Sunn Beta Lead Input Switching Issues

[gbono]

I have a Sunn Beta Lead that originally had an issue where the "B" channel would stop working (pulsing on and off) when the footswitch was connected - worked fine without FS.

I replaced IC103 (74C04 HEX Inverters) and now the amp powers up with "B" channel LED on (NO INPUT). The amp stays in this state for 20 sec and then turns the green LED (B channel) off. If you plug a jack into the B input the amp works BUT when unplugging from the B input the LED stays on for 20 sec. The "A" channel and "both" work as they should (LEDS go of immediately after removing input). Now when you hook up the footswitch to the amp,all inputs behave as normal (i.e. fixed original issue!!). This is a digital switch/control issue only since the amp passes an input (sig gen, guitar, etc) without issue. All voltage supplies look normal.

I'm assuming that IC106 (MC14506 expandable AND-OR-INVERT gate) is the issue: B channel LED is on without input because Q102 is getting turned on by PIN7 of IC106. Any other ideas?

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

also posted at ssguitar.com

[Drummer4gc]

I had basically this same issue with a Beta Bass I just worked on. A fault on the -15V line killed both IC103 and IC106, so replacing them got it up and running, but I was having the issue of the LEDs staying on after unplugging. The JFET switch would remain on too - you could hear hiss through the preamp until the light turned off.

C165 and C164 were soldered between pins 1/2 and 5/6 on IC103 on the back of the preamp board - I believe this was a late addition to the circuit, so it's possible that not all versions had it. You can see them on the schematic you posted. When I replaced IC103, I had removed them, and I neglected to put them back. I reinstalled them and the problem disappeared.

Hope this helps!

[gbono]

Thanks Drummer4gc - this amp is a early 79" and doesn't have the .01uF caps across the inverters. I'll give it a try.

[gbono]

Here's the issue - when you turn the amp on there is -13V at channel B stereo jack terminal - over a time constant of about 20 sec the voltage decreases to about -8 V and then the channel is off. The -15V line to the preamp board is stable. There isn't much between the input of IC106 and the channel B jack. Bad resistor?

[Drummer4gc]

Any luck? I've got some MC14506 if you think yours is no good, shoot me a private message if you're interested in one.

[gbono]

Update....

Finally back on this amp - I tried a simple experiment where I connected channel A input (lifted R132 and 134) into B and vice versa. Now the A channel remains on for 15-20 sec at turn on and when ever there is an input connector removed from the input jack. This seems to point to an issue with the -15v which supplies R111/R134. The emitter of Q102 and pin 8 of IC106 are the -15v node. I can't find any issue with Q102 so I'm baffled. What's causing the voltage a R111 to slowly rise to a negative voltage (-7) after 15-20 sec?

[g1]

Is pin 8 of IC106 holding steady at -15V at all times? Also pins 6 and 14?
All the jacks are TRS and none have been replaced with switching type?
The rings short to sleeve (ground) when a mono plug is inserted. So with nothing plugged in, I expect the -15V to show up at IC103 pins 9,11, and 13.

[g1]

Another thing to try is to completely disconnect anything connected to the ring terminal of Ch.B jack.
If it now turns on without the B LED lit, the jack is faulty. Otherwise, R111 is bad or losing it's feed.