MOSFET reverb buzz

[Ebwelds]

I got a lead 100 mosfet. The reverb tank had a broken piece so I ordered a MOD replacement. So plugged it in. And I’m getting a terrible hum when I turn the reverb knob up. It doesn’t seem to be pushing signal through the tank. If I touch the springs I can clearly hear that in the speakers. If I move the tank away from the amp the humming goes away. Like maybe some feedback from the amp when the tank gets to close. Any help would be appreciated or anything about where to look to see if I have a bad part that is blocking signal from going to the reverb tank. Bad cap or ground maybe? I noticed that it only makes noise when something is plugged into the input.

 

[wakjob]

Good luck...there's a LOT involved in that reverb circuit.

Start with the simple stuff like replacing the tank cables...
or just making sure you don't have them reversed?

You'll have to probe around that part of IC4 and all the parts connected to it.

Flip the board over and look for cold solder joints/reflow.

 

[Mitchell Pearrow]

Good luck...there's a LOT involved in that reverb circuit.

Start with the simple stuff like replacing the tank cables...
or just making sure you don't have them reversed?

You'll have to probe around that part of IC4 and all the parts connected to it.

Flip the board over and look for cold solder joints/reflow.

Plus 1 on checking that the leads are not reversed, the reason I know this, is I reversed them on a
AVT50H, and got the noise, other than that I have no clue, as I keep the reverb off on mine.
Cheers Mitch

 

[ampeq]

If all seems to be good with the cir-brd, you might try a ground and / or wrap the tank with a copper shield. Good luck.

 

[anitoli]

Check the supply voltages, make sure you are getting +- 15v to the IC and +- 34 to the driver transistors.

 

[Ebwelds]

Check the supply voltages, make sure you are getting +- 15v to the IC and +- 34 to the driver transistors.

I’m getting +15.8v and -16.2. I think I was running somewhere around 34-37v for that part. ( marked in yellow )
I measured the ground on the send and return leads. Return 0v. Send -17mV. Which makes sense because of the resister in series after that goes to 0v line. ( red marks )
I was thinking that something may be up with ( what I think is a feedback loop) marked in blue because it doesn’t hum until something touches the ground on the input jack.

 

[anitoli]

What is the model number of that amp?

 

[anitoli]

On the blue loop a direct ground connection there would kill the signal, but the arrow at JS1 usually means a breakable connection, how is the input jack configured?

 

[Ebwelds]

It’sa lead 100 mosfet 3210 from 1989 according to the inspection sticker

 

[Ebwelds]

 

[Ebwelds]

So would that blue line be like a silence loop that grounds any signal when the input isn’t plugged in? That could explain how there’s no noise when there isn’t a jack plugged in. But as soon as it is it breaks that silence circuit and the hum comes through...

 

[Mitchell Pearrow]

So would that blue line be like a silence loop that grounds any signal when the input isn’t plugged in? That could explain how there’s no noise when there isn’t a jack plugged in. But as soon as it is it breaks that silence circuit and the hum comes through...

Are you running a foot switch ?
If nothing is plugged into the footswitch the channels will bleed together, not sure if that has anything to do with what you’re hearing.

 

[mickeydg5]

Wondering, which MOD model tank did you buy?

 

[South Park]

Try to find what wire to the tank is causing the hum . Disconnect one at a time

 

[Ebwelds]

Wondering, which MOD model tank did you buy?

i got a direct replacement to the one I pulled out. 8DB2C1D

Try to find what wire to the tank is causing the hum . Disconnect one at a time

I don’t believe the wire is causing the hum. Although I most definitely could be wrong about that. But I don’t believe so.

 

[mickeydg5]

Two threads, one tank.

 

[mickeydg5]

Does it do the same with everything mounted correctly, amplifier box closed up properly and then plug into the input with volume?

 

[Ebwelds]

Are you running a foot switch ?
If nothing is plugged into the footswitch the channels will bleed together, not sure if that has anything to do with what you’re hearing.

i don’t believe so. I’m getting this issue with the master volume on 0.

 

[Ebwelds]

Does it do the same with everything mounted correctly, amplifier box closed up properly and then plug into the input with volume?

yes

 

[Emtbreid]

Two threads, one tank.

I see what you did there..