Princeton Reverb RI speaker jack question

[Randall]

http://support.fender.com/schematics..._schematic.pdf

Amp was cutting out and sounding bad intermittently. I quickly found that if I moved the speaker plug it would cut out. Cleaned all jacks and speaker plug. Then I found if I applied a bit of pressure around the speaker jacks, I could get it to sputter and act as described. I torqued down the jacks and it went away... for a while. So I pulled the jacks to have a look. I found that when off the chassis all is well, but if the threads of the int speaker jack touch the chassis, sound drops and it gets dirty. Can someone explain this, if the jack is fastened to the chassis why would this be the case?

I temp snipped the black wire from the jack to see if maybe I could find a partial short in the switching jack, but no. I must also say there is a homemade DI mod built into the cab with a sheilded wire running up to the ext speaker jack to a pair of resistors between the + and - lugs. It looks well done with an audio transformer and XLR in a box in the cab.

I am missing something here.

[Tom Phillips]

The PR RI uses original style open frame jacks wired in the usual Fender way with a shorting bar on the main "SPKR." output jack. There should be no problem with the jacks touching the chassis. Verify that the shorting bar on the "SPKR." jack is working properly. If it is bent such that it isn't opening properly when a jack is inserted that could cause the problem that you are describing. When you did your touch the threads to the chassis test you were plugged into the "SPKR." jack, not the "EXT. SPKR." jack. Correct?

[Enzo]

I occasionally found the shorting contact in the MAIN jack was too close so the plug didn't reliably push the tip contact aside far enough. Look closely at the cutout contact blade. Touching anything while plugged in? I usually could grasp the plug and flex it side to side and make the sound come and go. A little judicious bending and all is well. I had to do any number of warranty repairs on this on new from the box amps.

[Randall]

Nope, it's not the shorting leaf, that was the first thing I checked.

[g1]

Have you tried some other speaker cable?

[Randall]

Yes, I've tried a different speaker and cable.

[Randall]

OK. I disconnected the DI from EXT SKR jack, no diff. connected the out put transformer directly to a fresh jack with no switching, same thing. Here's where it gets interesting, I disconnected the neg.feedback wire and issue goes away. R10, R7, and R4 measure close enough in circuit, and I replaced C1 and V3, still does it with the NF wire connected, does not with it disconnected.

So am I chasing something that is not there? Is this actually how it is supposed to work? A drop in volume with NF in circuit? Could it be that the original jacks were not getting a good chassis ground and was making a change in volume and crackling as the connection was changing? I admit I do not have a strong comprehension of NF circuits. I don't see how a chassis ground or not can make a difference to a NF circuit when NF is connected to the hot side of the OT, and the SPKR jack is connected to the OT neg side with the black wire.

[Tom Phillips]

...So am I chasing something that is not there? Is this actually how it is supposed to work? A drop in volume with NF in circuit? Could it be that the original jacks were not getting a good chassis ground and was making a change in volume and crackling as the connection was changing?...

Yes and yes. Your original description gave the impression that there was a large volume drop when you touched the jack to chassis ground. However, if the drop was noticeable but not huge, then that is normal. If the ground was intermittent then it could cause crackeling.

...I admit I do not have a strong comprehension of NF circuits. I don't see how a chassis ground or not can make a difference to a NF circuit when NF is connected to the hot side of the OT, and the SPKR jack is connected to the OT neg side with the black wire.

The output transformer secondary winding connection to chassis ground is required to provide the ground reference for the negative feedback signal with respect to the rest of the amp circuit. Without that connection only the speaker itself has a complete connection and the negative feedback is disconnected.

[Randall]

Thank you Tom. I think I actually fixed it two days ago!