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‘79 Deluxe Reverb intermittent crackling

Discussion in 'Amp Tech Center' started by INFANT, Feb 25, 2021.

  1. INFANT

    INFANT Tele-Holic

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    Good day everyone. As the title indicates, my ‘79 DR is crackling very loudly but only every once in a while and only on the vibrato channel. This is the model with the pull boost volume control. I’m thinking that it might actually be the actual volume control as it feels a bit “flakey”.

    However, the amp only seems to start crackling after being on for 10-20 minutes. I don’t think it’s power amp related as it doesn’t make any noise on the normal channel. I’ve left it on with nothing plugged into it and sure enough, it started crackling. I turned down the vibrato channel and turned up the normal...it was dead quiet. Turned up the vibrato and turned down the normal, it started crackling again.

    I’ve swapped the pre-amp tubes between the normal and vibrato channels and the problem remained on the vibrato channel. Today I took the chassis out and sprayed cleaner in all the pots. I then sprayed Deoxit in the preamp sockets and inserted and removed the tubes a few times. After reassembling it I turned it on and started playing some guitar thru it and sure enough, 15 minutes later, it started crackling again.

    I had all the electrolytics and filter caps changed 2 years ago and my tech went over it with a fine tooth comb and it’s been great ever since. Then a couple of months ago, it started doing this every once in a while.

    Like I said, I feel that the push/pull volume pot seems flakey and if I wiggle it, there is a bit of movement and it crackles and cuts out a bit. Therefore, I’m thinking that I may change this next. However, I’m thinking of just putting a regular 1M ohm-A rather than another push/pull pot since I find the boost to be useless. My question is what do I do with the 2 wires that attach the the piggyback portion of the original pot? Can I just add some heat shrink on them and leave them be? Or should I be following these wires all the way back and desoldering at the source?

    Thanks in advance for any responses and help.
     
  2. goldguitarguy

    goldguitarguy Tele-Meister

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    Sounds most like a cracked solder joint near a tube socket or heat source or a cracked resistor. Strange for a pot to be affected only when the amp warms up. Let the amp warm up and do some chop sticking around heat sources and look for suspect solder joints.

    As for your wires you can shrink wrap the ends. If you get added noise then just remove them at the source.
     
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  3. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    That kind of thing is almost always a bad tube. Are 100% certain that it's not a tube? Pull V1, V2, V3, & V5. Then add V2. In other words, start with just the PI tube and the reverb recovery tube. Then, add V2 once you're certain there's no crackling.

    And if it's not a tube, this helps you narrow down where the problem is in the circuit.
     
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  4. Jon Snell

    Jon Snell Tele-Holic

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    It's a Fender. Check/replace the 100k anode load resistors with MO 1W types if replacing the 12AX7s don't cure it.
     
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  5. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    That amp will already have larger plate resistors.
     
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  6. schmee

    schmee Doctor of Teleocity

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    I agree with this. Check those heater wires on the tube sockets especially. They can look perfect but be just floating in a solder glob. Spent a couple months trying to figure that out on a Vibrolux once. Leo used 18 gauge stiff wire on those heaters, which makes them prone to coming loose.
     
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  7. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    @INFANT if you've completely ruled out the tubes, check to make sure there's no voltage drop across the pot (with the volume the volume all the way down), in case a tone stack cap is leaking DC. I have had to ultimately replace pots, mostly in old Traynors.
     
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  8. INFANT

    INFANT Tele-Holic

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    It can’t be the PI as that would affect both channels. I will pull all the others and check. As I said though, I swapped the V1 and V2 and the problem didn’t follow the tube. So it seems that it is going to be in the vibrato channel somewhere. But like you said, I’ll pull them all and see if that tells me anything.
    Thanks
     
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  9. INFANT

    INFANT Tele-Holic

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    Last night I turned the amp on and let it run for almost 3 hours and no noise until I touched the volume control. Its the main reason that I think it may be the actual pot. However, I’ll do the chopstick thing this weekend when I get a chance as I’m also working on my sons ‘73 Champ trying to figure out why his bass control isn’t working. I’ve changed the caps in the tone stack.except for the 250pf mica cap. I’m wondering if I may have overheated it.
     
  10. schmee

    schmee Doctor of Teleocity

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    My 65 BFDR had the volume pot go bad. It wouldn't make sound in some positions. But chopstik those green heater wires to the tube sockets right after you turn it on. It'll be a loud pop and crackle if you find it. My experience is that it may make noise chopstiking the board also.... but NOT be a problem on the board.
     
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  11. goldguitarguy

    goldguitarguy Tele-Meister

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    Yeah hopefully it is just the pot. If the noise persists after you swap it it’s most likely a solder joint or a cracked resistor. You’ve pretty well eliminated anything else already.
     
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  12. INFANT

    INFANT Tele-Holic

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    I’ve been wanting to change it for quite some time because like I said, I feels flakey and loose. Though I’m still going to check the tubes one by one and poke around with a chopstick..
     
  13. INFANT

    INFANT Tele-Holic

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    Thanks for the suggestions everyone.
     
  14. Telenator

    Telenator Doctor of Teleocity Vendor Member

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    Well, it is a dry winter, and you're up north. Have you ruled out static electricity in the pickguard of your guitar?

    I always start with the most simple / obvious things just in case it can be solved easily.
     
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  15. goldguitarguy

    goldguitarguy Tele-Meister

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    I live in northern Canada... Every guitar I own has shielding tape under the pick guard because of this...lol
     
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  16. INFANT

    INFANT Tele-Holic

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    Just to ensure that it wasn’t the guitar, I left the amp on with nothing plugged into it for a few hours the last couple of nights. Two nights ago, it was making noise with nothing plugged into it. Yesterday, I sprayed the pots and cleaned the tube sockets and when I turned it on and played through it, it started crackling again. I unplugged the guitar and it still kept on crackling. Last night, I went crazy and rotated the volume knob back and forth for a few minutes and then left it on for a few hours. I didn’t hear anything until I touched the volume control to turn it up, down or switch the boost on. Then it crackled at times as i moved it through its sweep.
     
  17. INFANT

    INFANT Tele-Holic

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    funny thing is that I don’t get static with my Fenders but my Gibsons snap, crackle and pop just by touching the body and neck in the winter. I think in Gibsons case, it’s the nitro finish that holds static. I keep Bounce sheets in my Gibson cases.
     
    Last edited: Feb 26, 2021
  18. jrblue

    jrblue Friend of Leo's

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    Well, if thew volume pot feels loose internally, that's likely it. I hope you've tracked it down! I would have bet on tubes/sockets myself, but from what you describe, it sounds like the pot's to blame. At least that's an easy fix! And lots of love to you Northern Candians... land of ice and static.
     
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  19. SoK66

    SoK66 Tele-Afflicted

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    If one the other receommendations isn't the cure, try a new reverb transformer. Way to tell, turn off the reverb with the foot switch and see if the crackling goes away. If so you've found the culprit.

    In those pull boost amps Fender changed the reverb driver circuit to put even more load on V3 and the transformer. In so doing the reverb transformers eventually get fried. The best fix is to eliminate the pull boost entirely. Replave the pull-pot with a standard 1K pot, revert the reverb driver circuit to the earlier AB763 component values, then install a new reverb transformer. Done.
     
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  20. INFANT

    INFANT Tele-Holic

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    Update: I took some of the advice posted here, swapped tubes with known good ones that I had laying around and also poked around with a chopstick. The tube swap did nothing to stop the crackling and I didn’t hear anything bad while poking around with the chopstick. The new pot showed up yesterday and I installed it this morning. The amp has been on for quite a few hours now and no crackling at all.

    After removing the old pot, I rotated the shaft with my fingers. Although I couldn’t feel it through the knob when it was mounted in the amp, the pot felt gritty.

    Anyway, thank you all for the suggestions and advice. It was much appreciated.
     
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