Egnater ie4 has an intermittent "blatty" gated sound on Ch3. Help please

[rosssurf]

I have a '96 silver faced Egnater ie4. It has served me well in my band for several thousand gigs. Over the last year or so I occasionally get a "blatty" gated sound on Ch3. I would often open it and poke around a bit but have never been able to touch anywhere that was evidence of a bad joint or anything. Can someone steer me where toward check voltages or something else to help diagnose this problem? Also if anyone has a schematic I would appreciate it.
For the record. I have read and understand all the safety procedures and have been dabbling with pedal builds and some minor mods in my older Fender amp on and off for many years, but I am by no means a tech or truly understand amplifier circuitry. My knowledge is limited to small paint by numbers kind of fixes and mods.
I have checked /swapped known good tubes and it does not remedy the situation. No caps appear to blistered or leaking. I do not see any obviously bad joints anywhere.

Of note: I did have Jeff Bober add (from Budda at the time) a 3 position toggle to ch1 to make the bass switchable so when I switch between my strat and Les Paul I can attenuate or boost the bass as needed. That as well as the other two channels function fine. The issue is only intermittent on the 3rd channel.

Thank you in advance.
Ross T
Towson, Md

[Dave H]

Egnator ie4 schematic.zip

[rosssurf]

Thanks Dave for the schematic. Let me know if you have any suggestion.

[Jazz P Bass]

If you have a spare 12AX7 try easter egging it one socket at a time.

[rosssurf]

Thank you Jazz P Bass. At least however I have tried that repeatedly with known perfectly functioning 12ax7's

[The Dude]

If you've tried tube substitution, I'd next check your supplies for ripple.

[Mick Bailey]

When I get amps with intermittent problems I try to recreate the problem on the bench. Trying to diagnose a fault with a working amp is very difficult and you can read all the voltages and they will be fine. Intermittent faults are usually (though not always) either mechanically induced (bad joints, dirty pots, bad switching contacts in sockets, etc) or thermal. So think of the conditions that cause the fault - is it at a certain volume, or only when the amp is up to a good temperature? Does it do it form cold sometimes, or is it truly random.

To eliminate the possibility of a thermal fault you need to get the amp up to a good temperature while a signal is going through it. A hairdryer, or very cautious use of a hot air gun (I also have a little infrared heater that's also used for craft activities) can reveal a faulty component. Pay particular attention to the optical switches - those 6-pin ICs. You want to get a good heat-soak on the board. If it shows a fault under those conditions then you can let it cool down until it behaves and then target the heat in specific areas to help pinpoint it to a group of components. I then use a can of freeze spray to target individual components. Get the amp in failure mode and then give a quick blast to each component until you find one that's failing.

There are plenty of connectors in this amp. You say you've poked around a bit, but did you take an analytical approach or just poke at random? I would mark each one I tested and make sure I wiggled and pulled on each termination to make sure it was good.