Members Bluescout Posted August 31, 2009 Members Share Posted August 31, 2009 So I bought a Peavy Bandit guitar combo for cheap just because I feel I may want to stay in music after altzheimers hits. It sounds very quiet and distorted but if I bypass the preamp by plugging into the "poweramp in" it sounds clean and loud. Anything in particilar jump out before I rip into this thing? Link to comment Share on other sites More sharing options...
Members Guttermouth Posted August 31, 2009 Members Share Posted August 31, 2009 So I bought a Peavy Bandit guitar combo for cheap just because I feel I may want to stay in music after altzheimers hits. It sounds very quiet and distorted but if I bypass the preamp by plugging into the "poweramp in" it sounds clean and loud. Anything in particilar jump out before I rip into this thing? have you tried the old "jumper in the effects loop" trick yet? Link to comment Share on other sites More sharing options...
Members bassthumpintwin Posted August 31, 2009 Members Share Posted August 31, 2009 That, and make sure all of your jacks are clear of any gunk or debris. This could simply mean plugging/unplugging an instrument cable jack into each of them repeatedly. Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 1, 2009 Author Members Share Posted September 1, 2009 Update: I tried the "effects loop" jump with no change. I pulled the head out and reseated all connectors and chips but it's still the same. Any more ideas or experiences? Link to comment Share on other sites More sharing options...
Members Guttermouth Posted September 1, 2009 Members Share Posted September 1, 2009 i wish i could be more help man,it's been years since i've been in a bandit.i would start by making sure i had continuity from the inputs to the board and then start checking voltages on the opamps in the preamp. if i was a betting man,i'd say you've got an opamp that crapped out somewhere in the front end. Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 1, 2009 Author Members Share Posted September 1, 2009 Is an opamp one of the ICs on the board? Link to comment Share on other sites More sharing options...
Members bassthumpintwin Posted September 1, 2009 Members Share Posted September 1, 2009 Is an opamp one of the ICs on the board? Yes. It probably looks somethign like this: A schematic might help you narrow them down a little better, although a quick search of any of the IC's serial numberes will tell you what they are used for most likely. Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 1, 2009 Author Members Share Posted September 1, 2009 Yes, I have 4 of those. It would be easy a probably cheap to replace all 4 just for good measure. Is that a prudent plan or would it change the tone or sound? Yes. It probably looks somethign like this:~snip~A schematic might help you narrow them down a little better, although a quick search of any of the IC's serial numberes will tell you what they are used for most likely. Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 1, 2009 Author Members Share Posted September 1, 2009 the number on them is RC4558P. Another question: There's a footpedal jack. Is there a way to jimmy that to switch to the other channel so I can test and see if it's one channel? Link to comment Share on other sites More sharing options...
Members bassthumpintwin Posted September 1, 2009 Members Share Posted September 1, 2009 Yes, I have 4 of those. It would be easy a probably cheap to replace all 4 just for good measure. Is that a prudent plan or would it change the tone or sound? It shouldn't have any effect on the tone or sound. Just make sure you replace them with the exact same IC, or a viable replacement. Often, if you can't find a replacement IC, it had simply become obsolete and been replaced with a basically identical chip with different serial numbers. Of course, whether or not this works obviously depends on if the problem is with the OP amp or not. This is where a schematic with listed voltages might come in handy. Also, if you're adventurous, you can turn the amp on for a couple of minutes and touch each of the op amps with your finger tip (or maybe more safely, a temperature probe). Occasionally, a bad IC will become hot to the touch, or at least be of a noticeably different temperature. Of course, you'll also often see burn marks on the chip or the board as well. Hard to say is that's the case here though. It wouldn't hurt to email Peavey for a .pdf of the schematic if you haven't already. They should either email it to you for free, or send you a hardcopy for cost of postage. Link to comment Share on other sites More sharing options...
Members bassthumpintwin Posted September 1, 2009 Members Share Posted September 1, 2009 the number on them is RC4558P. Another question: There's a footpedal jack. Is there a way to jimmy that to switch to the other channel so I can test and see if it's one channel? There should be an onboard switch (probably on the front control panel) that switches the channels without the aid of a foot pedal. I believe the footpedals for the bandit amps were simply for switching from clean to overdrive, and/or turning the effects loop on and off. Did you try repeatedly inserting/removeing a jack from each of the inputs, outputs, and footswitch jack? Sometimes these can get dirty, dusty, gunky, full of cobwebs, etc... and effect whether they function properly or not. As long as you have the amp open, I'd also double check that all of the jacks are makign a proper, firm contact when an instrument/patch cable is inserted. I'd also make sure that any cables you are testing with are working properly. Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 1, 2009 Author Members Share Posted September 1, 2009 It wouldn't hurt to email Peavey for a .pdf of the schematic if you haven't already. They should either email it to you for free, or send you a hardcopy for cost of postage. I'm going to give that a try. I'm fairly handy with tools and know about electricity and so on so I can probably get through this with a little help. Link to comment Share on other sites More sharing options...
Members bassthumpintwin Posted September 1, 2009 Members Share Posted September 1, 2009 Just for the heck of it, you might also consider running a magnifying glass over each of the board components (both above the board, and under the board) and check for any cracks, burn marks, lifted PCB foil (assuming it's a PCB) etc... in each of the components or solder points. Swelled capacitors would also be a no no (though this is likely not a contributing factor to your problem?). Another easy thing you can do is check any diodes (especially any running into the op amps) to make sure they are working properly. Aside from that, blow out any dirt/dust/debris, clean any gunk off the circuit board (carefully, and while unplugged), and double check any and all connectors, sockets, etc... Link to comment Share on other sites More sharing options...
Members bassthumpintwin Posted September 2, 2009 Members Share Posted September 2, 2009 Any progress with the diagnosis? Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 2, 2009 Author Members Share Posted September 2, 2009 Any progress with the diagnosis? I asked Peavey and in typical Peavey fashion I got 2 pages. One is the schematic, and one is a diagram of the circuit board with every component labeled and showing which trace they connect to. Peavey!!! I'm headed to a job near a huge electronics parts place. I'm going to pick up opamps and try them just because it's a quick and cheap el-testorama. It looks from the diagram, that if I ground the tip of the footswitch, it wll switch channels. There's no switch on the front or back for this. The more I stare at it the more I see the 2 preamps, and the power amp grouped. Thanks for asking and for your help. When I get down to it, I'll post more. Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 3, 2009 Author Members Share Posted September 3, 2009 Update: I got new opamps for the fun of it but don't know which way they go in so they are not in. The manual says the normal channel is only accessible via footswitch but the amp will put out the same crappy sound whether I turn up the lead gain or the normal gain. When I short the pins on the footswitch jack, nothing changes. That would make what I call the "switching section" suspect. The lead and normal channels seem to be parallel so if they sound the same, probably not the problem. They both lead into switching, then to the EQ which are my 2 suspects. Power section isn't the problem because it sounds fine playing into poweramp in and the reverb section is deuced by turning the control all the way down. Based on all of that, 2 of the opamps (U1 and U4) are likely not involved. I may try and jump out the EQ section and see what I get. I've attached a link to the schematic where I've segregated the sections. http://www.summit-pc.com/rob/bandit.jpg Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 4, 2009 Author Members Share Posted September 4, 2009 Link to comment Share on other sites More sharing options...
Members Guttermouth Posted September 4, 2009 Members Share Posted September 4, 2009 Update: I got new opamps for the fun of it but don't know which way they go in so they are not in. The manual says the normal channel is only accessible via footswitch but the amp will put out the same crappy sound whether I turn up the lead gain or the normal gain. When I short the pins on the footswitch jack, nothing changes. That would make what I call the "switching section" suspect. The lead and normal channels seem to be parallel so if they sound the same, probably not the problem. They both lead into switching, then to the EQ which are my 2 suspects. Power section isn't the problem because it sounds fine playing into poweramp in and the reverb section is deuced by turning the control all the way down. Based on all of that, 2 of the opamps (U1 and U4) are likely not involved. I may try and jump out the EQ section and see what I get. I've attached a link to the schematic where I've segregated the sections. http://www.summit-pc.com/rob/bandit.jpg i would start with u3b and work my way back towards the input checking all the opamps and jfets making sure i had voltage on all of them first.anything other than that and you're really gonna need a scope or alot of patience are there any visibly burned or broken components anywhere on the board? Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 4, 2009 Author Members Share Posted September 4, 2009 i would start with u3b and work my way back towards the input checking all the opamps and jfets making sure i had voltage on all of them first.anything other than that and you're really gonna need a scope or alot of patience are there any visibly burned or broken components anywhere on the board? Thanks GM. I'm reading up on opamps and how they work. I CAN get my hands on a scope, actually a very nice digital one. I assume the 3 readings I would get would be GND, straight DC, and guitar signal. If I compare scope patterns of the input and the Pre-out, I should see the difference between clean and busted. The more I stare at the schematic the easier it gets. I also thought I could put a cord into the poweramp in and find a few select places to touch the tip to bypass sections like maybe diode CR36 leading to just before U3B. The only thing that looks remotely bad is diode CR21. The lead is discolored. But that's in the power amp section which seems fine. Link to comment Share on other sites More sharing options...
Members Guttermouth Posted September 4, 2009 Members Share Posted September 4, 2009 Thanks GM. I'm reading up on opamps and how they work. I CAN get my hands on a scope, actually a very nice digital one. I assume the 3 readings I would get would be GND, straight DC, and guitar signal. If I compare scope patterns of the input and the Pre-out, I should see the difference between clean and busted. The more I stare at the schematic the easier it gets. I also thought I could put a cord into the poweramp in and find a few select places to touch the tip to bypass sections like maybe diode CR36 leading to just before U3B.The only thing that looks remotely bad is diode CR21. The lead is discolored. But that's in the power amp section which seems fine. one other thing you might try is starting at the power amp in and work your way back with a signal probe listening for where the signal stops getting louder. this will give you a better indication of where your failed component is and may help narrow it down without a scope. i do this with pedals alot and it can speed things up considerably. Link to comment Share on other sites More sharing options...
Members bassthumpintwin Posted September 4, 2009 Members Share Posted September 4, 2009 Diodes are fairly easy to check. Just turn the amp off and put a diode tester on it (or just an ohm meter if you don't have a diode tester). You should only get continuity one way. As far as which way the opamps go, there should be a small indent on one side that signifies which direction it goes. Note the picture of the opAmp above. it will be a small circle like so, or an indent in the side. Link to comment Share on other sites More sharing options...
Members bassthumpintwin Posted September 8, 2009 Members Share Posted September 8, 2009 Progress? Link to comment Share on other sites More sharing options...
Members Bluescout Posted September 8, 2009 Author Members Share Posted September 8, 2009 I pulled the board out of the chassis to get a look inderneath. There's no components on the bottom. All board etches and solder joints look good. I was hoping to find something obvious. I checked that diode, CR21, out by the big transistors and the reading is odd. one way it's 4 million ohms, the other, 2.5 million. Obviously a toasted diode but it's in the power section which sounds fine if I plug into the poweramp-in. I'm still waiting for access to my friend's scope. It's a snap-on but does very good o'scope as well. Edit: That diode is "special". The number on the sheet is 13886 which a search turns up MZ-2361, a dual diode with voltage drop characteristics. The diagram shows it as different as well. I don't know what THAT means to my readings but it certainly means it's probably not a standard diode test. Link to comment Share on other sites More sharing options...
Members Bluescout Posted October 4, 2009 Author Members Share Posted October 4, 2009 BIG OL' Update!! I got the scope here and started "probing" . I could see the nice smooth input signal, and see the chopped signal at the "preamp out". I started just testing the legs of the op amps and we found we were missing the +15v and -15v. We were getting measurements of like 1.5v and -3v. Looking at the schematic, at the power supply on the FAR right, there are +42.8v and -42.8v outputs which are within a volt of correct. I measure at the ends of resistors R86 and R87. The other side should be that -16 and +15 but the voltage again is -3v and 1.5. I can't figure what it is from here. EDIT: those 2 resistors, R86 and 86 get pretty hot. Help PLEEZE!!!!!! Link to comment Share on other sites More sharing options...
Members perrydabassman Posted October 4, 2009 Members Share Posted October 4, 2009 This may sound stoopid, but did you check your ground wires coming off the main input jack?............. Link to comment Share on other sites More sharing options...
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