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Sunn Beta Lead Input Switching Issues

Started by gbono, November 06, 2018, 03:20:29 PM

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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?


Probably an issue with leakage (on the PCB) or partial static damage on the input of IC103. IC106 will be difficult to find and expensive. Find a flux removal spray (or use isopropyl alcohol) and cleanup any flux around IC103. Best to install a high quality socket, then you can shotgun chips until you find one that will work. If you are having trouble with static discharge, use something to raise the humidity, or work naked.

A workaround if you can't find IC106. You will need to join the forum to see the attachments: https://forum.sunnstillshines.online/index.php?topic=6864.msg29374#msg29374


I did put in a socket for IC103 and tried several different versions of the 74C04 - problem doesn't track replacing different inverters. Always use a properly grounded wrist strap when working around these older CMOS parts.

The board I have doesn't have C164/5 across pins 1/2 and 5/6 on IC103 so I'll try adding these.

I like your "replacement" for IC106.


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?


Attached is the service manual with the logic/switching circuit description on pg.4


Interesting... if you look at the "Footswitch Circuit" part of the manual (page 5), it says that the voltage drop across the LEDs is what actuates the switching functions.  Maybe check the LEDs and their respective resistors (R140 and R141) for  flaky operation.  Sure they light up, but is the voltage drop still on-spec?



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?