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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 Paul Craig on January 19, 2018 at 8:02am

I'm thinking that the 2-wire ribbon cable from the RF converter to the board is where you need to wire your guitar input. I would unhook the cable from the converter and check for voltage(radio should stop working) and if okay, hook up guitar.

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Permalink Reply by Paul Cezanne on January 20, 2018 at 5:52am

Oh, I like that idea. I'm traveling now but will check it out when I get back home. Thanks!

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Permalink Reply by Paul Craig on January 18, 2018 at 12:06pm

I'm thinking that the last stage of the pot is actually a volume instead of tone. The cap could be a high pass filter.

The pot shown is a 3 stage pot. 1st stage for on/off, 2nd stage is obviously power distribution circuit. That would leave the 3rd stage for volume. Unless the 2nd stage is the volume that has shorted out to the power supply.

Do you have a schematic? What brand radio and model # is it?

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Permalink Reply by Paul Craig on January 18, 2018 at 12:07pm

More detailed pics could help.

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Permalink Reply by Timothy Hunter on January 20, 2018 at 6:01am

https://www.radiomuseum.org/r/sears_roeb_silvertone_700_1217.html

looks like its a silvertone 700. my first guess is that the input from the RF section will be to the base of T5, possibly biased via a resistor divider between gnd and bat + . also possibly also fed in through a DC decoupling capacitor...

That link shows the pic posted by Paul Cezanne .. its a super hetrodyne circuit but this only describes the RF side not how the amp section works so ??

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Permalink Reply by Paul Craig on January 20, 2018 at 7:20am

On the schematic and pic, T5(labeled-Driver) looks like the output tranny to the speaker. If so, that's not the place to hook up to. He needs to find where the Radio's signal inputs to the preamp.

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Permalink Reply by Timothy Hunter on January 21, 2018 at 11:31pm

TR6 and TR7 are the audio output transistors.

this is something similar to the circuit. i think i could be wrong( probably) but the amp output stage seems close..

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