Deluxe reverb channel 2 distorted

[delray]

I picked up a 70s deluxe reverb locally today. It has had a three prong cord added so I assume it has been recapped or maintenance at some point. The first channel sounds great, very clean all the way up to the too. Channel 2 plays, but it breaks up, especially on the lower strings. This happens on pretty much all the volume levels. I had all of the tubes tested and everything came out good… I have replaced V2 and the four with known good tubes just to check it and it didn’t make any difference. I will probably start with cleaning all the tube sockets, checking for cold, solder, joint, etc.. Could bad capacitors causing this issue? Could it be an issue issue with the Reverb tank? The noise happens Regardless of the reverb being on or off, same with the vibrato.


Could bad capacitors, cause this sort of breakup noise? I don’t hear any hum in the amp. 

[Latole]

First ; show schematic , witch amp model ? AB868, A1270 , A1172 .......

 " It has had a three prong cord added so I assume it has been recapped or maintenance at some point "
 - No you can't say that. You have to check

Break up could be a low voltage issue on preamp tubes . Check filter caps in the "dog house".
Check and replace , if needed, tube's plates resistors

[pdf64]

I picked up a 70s deluxe reverb locally today. It has had a three prong cord added so I assume it has been recapped or maintenance at some point...

From the documentation, Fender look to have fitted 3 prong mains plugs from 1969, eg https://el34world.com/charts/Schematics/files/Fender/Fender_pro_reverb_aa1069_schem.pdf

[mresistor]

Yes starting with the A1270 DR they installed power cord with ground. (3 prong)   So either the cord was bad and was replaced or his amp is pre-70s silverface.


Can't assume anything. Pull the chassis out of the cabinet.  Take the cover off the doghouse and see if the main filter capacitors have been replaced.  Do you have an amp caddy? 
Have any of the cathode capacitors been replaced?   Check the resistance of all the plate resistors. If out of tolerance replace them.  I like to use 1w metal film resistors there.


Be sure to check the ground connections to the brass plate underneath the back of the pots, I've seen alot of them with hairline fractures needed solder reflowed.
Yes capacitors can go bad.


Can you post up some pictures of the amp?  It would be helpful to actually see it.


Here's the A1270 schematic https://el34world.com/charts/Schematics/files/Fender/Fender_deluxereverb_a1270_schem.pdf


Here's the A1172 schematic https://el34world.com/charts/Schematics/files/Fender/Fender_deluxereverb_a1172.pdf

[mresistor]

If you think it might be the reverb tank then disconnect it and see if the problem resolves.

[delray]

I think its a 1973 based on the date code on the transformer.

[delray]

Replaced the 22uf power caps with the factory spec of 16uf. The old caps seemed to test ok but I have no idea on their age. That didnt seem to make any difference on my channel 2 sound issues. I also cleaned all of the tube sockets and checked as best as I can for cold solder joints, etc.

Some of the coupling caps seem fairly old but spot checking them in circuit with the tester they seem to be ok. Any thoughts on my next steps?

I can't attach an m4a file but here is a link with a demo of the sound. https://mobolr.com/amps/deluxrev.m4a

[AlNewman]

Silly question, but have you cleaned all the jacks and pots? 

[delray]

Silly question, but have you cleaned all the jacks and pots?

Yes.

[Latole]

Show schematic with preamps voltage you read

[mresistor]

So if the reverb/trem are both working, then the problem is isolated to the V2 and it's circuitry, the tone stack and V4a.


You say that 10pf ceramic cap reads ok?


And please do show the voltages for both triodes of V2 and V4A.  thanks. 

[delray]

V2 pin1 -224
V2 pin6 -230
(similar on v1)

v4 125 and 129

[pdf64]

So what's the V4 pin 3&8 cathode voltage and resistance?
And the V2 pin6 cathode voltage and resistance?

It's best not to use the '-' symbol unless indicating negative voltage.

[Latole]

V2 pin1 -224
V2 pin6 -230
(similar on v1)

v4 125 and 129

V2 pin 1  -224 volts !!!!!
V4 +125 and +129     

[delray]

Sorry about the dash. I didnt mean negative. While working on answering the questions i found a resistor that had broken off its solder joint. I was hoping this was the magic bullet but sadly no. I will remeasure the values with that resistor in place.

[Latole]

Those tremolo resistors are burn . 10 meg and 100 k