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Solid State Amplifiers => Amplifier Discussion => Topic started by: MCM1910 on July 21, 2017, 02:41:26 PM

Title: Peavey Bandit Novice repair
Post by: MCM1910 on July 21, 2017, 02:41:26 PM
Good afternoon. I bought a peavey bandit. The original owner explained that it had an issue where it smoked. I paid next to nothing for it thinking it'd be a good opportunity for me to try out another repair.

So here is what I know. Amp listed as not working and stating that it produced smoke at one point. So first inspection I notice the power cord has had the ground prong removed and one of the prongs remaining is damaged. I opened it up expecting a lot of burnt out parts but from what I can tell there is one cap that is damaged and pretty clearly spewed some liquid out of it.

So my plan of attack here is to replace the power cord and the cap and see if that takes care of the problem. Does anyone else have any suggestions? I'm really excited to get this thing going as I've heard so many good things about these bandits!
Title: Re: Peavey Bandit Novice repair
Post by: Enzo on July 21, 2017, 02:58:02 PM
The light tannish goo on the caps in the picture is glue.  They smear it on the larger parts to keep them from vibrating and breaking their wire leads.

Start with the power transistors, are they shorted?  Does this unit blow fuses?  Does it produce DC on the output?  Does it even light up?

No circuit works right without proper power supply, so we always look into that first.

Get your official model name from the serial number plate, and call customer service at Peavey for the correct schematic.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 21, 2017, 03:03:20 PM
Quote from: Enzo on July 21, 2017, 02:58:02 PM
The light tannish goo on the caps in the picture is glue.  They smear it on the larger parts to keep them from vibrating and breaking their wire leads.

Start with the power transistors, are they shorted?  Does this unit blow fuses?  Does it produce DC on the output?  Does it even light up?

No circuit works right without proper power supply, so we always look into that first.

Get your official model name from the serial number plate, and call customer service at Peavey for the correct schematic.

Thanks for the reply Enzo! The unit does power on but does not produce any sound. What I was referring to with the cap isn't the glue but the faint trail of orange where it appears the cap let out some liquid and it was hot on the board. You can see how it affecting the area around the leads of the components next to it.

Thanks again!
Title: Re: Peavey Bandit Novice repair
Post by: DrGonz78 on July 21, 2017, 04:19:07 PM
Yeah it does look like something nasty sticky fell in the amp. Try using a q-tip and some 99% isopropyl alcohol to give it a good cleaning. That 220uf electrolytic cap needs to go on a diet his body broke right out of his outfit.  :lmao:
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 21, 2017, 07:01:11 PM
Updates. Got schematic from peavey. All the voltages in the power supply seem to check out. Did some more fiddling around and found that if I take preamp out to another amp it works but unfortunately plugging into the power amp there is no sound. That capacitor is a part of the power amp so I'm still thinking that is my most likely problem.

Is there any thing else I should rule out other possible causes?
Title: Re: Peavey Bandit Novice repair
Post by: Enzo on July 21, 2017, 08:04:05 PM
Post your schematic or tell us the model Bandit, there were many Bandits.

Could it be the Bandit (Supreme) Transtube, 1995 schematic?

I don't see that as killing your sound, even if bad.  C71?  If that were bad I'd expect maybe distortion or hum or noise, not silence.

I see the speaker feed goes through a thermal breaker, is yours open or unwired?

Feed a signal into the FX return for the power amp, now follow it through the front of the PA.  You see the signal at the op amp outputs?  Looks like U3B, then U5A, then either side of U4, then U5B?  If it doesn't get that far, C71 has no chance to affect it.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 21, 2017, 08:49:38 PM
I'll try attaching schematic to this but I believe you are correct in your guess of the model. (Peavey is great about this by the way. They emailed it to me in 9 minutes!)

I will do some signal tracing and report back. I've got some friends flying in tonight so it may be a few days. Thanks so much for all your help!
Title: Re: Peavey Bandit Novice repair
Post by: Enzo on July 22, 2017, 03:23:14 AM
Look in the corner of the drawing, does it have the 1995 date?
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 22, 2017, 02:47:05 PM
Im sorry I'm having trouble posting PDFs but yes it is dated 1995.
Title: Re: Peavey Bandit Novice repair
Post by: Enzo on July 22, 2017, 05:27:50 PM
OK, then we are on the same page, I have that one here.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 23, 2017, 07:31:03 PM
Well here is the new development. I was thinking about the possibility that there was a speaker problem. So I though I'd plug in a small 8ohm speaker I had laying around and plug into the ext speaker jack. I did and there was indeed a hum when I turned it on and then a puff of smoke from the speaker. What in the world just happened besides probably ruining that speaker and discovering the Sheffield in the amp may be done for too...
Title: Re: Peavey Bandit Novice repair
Post by: Enzo on July 23, 2017, 09:22:53 PM
Your amp is producing DC, that will indeed damage a speaker.  Work with NO speaker and NO load on the amp until we know it is no longer making DC.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 23, 2017, 10:07:09 PM
Quote from: Enzo on July 23, 2017, 09:22:53 PM
Your amp is producing DC, that will indeed damage a speaker.  Work with NO speaker and NO load on the amp until we know it is no longer making DC.

Thank you Enzo. Can you give me any tips for how to trace down how it is producing DC? I guess I'm still wondering if c71 is involved. It is damaged but I guess I don't understand what would cause it to have problems. What I mean is did another problem create the DC and that damaged c71? Or is c71 the culprit?
Title: Re: Peavey Bandit Novice repair
Post by: galaxiex on July 24, 2017, 03:40:52 PM
C71 looks bad, did you replace it?

If C71 is shorted it could put DC on the speaker.

Disconnect the speaker and connect a multimeter set to DC volts to the amp output. (where the speaker would normally connect)

Switch on the amp. Is there a DC voltage reading on the speaker output? How much?
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 24, 2017, 05:36:58 PM
Quote from: galaxiex on July 24, 2017, 03:40:52 PM
C71 looks bad, did you replace it?

If C71 is shorted it could put DC on the speaker.

Disconnect the speaker and connect a multimeter set to DC volts to the amp output. (where the speaker would normally connect)

Switch on the amp. Is there a DC voltage reading on the speaker output? How much?

I just did this and it was holding steady with like 0.4-0.5mv and then all the sudden it spiked at 5.4v. It seems to spike when I tap on the amp, so I'm going to guess that is that cap.
Title: Re: Peavey Bandit Novice repair
Post by: phatt on July 25, 2017, 04:50:13 AM
That suspect C71 cap is right next to the heatsink alloy which gets very hot and over time the excess heat has shrunk the plastic wrap and so it has split which is not a problem in itself,,, BUT Electro Caps don't fair well if exposed to a lot of heat and will dry out so yes it might be stuffed.
I would probably just replace it But not before I found that intermittent issue you just mentioned :o,, that sounds like it might be a cracked solder pad on one of the components.
Remember that Large components like Electro Caps are the obvious solder pad breaks but also components that run hot are also prime candidates for solder cracks. Big resistors that run hot often stress the solder joins as they heat up and cool down each cycle the solder eventually cracks.
Look for hair line cracks around the wire on the solder points. you need good eyes and good light.
HTH Phil.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 25, 2017, 01:18:09 PM
Quote from: phatt on July 25, 2017, 04:50:13 AM
That suspect C71 cap is right next to the heatsink alloy which gets very hot and over time the excess heat has shrunk the plastic wrap and so it has split which is not a problem in itself,,, BUT Electro Caps don't fair well if exposed to a lot of heat and will dry out so yes it might be stuffed.
I would probably just replace it But not before I found that intermittent issue you just mentioned :o,, that sounds like it might be a cracked solder pad on one of the components.
Remember that Large components like Electro Caps are the obvious solder pad breaks but also components that run hot are also prime candidates for solder cracks. Big resistors that run hot often stress the solder joins as they heat up and cool down each cycle the solder eventually cracks.
Look for hair line cracks around the wire on the solder points. you need good eyes and good light.
HTH Phil.

thanks Phil I will get working on this stuff this week and report back
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 25, 2017, 01:28:45 PM
So one more question that has been bothering me.  A theory on the damage to this amp I have was that since the original owner broke off the ground lug of the power cord and because of the damage to 1 of the 2 remaining lugs, I'm thinking that he plugged this thing in backwards (or upside down however you want to think of it).  Is this a reasonable deduction?
Title: Re: Peavey Bandit Novice repair
Post by: DrGonz78 on July 25, 2017, 03:40:23 PM
No. Reversing neutral and hot plugs on the amp will not make any REAL difference in this case. The safety ground being removed just means if there is leakage somewhere in the amp, to ground, the primary path will now only be the possibly grounded human guitar player. 

Stop trying to make a theory of what happened exactly. There are trained pros in the business of repair that can't EXACTLY tell you a WHICH transistor finally died when an amp suffered a major failure.

Edit: Just divide and conquer. Troubleshoot the amp by looking for obviously bad parts and replace. Test things (Transistors, Diodes, etc) in circuit with meter and see what might be bad. If a capacitor has started bulging then it is time to replace that guy.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on July 25, 2017, 06:35:37 PM
Quote from: DrGonz78 on July 25, 2017, 03:40:23 PM
No. Reversing neutral and hot plugs on the amp will not make any REAL difference in this case. The safety ground being removed just means if there is leakage somewhere in the amp, to ground, the primary path will now only be the possibly grounded human guitar player. 

Stop trying to make a theory of what happened exactly. There are trained pros in the business of repair that can't EXACTLY tell you a WHICH transistor finally died when an amp suffered a major failure.

Edit: Just divide and conquer. Troubleshoot the amp by looking for obviously bad parts and replace. Test things (Transistors, Diodes, etc) in circuit with meter and see what might be bad. If a capacitor has started bulging then it is time to replace that guy.



Ok, great advice I will stop over thinking it and break out the multimeter. I'm stuck here at work all day wanting to be working on it!
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on August 04, 2017, 04:16:01 PM
Quote from: phatt on July 25, 2017, 04:50:13 AM
That suspect C71 cap is right next to the heatsink alloy which gets very hot and over time the excess heat has shrunk the plastic wrap and so it has split which is not a problem in itself,,, BUT Electro Caps don't fair well if exposed to a lot of heat and will dry out so yes it might be stuffed.
I would probably just replace it But not before I found that intermittent issue you just mentioned :o,, that sounds like it might be a cracked solder pad on one of the components.
Remember that Large components like Electro Caps are the obvious solder pad breaks but also components that run hot are also prime candidates for solder cracks. Big resistors that run hot often stress the solder joins as they heat up and cool down each cycle the solder eventually cracks.
Look for hair line cracks around the wire on the solder points. you need good eyes and good light.
HTH Phil.

I replaced c71.  Turned on amp and the cap blew on me.  I double checked thinking maybe I had installed backwards but I don't believe I did.  There is some bigger problem here.  I'm frustrated but hoping I can get some guidance on where to go next.  I don't see any other obvious visual signs of problems.  What kind of voltage should be present at the negative lead of c71?

Again thanks in advance for helping me to learn
Title: Re: Peavey Bandit Novice repair
Post by: Enzo on August 04, 2017, 06:43:15 PM
Visual inspection is important, because if we see something burnt up, we need to deal with it.  But mostly, bad parts look like good parts.  What was the last time your car looked different if the water pump quit?  Or the battery went flat.  So we inspect with volt meters more than eyes.

Look at the layout, the + end of C71 faces the power transistor, the negative end closer to the end of the board.  The negative end of C71 connects to speaker hot basically.  The cap should not have more than 100v across it.  What are the power rails, 45v?  That means it ought never exceed 90v.  But caps hate reverse voltage, so if one of the main rectifiers shorts, it can put AC on that cap.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on August 04, 2017, 09:43:31 PM
Quote from: Enzo on August 04, 2017, 06:43:15 PM
Visual inspection is important, because if we see something burnt up, we need to deal with it.  But mostly, bad parts look like good parts.  What was the last time your car looked different if the water pump quit?  Or the battery went flat.  So we inspect with volt meters more than eyes.

Look at the layout, the + end of C71 faces the power transistor, the negative end closer to the end of the board.  The negative end of C71 connects to speaker hot basically.  The cap should not have more than 100v across it.  What are the power rails, 45v?  That means it ought never exceed 90v.  But caps hate reverse voltage, so if one of the main rectifiers shorts, it can put AC on that cap.

When I measure across the c71 connections (with cap removed) I get around -46v of Dc. When I switch to AC on the meter it bounces around all over the place.

Title: Re: Peavey Bandit Novice repair
Post by: Enzo on August 04, 2017, 10:46:41 PM
Is music playing while you do?  This is a flying rail amp.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on August 04, 2017, 10:51:18 PM
No unfortunately nothing playing or plugged in. Speaker unhooked because of the voltage I was getting on the speaker wires.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on August 05, 2017, 02:53:30 PM
Quote from: Enzo on August 04, 2017, 10:46:41 PM
Is music playing while you do?  This is a flying rail amp.

I've been trying to understand this term "flying rail" and I ran across your post here. https://peavey.com/forum/viewtopic.php?t=42222

With that in mind I started to look at the transistors. I'm not really sure how to check them but they have continuity between all lugs but then I got to q11 and 1. I don't know where to check, are the lugs under the heat sink and curiously it is missing one of the nuts holding it down.
Title: Re: Peavey Bandit Novice repair
Post by: MCM1910 on August 05, 2017, 03:16:10 PM
I took out the board so I could see what q11 looked like and sure enough the board is burnt there as well as on one of the leads on q13...

The more I've looked at it the more I'm seeing signs that I'm not the first person to open this thing. Lots of scratching on the chassis around screws and bots that hold board in. The person I bought it from did not mention any of this.  Anyway I feel like I at least have a lead now.

I'll start looking up how to test transistors...