Saturday, October 1, 2011

Repairing a Peavey Bandit 112 Guitar Amp

Today were going to look at a fairly common and easily performed amplifier repair.

The amp in question is a 1980's vintage Peavey Bandit 112 Soloist Series 100W guitar combo and belongs to the singer/rhythm guitarist in my band. Unfortunately at some point between our latest gig and its arrival back at our practice room the control knob for the lead channel level has been snapped off. This is an unfortunate but not too uncommon occurrence which can be attributed to careless handling or bad loading of the van after the gig. On these occasions tiredness / the euphoria after a successful show / overzealous helpers and perhaps a small quantity of alcohol can lead to these sorts of careless mishaps !

So now on to the repair.

Peavey Bandit repair - Step 1: Remove the Amp chassis from the Combo.

IMPORTANT - Before working on the amplifier we double check that it is disconnected from the mains electricity supply ! (ie. Make sure you've unplugged the amp from the wall!)

Next we remove the back of the combo ( undo the 4 screws ) to allow the amp chassis to slide easily out.

Then  we undo and remove the power cord retaining clip and unplug the cables that connect to the speaker and the reverb unit (making note of how they need to be reconnected to avoid any errors later on).

Now the combo is laid face down and the 4 screws which attach the amplifier to the speaker housing are removed and the amp chassis is lifted out. All screws are put to one side to avoid misplacing them.





Peavey Bandit repair - Step 2: Remove the circuit board from the amplifier chassis

Next we remove the plastic control knobs from all the potentiometers and remove the black retaining nuts (11 in total)

We also remove the retaining nuts and washers from the 4 input/output jacks on the front panel.

We then unplug the connecting cables which run between the metal housing and the circuit board (4 connectors in total) taking note of the correct positions for reconnecting later.

Once this has been done the 6 black screws which attach the back cover to the amplifier chassis are undone and the circuit board can be slid out.

Peavey Bandit repair - Step 3: Remove the faulty potentiometer.

As can be seen in the photo below the potentiometers are mounted directly on the circuit board and are supported by a small metal bracket which is also soldered to the circuit board. This is so that the circuit board can be assembled as a whole in the factory then easily mounted in the chassis as a single unit.

To remove the pot from the circuit board we carefully desolder the 7 tabs using a soldering iron and a solder sucking pump. The pot can then be lifted off the circuit board.

Once the level pot has been removed from the circuit board we confirm that it is a 10K linear pot.


Peavey Bandit repair - Step 4: Buy a replacement potentiometer.

Unfortunately my local electronics store didn't have the exact replacement part.  Normally I'd purchase the original replacement part online if available. But in this case the amplifier needs to be fixed before our next practice session so there isn't time to wait for delivery. This leaves the alternative of buying a standard potentiometer of the same value, size and type and mount it directly on the amplifier chassis with 3 wires connecting it to the circuit board. As can be seen in the photo we also need to cut the shaft down to size. It's a little bit more work but saves waiting, a little money and will be not be noticeable once the amp is reassembled. Obviously we have to use a pot of the same value otherwise the amp's performance will be affected !


Peavey Bandit repair - Step 5: Connect the replacement potentiometer to the circuit board
First we solder three short, insulated wires to the connections on the circuit board. As you can see in  the photo I've used different coloured wires to avoid confusion when connecting them to the replacement pot. I've also made sure that the wires aren't overly long , they just need to be long enough so that the pot can be fitted to the amp chassis in such a way that the connecting tags won't touch any other components or the metal chassis itself. I then solder the coloured wires to the corresponding tabs on the pot.

Peavey Bandit repair - Step 6: Mount the Circuit board in the metal amplifier chassis.

This is just the reverse of the removal sequence with one additional stage which is to connect the new potentiometer to the metal housing. Care must be taken to ensure that the pot doesn't turn while the retaining nut is being tightened. The photo below shows how the final repair looks - all neat and tidy on the inside and totally invisible once the amp is reassembled.



Peavey Bandit repair - Step 7: Re-fit the amplifier into the speaker housing.

Once again this is just a case of reversing the removal process, taking care that everything is reconnected correctly.


Peavey Bandit repair - Step 7: Check that everything works OK.

Once everything is back together and we are happy that all the parts are connected as they should be we plug the amp in, connect a guitar (in this case a Fender Telecaster) and make sure that the amp works as before its accident.


11 comments:

  1. Hey thanks for posting the guide! I'm looking at repairing a newer TransTube Bandit 112 (Silver Stripe) with the same problem, a broken knob. Any idea if the layout is similar and just as simple? Also, what exactly is the "main's electricity supply" that you have to disconnect first? Thanks again!

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    1. Hi Blue,

      Thanks for your comment I'm glad you found the post useful.

      I'm not familiar with the newer Peavey amps I'm afraid but a simple pot swap like this is usually pretty obvious when you take eveything apart. Otherwise theres plenty of info on the Peavey website. Generally older devices are easier to work on tho because there were less surface mounted components etc but you can often adapt things. I was also supprised to find that the original parts are still available online. (I didn't use original parts for this amplifier repair tho cause we needed to get the amp fixed super fast and couldn't wait for them to arrive.)

      The "mains electricity supply" is the power cable - connected to the outlet. To be honest and I hope I don't sound patronising here, if you are unfamiliar with basic electrical safety I recomend that you take the amp to a qualified technician because it is possible to injure yourself when working on any electrical appliance if you don't take the correct precautions (please read this blogs disclaimer in the links section on the right hand side)

      Good luck

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    2. I've got same amp, same problem, exactly the same knob! So thanks very very much, I'll be ordering my new part and following your instructions. Thankyou very much, and the pictures are awesome help.

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  2. Thanks for your comment, I'm glad to be able to help. If you order the original Peavey replacement pot its even simpler than the way I describe since you can just swap it out without the extra short wires.

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  3. Thanks for the info. I know the capacitors can hold residual voltage, not much though in the newer Bandits thankfully. I figured that's what you meant by the main's electricity supply. Most people just say: "make sure it's not connected to the wall" which is clearly more than obvious. Just making sure I didn't miss something since we have similar amps. I opened up the amp and found that there were more problems awaiting me inside. So I decided to can the project. Oh well, I found an exact working replacement a few days later for cheap. No sweat off my back! Thanks again!

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    1. Hi again, thanks for the update glad you've got things sorted.

      I agree that "make sure its disconnected from the wall" would probably have been a more obvious way of phrasing things! I tend to get a bit longwinded sometimes, especially when writing. I'm more of a doer than a writer.

      I agree that it seems pretty obvious but I think it has to be stated just to be on the safe side.

      Residual charge is well worth mentioning here too,(thanks for that). As you suggested, its more of a risk with valve amps than solid state but we should all be aware that capacitors store charge even after the amp is unplugged.

      Cheers!

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  4. No problem, this is a great guide! Keep up the good work!

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  5. you have a great post here.. can i put your post as reference to my blog http://theguitarlearner.blogspot.com/ ?

    thanks

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    1. Hello,,

      Thanks for the compliment, I'm glad you found my post interesting.

      You're more than welcome to link to it from your blog.

      Good luck with your blog, I look foward to reading more posts.

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  6. I, too, was confused on the "mains electrical supply" instruction, thinking it was something internal concerning a residual charge. However, once I figured out what you meant there, I went through this step by step on my Bandit (same teal stripe model - think mine was the same knob, too (may have been one to the left of yours)), and found it to be very helpful. Thanks. It was quite easy and did the trick. My biggest issue was that I didn't have a solder-sucking pump and had to use a braided wick, which was more tedious. One thing that seemed odd - on both mine and yours, the pot that needed replacing didn't match the other pots - almost as if it had already been replaced. Wouldn't think much of it if yours wasn't the same way. Thanks for the help!

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    1. Hi,

      Thanks for the feedback, glad this post was useful.

      Since the bit about mains electricity seems to be confusing people I've edited that sentence to (hopefuly) make it a little clearer.

      As for the pots not all being the same. I'd assumed that my friend's amp had already had it swapped out at least once. Maybe they came like that from the Peavey factory. Its hard to know when working on gear thats been bought second hand.

      Cheers!

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