Peavey Bandit 112 having Weak Signal

[emilan87]

Hi, thanks for receiving me in this forum, I joined today because In need some help with my Peavey Bandit 112 amp

Here's the thing... Some weeks ago I accidentally dropped the amp from a minimum altitude and it fell on the front panel. Since then I have been having problems when I connect the guitar to the front input (High or Low gain, the problem its the same on both) The sound is very weak and some times I get no signal at all

When I connect the guitar to the Power amp In or Preamp Out on the back the problem goes away and it sounds normal. I have taken apart the amp already and checked for bad soldering joints but all looks cool, nothing moves and everything its nice and shinny

Does anybody can guide me to solve this problem?

[The Dude]

Welcome to the place!

If there was a guitar cord plugged into the input jack when it dropped, I would look for cracked solder on the jack. Or, possibly even a broken jack. Plug a cord into the jack and check for continuity from the hot and ground of the jack to the pads where the jack solders in.

[g1]

Try putting a patch cord to connect Pre-out to Power amp In.
With a second cord, plug your guitar into the regular front input. Do you still have the problem?

[emilan87]

Try putting a patch cord to connect Pre-out to Power amp In.
With a second cord, plug your guitar into the regular front input. Do you still have the problem?

I tried as asked and the guitar sound comes out of the speaker but at a very weak level... When I take out the patch cord on the back the sound gets muted or can be heard at a very low level but the sound it's pretty bad, like a guitar with dead strings.

What does this mean?

[g1]

It means the problem is in the preamp section, as you said the problem is gone if you plug the guitar into 'Power amp in'.
Did you check the items that The Dude mentioned in post #2 ?

[cjenrick]

wiggle the vol pots with it plugged in, maybe the pot got smashed,

check for free movement of spk cone sometimes magnets move and rub voice coil, probably not the problem as you get sound when using amp in jacks but this at least eliminates the speaker as the problem,

[emilan87]

It means the problem is in the preamp section, as you said the problem is gone if you plug the guitar into 'Power amp in'.
Did you check the items that The Dude mentioned in post #2 ?

The Jack and solder joints look fine. There's no continuity between the ground and hot

I resoldered everything just to be sure but the problem is still there.

Would you guys can recommend a good starting point when checking the preamp section?

Tell me if you need photos of the circuit board

[The Dude]

Ok, there's no continuity between ground and hot. I expected that. Is there continuity from the guitar cord plug to the pads that the jack solders to- both hot and ground? You need to verify that the jack contacts are actually reaching the circuit board. It could be broken or bent internally.

[emilan87]

Ok, there's no continuity between ground and hot. I expected that. Is there continuity from the guitar cord plug to the pads that the jack solders to- both hot and ground? You need to verify that the jack contacts are actually reaching the circuit board. It could be broken or bent internally.

Ok I got it. I checked the continuity between the cord and the pads that are on the circuit board and yes, it has continuity hot to hot and ground to ground, everything is cool

Since I took apart the circuit board the sound doesnt get muted anymore, now it just has very low volume even if I put the pre volume at max

Thanks in advance for all your help guys

[The Dude]

Another possibility is that, as cjenrick mentioned in post #6, you could have a speaker with a shifted gap/magnet. If that is the case, the cone will appear to be frozen if you try to move it with your fingers. It may still make sound, but not much because of it's limited movement, which is much like what you are describing. You could remove the speaker and see if the cone moves freely. You could also try a known good speaker. Just remove the wires from the existing speaker and use clip leads to temporarily hook up another one.

[cjenrick]

this might be a good time to rip out the plastic input jack and attach loose leads to a switchraft 12A jack, about $2.50 from digikey or your favorite vendor, it can be a tough fit depending on the type of jack PV used, the ones that sit lower are the toughest, the standard Cliff jacks are cake, you can file the hole upwards if you need more room, the washer will cover up the daylight, we solder the leads from the bottom of the board, trim the tinned wires to pc board thickness so they do not protrude past the top of the board and contact the jack, trimming with dikes will leave a burr. put a few layers of black tape over the holes to make sure there will be no contact.

if you plan on keeping the amp yourself and do not use the effects loop, you can solder the tabs for hot and gnd on the send/return or any preamp in/out jacks to their respective shorting contacts to insure that there will be no future problems of vol loss. you can also just tack on jumpers on the bottom of the board if you ever want to remove them to sell the amp.

i would resolder all the pots as long as the board is out, sometimes the solder does not wick all the way up into the holes which can make a weak joint, they have to set the heat on the solder tank so that it does not cook or warp the pc board. this can mean the parts with the most metal do not get up to optimum solder temp to get a good flow,

the speaker idea is a long shot but it did happen to me with an Altec 417 8C that dropped off the tailgate,

if you put the board back in and still have problems, tap on the board at various locations with some chop sticks or a pencil with the amp on and listen for vol changes, gently press down on the board to see if you might have some broken traces. a lot of times, these problems are mechanical in nature, especially with PV who uses pretty solid circuits.