SX-28A Audio Output transformer needed

[Richard Post]

I agree with Scott Robinson's assessment.  Running an output transformer
unloaded causes VERY high voltage spikes in the primary. (Yes, I found out
by experience many years ago.)  Maybe with todays technology, the right MOV
or back to back zeners on a secondary tap might prevent the problem.  A
resistor also works and was a common solution in older communication radios.

The SX-28(a) has a resistor that is always tied in to the secondary side.
Make sure it is intact and of correct value and not disconnected somehow by
the headphone jack contacts.  My SX-28 had a much smaller universal
replacement transformer in place of the original when I bought it.  Because
of the smaller size, only one leg of the replacement transformer was bolted
to the chassis. (At least they didn't drill a hole for the other leg!)  I
found a size match and primary impedance match in an old parts Magnavox
phono.  As a result of the transplant, I only have the low ohm impedance
output, no 500 ohm out. That's not a handicap as I do not use the 500 ohm
and could easily add an outboard matching transformer if I did need it.

Rich KB8TAD

>Date: Mon, 08 Dec 1997 23:13:24 -0800
>From: Scott Robinson <spr at earthlink.net>>
>Folks,
>
>The usual cause of output transformer failures is running them unloaded,
>that is, no speaker or equivalent load connected.  This can cause HV spikes
>on the primary side that fry the insulation, which sounds like what's
>happened to that SX-28A unit.
>
>Regards,
>
>
>
>Scott Robinson
>spr at earthlink.net
>
>Junque is GOOD for you!
>


Boatanchpor Pix website
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mirror site
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visit the Museum of Radio and Technology website
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