I am working on this Realistic 12-689a SS radio.

no schematic... any help appreciated

There are 3 wires going to the 2 speaker terminals.

Why?

There are two coming from the output transistors..
( no output transformer)

But also there is another wire from a 2.2meg resistor connected to one of the 2 AC input leads to the 120/12v power transformer.

Why?

Is this some attempt at hum canceling?

The 2.2 meg resistor is mounted on a separate little 2 sq inch board on top of the transformer.

This seems to be primarily used also to inductively couple the FM antenna to the AC line.

The two ( non polarized plug) AC lines connect to the little PC board.

One AC input lead connects directly to one input side primary of the 120/12v power transformer and at that junction is the 2.2 meg resistor with a wire going to one term on the speaker.

The other AC input lead goes to a PC trace that forms a 3 loop spiral getting smaller circling inward and then soldered direct to the other side of the Power trans primary.

There is another larger 3/4 circle loop trace that wraps around the 3-loop inner spiral but not physically connected. This outer loop has a lead that connects to the FM ant.

So that part seems to simply form a little transformer loop pick-up to inductively use the AC line for the FM antenna.

But its that 2.2 meg resister on the board that confuses me. It just comes right from the raw AC and then feeds over to one speaker terminal.
Wonder what it's doing?

Any ideas?

Does any part of the speaker wiring connect to the common part of the circuit wiring (equivalent of B- or chassis in a tube radio)? If so, the resistor might serve to leak off any charge that might build up between the radio and the AC line. You may recall a resistor similar to this in AC/DC radios with a floating ground, where the chassis is connected to B- through a capacitor and resistor in parallel, the resistor serving to assure that a DC charge does not build up on the chassis.

T.

Thanks Thom:
No. That" mystery" lead from the resistor connects to one speaker terminal. That speaker terminal is also fed directly from a wire that goes to the main board audio output section to the collector of an NPN transistor.

The other speaker terminal has a lead going to the main board output section to the positive end of a 10 vdc electrolytic cap of a capacitance value which is unknown as I cannot get in there to read it.

If you can delve into your varied SS experience Edd and come up with a clue.. I'd be delighted!

Are you sure it's a resistor?

That spiral trace is both an inductive and capacitive coupler.

The resistor may still serve to eliminate spurious charges, though that is an odd way of connecting it.

I have an RCA pocket radio that has push-pull output and the speaker has a tapped voice coil. Unfortunately the speaker got destroyed somehow, and I cannot find one like it. Anyway, probably not at all what you are looking at.

T.

Hi Thom:
Yeah.. I measured it too... (red red green) 1/2 watt.. and it measures 2.2 meg.

Hi Thom and Peter,
I think you have it. One could connect pretty much connect the 2.2 meg. resistor anywhere on the output. This may have been chosen as a convienient point.

All the Best,

Bill Grimm

Someone over at ARF offered this helpful info:
Here's a similar circuit that is used on a wall-wart to do the same sort of thing: