How to replace pots on vintage silverface amp

[jjk1407]

Hi,

A tone pot on my 73 Twin Reverb is pretty scratchy. Tried Deoxit but no luck. So I bought a new pot thinking of replacing it. But seems there are 2 nuts, one on the face plate (#2 in the pic below), and the other one on the brass plate (#1 in the pic below)?

Do I have to remove the brass plate in order to replace the pot? Is there an easier way?

Thanks,
Jay

 

[Lowerleftcoast]

Is the other nut in between the brass plate and the chassis?


Are you sure the scratchy noise is not DC voltage on the pot?

 

[jjk1407]

Hmm... seems there's a nut between the brass plate and the chassis, guess that's how they kept all the pot lugs facing up? e.g. first fasten the pot to the brass plate then insert the plate to the chassis?

I thought it's the pot, since only this particular pot gives that scratchy sound when being turned... how can I know if it's due to the DC voltage?

 

[Lowerleftcoast]

DC is supposed to be blocked by the tone caps. (IDK which tone pot you are dealing with.) On the board are 250pF, .047uF and a .1uF tone caps. One (or more) of the caps may be leaking DC. Use all safety protocols and meter for VDC on the scratchy pot. Just a few volts DC can make it scratchy.

The 250pF is highly suspect.

 

[Intubator]

I'd put a meter on that pot before pulling it out. With the meter leads on and set to the value of the pot, If you slowly rotate the shaft the values will rise appropriately and then bottom out or spike when you hit the flat spot/short if that's the issue. As others have said maybe a bad cap.

 

[jjk1407]

It's the bass tone pot for the vibrator channel. So I guess the 0.1uF one might be the issue?

 

[Phrygian77]

They are all connected together through the tone stack pots, so DC from one can show up on another. If you're asking these questions here, then I personally think you should probably take it to a tech.

 

[jjk1407]

Thanks for all the helps folks. Since I ordered a new pot anyways, I disconnected the old one and soldered the new one in (but certainly did not pull the old pot out...). Now everything works perfectly for over an hour. Also measured the DC on the pot, it's around 17mV. So I guess it's an evidence that the issue is only on the pot itself? If so... looks like I still need some suggestions on how to replace the pot on the chassis, or maybe I do need to take it to a tech?

 

[Lowerleftcoast]

Assuming there is a nut between the chassis and the brass plate, is there enough room to unscrew the pot from the inside of the chassis? You would have to be careful not to lose the nut. Maybe follow the shaft out with a straw to keep the nut in the proper location.

 

[Dacious]

That chassis grounding plate should be hard against the chassis. You should just be able to undo the outside but and it will come out.

 

[jjk1407]

Removed the outside nut already (the one on the chassis) and the pot does not budge... checked other pots, same. Maybe I need to remove the brass plate?

 

[milocj]

I don't recall there being a gap between the grounding plate and the inside face of the chassis on any of my Fenders and that is what would happen if they placed a nut on them. I do recall something about some silverface amps needing to remove the faceplate to replace pots but I can't find any reference to that now. Seems like that would require the actual chassis to be dimpled to get a flush outside surface, though

 

[Dacious]

Undo all the nuts and see if you have rearwards play. It may just be the hole is interference on the pot base. When they tightened the nut it pulled the pot into the hole in the brass. Or its corrosion. The brass plate can corrode where it contacts the chassis.

Maybe just give the pot shaft a tap with a bit of wood. Not a 15lb chrome plated mallet, a small hammer.

On the 78 Vibrolux and 68 Vibrochamp I had the pots were loose as soon as you un-nipped the outside but.

 

[jjk1407]

Alright... I got it done. It's interesting to think that I had to spend this much effort to deal with such a "genius" manufacturing design 50yrs back...

Just for future reference, first of all there are indeed nuts between the brass grounding plate and the chassis on my amp (The amp is a 73 Twin Reverb, serial number 62xxx). The gap between the brass plate and the chassis is... well, just as wide as the thickness of those nuts. Genius design, everything is seamlessly put together like a tank.

The first step I took was to widen the gap, by pushing the brass plate back a little bit and then inserted a hex key there to keep it that way.

To loosen the nut I put a long nail against the opening behind the pot (blue circle below), and used a hammer to make the pot rotate counterclockwise, about 60 degrese. Then I pulled the pot back clockwise, back and forth until it's loosened. Now with another smaller hex key reaching into the gap I was able to rotate the nut a little bit every time, and eventually got the pot out. Key thing to remember, as someone pointed out earlier, is to use a stick (I used another hex key) to go with the shaft when removing the pot, so that the nut is not dropped. The rest is pretty straightforward - just reverse the steps to install a new one.

Now I have a all-but-one-pot-original 73 TR with no scratchy sound! Thanks everyone for your help!

 

[corliss1]

I've never seen a factory fender with pots like that. It's always just the brass pressed up against the chassis face. I'm guessing this was a later change by someone in an attempt to do...........................something.

Does anything else looked disturbed in there?

 

[Boreas]

Alright... I got it done. It's interesting to think that I had to spend this much effort to deal with such a "genius" manufacturing design 50yrs back...

Just for future reference, first of all there are indeed nuts between the brass grounding plate and the chassis on my amp (The amp is a 73 Twin Reverb, serial number 62xxx). The gap between the brass plate and the chassis is... well, just as wide as the thickness of those nuts. Genius design, everything is seamlessly put together like a tank.

View attachment 882155

The first step I took was to widen the gap, by pushing the brass plate back a little bit and then inserted a hex key there to keep it that way.

To loosen the nut I put a long nail against the opening behind the pot (blue circle below), and used a hammer to make the pot rotate counterclockwise, about 60 degrese. Then I pulled the pot back clockwise, back and forth until it's loosened. Now with another smaller hex key reaching into the gap I was able to rotate the nut a little bit every time, and eventually got the pot out. Key thing to remember, as someone pointed out earlier, is to use a stick (I used another hex key) to go with the shaft when removing the pot, so that the nut is not dropped. The rest is pretty straightforward - just reverse the steps to install a new one.

View attachment 882160

Now I have a all-but-one-pot-original 73 TR with no scratchy sound! Thanks everyone for your help!

You schooled us - Thank YOU!

BTW, what actually holds the brass plate? Just the pots? When you removed ALL of the face nuts, was there anything keeping the pots from being pushed out the back WITH the plate? I am trying to figure out how the assembly was ever installed.

 

[jjk1407]

Hmm.. nope, a

I've never seen a factory fender with pots like that. It's always just the brass pressed up against the chassis face. I'm guessing this was a later change by someone in an attempt to do...........................something.

Does anything else looked disturbed in there?

Hmm.. nope, aside from those interesting nuts everything looks quite normal. Can't find any sign of modification. See pic below, if anyone see anything suspicious...

 

[jjk1407]

You schooled us - Thank YOU!

BTW, what actually holds the brass plate? Just the pots? When you removed ALL of the face nuts, was there anything keeping the pots from being pushed out the back WITH the plate? I am trying to figure out how the assembly was ever installed.

Tried that too. Even after everything removed the plate doesn't budge. Guess it's soldered to the chassis. But I didn't push it really hard, afraid of breaking any grounding point.

My guess is that they mounted all the pots to the brass plate first, then soldered it on to the chassis, then did all the wiring etc.

 

[Boreas]

Tried that too. Even after everything removed the plate doesn't budge. Guess it's soldered to the chassis. But I didn't push it really hard, afraid of breaking any grounding point.

My guess is that they mounted all the pots to the brass plate first, then soldered it on to the chassis, then did all the wiring etc.

I'll bet that procedure didn't last long!

 

[wangdaning]

I wonder if it was a batch thing where they ended up with a batch of pots where the shaft was too long and improvised.