30 Nov
2005
30 Nov
'05
4:51 p.m.
On 11/30/2005 at 11:25 AM Dwight K. Elvey wrote:
I find those sort of tips totally useless!. There could be many causes of
'no vertical deflection' (or whatever), the fact that _once_ it was
caused by a particular set of components doesn't mean it always is.
The first time I came across soemthing like this was over 20 years ago. I
had to fix a tape recorder for a teacher at school. The user manual was
Many IC's and transistors show little to no
visual
indications that they are shorted inside. I don't
know how one can find these without understanding
where the resistors are in the schematic and how
the resistors are being used.
Actually, many servicepeople rely on compiled tips for servicing.
Something like "If there's no vertical sweep, replace R302, Q401 and C213".
from the time that such manuals were useful and
contained not only a
schematic but also a list of faults and their cures. For
'low output and
distortion' it told me to replace 3 capacitors (the decoupling capacitors
on the cathodes of the amplifier valves). I did, and it made no
difference. So I sat down with the 'scope and meter and found the real
fault was a leaking coupling capacitor between the anode of the phase
splitter triode and the grid of one of the output pentodes. Said grid was
runing several volts +ve wrt that valve's cathode (!).
I supose such fault databases are useful if you have many similar units
to repair in that you'll get most of them off the bench quickly. But it's
not a substitute for knowing how to trace the fault properly.
-tony