Old Blackface Showman / No Sound Except Knob Crackle

Discussion in 'The Workbench' started by iron broadsword, Apr 20, 2018.

  1. iron broadsword

    iron broadsword Well-Known Member

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    It's a '65, I think. Transformer codes are 606551 and 606542. Alright, so last time I fired this thing up it worked fine but it's been sitting for about a year and now it makes no sound when you play through it. It does however give off some volume knob crackle when you rotate those, so I'm assuming the problem is somewhere early on in the preamp. This is all through the normal channel.. in the vibrato channel it seems completely dead (or maybe it's just that that volume knob doesn't have a crackle issue)

    What I've done so far:
    - Tried all the tubes in another amp, and they work fine there
    - Cleaned all jacks (cleaning all pots is next on the todos)
    - Cleaned all tube sockets
    - Tried obvious stuff like different guitar / cables

    It's never had a full cap job.. so yeah I'm thinking it might be time for that but the big electrolytics were replaced 2 years ago. The preamp tubes don't get very warm, and I can hold V3 with my bare hands while the amp is operating so it could be that the heater supply voltage is the issue.

    Where do I go from here? Thanks in advance
     
    Last edited: Apr 20, 2018
  2. iron broadsword

    iron broadsword Well-Known Member

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    Checked the preamp heater voltages at pins 4/5, and yep it's about 5mV so that's definitely the problem. Plate voltage was on point though at 469v so the thing isn't completely dead. Going to try to hunt down a schematic, the code stamped on the chassis beside the board is TB 3 66 if anybody's got a schem for it.
     
  3. chuckharmonjr

    chuckharmonjr Well-Known Member

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    That'll do it every time. Great catch!
     
  4. iron broadsword

    iron broadsword Well-Known Member

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    SS rectifier. Alright, I might even have replacements for those on hand. Gotta find a damn schem before I can mess with it though.. google is being difficult.
     
  5. johnfv

    johnfv Well-Known Member

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  6. iron broadsword

    iron broadsword Well-Known Member

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  7. iron broadsword

    iron broadsword Well-Known Member

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    Alright, got some pretty wild readings on the preamp tubes. (Btw my V1-V4 is assuming V1 is furthest away from the power tubes).

    I got 6.3V across green wires at pilot light.

    V1:
    6 - 615mV
    8 - 18.1mV
    1 - 622mV

    V2:
    6 -618mV
    8 - 12mV
    1 - 621mV

    V3:
    6 - 367V
    8 - 33mV
    1 - 373V

    V4:
    6 - 299V
    8 - 73V
    1 - 301V

    I got 2.9V on Pins 4/5 on all 4.

    So V1 and V2 are both clearly not getting enough voltage, V3 is very high on pin 1 and very low on pin 8, and V4 is very high on pins 6 and 1 and a little low on 8. And the heater voltage is very low on all. Wtf.

    Here's the schematic: http://schems.com/schematicheaven.net/manu/fender/showman_ab763_schem.pdf
    Where do I go from here?
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    showmanB+rail.png

    showmanpreamp.png
     
    Last edited: Apr 20, 2018
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  9. iron broadsword

    iron broadsword Well-Known Member

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    Jesus, I can't find that 4.7k to save my life
     
  10. iron broadsword

    iron broadsword Well-Known Member

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    Alright, so the wire that connects D to the 4.7k had come loose. Fired it up and it's ALIVE again, but there are still some issues. Gonna grab voltages again tomorrow.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    It's under the capacitor cover.

    [​IMG]
     

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