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Fender Bronco- Barn Find!

Discussion in 'Amp Central Station' started by SanAnTex, Jan 5, 2018.

  1. SanAnTex

    SanAnTex TDPRI Member

    Age:
    30
    45
    Nov 18, 2017
    Texas
    It could be wax.. I didn’t think of that. I was thinking of replacing the entire board and all new caps\resistors\ wiring. This amp is quieter(hum) than my near mint 75 champ.. go figure. I hope it stays that quite after I change some parts out.
     

  2. SanAnTex

    SanAnTex TDPRI Member

    Age:
    30
    45
    Nov 18, 2017
    Texas
    460B1202-328B-429E-ADA0-38150611640C.jpeg here Is a picture of this rebuild. I plugged it in to the wall and connected it to a speaker to test it out. No sound came out of the speaker and it appears that only the 5y3 was heating. Can anybody spot my error from the picture? Thanks!
     

  3. dkevin

    dkevin Tele-Meister

    147
    Mar 13, 2011
    Seattle, WA
    It appears your a/c wiring may be in error. The black a/c wire should travel to the fuseholder tip. The side of the fuseholder should have a jumper wire running to one tab of the slide (off/on) switch. The other off/on switch tab should connect to one of the power transformer input wires (one of the two wires now twisted together and soldered at the slide switch). The white a/c wire should be soldered and shrink wrapped to the remaining power transformer input wire. I don't know if the wiring as/is would cause the symptoms you describe but the amp would definitely not function normally.
     
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  4. jmp81sc

    jmp81sc Tele-Meister

    220
    Feb 6, 2015
    Orange County
    It looks like the tube heater wires were not put back, they are the green wires that come from the pilot light to the tube sockets. Pins 4 and 5 on the 12AX7 and 2 and 7 on the 6v6.
    Fender grounded these to the chassis, most builders now do a twisted pair coming off of each leg of the pilot light for lower noise.

    Search on the shock brothers section for heater wiring.
     
    BobbyZ likes this.

  5. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    Unless my eyes are playing tricks on me, I don't see any heater filament wiring on the other tubes. They're grounded on the preamp tubes, like Fender did in the '60s on the Champ line, but you don't have the other end wired up.

    If you used the layout for your guide it usually only has a small note, like 'fil' at those connections, but doesn't show the wiring as its basically understood for tubes.
     
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  6. Finck

    Finck Tele-Holic

    Age:
    52
    921
    Oct 11, 2017
    São Paulo - Brazil
    I'm going to say the same, can't see heater wiring in the preamp tubes...
     
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  7. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    Go back and look at your first picture in this post. You can see the light green heater wires on the amp at that time.
     
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  8. SanAnTex

    SanAnTex TDPRI Member

    Age:
    30
    45
    Nov 18, 2017
    Texas
    CDA6A63D-1674-458F-88AA-A6C3297E7BD9.jpeg Thanks for the tips every one.. I soldered in the heater wires to pins 4 & 5 on both 12ax7 and pins 2 and 7 on the 6v6. I plugged the amp in and still the same problem occurs (no sound and only 5y3 heats up) but now with the new heater wires the pilot light didn’t come on. I think I need to switch the ac cord like dkeven has suggested.
     

  9. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    I think you're still missing a wire. Is there one from the pilot lamp/6.3 v lead? Check your first pictures to verify since you don't seem to be able to read a schematic. I think you're only a wire away.
     

  10. SanAnTex

    SanAnTex TDPRI Member

    Age:
    30
    45
    Nov 18, 2017
    Texas
    Is the wire you’re referring to go from the 6.3v lead to ground? I do have the schematic but can’t figure this part out.
     

  11. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    I could see what you did better in your last picture in a bigger screen. It looks like you have mixed up a variation of the "twisted pair" wiring with the method that the Bronco used. The way that the Champ/Bronco was wired only used one lead to one side of the filaments and another lead to ground.

    You need to take out the twisted pair from the pilot lamp, along with the jumper across the 6V6 heaters. Then add a single wire from the same post of the pilot lamp that has the green wire from the power transformer and connect that to the 6V6 filament. Leave the other filament (pin 7?) grounded. You should have one "hot"wire to the pilot lamp and tubes with the other side grounded.

    Your first picture in this post shows it pretty well with the light green and dark green wires. Also, make sure that pins 4 & 5 of the preamp tubes are tied together when you move from one tube to the next.
     

  12. SanAnTex

    SanAnTex TDPRI Member

    Age:
    30
    45
    Nov 18, 2017
    Texas

    Thanks a bunch for this info.. unfortunately I had to leave the country for work but looking forward to finishing this amp up. Thanks to everyone for all the info and help. I need to learn to read schematics! I’ll post a picture when I’m done with this thing. Have a good weekend!
     

  13. Milspec

    Milspec Tele-Afflicted

    Feb 15, 2016
    Nebraska
    If it is mold, your nose will tell you more than your eyes ever could. Personally, if the transformer isn't a rust bucket, I would convert it to a head unit and clean up the wiring. I never find a lot of magic in a cabinet that small in the first place so with the head you could run it to the cab of your choice or choices for that matter. You might find as I did that you like it better running through a 12 inch speaker which is not going to fit in the original cabinet. I love barn finds and that is a good one, but be realistic about what you want to achieve in the end or you will be disappointed.
     

  14. 2blue2

    2blue2 Friend of Leo's

    Jul 20, 2013
    Island of Oahu
    Sounding like a good idea.


    It's cool, glad your fixing it.:)
     

  15. ecosse

    ecosse Tele-Meister

    398
    Jul 24, 2011
    Nova Scotia, Canada
    Did you ever get this thing going?
     

  16. Mike Eskimo

    Mike Eskimo Doctor of Teleocity Ad Free Member

    Nov 9, 2008
    Detroit
    Wait - what ?

    That implies amps belong in kitchens.

    I know a certain somebody that has not 1 but 2 Vibro Kings in his kitchen which I only think is mildly eccentric but mainly awesome !

    Nice barn find - VibroChamps/Broncos are crazy cool.
     

  17. SanAnTex

    SanAnTex TDPRI Member

    Age:
    30
    45
    Nov 18, 2017
    Texas
    Yes, with the help of this forum I did get this amp running. I think it still needs some tweaking because past 7 on the volume knob the amp breaks up quite nasty/ unproportinal/ rough if that makes sense. It could be due to the cathode resistor value? Anyways, this amp was loads of fun to put back together! Here’s some before and after pictures. Cab is still unfinished...
     

    Attached Files:


  18. adamkavanagh

    adamkavanagh Tele-Holic

    Nicely done! Any reason you completely re-made the cabinet, was the original too far gone?

    I have a bronco just like yours and i never turn it up past 5, usually I keep it at 3.5 / 4 otherwise it gets nasty.
     

  19. zombiwoof

    zombiwoof Tele-Holic

    723
    Oct 17, 2008
    Tujunga California
    Answered in post #6.
    Al
     
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  20. adamkavanagh

    adamkavanagh Tele-Holic

    Ah yes, there it is
    Tapatalk has been glitchy for me lately
     

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