Check you have continuity through the Driver transducer as well as the cable going to the tank.
Check you have +7volt & -7volt rails at the 2 boost transistors
BTW, D11 & D12 are plain diodes not Zeners.
Phil

Thanks, those are helpful. I think my zener is bad, unless something else in the circuit could be dragging the voltage down somehow, but not sure what that could be. That the output still works points away from that, but not sure. Maybe it still works well enough for the output but doesn't have enough juice to feed the two transistors?

Anyway, I'll have another look this evening.

Hi everyone!
I have a Baldwin C-1 Professional amp and am wondering how to get the most out of the 'EXTER. SPEAKER' jack in the back to potentially push another speaker or cabinet.
After researching on this forum, it's my understanding that this amp is putting out 2ohms into it's internal two 4ohm 12" speakers which are wired in parallel.
It's also my understanding that the external speaker jack is wired in series with the internal speakers.
So I'm wondering what the ideal setup might be for a two speaker cabinet (or possibly a single additional speaker) to give some different tone to the amp.
I'm primarily confused about how an external speaker/s that in series with a pair that's in parallel would work together.
I only have a basic understanding of parallel vs series and how the ohms of speakers in those scenarios affect the amp so the combination of two speakers in parallel and then adding one or more speakers in series with that has me confused as to what my various options are.
Any help would be greatly appreciated!

These might help you a little bit in figuring it out, or at least help you in the right direction.

https://sound-au.com/project34.htm

https://sound-au.com/project211.htm

Hi all, I recently got a Roland Bolt 60 that a friend found on the side of the road. It was a bit beat up but not too bad.the power switch had failed, some cosmetic stuff. It sounds pretty good really.

I'm having problems with the reverb though. If I turn the reverb up and smack the cabinet I get a nice splashy reverb sound so that means the output part of the circuit works. As I'm writing this I think I'm answering my question, but here I am anyway.

There's a dual opamp, a 4558, that is used in the reverb circuit, one side before the tank and one after. I used an audio probe and the guitar sound gets to the opamp, but I get no output to the tank. I was concerned about the voltage as the schematic made it look like it should be 30v and I only have about 8v. But the output works, so that can't be right.

I assume the opamp is the issue, but if anyone who remembers how zener diodes work wanted to look at the schematic id appreciate it. Here's a link. https://el34world.com/charts/Schematics/files/Roland/Roland_bolt_60.pdf

Thanks!

I agree it's negative feedback, with some equalisation. I just couldn't figure out here the constant amplitude 50Hz hum is coming. It's present even with the input disconnect from the pre-amp output.

Maybe it's gain is too high, and it's picking up EMC from the transformer.

Usually power amps have low gain, no feedback, and no equalisation, so I am baffled by this design.

The way I interpret the speaker outputs signal return back to input, as a type of negative feedback. Thus reducing the signal upon itself using an inverted out of phase signal. In PA systems each gain stage is filled with negative feedback to keep the signal stable and less distortion. Well that's just a thought I had looking at this. I see where it might be positive feedback but I am pretty sure that the signal is phase-inverted which cancels out the source signal. To make the signal in phase with the source input signal would be positive feedback. I am guessing that would make the signals a tiny bit time delayed, milliseconds. Hence you delay the positive feedback more and more you can create a chorus effect from the two signals.

Attached is the power amp portion of the Crafter DSP-1 30W guitar amplifier.

It's an unusual design, as there's a long tail pair at the input. One input is the signal input and the other input is a compensated/equalised signal from the speaker output. I guess the latter is to balance the enclosure, but I suspect it's also providing positive feedback that amplifies the low frequency which is creating a constant 50Hz hum at the output. Am I right or wrong? Please advise.

I haven't opened the amp to investigate the source/cause of the  hum yet. It seems that it's a common problem with this Crafter amp because another one currently listed ebay UK has the same fault/problem. I am thinking of modifying the inout circuitry into a single input one without feedback, ie the classical design, or replace the entire power amp with a Class D mudule. Thoughts anybody?

Quote from: Synctime on April 08, 2026, 05:17:55 AMSince it's a 50Hz hum, it's either from mains leakage or from the transformer EM radiation.

Not sure how you verified the hum is 50Hz, but if you are sure, then the cause could also be a grounding issue.

Verify that both sides of preamp are ok by taking the L and R line outs to another amp.
If they are both ok, I would move straight to the output IC's.  Check the supply voltages at each IC and whether signal is getting to the IC inputs.  If that is all good replace the defective output IC(s).
Finding real TDA2050L-TB5-T may be difficult.