Beta Lead 'both inputs' -> No Sound / No LEDs

[marvin]

Hi folks,

first of all I am new to Sunn amps, but I had the chance to get myself a cheap beta lead combo, so I took it without thinking twice or testing the amp.
Both channels and their LEDs work fine separately, but when I use the 'both inputs' jack, there is no sound and both LEDs are off.
However, when I use a second cable to connect the channel A and channel B jacks, the channels are combined, both LEDs are on and everything (sound, knobs) works as expected.
But thats not how it is supposed to work, right? 

I used some contact spray on the input jacks, but that does not seem to solve the problem. 

Any ideas, what might be the problem here? Thanks 

[Isaac]

My first thought is a bad connection.

[loudthud]

The input jacks on the Beta series amps are actually stereo jacks like you find on the input of just about any pedal or stomp box. The contact that would normally contact the "Ring" is grounded when a mono phone plug (normal guitar cord) in inserted. This is how the Beta amp detects which jack you plug your guitar into. Either there is a defect in the internal circuitry, or the jack is just dirty and the contact in not grounding to you guitar cord. If you do not have the training to safely work inside the amp, I strongly suggest you take the amp to someone who is qualified to work on it.

[marvin]

I verified that there is no bad connection between the jacks and the circuit board. Also the Ring of all 3 input jacks is grounded when a cable is plugged in.

[marvin]

I took a closer look at the 74C04 Hex Inverter (IC103), which implements several NOT-gates. In particular the one that is needed by IC106 to sense that a plug is present in the 'both' inputs jack (Pin 11 = input and Pin 10 = output). So when a plug is present, this grounds the contact on Pin 11 and I verified that this is the case, since there is 0V on Pin 11, which means "high" or "1" in logic terms for the Hex Inverter. If this part of the Hex Inverter would work correctly as a NOT-gate, there should be around -15V on Pin 10, right? But this is not the case, it is also 0V. In fact it is 0V for every possible input jack combination, but it only matters if you use the 'both' inputs jack alone, since only then you need a logical 0 at Pin 10. If you look at the logic of IC106, this explains the behavior that I explained in my first post, and suggests that IC106 works fine.

So my best guess is a defective IC103 that does not invert the input at Pin 11, if it is a logical 1, so that Pin 10 is always logical 1. What do you guys think?

Here is a schematic, where I marked the NOT-gate in question:
https://ibb.co/fr4XZn

[loudthud]

I think you are correct in diagnosing a malfunction of IC103. Replacing that IC requires considerable skill and proper tools. I strongly suggest that you install a socket on the PCB. Be advised that only a 74C04 will work in this circuit because of the 15V power supply. A 74HC04 will just not work