The sunn Forum

TOP, thanks a lot!

I would return it to Guitar Center and try to get them to repair it under some kind of implied warranty. They may have some kind of "As Is" policy that precludes this. IMHO you paid too much for an amp not in 100% working order. It could cost $300 or more to get the amp into reliable working order. It might be better to look for one that you know is broken and get it for $100.

These amps are getting to be 30+ years old and they need lots of TLC. If there is a professional band using these amps on the road, you can bet they have two spares for every amp on stage and they have had their amps completely rebuilt. In Beta amps there are parts that are obsolete or hard to find. It's getting harder to even find someone who knows how to work on them. One company is making clones of Beta amps, they might be your best option.

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

I connect the “to” to the “from” on channel B with no luck. :/

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.