low supply voltage, buzz in output,audio power amp

Discussion in 'General Electronics Chat' started by dsweetland, Nov 21, 2008.

  1. dsweetland

    Thread Starter New Member

    Nov 21, 2008
    I'm working on an old Peavey power amp that's working but has a significant buzz in the audio at the speaker. It's a 200H 100 watt audio amp made by Peavey in 1979. It's supposed to have +42V on one side and -42V on the other, the +42 is there, but the minus voltage is -26V, no blown fuses and the circuit works except for the noise. I've replaced the output power transistors Nte130's (4) and nothing else is heating up including the transistors. The power supply filter caps seem to be OK as I've pulled them and checked them OK. I think there's a part shorted but don't know where to continue, I'm stuck. Any help would be appreciated...Dale
  2. mik3

    Senior Member

    Feb 4, 2008
    I believe its the power supply filter capacitors because electrolytic capacitors loose capacitance due to ageing. Another think in my mind is that the half of the winding of the center tapped transformer is partially shorted.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    The rectifier bridge is worth checking, too. One diode open can give similar symptoms. And the electrolytics are cheap, go ahead and replace them.
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    It's unfortunate that you replaced the output xstrs, for they are typically under rated for max signal output, especially in guitar amps where heavy distortion is often desired. In giving the NTE130 datasheet a quick scan, they appear to be a commercial version of the 2N3055 which is the xstr used in the design (so, at least you got something as good and not worse). The PS voltage is 84V (and could be higher), while the 2N3055 is rated 60V 115W. This is okay at lower volumes, and places dependency on the .33 ohm emitter resistors, but they are under rated for design maximums and is often what causes Peavy and other similar amps to "break" by taking other components (usually under rated as well) with them. So if you should change them again in the future, the MJ802 is a much better output xstr (Q7/8/10/11) for these designs and is still made by On-Semi. If you replace any other transistors (Q4/5/6/9), make sure they are rated at >90V or so, with similar or better wattage/current capacity as the present ones, and the design will be more bullet-proof.

    Refer to the attached schematic, and you will see that Q4/Q5 and everything to the right is suspect. Ohm out all the resistors by lifting one end, check/replace the biasing diodes CR5, and other diodes (again, lift one end to be sure). Especially check or replace R26, if this is bad, hopefully it's open or high resistance. If it shorted, then it probably took out the pre-drivers. Note that Q4/Q5 are a common design for output current-limiting ("our amps are wonderful, they have protected outputs"), but very often they just blow out and then cause everything else to fry (cascade effect).

    You could also disconnect both the +/-42V PS wires from the board, and check the PS independently by CAREFULLY loading and measuring it. Do NOT connect only one voltage side of the PS (+ or -) to the board and power it on.

    Also, with the power supply connected to the board and no input signal, you should measure <0.5VDC between the output & GND (generally even less). This voltage could be either negative or positive -- it doesn't matter -- many amps have a pot to adjust for this, it would be near Q2, but I don't see one here.

    I hope the schematic helps you out, if don't already have it. Also, changing C7 to a 2.2uF non-polarized can help some.
    Last edited: Nov 21, 2008
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    I note you say buzz. This is significant as the ear hears 100hz or 120 Hz as a buzz.
    This is indicative of power supply problems.

    50/60 Hz is heard as hum and indicative of earthing problems.

    That is so long as you also get signal.

    A (loud) buzz or hum with no signal is indicative of shorted output transistors.

    Hope the rough and ready guide helps.
  6. dsweetland

    Thread Starter New Member

    Nov 21, 2008
    Thanks to all for their help, the amp is fixed and working properly. You guys are too good, the "buzz" was the correct description as 2 of the output transistors were shorted taking with them 2 of the .33 ohm power resistors and a couple 1/4 watt resistors, and a couple pre-driver transistors. The big power supply electrolytics were OK as were all the zeners and diodes. The weirdest thing about this amp was that the diode checker part of the multimeter checked those transistors out as OK, not shorted, I guess they only shorted at "power on" I couldn't have fixed it without your help, I did get a little tip from an old troubleshooter here about the 2n3055's, he said to replace them with NTE388's, they're way more money but a much better componant. The amp is quieter than it's ever been, it always had a little "hum" in it, but no buzz. I had replaced those 2 before I realized the resistors were shorted as well, and there was also 10 volts DC across the output, another clue that I forgot to quote. Anyway, thanks again to all, I can sleep tonight, Best Regards, Dale
  7. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    Note that the NTE181 is their cross-reference to the MJ802, and is half the price of the NTE388 (which is over-rated for this design).

    If you want a little more voltage headroom, the 2N5631 can be used. It crosses to the NTE60 and is between the other two in price (from Mouser).
    Last edited: Dec 2, 2008