J99 not functioning on gain steps 11 and 12

[andywho]

the problem I was having the the J99 referenced in this thread...

http://seventhcirclestudios.com/SCA/phpBB/viewtopic.php?t=1212&highlight=

...has now morphed into the J99 passing a distorted signal on gain steps 11 and 12. on step 11 I also get some kind of slowly oscillating interference as well, and flipping the phase switch on those two gain steps makes things even wackier.

I checked and reflowed all the solder on the grayhill switch and the resistors near it, with no luck. all other joints look ok to my eye, but I could be missing something.

is this some kind of short? any help would be greatly appreciated--this gremlin is getting me down. I'm at a loss for how to diagnose the problem.

btw, flipped C12 on my N72's and it worked like a charm.

[andywho]

after all kinds of searching and troubleshooting, I came to realize that BOTH my J99 modules are exhibiting the exact same symptoms (gain switch at 12 creates nasty noises), and furthermore, the following applies to both as well:

- when I plug the modules into the wiring harness in spots 1 & 2 or 2 & 3, they work perfectly

- when I get further out (~4+), gain switch at 12 goes wonky

- this seems to occur whether or not any other modules are plugged in as well

- I have two wiring harnesses (an AMP one I made, and one of tim's), and both the J99's do the same thing with both harnesses

- I tried every combination of my 990c's and ruled them out. also, I usually run them at +/-22V, but just to rule things out, I tried running them at +/-18V and +/-24 -- same thing happens

also, I realize I could cross my fingers and mount the J99's leftmost in the chassis (and/or never switch the gain to 12), but I'd rather figure this one out and avoid future surprises.

any help would be greatly appreciated...this one is really leaving me scratching my head.

[tpryan]

Sounds like instability and oscillation. You should:

*make sure the compensation capacitors (C6 and C12) are the correct value
*trim all component and transformer leads properly
*deflux the board
*make sure the rotary switch bushings are well-grounded
*test with the inputs and outputs properly terminated

[andywho]

quick update: the problem seems to have solved itself after putting all eight channels in the chassis.