Nasty, loud crackling static coming from my old combo amp

Discussion in 'Amps and Cabs [BG]' started by Philthy, Mar 7, 2005.

  1. Philthy

    Philthy Supporting Member

    Jun 12, 2001
    Wallington, NJ
    I think my good, old Peavy TKO 115 combo of about 13 years has finally had it. One day while jamming, I totally lost all signal and a loud, staticy, hissing/crackling started coming out of the amp. I pulled out my cable and the crackling continued. I shut the amp off and didn't use it for a couple of days. I turned her on, immediately there was no crackling/static/hissing but once the amp warmed up it started again.

    Anyone out there ever experience something like this? Any ideas what it could be? Is it even worth fixing? I know I've got my money's worth out of it and it's been good over the years. Maybe it just bit the dust.
  2. IMHO there's two possibilities:

    a) A gain or volume pot is getting worn out/dirty....does the situation improve when you try turning the knobs?

    b) An output transistor is going out. This is consistent with your statement that it was OK cold but started happening when the amp warmed up.

    Dirty pots can be replaced or temporarily improved by spraying cleaner in them.

    Output transistors are relatively cheap. Peavey used a bunch of 2N3772 transistors in a lot of their amps. They're about $3 each from Mouser. They are easy for a tech to replace.
  3. Philthy

    Philthy Supporting Member

    Jun 12, 2001
    Wallington, NJ
    Thanks for the quick reply. By the way, nothing improves when I turn the knobs. So, it's definitely something internal like a transistor.
  4. Of course, there could be something else, like a dying coupling capacitor or a preamp transistor, but if it does it without an input signal, IMHO that points to an output transistor. How comfortable are you with your electronic/mechanical skills?
  5. Philthy

    Philthy Supporting Member

    Jun 12, 2001
    Wallington, NJ
    I'm most definitely comfortable when it comes to soldering and guitar electronics. I do all my own work. If I could identify the transistor, then I could definitely install a new one. No doubt.
  6. Well, open the amp up and look. The output transistors will be the big transistors mounted on the heat sink...some old amps didn't use heat sinks, but mounted the transistors on the chassis (so the chassis acts as a heat sink).

    They will either look like this:

    (note, the URL is a 2n3055, the world's most common BJT power transistor

    Or this:

    Note, this is a voltage rectifier, not an output transistor, I just wanted to show you the relative size/shape.

    Peavey uses their own "house numbers" instead of common designations, like 2N3055 or 2N3772, but on their web site, under their tech support section, they have a chart to cross reference them to common numbers.
  7. Oh, one note of caution:

    The TO-3 case must be insulated properly. Notice carefully, there's a thin clear mica insulator between the case and the mounting surface. DO NOT break this! Also, there are insulating bushings where the screws go through the case. Be sure to save these.

    Note the orientation of the connector. The TO-3 transistor will only mount one way--the two leads are off center.

    On the mica insulator, there's a thin film of "grease". Do not wipe this off--it's heat conductive compound. Better yet, buy new silicone rubber insulators (Silpads) to replace the mica insulators. Silpads conduct heat almost as well as the mica, but they're easier to use. They don't need the thermal grease.
  8. Philthy

    Philthy Supporting Member

    Jun 12, 2001
    Wallington, NJ
    Thanks!!! I did open the amp and found the two resistors attached to the chasis. They have Peavey ref #'s of 70483100 & 70473100. However, when I check the Peavey website, I cannot find this cross reference chart that you spoke about. Also, between the resistors, is another piece that is the same shape as the resistor, but a little smaller, and it has a round, black piece on it with two wires attached to it. Any idea what this might be and if it's something that need changing?
  9. They may have taken the cross-reference chart off their site....I have printed a hard copy but it's at work so I'll look tomorrow.

    The piece between the transistors is most likely a diode used for temperature compensation. As the output transitors heat up, they change slightly in operation, so this diode is used to adjust for the change in temperature.
  10. I found the numbers on their forum:

    6392 - factory part 70483100 - industry type MJ15015.

    6505 - factory part 70473100 - industry generic MJ15016.

    Mouser has the NTE equivalent of the MJ15015....*&Dk=1&Ns=MfgrPartNumber||SField&N=0&crc=true

    They also carry the NTE equivalent of the MJ15016. So for less than $20 you can replace them....

    You may also want to order a schematic from Peavey, it's like $2.50 directly from them.
  11. Better yet...order these from Digikey

    They are less than $3 each. Much cheaper than NTE for the same thing. :)
  12. Philthy

    Philthy Supporting Member

    Jun 12, 2001
    Wallington, NJ
    Thanks for the help. I ordered them from digikey along with some sil-pads. They should be here in a couple of days. I'll install them and let you know what happens. Thanks again!!!!
  13. Just be sure to a) pay VERY good attention to the insulating, and b) be sure to put the right transistor in the right spot. Oh and c) put the two leads on correctly, draw a picture if you need to.

    With the silpads, you don't need the thermally-conductive grease but you will still need the insulating bushings at the screws. Make sure the two leads out of the center of the transistor don't touch the sides of the holes: if they do, the transistor needs to be turned around--look carefully and you'll see the two leads are off center so the transistor can only be oriented one way.

    Good luck!! If you still hear crackling, also check for loose solder connections going to the output jack. Then from there, it can be difficult to trace intermittent crackling....