Blues Deluxe Modern SS Reverb Problem

[captntasty]

What would cause a reverb circuit in a modern Fender amp (specifically a 90's Blues Deluxe - not reissue but the circuits are pretty much the same) to oscillate and feed back on itself? I've tried everything I can think of (with one exception) - different tanks, tested every component (resistors, opamps, diodes, etc) remotely connected and even replaced a good many of them as a shot in the dark. The one thing I haven't tried is replacing the 9 pin power amp out jack as I had to order a few - I seem to use a lot of those! When I jump from the preamp out to the power amp in it has an effect on the ringing and feedback - oscillation and feedback are reduced when jumpered but not eliminated. I'll be putting that jack in tomorrow but after that I'm stumped. Could a bad switching jack create a loop and cause the reverb driver to become an oscillator?

http://www.blueguitar.org/new/schem/...ues_deluxe.gif

[g1]

Have you tried isolating the tank from the cabinet? These reverb feedback/oscillation issues are often physical and related to the vibrations from the cab. or proximity to the speakers. Get the tank away from the rest of the amp and see what happens.

[captntasty]

Good point - that was my first thought when I became aware of it... for the sake of brevity I guess I didn't explain the symptom very well. The chassis and tank are out of the cabinet... this is one I had to replace the low voltage dropping resistors 470R 5w which is common in these as they run hot. The traces had to be reconstructed they were so fried. Everything was fine until I realized the reverb was no longer working - one side of the tank was shot so I replaced it with a new MOD tank same model except for dwell which is medium instead of long - that was all I could get but the input and output impedances are the same. When I fired it up with the new tank is when the reverb problem became apparent. So I took it all out... there is a constant tone (no scope but I would say it is sinusoidal - very smooth medium pitch frequency) running through the reverb circuit - I can hear it even with volume down - most pronounced in drive mode (I have confirmed all the switching is functioning). When the reverb control is turned up with any volume up (no signal from an instrument) the tone becomes audible starts to feedback, I can actully feel it through the reverb springs... it feeds back on itself, the same as a guitar held up to the speaker does. If the original oscillation could be eliminated I think that would solve the problem. I tried another tank, same but long decay with same result. I also tried a completely different spec tank and that did tame the problem but due to the input/output specs there was very low output from the reverb circuit.... so the problem is not the tank or vibrations but the fact that the reverb driver or recovery circuits has become an oscillator... I'm stumped! I've searched for similar problems throughout the usual sites and found one post that described a blues deville(pretty much identical circuit) with the same problem but the thread died without much discussion or resolution. Any ideas from you guys with more experience to point me in the right direction would be greatly appreciated...

[Jazz P Bass]

You need to isolate where the oscillation is coming from.
What happens when you disconnect the input cable to the tank?
What happens when you disconnect the output cable to the tank?

[captntasty]

I don't have the amp near but if memory serves me correctly, the ringing tone that causes the feedback ceases when the input to the tank is disconnected, indicating it is somewhere at or before the reverb driver. I'll have to double check tomorrow but I don't think removing the output from the tank stops the ringing oscillation but does stop the feedback created by it. I will isolate it better tomorrow. I will be putting in the new power amp in jack tomorrow and hoping that may do it as the jack is connected directly before the reverb driver and the phase driver section. I guess my original question was could a defective jack with multiple connections in this circuit cause the driver section to become an oscillator? Then again, am I over thinking this? Thanks for the replies.. until tomorrow.

[captntasty]

I had that last bit backward... removing the tank output stops the ringing... which brought me to a fix... when I was checking what happens removing each RCA from the tank I noticed that when the output was removed enough to disconnect the ground/shield the oscillation stopped - snipped the shield on the board and everything works as it should and not a big difference in noise without the shield. So final analysis... I ordered the wrong tank. I ordered the same code as the tank that was in there which was apparently the incorrect one... I used a 4BB2A1B, the original would have been a 4BB3C1B A = Input Grounded / Output Grounded C = Input Insulated / Output Grounded. Live and Learn!... thanks all.

[captntasty]

And one final tidbit for reference on MOD reverb tanks... the input and output RCA jacks are mounted on a small PCB inside the tank. On the PCB may be a small solder blob that is a jumper - desolder that blob and the respective jack is now insulated! My 4BB2A1B is now a 4BB2C1B... and works properly in this circuit with the input shielded.

[Jazz P Bass]

Nice find!