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.
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.Originally Posted by JimmyTolson
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?
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.
Last edited by JimmyTolson; 04-13-2012 at 10:19 PM. Reason: add pics and text
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).
The farmer takes a wife, the barber takes a pole....
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.
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.
Education is what you're left with after you have forgotten what you have learned.
^^^What he said^^^
The farmer takes a wife, the barber takes a pole....
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.
@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.
Ah. Well, no, different op amps wouldn;t cause hum. They might increase or decrease noise, as in hiss.
Education is what you're left with after you have forgotten what you have learned.
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.
Last edited by JimmyTolson; 04-14-2012 at 07:15 PM. Reason: added text
Are the resistors 'open'?
You can get just about any Peavey part direct from Peavey.
Great people to work with!
no there not open but getting from mouser is easier for me as im not to tech savvy and peavey may mis understand me.
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.
Education is what you're left with after you have forgotten what you have learned.
Why are we replacing good resistors?
Was the initial fault ever located?
Like maybe shorted output transistors.
on the 3 47ohm resistors (the small brown ones with strips beside the heatsinks) in line with the output transitors, 2 show 0.00 on the ohm meter while one moves up and down in the numbers but slow. all are still mounted to the board tho. if this one is suspected being bad what would the part number be on mouser.i agree with Jazz p bass on the shorted output transistor but thinking the one little 47ohm one may have caused it.
Generally speaking, resistors do not 'cause' failures.
The 47 ohm resistors that you are referring to will burn up if the output transistors go shorted.
Testing them in circuit can be misleading as other parallel paths exist.
The one that shows '0.00' is most probably pointing to a shorted transistor.
Labeling the parts that you want to talk about is a good idea.
You can use the part designator that is labeled on the schematic.
Can you please list the test equipment that you have available?
The transistors can be checked with a decent multimeter.
Preferably one with 'diode check' function.
If you do not know how to check a transistor, Google it.
Okay i changed the 3 driver transitors 5331 and 5332 + bought new output transistors to make sure i had the right ones and there is no change in hum or speaker standing out. i made the lightbulb limiter as suggested to hook between amp and electric and without the speaker hooked up the lightbulb glows real dim but when speaker is hooked up lightbulb glows bright.the 3 resistors that are beside the little heatsinks beside the driver transistors (2 on one and 1 on the other) all check out to 47ohms.
Now it's time to take some voltage readings.
With the amp hooked through the limiter & the speaker disconnected.
Look at the schematic.
There are test voltages galore.
What you want to find is something way abnormal from the schematic voltages.
It may help if you keep in mind what this output section is doing.
It is a balancing act between the + & - rail voltages.
The output transistors should be seeing around .500 - .600 volts on there bases. In other words 'barely' turned on.
That will place the driver transistor bases at around 1.1 volts.
Start there.
Have we ever found out how much and what kind of voltage is on the output of this amp?
I'm really afraid that if Jimmy keeps throwing parts at this amp there will be far more to repair than what was originally wrong with it in the first place.
I POSITIVE he's going to be buying a new speaker soon!
As 52Bill said in the very first response to your question.Do not leave the amp on with the speaker connected, as it will cause damage to the speaker.
Seriously, DON'T connect the speaker until you have resolved the problem. If you want to know if there is still DC at the output, use a meter to check NOT the speaker!
The DC on the speaker is the hum, so measure that w/ meter. Do not think that the meter will show something different than when you plug in the speaker to take a listen. Also, I was wondering if you had tested the rectifier diodes? In the original picture this diode looks funny to me... Probably just the shadow or something, but it looks chipped or something. I bet that is just the shadow, but I would still test those diodes. Also, test all your voltages foremost...
I looked at the rectifier diode you circled in the pic above and there is a raised line like crack on the side toward the edge of the board. the numbers are hard to read but it looks the same as the one below it and the 2 between the big blue capacitors. the 2 beside the white square thing are smaller with different numbers. ill post closer pics of it.( the meter pic is of what i have to check things with.)
Last edited by JimmyTolson; 04-20-2012 at 06:39 AM. Reason: added pics
I have read a post of these diodes creating hum before and the guy had changed so many things before the rectifier diodes. To me it looks like there is maybe a crack in it and then someone put a dab of glue on it...? Not sure if that is what I am seeing here. Did you test the diode? Now if all the diodes have this hair line crack then that is how they will look, so make sure I am not leading astray. If it looks like more than hairline then I would probably be questioning that part more. I was just seeing something that was in the picture and that may(probably not) not be the problem here...
Put your multimeter on the diode checking feature and test it both ways with the amp off. One way will read I or OL and the other will read .500 or .700 or something around those types of readings. My feeling here is that they are probably fine but you should always check. If you get funny readings then sometimes you need to lift a leg on a diode to pull it out of circuit to get definite readings. The fact that the amp is passing the signal to the output make me think that these are probably working just fine.
Edit: Get on track w/ the light bulb limiter running the amp and get those output transistor voltage readings foremost.
Last edited by DrGonz78; 04-20-2012 at 11:12 AM.
i pulled one leg of the diode in question and checked it. one way does nothing on the meter as it stays at 1 and when i reverse the wires it shoots to 620. this is using the diode check on the meter in pic above (->l-) .
So it looks good. Sorry to get you going in another direction, but that is good to know that the part measures correctly. How is it going w/ your voltage measurements? Let us know when you test those output transistors. Best regards
ive done everything to my scope on this and im having no luck at all.the nearest place to me is over a 2 hour drive one way that can even look at it. would be willing to u.p.s it to someone on here that knows about these amps to get it fixed. would be glad to pay plus pay for return shipping if anyone is interested.
You still have not posted any voltage readings.
Basic stuff.
Like the power supply rails, the base voltages of the output transistors.
That would basically require a volt meter.
(unless the scope was set to read Vdc.)
Sorry Motumoyo, don't get you.
What are you trying to say?.
Thanks.
Juan Manuel Fahey
In a galaxy far, far away... C3 PO and R2 D2 were in a search window trying to save us all from bots...
Yes, it *did* sound like "Botglish".
But his earlier post about master volume sort of made sense.
Yes, I also saw his manga-pushing signature.
Which is about Samurai fighting Aliens in Medieval Japan.
Swords against Laser and Nuclear weapons.
Yea, sure, any day
Juan Manuel Fahey
I actually have some spare parts sitting on my windowsill, so it was kinda of a moment where Botglish made sense too. I read his other posts and thought they were even better thought out. Isn't it funny how someone takes all this time to clutter up these boards as to gain a link pointing to their commercial websites. I understand why it helps search placement in google etc., but it is funny when they try to talk on topic. Not sure if it generated by string data that it collects from postings on this site or if someone actually typed that junk.
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