THORN 3500 CHASSIS

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The line output transistor was replaced (had been found
short-circuit) and the set e.h.t. control was then adjusted for
minimum e.h.t. consistent with sufficient width (h.t. at 60V,
I can’t measure the e.h.t. itself). The set was o.k. for a time,
then on switching on the final anode kept arcing loudly to
the metal shield. I cleaned around the anode connector and
the insulated metalwork, and reduced the h.t. slightly (width
still adequate) but there’s bad frying to the outer tube
coating, so the e.h.t. is obviously too high – increasing the
h.t. above 58V makes it worse. The tripler has been
replaced and the preset controls set up. The voltage across
R907 in the beam limiter circuit is slightly high at 1.7V.
We feel that it’s essential to be able to check the actual
e.h.t. voltage. If this is not in excess of 25.5kV, cleaning and
polishing the appropriate areas of the c.r.t. bowl should be
sufficient. If the e.h.t. is excessive, suspect the flyback
tuning capacitor C518 (0.028,uF). Note that there are
alternative line output stage transformer connections and
that these affect the width – details are given on card 9 in
the service manual.

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