Hi Joe,

I have spent some years in designing oscilloscopes (Frankfurt,
Germany...), and I have observed this behaviour on some units that came
in for servicing. This kind of error (focus adj. at one end) is most
probably due to the high voltage's resostors aging. Don't laugh: The
resistive material undergoes some influence due to the high electric
fields present inside the resistive material, and very very slowly the
resistive material exhibits changes in composition, molecules tend to
wander. This is what happens in "lower" quality resistors for high
voltages. If your resistors are molded into the transformaer plastic
case, then you are up the creek without a paddle.
If you have ever wondered why a 10Meg or 22 Meg resistor "off the shelf"
is rated at 0.25 or 0.5 Watts, but only for 200 Volts (this would never
give the power dissipation) here is the answer.These resistors exhibit
this phenomenon even more than high-voltage resistors.
Some 1 MEG high voltage resistors could have up to 1.6 Meg after some
years, and they were rated at 1 % !!!

Best regards
Jochen Feldhaar

Jinx schrieb:

> I've rescued a couple of good 12" colour monitors due to be
> biffed out because the focus can't be adjusted anymore. The
> pot on the transformer is at the C/W limit. Is there a reason why
> I can't add a resistor in series with the lead that goes to the tube
> PCB or will this just make the focus even more O/S. Is the
> problem that there's too much resistance already ?
>
> If not, any considerations re the choice of resistor(s) ? Several
> meg in parallel-series ? TV  focus pots look pretty shabby after
> a long time with HT across them. New transformers would be
> ideal but if resistors will do I'd be happy with that
>
> TIA
>
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