Results 1 to 6 of 6

Thread: 50s jukebox oscillator input circuit

  1. #1
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    1,587
    Thumbs Up/Down
    Received: 35/0
    Given: 2/0

    50s jukebox oscillator input circuit

    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

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  2. #2
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,173
    Thumbs Up/Down
    Received: 268/1
    Given: 0/0
    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.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    1,587
    Thumbs Up/Down
    Received: 35/0
    Given: 2/0
    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.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  4. #4
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,173
    Thumbs Up/Down
    Received: 268/1
    Given: 0/0
    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?

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  5. #5
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    1,587
    Thumbs Up/Down
    Received: 35/0
    Given: 2/0
    "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.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  6. #6
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,253
    Thumbs Up/Down
    Received: 18/0
    Given: 2/0
    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.

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. 50s AMi Jukebox Amp
    By paulmolive in forum Maintenance, Troubleshooting & Repair
    Replies: 8
    Last Post: 02-21-2018, 06:53 PM
  2. Digitech RP500 Input circuit
    By predrag152 in forum Schematic Requests
    Replies: 2
    Last Post: 07-30-2015, 08:40 AM
  3. 5D8 Twin input hi/low switching circuit
    By octal in forum Theory & Design
    Replies: 9
    Last Post: 11-03-2013, 01:58 AM
  4. bad input jack/circuit board on my Marshall mg102fx
    By topzpin in forum Music Electronics
    Replies: 6
    Last Post: 06-10-2011, 04:22 PM
  5. Input circuit differences 5D4 vs. 5E4A
    By acorkos in forum Tweed Builders
    Replies: 3
    Last Post: 01-21-2011, 06:29 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •