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Thread: Sunn Beta Lead Input Switching Issues

  1. #1
    Supporting Member gbono's Avatar
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    Sunn Beta Lead Input Switching Issues

    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?

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    also posted at ssguitar.com
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    Last edited by gbono; 11-06-2018 at 09:21 PM.

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    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!

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  3. #3
    Supporting Member gbono's Avatar
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    Thanks Drummer4gc - this amp is a early 79" and doesn't have the .01uF caps across the inverters. I'll give it a try.

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    Supporting Member gbono's Avatar
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    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?

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