6 Aug
2012
6 Aug
'12
1:42 a.m.
On Sat, 4 Aug 2012, Tony Duell wrote:
You were somewhat lucky. Sometimes the horizotnal
output transsitor just
fials (as yours did), but sometimes it's killed by some other fault
(shorted turned in the flyback transoformer being the worst, but
shorted/leaky diodes and capacitors in that circuit can do it too). I
haev learnt by bitter experience to always order a few spare transistors
when fixing this kind of fault in case the new one fails instantly at
switch-on.
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
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).
Christian