More parts I need to replace on the VT100

[Tony Duell]

> 
> On 11/30/2005 at 11:25 AM Dwight K. Elvey wrote:
> 
> > 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".

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 
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