fit a new transistor.
The otehr common cause of problems are dry joints in the area. They will
work find and then open at just the wrong time to blow the tranistor. A
good resodler sesssion helps.
with a modern isolating silicone part. Since then, no
more failing HOT.
So my guess is that the mica became somehow sensible to high voltage
differences and sometimes shorted the collector to the chassis.
For now, the TV's running fine (it's 25 years old now).
Ah, a relatively new TV...
Some devices, particularly SMPSUs, get round the problem of mica washere
brakedown by conencitng the heatsink to the collactor on the PCB. This
has the mild disadvantage that said heatsink is then live -- very. Do not
touch random heatsinks in SMPUS circuits if you want to stay in this world...
-tony