Peavey Bandit (1981) loud hum

[JimmyTolson]

I have a peavey Bandit made in 1981 (the one before the 65) and when i turn it on there is a loud constant hum from the speaker.this happens when all knobs turned to zero and nothing pluged into it,and nothing changes to the hum when knobs are turned up or down.did notice the speaker moves outward and stays there. any help on this would be great.

[52 Bill]

did notice the speaker moves outward and stays there. any help on this would be great.

Welcome to the place. When the speaker pushes in or out and stays there, that means that the amp has a fault that is putting dc voltage across the speaker terminals. Do not leave the amp on with the speaker connected, as it will cause damage to the speaker.

This problem is usually an indication of a failed output transistor in the power amp. What sorts of skills do you have? Is this something that you can fix yourself?

[JimmyTolson]

hi and thanks for the welcome. i'm not a amp repairman but can figure things out pretty quick.i posted the schematic in hopes someone could point me in the right direction.it has the schematic plus a drawing of the location of the electronics on it.i have a pretty good testing meter for testing and all electonics stick through and sorder to the backside of the board.

added pics of the amp's board. the 4 items in the last pic i changed as told but there was no change in hum or speaker standing out.

[Gtr_tech]

Just to elaborate on the DC on the speaker thing.....the speaker can also suck in and stay there. That would indicate the output section stuck to the negative rail (providing the speaker is wired with correct polarity).

[JimmyTolson]

speaker is wired right reguarding polarity and goes outward and stays till amp is turned off then it goes back to where its suppose to be.
on another forum about steel guitar amps they change the op amps in there mods to the bandit to these " OPA2134PA Texas Instruments Op Amps " would these help my problem.

[Enzo]

No No No.

The amp needs repair, NOT modification. They may like those other op amps better for whatever reason, but the fact you have the standard op amps is not what is wrong here. FIX the amp first, THEN think about any changes you want to fiddle with.

In other words, if you are in the emergency room with a massive heart attack, then is not the time to suggest plastic surgery to your nose.


If you have already replaced the power transistors, we also need to check the drivers, and they are the smaller transistors on their own little 8-finned heatsinks. Without me looking, I think they are probably 5331 and 5332 types. Not only that, make sure none of those rectangular tan cement power resistors have gone open. And look in the schematic for resistors associated with those drivers, PV likes to use 47 ohms a lot. See if any of those are open.

Make sure both positive AND negative power supplies are present. And I mean both the 15v ones for the ICs, and the higher voltage ones for the power amp. A missing -40v can slam an output to +40 just as easily as a shorted +40v transistor can.

[Gtr_tech]

^^^What he said^^^

[J M Fahey]

And *DISCONNECT* that speaker NOW.
Also search for "bulb lamp limiter" , build one and use it.
Plug your amp into the current limiter until it's fully repaired.
It will save you $$$$$ and bad mood.
And follow all of Enzo's instructions, they are meant to drive you safely to a happy ending.

[JimmyTolson]

@ENZO I wasnt meaning modification but more to would those cause the hum.I'll check the things you said to and post back,thanks for everyones help on here.

[Enzo]

Ah. Well, no, different op amps wouldn;t cause hum. They might increase or decrease noise, as in hiss.

[JimmyTolson]

Is there a replacement part # for the rectangular tan cement power resistors that i can get from Mouser.
they have " cp-5-25 Dale 5w
400u 10% 8136

and " cp-5-25 Dale 5w
.33u 10% 8136

and " cp-10-25 Dale 10w
.22u 10% 8103

thanks again.

also on the resistors they are 47 ohms is there a part # for these i can get from mouser.
altho i replaced the 2 larger caps on the board with ones i found at radioshack 2200 50v is there a better one to replace it with i can get from mouser or are the ones i put in it alright.
dont mean to agravate with questions but where you guys work on these alot thought u might already know.

[Jazz P Bass]

Are the resistors 'open'?
You can get just about any Peavey part direct from Peavey.
Great people to work with!

[JimmyTolson]

no there not open but getting from mouser is easier for me as im not to tech savvy and peavey may mis understand me.

[Enzo]

Agreed, if the resistor is not open, it is OK.


Mouser stocks them all. These are wirewound power resistors. If you enter CP5 in their search window then select resistors under the passive parts section, you sill find them. 0.33 ohm will be listed as 330mOhm and 400 ohm they have too. You can also search CR5 for very similar parts. In the CR line they stick to standard values, so the 400 ohm would have to become 390 ohm.


I have never seen that 10 watt 0.22 ohm one open before. But search same way for it, except looking for CP10.

[Jazz P Bass]

Why are we replacing good resistors?
Was the initial fault ever located?
Like maybe shorted output transistors.