Fender Deluxe Reverb AB763 reverb pot adding gain/mids?

[52 Bill]

mid 70s silverface, no master volume or boosts
unplugging the reverb tank and grounding the reverb return jack eliminates the problem...same as turning the reverb pot to zero

With all wires back to normal, pull out the reverb driver tube. What happens?

[acorkos]

With all wires back to normal, pull out the reverb driver tube. What happens?

no reverb, output sounds normal, like turning the reverb pot to zero

[Richard]

Is it possible that some of the reverb send signal is getting fed through, bypassing the reverb?

[52 Bill]

Is it possible that some of the reverb send signal is getting fed through, bypassing the reverb?

That's what we are trying find out.

Mid 70s Fenders sometimes had a real problem with lead dress. Or it could be another case of a conductive eyelet board.

Are the reverb send and return wires inside the chassis separated? Can you move the wires around so they are not close or are at 90 degree angles from each other? Can you post a photo of the chassis?

[Richard]

Mid 70s Fenders sometimes had a real problem with lead dress.

I heard they were banging out 20+ a day.

[acorkos]

could there be a problem in the vib chanel preamp? when I touch the input wire at the junction of the 68k resistors with a metal probe tip, there is VERY loud static noise, much less when I touch the same point on the normal channel

[52 Bill]

I don't see anything obvious in the photo that would be a problem. Try and pull the red reverb transformer wire away from the plate wire at pin 1 of V4. Make sure that the grounds on the reverb RCA jacks are all clean and tight.

Earlier on it was recommended to check the filter caps, did you do this?
Have you checked the tubes?

[acorkos]

I don't see anything obvious in the photo that would be a problem. Try and pull the red reverb transformer wire away from the plate wire at pin 1 of V4. Make sure that the grounds on the reverb RCA jacks are all clean and tight.

Earlier on it was recommended to check the filter caps, did you do this?
Have you checked the tubes?

I swapped in a new filter cap at the "D" location, made no difference. Swapping tubes made no difference either.

[52 Bill]

I swapped in a new filter cap at the "D" location, made no difference. Swapping tubes made no difference either.

Try the filter that feeds the reverb driver transformer.

[acorkos]

Try the filter that feeds the reverb driver transformer.

thanks, I swapped it as well, but it made no difference

how might a scope help me track down this problem?

[Jazz P Bass]

Quote:"the reverb pot adds gain/mids when turned up"

How far are you turning up the reverb control?

A lot of Fender reverbs get weird above 6.

Check the coils on the tank.
Are they flopping around?
If so, apply a small dab of glue to the side of the coil where it meets the laminated frame.

[52 Bill]

Well, you have proven that the problem is, that output signal from the reverb drive circuit is directly getting to the input of the reverb recovery circuit.

Scope the input of the recovery circuit and see if you can see the delayed and the non-delayed output together. Then see if you can find out where the non-delayed signal is coming from.

[acorkos]

Quote:"the reverb pot adds gain/mids when turned up"

How far are you turning up the reverb control?

A lot of Fender reverbs get weird above 6.

Check the coils on the tank.
Are they flopping around?
If so, apply a small dab of glue to the side of the coil where it meets the laminated frame.

I tried a new replacement tank...same problem
no reverb effect until about 2.5 on the pot, but the change in tone (mid boost) occurs at that point on the pot as well, and increases along w/overall volume.

[acorkos]

Scope the input of the recovery circuit and see if you can see the delayed and the non-delayed output together. Then see if you can find out where the non-delayed signal is coming from.

I will try this tonight

[acorkos]

solved


but i'm not sure why. I accidentally stumbled onto the solution when I tried subbing in a new reverb transformer. I unsoldered the 4 leads from the existing reverb tranny, and alligator clipped a replacement to the connection points....the black ground wire was clipped to the lip of the chassis instead of the solder point, a solder lug attached to the reverb return cable. And the reverb problem was solved. My first thought was a bad reverb tranny...I soldered in the replacement, but the problem persisted. I disconnected the leads one at a time and reconnected each one at a time via alligator clip leads. None made any difference until I got to the black ground wire. when the clip was attached to the original ground lug on the reverb return RCA jack, the problem persisted. When the clip was moved to a different ground point, problem solved. I eventually put the original tranny back in and moved the ground connection to the ground lug on the trem switch jack....problem solved. Something is up with the original ground lug on the reverb return RCA jack, but I don't know what....I tested for continuity between the lug and chassis, tested ok, I tested for no continuity between the tip and ground on that RCA jack, tested good. So it's fixed, but I can't explain why.