Peavey Max115 Bass amp fades away

[allante666]

Hi, wondering if anyone has had this prob? Its a brand new Peavey Max 115 Bass amp. Its fine for the first ten minutes then just starts to fade away with a little distortion that sounds like its underwater!
It has a standard + and - supply but they do seem to drift around a little. I disconnected the circuits after the filter caps and the supplies stayed stable.
Im guessing at the LM3886 output IC but just wondered if anyone had any other ideas? Ive checked just about all the components on the board, theres not that many actually.

If i give one of the smaller electrolytics a blast of freezer spray it cures it for a bit but ive replaced the cap and it didnt fix it, i think though that the cap is subject to the same drifting supply as the rest of it so its probably just temporarily altering its characteristics and making it work.

[Enzo]

Brand new, and we are soldering on it? Didn't it come with a warranty? Peavey would fix it for you. That is what warranty repair shops like mine are for.

DOn;t guess, know. Guessing leads to lots of needless parts replacing, when the problem could easily be a connection or a solder joint. When you have reason to suspect a part, then is the time to change it.

Drifting powr supplies? Like what drifting? The high voltage for the power amp? Or the low voltage for the preamp? Drifting a quarter volt or drifting 50%? Regulated supplies drifting or only unregulated?

Isolate the problem.

Does it do this through the headphones too, or are they OK?

Connect a cord from FX send to FX return, any help?

Plug the guitar or other test signal into the FX return. Does it come out the speaker OK, ir still have the problem.

Likewise, connect the FX send to some other amp for a listen. Hpw does the signal from the FX send sound over there?

Look in the schematic at R21, R22. They are in the ground circuits near the speaker and near the FX loop jacks on the drawing. I don;t know where on the board. 47 ohms, though they may measure zero ohms due to ground connections elsewhere. Make sure neither is open.

WHICH cap did you freeze?

You have +/-35v for the power amp, you have +/-15 for the op amps, and also a +30 for the transistor stages. I don;t care if they are dead on or not, but when the amp fades away, did any of these go with it?

When the problem occurs, do you have anything other than the desired zero VFC at the3 output pins of all the op amps?

And follow the signal through the amp stage by stage. Where does it break down?

[allante666]

Hi Enzo. Yes its new and no it isnt under warranty, It was a shop damaged unit which had been dropped BUT, apparently it had this fault before it was dropped.
Measuring across each of the filter caps I get around the required 35 volts if there is no circuits connected beyond that. However, when it is all connected, one might measure say 50 and the other 25 but as you watch it one will slowly creep up and the other one down. they dont stablise when they are equal, they sometimes cross over and the higher one will become low and vice versa.At some points one will drop to almost nothing. Because of this the 15 volt supply is also drifting. The cap i froze was c116.
It does the same thing through the headphones, also playing into the fx socket. strangely enough, when it does it, the voltages dont seem to go with it, i.e. it can do it with the voltages high or low. C116 was the only thing that made a difference.
Ive only just got my hands on a schematic today though. Havent checked R21 or 22.

[Enzo]

Are you sure the fuse F1 is OK? SOunds like your ground is missing, and the power rails are floating. Or if the tuse is OK< and I mean measure its resistance when removed from the clips< look for some other break in the ground connection between power transformer center tap and ground

[allante666]

Ah, well, funny you should say that. I went back to it to measure the resistors you mentioned (which were ok) and found no sound output. F1 had blown as well. So, after replacing F1 and checking around (it may have happened when I had one of the ceramic resistors disconnected to measure it, there was a spark as i reconnected it, there must have been some residual voltage in the filter cap) I switched back on. I found that the voltage across C120 was 30 volts and stable yet nothing across C121. I traced this to a shorted 15 volt zener diode D8. Im wondering if this could have been the cause of it in the first place? the zener going leaky? The reason I dont know the answer to this is because it was only then that I discovered I didnt have a single 15 volt zener in the place! So, as soon as I get some more Ill let you know!

[Enzo]

You have any dead audio gear around. If you are in a hurry, you could probably find a 15v zener hiding in there.

[allante666]

Yes I probably could find one but Im in no particular hurry. I only do this for resale so I have the luxury of no impatiently waiting customers!
You know the type- 'it only lost its volume so therefore it just needs a new volume knob, why's it taking so long?' or 'back when i was in the Army i used to fix that problem with silver foil and chewing gum' etc etc..

[Enzo]

"My friend Bob knows about this stuff, and he says it just needs a new resistor."


My reply, "If Bob knew what it needed, why didn;t Bob fix it?"

[allante666]

Im beginning to think Bob should have fixed it now! Replaced the zener and still no sound. One rail at the now correct 35 volts (it was 30 before) and the other was down to about 20 and one of the 15 volt rails is right down . found out the CA3080 op amp seems to be dragging it down. If I remove it from its socket both rails come up to 35 and are perfectly stable. both 15 volt ones come back too. And no, I dont have a 3080 kicking around either....
I checked around the IC socket for resistors etc gone short but all seem ok.

[allante666]

This thing is never ending!! put a new 3080 in it and up came the power rails, all as they should be, no drift. great..apart from no audio either! So.. after a lot of signal injecting and tracing I found that one of the op amps had gone bad. the last one before the power amp board. so as i didnt have one i swapped this out with the one at the very front end of the amp, figuring it didnt need it as i could still plug a guitar in to the fx socket for diagnostic purposes. Nothing! still no audio, still seemed like it was the opamp, i could stick a tone into its output pin and get full volume, put it on its input and get a weedy low squeak! gave up for the night. However, it bugged me so just as an absolute last resort I swapped the chip again, this trime for the one just before the fx socket halfway along the preamp chain and voila!! we have sound! looks like murphy's law struck again and took out two opamps, one at each end of the amp just to keep it confusing!
more on order, hopefully that will be all!

[allante666]

FINALLY! replaced the op amps and all is well. stable power, good amplification. Nice bass amp actually. Thanks to all who contributed.

[J M Fahey]

Glad you could repair it, because this is a confusing little bugger.
Enzo was very close when he pointed

SOunds like your ground is missing, and the power rails are floating.

I don't know this particular amp, but Peavey is very fond of the "grounded output" topology, where they ground the amplifier "hot" or "output bar" and the speaker is connected from chassis ground to the ungrounded PS center tap, more precisely to the point where the big filter capacitors join.
The power transistors drive a short but the current necessary to do so comes from one of the filter capacitors, and closes the circuit through the speaker.
The voice coil itself only cares about the current through it and couldn't care less about where does that current come from.
That topology is used to allow a +/- 15V powered op amp to drive a +/-40 to +/- 70V rails power amp.
Any problems?: not much in mono power amps (such as this one), because you have full, speaker-level audio voltage riding on your DC rail voltages, but you can't power a stereo amp with that same PSU.
In fact, QSC big power amps, which often use it, have dual secondary dual PSU.
Another problem: if +/-15V for preamp is derived from power rails, through power resistors, zeners and some electrolytics, you only get that with no signal, but not when +/- B follows audio wildly.
It works, sort of, because the RC time constant of said filters is much longer than any audio signal, but if for any reason output voltage stays too long (a couple seconds) too positive or too negative, you lose either +15 or -15.
That's what happened here: a drifting, bad op amp, (specially "the last one"), being DC coupled shifted the DC level on what on another amp would be the hot speaker out.
It can't drift, being tied to ground, so +/- rails do, killing + or - 15V in the process.
Never saw this specific schematic, but please check Basic 60 and a few similar ones; also QSC 1200.

[allante666]

I think the very first original fault of fading volume and drifting rails was down to a leaky zener pulling one rail and one 15 volt rail down. Dont forget at the outset my audio was fine (until it faded). It was only when I found the fuse blown that I lost all audio. I dont know what I did to cause this but as I said earlier I disconnected part of the supply to take some measurements and when I reconnected it I got a spark off one of the resistors. This must have been due to residual current in the caps but I think it must have been the cause of the blown fuse and blown op amps.
I can see your theory but im pretty sure that at some point I was taking measurements with the pre amp board disconnected and still getting drifting rails so that would rule out a faulty opamp. I think the opamp thing was my own fault for not being systematic and looking too deep to start with. Thats my theory anyway, Im just glad its sorted, it kept laughing at me every time I saw it!

[J M Fahey]

Maybe it happened that way.
Anyway, being DC coupled, a failure in any element in that block will affect DC voltages in other points.
For lack of the proper schematic, I was referring to:

There you can see that U4A is not a "preamp" IC but within the power amp itself.
Also notice the "grounded output", the midpoint between R45/R46 and that the speaker "hot" terminal is driven from the ungrounded PSU center point, between C33/C34 and also the "double filtered" +/- 15V, with those big 1000uF electrolytics, far beyond the classic 22uF used for audio bypassing proper, across the Zeners.
Really can be very confusing at first.
How do I know? ....... don't ask