SB-104A pops when PTT is pushed. Why?

[Ron Eisenbrey]

Rob:

Heath wrote a service bulletin on this problem, SB-104-7, dated 1/29/75.
"Pop" heard in the transmitted signal when the VOX "Drops out".
Change C1, .1mfd to 10mfd, +end to P7.   This cap may not appear in the
schematic.  It goes from connector pin C7 to ground.  NOTE:  The .1 cap
can be left in and 10 mfd added across.

As a follow up I tried this on my SB104 and it did help somewhat, but
still have the pop.  I kept increasing the size of the cap and finally
decided a 100 mfd worked best.  Much bigger than that and the VOX circuit
is too slow on transition back to receive.  The 100 mfd cap worked much
better than the 10.  None the less, I still have the pop, although not
quite as loud.  Started tracing the source of this "spike", but never
found it.  Even tried a bi-directional silicone avalanche diode to clamp
the spike, but no help.  If you ever find the ultimate solution, please
let me know.

Good luck.

73

Ron, AB5WG

Rob Frohne wrote:

> Hi All,
>
> I just acquired an SB-104A that pops when the PTT is keyed or even if
> the VOX is keyed.  It is coming in after the RF gain and before the
> AF gain (AF gain has an effect, but RF gain doesn't).  Anyone have an
> idea how to cure this problem?
>
> Thanks & 73,
>
> Rob, KL7NA/W7
>
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