Dick,
I would like to have a copy of this service bulletin also. The
only one I have heard of dealt with fusing the, otherwise unprotected,
filament transformer primary. Of course, this change should also be done
and, if done properly, it will works with both 120 and 240 line voltage.
But, this is not the problem that I was talking about. With the original
HeathKit transformer, this winding is only rated at 70 ma (.070 A). Drawing
current in excess of this rated value, as with a leaking electrolytic,
caused the winding to overheat and eventually break down, usually shorting
to the primacy windings. It has nothing to do with protecting the primary
with CB's or fuses.
A bit more explanation may be in order. In the original HeathKit
design, the filament transformer draws very little power, something in the
range of 150 to 175 watts. Its has no dedicated fuse. Its primary is only
protected by the main 10A CB's. So, basically, it has no protection, at
all, because it will burn up way before the 10 A CB's open. The simplest
answer, that I have been able to come up with, is to re-wire the primary of
the filament transformer for 120 V operation, all the times. Pay attention
to which side of the line you re-wire it to. Then add a small fuse in this
120 V line, usually 2 to 2 1/5 A is sufficient. Now the transformer will
have real protection and will not care whether the amp is being run from
120, or 240 VAC mains.
73 de Lon, K5JV
1110 Golden Bear Ln.
Kingwood, TX 77339
281-358-4207
281-358-4234 FAX
281-795-1335 CELL
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