73 SFDR half volume & crackle

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Norse, Sep 24, 2006.

  1. Norse

    Norse Member

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    My 73(?) Deluxe Reverb (AB763) has been going to half volume and crackling at (in-opportune) times. It does this using either channel, and will usually recover if I bonk it on the top a few times... the last few times that I turned it on were to trouble-shoot, and it has acted up each time.

    Heres' what I've tried so far:

    1) replaced the 12AT7 coupling tube with new GT 12AT7 (no affect).
    2) swapped preamp tubes with a spare (put back to orig config now).
    3) replaced power tubes (GT 6V6-C)& Checked bias voltage (-35v).
    4) checked the 470 ohm resistors at the power tubes (one is 500, and the other is 450 - explains why one of the tubes runs hotter (brighter blue glow) than the other(?)).

    I have an oscilloscope (DSO) on-hand to mess around with and could use some suggestions (I am aware of the high voltages and practice the "one-hand" in the circuit rule) Your thoughts are greatly appreciated!!!

    "Help the poor" :O)

    Greg
     
  2. Dave C

    Dave C Gold Supporting Member

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    Sounds like a bad connection internally , possibly a cold solder joint or something similar on the board or tube socket.....especially since you can "bonk"the amp and have it come back. It could be a component going south , but it sounds like a bum connection to me.Let us know what you find.
    Dave C
     
  3. AL30

    AL30 Member

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    When is the last time that amp has had a tune-up?

    1) Replace all tubes - it's just the easiest thing to do and you've already done it.

    2) Replace those 470 ohm resistors with 3Watt flameproof. While you're there replace the 1.5K across the tubes as well. These take a lot of heat.

    3) CAP JOB !!! Replace the electrolytics - all of them. Filter caps, filter board cap and main board caps. Two schools of thought on this but if you want to keep playing the amp it's better to replace them now than to replace them (and a transformer) later.

    4) Check the plate load resistors - 100K's. They are a source of noise - crackles and hisses and all those buzz words.

    5) Check all the connections like Dave C said.

    AL
     
  4. brad347

    brad347 Member

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    I'd like to put in a vote as a dissenting opinion on this point.
     
  5. AL30

    AL30 Member

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    Fair enough. Brad347, I have zero issues with that comment but I would be interested in hearing your reasoning.

    I'm assuming you're thinking it may be an un-needed expense that may or may not solve the problem. From my end - I've usually got enough tubes lying around to swap things in and out at no extra expense.

    What are your thoughts?

    AL
     
  6. brad347

    brad347 Member

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    well, perhaps I misunderstood your post. Swapping tubes in and out to see if it solves the problem, I'm a major advocate for.

    Blindly replacing all tubes and undoubtedly throwing away plenty of perfectly good tubes with lots of life left in the name of a "tune-up," or "re-tube," I am not.

    I wonder how many vintage fenders have had their original RCA and Mullard tubes with HOURS of life left in them replaced with G***** T**** without even listening, in the name of "maintenence?"

    The originals thrown in the trash to be crushed by the compactor and never to be heard from again?
     
  7. Blue Strat

    Blue Strat Member

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    Why not do the easiest thing first, and something that ANYONE can do?

    Clean all tube pins and sockets as per the FAQ on my website under resources. It's amazing how often this will cure a problem like yours. Doing anything else before this is a waste of time.
     
  8. AL30

    AL30 Member

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    Brad347, after re-reading my post I don't think you misread it - I think I didn't explain myself very well. I guess the coffee hadn't kicked in yet.

    Swap out the tubes. Yes. Throw away the other tubes. No.

    Sorry about that.

    AL
     
  9. Blue Strat

    Blue Strat Member

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    BTW, bias it probably not related to your problem, but measuring/adjusting the -35 grid voltage is virtually useless. Best bet is to get a bias probe or Google "transformer shunt method".
     
  10. Norse

    Norse Member

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    I've done a bit more testing and have found that the power supply and all pin voltages are close to the schematic. Probing for cold solder joints turned up nothing but I could see that the checks were well warranted (btw, I found a bad connection at the fuse on my HR Deluxe - fell right off the eye terminal when I probed it).

    It's going to be a while, but I am going to take your advise and order up some caps - I have had the amp for 20 years and never touched them.

    I am also going to play with bias readings a bit just for kicks - I have a very accurrate low amp probe that works great with the scope! The links that turned up on the google search are great so I'll be digesting some of that for a while too... Thanks much for all your great advise!!!

    Greg
     
  11. Blue Strat

    Blue Strat Member

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    Probing for cold solder joints is pointless. Get in there with a soldering iron and breathe some fumes! :)

    Don't discount cleaning the sockets and pins. People are always amazed when this works, and it very often does in situations like yours.
     
  12. Norse

    Norse Member

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    Good idea, Mike - it's been quite a few years since I looked at the sockets and tightened the pins. Do you use contact cleaner on them?

    Also, when I change-out the big caps, is there an issue going with larger values? I didn't have the amp in fron of me when I ordered them, and the schematic showed 16-450... I think I ordered 22-600's.
     
  13. Blue Strat

    Blue Strat Member

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    22s are ok. Worst case, the lows might be a bit tighter.

    Yes, tension and clean the sockets with contact cleaner. This cures LOTs of intermittant and/or noise problems.
     
  14. Texasamp

    Texasamp Member

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    Look at the input jacks on both channels. Get a burnishing tool and clean between the shunt and the tip of the jack. Only the #1 jacks on each channel have a 12a shunted jack. Make sure there is enough pressure against the shunt to where the tip grounds out when you unplug your guitar cord.
    You may need to pull all the pots back enough to pull the brass ground buss to where you can clean the corrosion between the buss and chassis. Also, remove the star washers on the pots and clean those and where they ground out to the buss. Use a brass wire brush. Replace any star washers that are bad. Use nickle or stainless star washes (.5" id).

    Good luck,
    Darrell S.
    Texas Amplification
     
  15. Norse

    Norse Member

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    Well I got the filter caps replaced, cleaned & tightened the tube socket connectors and tried the amp - it was back! .... but still had the crackle... ended up finding a broken ground wire on the chassis right behind the vibrato nob. Now she's as sweet as ever! The kids are asking me to turn down with the volume on 3... I like it :0)

    Thanks much for all the great input!!!

    Greg
     

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