50s jukebox oscillator input circuit

[Randall]

I rebuilt the power supply in this 50s Wurlitzer juke box, but now it won't pass sound through the input stage. From the Aux in jack it works great. At the input I see signal at the input side of C2, but at the C3 side I get almost none. C1 was leaky and since it is the same value and type as the questionable C2 I replaced them both. Each is listed as 51 MMFD which I believe is 51 pF, and they look like big resistors. I replaced them with 47pF, but it still doesn't pass signal past C2.

Then I saw in the manual that the first tube is an oscillator circuit, but I don't really understand the theory of this, nor have I ever heard of this. But, I think, oscillator or not, shouldn't I see signal on the grid of that tube? Or is L1 shunting it to ground? And why such small value caps? I get a pop when probing the plate, but not like you would expect on the grid.

Is there anything I can do to determine what is the issue here, or I am I trying to fix something that is not broken? Again.

Whurly schematic.pdf
theory of operation.pdf

[Enzo]

That amp expects the Cobra pickup. It is not a common pickup that produces a small signal voltage.

The first stage is indeed an oscillator at some high freq I forget. The pickup has an inductor in the tuned circuit. The moving needle detunes the oscillator as it moves. SO the input stage is almost like FM radio. L1 is part of the tuned circuit. I have a CObra tonearm for the shop, so I can plug it into the amps. I inject audio into the second stage for testing. To test the first stage, I plugged my CObra into it, and dragged my finger across the needle. If that came through the amp, the stage was OK.

Your cap values are so small because they are in a circuit operating at way high freq, not audio.

When stereo came along, CObras went away as they could not make stereo.

[Randall]

YAY! I was hoping you would chime in Enzo. So, are you saying you would not expect any audio from a tone generator or music source plugged into the input jack?

Injecting audio into the second stage produces sound, so I guess it is down to installing this amp into the console and see what happens.

Another question, I put 47pF in the original 51pF slot. Since you say this is a tuned circuit, would you be OK with being ~7% off when the schematic calls for 5% there? I have no way to test caps under 1uF.

[Enzo]

It is a tuned circuit, but it is not tuned to something else, so it shouldn't matter. yes, I do not expect the first stage to pass audio.

I would not normally expect those small ceramic caps to need changing.

Have you been through the rest of the amp to check for cap leakage?

[Randall]

"I would not normally expect those small ceramic caps to need changing."

Small in value, large in size. I 'think' the one on the plate was leaky, I'll have to recheck that.

"Have you been through the rest of the amp to check for cap leakage?"

I am doing that presently, found one coupling cap already. Most of the rest were replaced already, but it looks like at least one of those looks leaky in my quick check. This board is not labeled, and I don't see a board layout in the manual, so it's a long process. It does start out quiet, but a hum develops the longer it is on, and some V's don't look right, so I'm not done with this yet.

[Mick Bailey]

That's a fascinating input circuit. I'm glad I opened the theory of operation PDF and never would have remotely thought a 50s jukebox would be anything other than a signal amplifier. That's an invention of pure genius to have a tonearm that reduces the stylus loading so much and yet doesn't produce any signal output.