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The width slowly decreased, then the raster disappeared. On
checking, we found that the spring-of resistor feeding the line
output stage had opened. This was resoldered and the PL509
and PY500 valves replaced. On switching on however there
was sound but no raster, with the PL509 glowing bright red
inside. A new coupling capacitor to the line output valve was
tried, but without success. Just after switching on, the voltage
across the boost capacitor rises to 370V: it fals to only 20V
when the PL509 starts to overheat.
Oh dear! These symptoms strongly suggest a defective line
output transformer in this chassis. But check for drive at the
control grid (pin 8) of the PL509 — there should be around
—60V here. Even a new valve can be duf, so another should
be tried. If the voltage at pin 8 is lower than it should be, check
the values of R4088 (anode load) and R4087 (screen grid
feed) in the PCF802 line oscillator circuit — they tend to
change value, giving wrongly shaped or reduced drive. If the
drive is o.k. however, the PL509’s anode circuit is being
excessively loaded. The usual cause is a defective line output
transformer overwinding, but another possibility is shorting
turns in the desaturation choke L5502 — this will be
accompanied by a smell of burning plastic and smoke from
the coil. A less likely possibility is a defective PD500 shunt
stabiliser valve.

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