[Boatanchors] SX-28 Help Needed

[Drew Papanek]

Joe Connor  wrote:

>I replaced all the electrolytics and paper caps in my SX-28 several
>months ago. It ran great until tonight, when smoke started coming from
>the audio-output transformer. I need some help in diagnosing and
>repairing the problem:

>1.  The only original cap I see near the trouble area is a 2,000 pfd mica
>cap (C-76) across the primary of the audio transformer. Is that a likely
>suspect? If so, I'll replace it. Is a .002 mfd orange drop a suitable
>replacement?

An Orange Drop would work fine here.  Just make sure that it has a high 
enough voltage rating (1200V or more).  Failure of that cap is not likely to 
kill the output transformer, however.

>2.  Any other likely suspects (other than a shorted audio transformer)?
>The weird thing is that this set still plays great even when the smoke
>starts. (I kept it one only long enough to pinpoint the precise source of
>the smoke).

Seems odd.  Perhaps one of the output tubes (6V6) has shorted.  If that were 
the case I would expect the common cathode resistor for the 6V6's (220 ohms, 
2 watts) to be somewhat cooked also.

>3. If I need to replace the audio transformer, what would be a suitable
>replacement from AES or Radio Daze? I'm currently running the set through
>a PM-23 speaker. I'd like to keep using that speaker (if possible).
>However, the 5,000-ohm impedance may be a problem. I suppose with a new
>audio transformer I could always disconnect the matching transformer
>inside the PM-23 to make the speaker 8 ohms. Any thoughts or suggestions?

Peter Dahl makes an output transformer for the SX-28.  It ain't cheap, but 
is good quality and an exact replacement and will enable you to keep on 
using the PM-23 with no modifications.

Drew