NAD 1976 (1977) Vibro Champ

Discussion in 'Amp Central Station' started by HD28, Sep 16, 2021.

  1. HD28

    HD28 Tele-Meister

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    Just got this time machine. It sounds so clean and crisp and has a wonderful tremolo. Newer tubes, new Weber speaker, but other than that, as-built.
    Tomorrow I go under the hood.
     

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  2. gridlock

    gridlock Poster Extraordinaire

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    Congratulations, I put the same Weber speaker in my ‘68 VIbro Champ.

    Nice amp.
     
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  3. SnidelyWhiplash

    SnidelyWhiplash Friend of Leo's

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    Great lil amp.
     
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  4. 57joonya

    57joonya Tele-Meister

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    Sweet - I like it. A lot .
     
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  5. Crobbins

    Crobbins Friend of Leo's Silver Supporter

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    Nice score.
     
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  6. mojek

    mojek Tele-Meister

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    Congrats:)
    Have all original 77 as home amp. Surprisingly a lot of bass and nice clean/crunchy tube sound.
    The only thing was to clean and fasten jack connectors;)
     
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  7. Old Plank

    Old Plank Friend of Leo's Gold Supporter

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    Nice one, Congrats!
     
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  8. HD28

    HD28 Tele-Meister

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    I had a quick look under the hood today because my DigiKey order arrived so fast. I ordered it late Friday, it arrived around noon Monday. Not bad kids.
    I think the circuit is mostly untouched, but there are 2 things that draw my attention.
    Firstly is the shoddy soldering on the input jacks in the photos.
    Secondly is the uniquely Canadian addition of the thermal fuse which is attached using Canadian Robertson square head screws.
    I don’t see any issues and will post once I replace the e-caps and resistors.
    Btw, Robertson screws are so far superior in ease of installation over Phillips that I still can’t understand why anyone still uses Phillips. Don’t get me started…
     

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  9. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    Looks like the jacks have been replaced and the soldering is just bleh, no big deal.
     
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  10. Boreas

    Boreas Poster Extraordinaire

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    Great find!
     
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  11. Wally

    Wally Telefied Ad Free Member

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    I would rewire the AC. Congrats on a clean VC.
     
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  12. HD28

    HD28 Tele-Meister

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    I’m on it.
    I have a question about the original wiring on the PT.
    The 1973 Champ I recapped a month or so ago originally had the red/black and yellow/black connected to neutral.
    This 1977 Vibro Champ has the red/black and yellow/black connected to hot.
    Can I assume the electrons don’t care which way they spin?
     

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  13. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    Correct - there is no polarity in the power transformer.
     
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  14. Wally

    Wally Telefied Ad Free Member

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    +1 with Corliss. My problem with that circuit is that the hot should be going through the fuse to the switch to one end of the primary…for simplicity’s sake in this amp to that blk/blk green joint that is at the switch now. The neutral should go directly to the other end of that primary winding. This would necessitate extended the length of the neutral and rearranging things a bit for the hot feed from the fuse to the switch.
     
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  15. HD28

    HD28 Tele-Meister

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    That's my plan.
    I will also pass the neutral through the thermal fuse before the primary.
     
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  16. HD28

    HD28 Tele-Meister

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    Good eye. The jacks are stamped "Japan"
     
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  17. HD28

    HD28 Tele-Meister

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    I replaced the 40/20/20 cap can with a new 40/20/20, 3-25/25 e-caps with 25/25’s and the 10/25 with a 10/50. The carbon comp 470 Ohm, 1k Ohm and 10k Ohm resistors we’re replaced with 2W flameproof ones. Powered it up slowly using a variac and all was well.
    Wall voltage was 121.4V AC.
    All DC voltages we’re just a bit higher than the schematic.
    Plate voltage was 324V DC, plate current was 40mA, for a cool plate dissipation of 12.3W, 108%.
    The amp had new tube store preamp tubes, a JJ 6V6 and the original 1977 rectifier tube when I got it. I will put it back together and take it for a test drive.
     
    Last edited: Sep 25, 2021
  18. HD28

    HD28 Tele-Meister

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    As well as the death cap desoldered at the fuse, the hot leg routed directly from the fuse to the switch. And the neutral routed through the thermal fuse to the primary.
    Japan input jacks replaced with 12A’s.
    Deoxit to all pots, switch and sockets.
     
  19. HD28

    HD28 Tele-Meister

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    The Champ tremolo always is on by default. The shorting cap turns it off.
     
  20. Wally

    Wally Telefied Ad Free Member

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    That is some low plate voltage compared to what the schematic calls for. The B+ in the AB764 schematic calls for 420vdc with a wall supply at 117VAC. That would give a plate voltage of maybe 415vdc or so. Did you mean to type 432vdc??? What is your cathode voltage? What is your HT voltage…pins 4 & 6 on the rectifier to ground in AC mode? If the HT to each of those pins is not 300vac or so, something is not correct. With 300vac there, one might expect to see about 420vdc off of pin 8 of the rectifier….B+. With only 324vdc on the plate, one is thinking maybe there is 240vac on the HT supply…P4 and P6 of that 5Y3.
    I have never seen voltage that low on any BF/SF Fender Champ. With modern wall voltage, it is always over 400vdc. But…if that is what it is, that is what it is.
     
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