THORN 1400 CHASSIS

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The fault is sound but no raster. The PY801 boost diode
gets hot, and the line output valve’s screen grid resistor
R138 also overheats – its soldered connection melted. The
line output valve and R138 were replaced, but the PY801
and R138 still get hot.
There is a harmonic tuning capacitor from the PY801’s
cathode circuit to chassis. This is C114, 220pF, a pulse
type. It could be leaky, placing the PY801 across the h.t.
line and diverting the PL504’s cathode current via its screen
grid. If not, check for continuity between the top caps of the
two valves to make sure that L36 is not open -circuit. The
line output transformer is generally reliable but could be
faulty. It would be as well to check C115 (0.1µF) which
decouples R138.

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