?

I have not read all the posts on this thread so may be repeating
something. Tom, I do not have an SX-28, In normal operation what does
the bass switch do? Does the bass increase or decrease when the switch
is in the IN position? The handbook is confusing. The schematic suggests
the BASS switch boosts the bass by adding a resonant choke on the output
of the first audio amplifier. The schematic shows the switch cutting off
the choke in ON, that looks backward to me. The additional TONE control
is just a conventional high roll off. It appears that both of these tone
controls are independent, is that correct.
It seems to me the bass boost should INCREASE hum.
The plate transformer of the output stage connects to the input of
the B+ filter. This is not raw AC since there is a fairly large cap
across it. The balanced circuit should remove any residual hum. The
rest of the amplifier is fed by filtered B+ and seems unlikely to have
significant hum on it. Try shorting out the tone control choke, if its
getting induced hum that should eliminate it.
Since you practically rebuilt the receiver perhaps some wiring
error was made or some new part is actually bad. Worth going over again.
It is always frustrating to me to try to trouble shoot problems
where I can't just jump in an make measurements.

On 2/13/2025 4:21 PM, thoyer via groups.io wrote:

I just finished pretty much a complete overhaul of a nice condition
(physically) SX-28A. Replaced just about every resistor and all caps.

Radio is working well except for a 120hz hum. When I switch the Bass in,
the hum goes away.

Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that
is there independent of the bass switch position. The output side of the
HV filter is clean, no ripple. I paralleled another 47uf 450v cap across
the choke input cap and there was no difference.

I double checked the wiring against the schematic along with component
values. Is the schematic correct in this area? I found once schematic
error maybe there is another?

Thoughts?

Tom

W3TA

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998