jakdedert wrote:
Sorry, hit send too quickly....

JohnC wrote:
I sprayed cleaner in the volume control, hum still present.
Something else.. the hum is audible no matter what input is
selected from the input switch, FM, AM, Phono, tape/aux. But there
is a Monitor switch and when switched from Source to Monitor, the
hum ceases.

John C.
John,

Have you tried feeding a source into the monitor input and see if the
unit sounds OK? I'm guessing it will. While I don't know the
HK-230A, I have seen receivers that the tape monitor switch feeds the
volume control directly (or nearly so) eliminating the the tone
control amps.

I strongly suspect that you have a low level (regulated) power supply
failure that is causing the hum. Looking at the power supplies that
feed the preamp circuits with an oscilloscope is going to be the hot
ticket here.

Tim Schwartz
Bristol Electronics

Tim,
Yes, when I plugged a source into the monitor input there is no hum.
All controls, bass, treble, balance, volume work for the monitor
source. Another item of interest- one of the channels begins to cut
out after about half a minute while using the monitor input. I can
unplug the RCA jacks and reinsert and both channels come back, only to
have one of the channels drop out again after about half a minute.

John

This is starting to sound like the possibility of either bad solder
joints, corroded internal connectors....

Also, it's been reported here that some older stuff develops loose
circuit board to chassis connections which can be solved by merely
backing off and reseating any screws which hold them down.

Clean EVERYTHING--controls, switches, jacks etc. Pull and reseat all
connectors. If the problem still exists, poke around the inside with an
insulated probe and see if you can cause any change in the amplitude of
the hum...or in the case of the tape input, can get the bad channel to
either cut out or in.

Check back....

jak

Sorry (again)...should have read the OP more carefully and looked at the
picture. My advice about probing was not very timely, having already
been attempted by the OP.

That said, to be clear, it should have been done with the unit powered
up. It's not obvious whether that was the case, although I'd assume so.
I didn't make that clear in my post, either.

Having looked at the pic (finally), it's striking just how old this
relic is. It obviously wasn't all that well-built to begin with, IMO.
I'm surprised by the diminutive size of the power transformer and lack
of heat sinking (Just where are those outputs, anyway?)...not to mention
the amount of point-to-point wiring employed.

Nonetheless, my advice is reinforced by the view. There's a lot of
potential for problems caused by strictly mechanical aging. While I
wouldn't expect this unit to develop a lot of heat; heat-related
electrical issues could still develop. Those would manifest as dried
out capacitors, or fatigued solder joints...possibly defective transistors.

I think if this one crossed my bench, I'd look at it as much a
'restoration' as a repair. It's from an time where both mechanical and
electrical design were still in flux from the tube era. Looking at it I
would guess it to be older than stated. It looks more like something
from the mid-60's, as opposed to early 70's.

jak