Fender Deluxe 112 Plus Buzzing/Squeal in Reverb

[misterc57]

I am trying to fix a Deluxe 112 Plus solid state combo amp. It makes a lot of buzzing noise when the reverb pot is turned above zero.

With the reverb pan connected, and rotating the reverb pot from zero to 10 the following happens;

Zero no noise, 1 to 3 is a squeal noise, then a thump sound, 4 to 7 buzzing, then a thump sound, 8 and above louder squeal.

I have cleaned the pot. I have measured about 0 to 48K resistance to ground at R57 when rotating the reverb pot.

I suspect the pot but I think it is possible there could be other issues.

The reverb pan measures 50 ohms at the input, 210 ohms at the output.

With the reverb tank disconnected I get only buzzing noise from 1 to 10 on the pot, getting louder as I go up the scale.

I do not have a spare pot that fits this amp but I do have another 48K pot I could use as a test. I was thinking of running jumper wires to the new pot (leaving the old pot in place set at zero).
Does this test make sense? If so I am not 100% sure where to connect the pins of the new pot. I suspect one wire to ground, one to R57, one to C31. If those are correct I am not sure what pins on the pot go where.

Any guidance and suggestions are greatly appreciated. Schematic attached.

Thanks! MC

[nickb]

With a steady increase in buzz with no pan I would think the pot and recovery stage are probably OK.

Reconnect just the output white (P3) of the pan, leave the red input cable off. What happens?

Disconnect the white cable and short the center to outer. What happens?

[g1]

That schematic link doesn't work. But that's a good thing because it's for a Princeton .
Here's the Deluxe 112 plus:

[misterc57]

Sorry I uploaded the wrong schematic from my pc. Thanks for the correction.

BTW, all the noise will happen even with the volume pot set to zero.


>>>Reconnect just the output white (P3) of the pan, leave the red input cable off. What happens?

Buzz and squeal as I rotate the reverb pot.


>>>Disconnect the white cable and short the center to outer. What happens?

Buzz as I rotate the reverb pot. But not as loud as when the cable was connected to the pan.

[nickb]

Earlier you said "zero-no noise". Has this changed now?

Seems either the pan is bad or it's picking up something. Check the continuity between the ground on the pan output connector and the pan chassis.
Do you have another pan to try?

[misterc57]

Zero on the reverb pot = no noise

Nothing has changed. I am talking about two different pots. Just adding that the noise (buzz and squeal) happens even if the volume pot is at zero.

Continuity between the ground on the pan output connector and the pan chassis = open. Same with the input ground connector.

I do not think I have another working pan to try but I will look around.

Thanks!

[bsco]

I am no expert here but isn't one of the reverb pan connectors supposed to be grounded to the metal pan itself?? and the other one isolated???

[52 Bill]

I am no expert here but isn't one of the reverb pan connectors supposed to be grounded to the metal pan itself?? and the other one isolated???

Yes, you are correct the input must be isolated and the output must be grounded to the case.

The input transducer is connected to the feedback loop of the driver chip, so it must not be connected to ground. Be sure that you have the tank plugged in correctly.

[g1]

Do you know if the amp has ever worked properly with this exact tank? Does the tank say 8EB2C1B on it?

[misterc57]

The tank is good. I took the tank out and placed it inside another Fender SS amp that is missing its tank. The reverb worked perfectly. It looks like a 8EB2C1B, hard to read.

The cables going to the reverb pan also test good.

So we can rule out the tank.

The history on this amp...

I picked up this amp knowing it was making noise. The prev owner said it worked great for a long time, then would only make buzzing noises. I cleaned all the pots (they were pretty dirty) and jacks. Input jack one does not work, causes noise in the amp when a cable is inserted. Input jack 2 works fine. I pulled the board out and all the solder joints underneath look good. So currently the amp works fine using input jack 2, and keeping the reverb at zero. As soon as the reverb is turned up all the noises start (squeals and buzzing).

I am still suspicious of the reverb pot. Is there a way to bypass the pot and test the reverb full on?

Thanks!

[misterc57]

Off topic question... why does this amp use a 9 pin input jack. Isn't that a stereo jack?

[bsco]

I think it is a stereo jack with built in switching....somebody out there in repair land will correct me if I am wrong...I could be wrong here but it seems like you might have two problems?? I think the noisy input jack is just that.....The reverb squealing when you turn up the reverb control sounds like what was already stated....it must be picking up something......Just for the fun of it....did you try the tank outside of the amp??? maybe with longer rca patch cables so it is away from the chassis and speaker?? It is just an idea which would be easy to try.....I am sure you will get it figured out...the guys on the forum here are great...and they are very experienced................and helpful...

[misterc57]

I think a bad reverb pot could cause the noise I am hearing. I'd like to rule out the pot. If I can bypass the pot and run the reverb full on, it will tell me if the pot is the problem or not.

I believe the pot connects C31, R57 and ground. I need guidance on how to bypass. The last thing I want to do is jump something wrong and cause more problems.

Yes, we will get it figured out. The folks on here are very helpful!

As far as reverb and chassis location I have been testing with the chassis out of the cabinet and the reverb pan still in the cabinet.

Thanks

[52 Bill]

Let me understand the symptoms better. I thought that you had stated that with the tank unhooked, you can turn up the reverb control and not get the feedback buzzing noise. Is this correct?

[bsco]

If you had the tank disconnected and rotated your reverb control up and down and the only thing you were getting was some noise that varied with the rotation of the control I would tend to think that the control was fine.as the control is located on the output side of the tank........which tends to lead me to a problem on the input side of the tank......the driver op-amp or it's surrounding components.....you said you had the chassis taken out of the cabinet but the tank was in the cabinet under the speaker...just take the tank out and try it again just to see what happens....electronic problems can sometimes be hard to figure out...I usually try the easiest things first and this one is very easy....it may not make any difference but it is best to eliminate as much as you can....