Lead 12 owners - Troubleshooting weird break up / drive 'fizzle'

Discussion in 'The Workbench' started by Ubik, Sep 25, 2020.

  1. Ubik

    Ubik New Member

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    Hey all. I had this over in the 'amps' thread, but it probably makes more sense here.

    So, I got a Lead 12 combo a month ago, and cleaned it up (see my other post). I love it, but I have noticed something slightly odd in the break up of the distortion. If you sustain a chord or note, especially at lower gain settings, it kind fizzles out in a weird way as the sustained note begins to decay, kinda like a voltage starved fuzz would.

    A made a short recording to demonstrate it here, you can hear the issue very well at the end of the more dissonant chords as they decay. Kinda like a quick fizzle.

    Is this normal? Or do I need to check over the PCB? No such issues when dialed in clean, so I am guessing this may be a characteristic of the gain structure? But it doesn't seem right as such. The speaker is fine, as the issue is the same when hooking an external speaker cab.

    I'll go check for cracked or cold solder joints tonight. But might be the output caps? I have no real experience troubleshooting amps, but I can use a soldering iron and have a multi-meter.
     
    Last edited: Sep 25, 2020
  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    That's just the nature of Lead 12s. Those simple transistor circuits clip very abruptly at a specific voltage point. There's not much subtlety about them and they don't do an "edge of distortion" tone well. I have two Lead 12s.

    The way to play a Lead 12 is balls out. Then their shortcomings at lower drive levels just don't get noticed.
     
    Wildeman, Jethro Rocker and scozz like this.
  3. Ubik

    Ubik New Member

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    Oh, that's great to know! I was about to spend a day reviewing the entire PCB! So that weird little 'fade out / fizzle' you can hear in my mp3 clip at the end of notes is totally normal?

    On a side note, this was a quick recording with an SM57 via a Joe Meek VC3 and i like the tone!
     
  4. Bjon218

    Bjon218 New Member

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    Interesting. What were your settings on your amp so I can try to duplicate it here on mine. I have not noticed that on mine before.
     
  5. Ubik

    Ubik New Member

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    Gain on full, volume at about 2, Bass, Treble and Middle all on 6. This also occurs with less gain. It's just when a note is dying out as you can hear with the sound clip I posted.

    LP style guitar with 2 humbuckers, middle position, tone rolled off a bit.
     
  6. Bjon218

    Bjon218 New Member

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    Ok I’ll try this to see if I get similar results. My normal settings are preamp dimed, all tones midway and volume will depend on the day and time of day as I’m apartment bound.

    I did notice on mine that once notes decay to a certain level, it sounds like I have a gate in with a soft knee and a medium attack and long decay.
     
  7. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Totally agree. Volume on 2 isn't really for those amps. I like gain but have mine on about 8 before the really fuzzy stuff starts and although I rarely play mine I had it at a jam last yea with the volume about 8. New speaker more sensitive too. Sounded quite decent. Never listened for that type of thing though, I would think an ol SS would do that.
     
  8. Bjon218

    Bjon218 New Member

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    I couldn’t duplicate your issue on my amp. I just had the sustain cut off, but that is probably due more to low volume tan anything
     
  9. Ubik

    Ubik New Member

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    Hmmm, perhaps I do need to have a look at the PCB for any bad solder joints.
     
  10. Wildeman

    Wildeman Well-Known Member

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    Yes, balls out, on or off. They aren't terrible clean at all but......
     
  11. Ubik

    Ubik New Member

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    Pretty convinced this is a fault and not an issue with running it flat out or not. I cranked it today, and the issue persists. I'm really not running it clean in that sound clip. Sure, I don't have the volume dimed, but gain is maxed out. the weird decay just seems off to me. Sounds a bit like a component starting to fail.

    I've since watched a few other Lead 12 Youtube vids and none of them have that weird fizzling battery sound on the tail end of notes. Even at lower volumes.

    People who troubleshoot amps, what's the most likely source of this issue? Filter caps? IC Chip? A diode? Bad solder joint? Defo having a poke around this weekend. I might socket the IC whilst I'm at it, so I can have some fun changing out IC chips.
     
  12. jon1917

    jon1917 Active Member

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    Download the schematic. That should give you the expecred voltages around the transistors. Check that yours match up. May just need to be rebiased. Amp on, volume on zero.
     
  13. MarshallLover12

    MarshallLover12 Member

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    Did you ever get this fixed? My lead 12 doesn't behave that way.

    Long shot but it might be the op-amp. Especially if its the stock one from the 80's, could be dying. My Lead 12 had a volume issue where it was scratchy and would also die out very similarly to what yours is doing gain-wise. I replaced the op-amp with a socket, and experimented with 3 different op-amps, all which did not have the volume issue of the first one.

    Stewmac sells the socket and a few op-amps for like $0.30 plus $5 shipping.
     
  14. Ubik

    Ubik New Member

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    Thank you, super helpful. I have got some sockets, and I got a replacement op-amp. Just need to find time now to install it all. Was going to build a capacitor discharge tool first.
     
  15. Ubik

    Ubik New Member

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    I will also check this. How do I go about re-biasing a transistor though!?
     
  16. jon1917

    jon1917 Active Member

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    Just had a look at a schematic. It looks like the voltages to the main transistors in the power section are sent via the op-amp. Positive to one side and negative to the other. If you are comfortable doing it, check that the expected voltages at the points on the schematic match up with your own. That could give a good indication that something is out. For now just measure pin 8 and pin 4 on the op-amp. Looks like 8 should read around +14v and 4 should be -16v.
     
  17. Bjon218

    Bjon218 New Member

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    Ubik, here are notes for voltage readings I used when I was working on a buddy’s Lead 12. They are just notes on what to look at and for what voltage. Ymmv

    DVM across the speaker and set to DC Volts to verify there is no DC across the speaker. Next set the meter to AC volts. Is the reading almost zero ? You are looking for an ultrasonic oscillation that may read several volts depending on your meter.


    you should have ~20-0-20VDC for power amp and about ~15-0-15VDC for preamp.


    The test point voltages are written on the schematic for the power amp section.

    The Emitters of Tr1&Tr2 should be -.7Volts,, Collector of Tr1 should read +18Volts.

    Junction of R15&R16 should read -9Volts.

    If one of those readings is way off then that will help folks work out what'sup.



    Check that pins 1 and 7 of the IC are near zero Volts. If not, that indicates a bad IC, component or broken connection problem.



    Lift (unsolder) the input side of C8 (it's the input to power amp) now use your walkman or any audio device that has line/headphone output.

    You should hear a reasonable medium level CLEAN sound.

    If the sound is severely distorted or very low volume then it's a fair bet the power transistors are shot or TR3 is dead.



    measure the DC Voltage across R26 and R27, the two 0.33 Ohm resistors. The Voltages should be close to the same.



    Rod Elliott's page on trouble shooting:

    https://sound-au.com/troubleshooting.htm#volts1



    According to the schematic you should get +16 volts dc and -16 volts dc.

    It may not be exactly that ie 15.3 v or similar is in the ball park.

    As I would suspect R 11 & R 6 check they have aprox 19 volts on one side

    and 16v on the other or another way measuring across the resistor should

    give a reading of 3 volts the amount "dropped" by the resistor.


    If thats all ok the next parts I would check are the emitter resistors R26 &

    R 27 which should measure like a short circuit as they are only .33ohm.

    also make sure the IC is a 1458. Apparently this circuit can be testy and not like newer iC versions of that op amp on certain board revisions.

    make sure to check for cold solder joints and or cracks around all jacks and pots

    the BC184 can be replaced with an NTE123AP
    The BC212 can be replaced with theNTE159
    The MJ2501 and MJ3001 are available from Newark.com as are the other transistors.

    Keep in mind if you have to replace any of the transistors the replacements I posted above will have the Emitter and Collector swapped sides from the stock parts. Meaning the flat face of the transistor will install the opposite direction than Stock.

    Also be aware that when following the schematic, the test points may show not the exact voltage but be within a volt or two and they might also be of reversed polarity compared to the schematic. My buddies amp showed a negative voltage where supposed to be positive in a couple places but on my lead 12 were reversed. And point for point both amps were within 2-3 volts of each other.

    Also you’re going to want to socket the iC when you can.

    I hope some of this or any might help you. These were taken from my notes I had collected across 5 different forums and probably about 20-25 threads. Usually a thread might go a few posts the dead end. But these were all the test areas I was able to find to troubleshoot the amp I was working on which needed almost all iC, transistor, capacitor and half the resistors replaced. I slipped once taking a voltage reading on the transformer and took out the diode bridge and main filter caps after having the amp just about done. Which added a 10 day parts wait for my screw up there.
     

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