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Trouble converting an old radio to an amp

I've found the volume pot, that was tricky since this radio has volume and tone on the same shaft. But by measuring the resistance I could see it change on the "back" pot when I turned the volume knob, without power of course.

I can see 5 taps off the volume pot, but I cannot get the guitar to play when I tap off of any of them. Here's what I have.

Tap 1 has 9v when the volume control is clicked on, 0v when off. Tap 2 has 9v all the time.

On the bottom we have 3 more taps.

Taps A, B and C also have 0v when the volume is off, and 9v when the volume is on.

I pull the batteries and measure resistance (and yeah, I know, the pot is in the circuit so it isn't accurate but I'm just trying to find the "input" lead.

A always reads 4k Ohms, B varies from 2k to 4k, C is at 2k no matter what I do with the volume knob.

1 in always at 2k Ohms, 2 is 0 Ohms when off, 2k when on (which makes sense since it is connected directly to the battery)

So it seems to me that I should be hooking into tap B. So when I hook the rod-piezo to B and to ground I still hear the static of the radio and when I pluck the string, I do not hear the note out the speakers.

All the online tutorials make this look easy, but I'm baffled at this point.

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Permalink Reply by darryl kernaghan on January 17, 2018 at 3:22pm

I'm no electro guru, and someone else might help more, but, i think term B will be signal out, so you still have the radio going, even if you connect the guitar to there, I would guess, terminal A is currently radio input to volume pot, and you would need to connect there and either add a switch to change from radio- guitar, or just disconnect radio totally . Looking at your pic again, it could be that A is ground as it appears it might be 1 piece with the terminal strip going to the rear, but hidden by the other component. I've probably confused you more than helped you, but someone will set you straight

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Permalink Reply by Timothy Hunter on January 17, 2018 at 11:22pm

i think this is what you are trying to do. i think that volume pot is between the preamp and power amp sections. your rod piezeo doesnt have enough to drive the amp section and you havent isolated the Rf or radio section. radio was not my field in electronics so i only have basic knowledge.

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Permalink Reply by Paul Cezanne on January 18, 2018 at 11:55am

I tried a 3-pole magnetic pickup just now. No joy.

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Permalink Reply by Paul Cezanne on January 18, 2018 at 11:56am

And how can I isolate the RF section? I'd love to not hear the radio...

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Permalink Reply by Paul Craig on January 18, 2018 at 12:12pm

If this is the layout, wouldn't it be better to disconnect the RF stage signal and make the wires to the preamp into the new input? Assuming it's not high voltage.

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Permalink Reply by Paul Cezanne on January 18, 2018 at 12:34pm

It isn't high voltage. I don't mind doing that but I don't know how.

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Permalink Reply by Paul Craig on January 18, 2018 at 8:31am

Lugs 1 & 2 on the back are the on/off power for the radio.

Lugs A, B, & C on the bottom are probably power on circuits to the rest of the components since they have a 9volt reading.

What about the 3 lugs on top with a cap? Trace those 2 wires coming off the top and get a volt reading on them.

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Permalink Reply by Paul Cezanne on January 18, 2018 at 11:55am

The three lugs with the cap are for the tone control, but I just tried it. No joy.

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Permalink Reply by Paul Cezanne on January 18, 2018 at 12:24pm

Silvertone, from 1961- or 1962

I'll run the resistance test right now.

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Permalink Reply by Paul Cezanne on January 18, 2018 at 12:33pm

There are two lugs on the tone pot. (The third is just stick up in the air, not attached to anything.) One of them varies the resistance from 0 to 40k Ohms when I turn the tone pot. The other is ground.

The resistance does not vary with the volume knob turning.