Intermittent normal output hi gain channel SWR Studio 220 Bass Amp

[ca7922303]

Have intermittent normal output on high gain channel only. Tried switching with low input jack and speaker Jack. No change. Any help appreciated.

[Jazz P Bass]

Trouble shoot the problem.
Where exactly is problem happening?
Preamp?
Output section?

[ca7922303]

Efx return and power amp in have normal output, so I'm gonna say preamp is the issue.

[ca7922303]

Also, preamp out from SWR to another amp produces same result: no high gain output. Let's call it no output because at this point it is not intermittent.

[Jon Snell]

Check the 12AX7, check the 100k resistor on pin 1, the 100p capacitor also from pin 1. Check the 1k5 on pin 3 and last but not least the 1M on pin 2.
You may find the 100k is open circuit, may even be a bad connection on the pins. Give the valve a wiggle first.

[J M Fahey]

That.
High input goes through an extra gainn stage, based on half 12AX7.
Besides "electronic" component problem, you might simply have a bad/worn input jack problem.
Here´s the schematic:

Proper procedure is to inject signal at input jack, say 100mV 440Hz or 1kHz, and scope after thatb stage, say at the "Direct/Out" line outb switch where both input channels converge and if missing go backwards towards input to see whereb it disappears.
Tube may nbe bad or with dirty contacts as Jon suggests, you may also have cracked/broken tracks or solder pads, very commonn where jacks are involved because people yanks them hard, step on cables, etc.

[ca7922303]

Appreciate your help. I started out swapping high gain output for low and then one of the speaker Jack's, so I was hoping for an easy fix and initially I got output thru high gain, but it turned out to be temporary for some reason, but now seems to be no output thru high gain position? Will use is open and signal generator as you suggested shortly.

[ca7922303]

Oscope and signal generator*

[ca7922303]

Checked all traces with oscpoe and signal generator with no issues found. Had recently change the 12ax7 with a new JJ and assumed it was good because it was new. Changed it just now and now have output thru the high gain input. Should have checked the obvious. 1 down, more to go. AporeAppre the help.