1976 Fender Twin Reverb Rebuild Problem

[keithb7]

Hi folks, I had a local musician bring me a 1976 Twin Reverb that was heavily modded. The owner wanted all the mods out and returned to a non-Master volume Twin Reverb. I picked the AA270 circuit and got to work.
After everything was re-wired, I checked all my work and powered up the amp. I used all original AA270 spec parts that needed replacing.The voltages appear to be good to me. I am no Pro, I do this as a hobby. Channel 1 seems to work fine. Channel 2, reverb channel is not right. For some reason the reverb pot, is also acting as a volume control. Reverb effect is there and seems to be working, but you have to turn reverb to max to get any considerable guitar volume through the amp. The reverb is also a little buzzy. I have double and triple checked all my connections against the AA270 layout and compared what I can, to the schematic. I suspect I may have a grounding issue. I lifted the board tonight to look at the underside with a mirror, for any possible hidden connections. There does not appear to be any. Tubes seem to test fine. I feel I am close to solving this, but just can't seem to find the problem. maybe I have a wire mixed up somewhere. I just can't seem to spot it. A chopstick tap test turned up no suspect areas. I can post pics, close ups, and voltage measurements if needed for anyone to review. I am about stumped. Thanks for any tips. - Keith

[52 Bill]

For some reason the reverb pot, is also acting as a volume control.

The reverb pot is supposed to be isolated from the input grid of mixer stage by a 470K resistor. If the wiring is incorrect or if the 470K resistor is the wrong value, turning the reverb control down will also kill the straight signal.

[keithb7]

The reverb pot is supposed to be isolated from the input grid of mixer stage by a 470K resistor. If the wiring is incorrect or if the 470K resistor is the wrong value, turning the reverb control down will also kill the straight signal.

Bill thanks for your comment. I put a .470K resistor in there, not 470K. I will swap it out now. Missed the decimal on the digital multimeter when I double checked it.

[Enzo]

A .470k??? You mean a 470 ohm?

[52 Bill]

A .470k??? You mean a 470 ohm?

I remember when I was a kid looking at TV schematics and seeing things marked that way, all resistances were like a .5M or a .25M. And caps were all marked in Picofarads, like 560000pf.

[keithb7]

Enzo, my DMM reads 470 ohm and displays it at .470K ohm. It reads 470K ohm, and the display shows .470M ohm
As you can see, missing the decimal can easily happen.

[Enzo]

That is unusual. Does your meter have switchable ranges?

[J M Fahey]

Maybe itīs a cheesy way to display many values in an autoranging DMM
I always preferred to set manually what I want to read, *instantly* and without thinking I detect a way too high value (it over ranges) or way too low, which is self evident because of the very small value shown, often single digit, while an autoranging one may easily cheat me showing, say,"170" and if I am expecting something between 150 and 200V I think "fine, letīs check something else" while it my actually be 170mV .
Just one made up example, of course.

[The Dude]

Agreed, Juan. I'm not a fan of the auto-ranging meters.

[keithb7]

Thanks folks. The amp turned out great. I had one other problem to solve too, but settled it elsewhere. I had a 100K resistor where I was supposed to have a 100 ohm. In the long tail PI circuit. I will be paying closer attention in the future. It can get dicey, as a hobbyist with a bag of resistors, caps, pots, etc. Things can get mixed up. I promise to be more diligent. My meter is an AMPROBE AM-240. It does have a manual range setting. I will consider using as such going forward.