[Amps] [HeathKit] The SB-220

[kingwood]

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