We've gotten spoiled now, used to be that all you could get was photo-copies, which were usually at best, on par with this one.   
This one if from '98, so probably taken from a copy or early tech scanner and that will be as good as it gets.
The only thing I have any issue with is the labeling of components on the 'pictorial'.  If they are not labeled on the board itself, that can make things more time consuming.

Not sure how this amp is set up, but are the jacks wired or board mount?
Sometimes jacks that have insulators get replaced with non-insulated, or the isolation washers get lost.
Then you can get weird ground loops, maybe because pot bodies are also grounded.
If you can dismount the jacks but leave the pots touching the chassis it's worth a try.  Then if it helps you just need to get insulating washers for the jacks.

The trimmer is not for bias, it looks to me like offset or symmetry adjustment but I'm not familiar with this amp.
There is a note about reducing quiescent (idle current) by adding 560 ohm resistor across one of the bias diodes.
You can check what quiescent for the power transistors is right now by measuring voltage across the 40411's emitter resistors (shown a 0R33 and 0R25) and calculating current.
It's possible with lower idle current the trimmer will do enough to get rid of the offset.  400mV or even 200mV that you have for offset is too much.  50mV is a better target, or maybe 100mV in worst case scenario.

As far as hfe vs beta, they are the same thing.
Here's a link to a good article on it, note what they say about hfe of circuit.  Usually for substitutions we need a beta greater than what the circuit requires.  Knowing beta of the transistor does not tell us the circuit gain.
https://www.ovaga.com/blog/transistor/what-is-transistor-hfe

Quote from: saturated on May 03, 2025, 02:28:35 PMTurns out that a variable resistor turned all the way to zero is a dead short 


fortunately my power supply kicked to zero 

Did you connect the supply to something other than the diagram in the picture?

Do you have any DC at pin 1 of U5 ? 
Schematic attached.

As far as I know that is the Acoustic number for the 40410.  If you search for that number you will find a lot of posts about different subs people have used. 
The issues are finding something with the same pinout that will physically fit and have a heatsink. 
If you can find a 2N5679, that should be able to work if you reuse the existing heatsink.  Seems like Mouser has stock on them.
Otherwise, something in a TO-126 package can be made to work with the existing heatsink.

Yes, it must be insulated from the chassis. 
The shielded jack you posted must have been wired wrong or defective.  It should have acted the same as the other did when it was grounded to chassis.
Also, the noise you experienced must be due to the dress of the wires.  Where you routed it through the back of the chassis must be a sensitive area.
An insulated jack mounted where the original speaker wire went through the chassis should not pick up any more noise than the original wires did in the same spot.

What do you mean by 'shielded jack'?  You should use a plastic type insulated jack so the bushing does not have any continuity with the chassis.
There shouldn't be any more problem with EMI than the existing speaker wires would have.  It may have to mount where the speaker wires route out of the chassis.
Not sure if the amp is capable of driving an additional speaker without disconnecting the internal speaker.

The 'original' zener was just a defective regular diode.  Someone very smart realized it could be controlled and utilized.  Like a lot of discoveries, I guess. 
Always analyze your mistakes. 

Those resistors are shown as 1 ohm on the schematic.  If so, you have idle currents of around 88mA per output device.  That seems kind of hot, 10 to 20mA would be a better target.  Adjust bias to get around .01 to .02V across each of those resistors.
Then again check for DC at the output.  If it stays around the 10mV you have now, that is good.

With no load connected, is there any DC at the output?  What is the DC voltage drop across each of R737, R738 ?

How much each output device conducts is related to the bias circuit.
As you have made modifications, the stock schematic does not show the whole picture.
Post a schematic that includes all your circuit changes.

Now the edits have rendered my question redundant. 
In any case, your answers in posts #2 and #4 are all correct.

Yes that is it, touching 2 component leads with the same probe.
But also, the meter is reconfigured as a short when you are in the current (Amps or mA) ranges.  That is why we must 'break' the circuit and insert the meter in series to measure current.
If you do a current measurement, then forget and try to measure voltage with the meter still set for current, you are essentially putting a piece of wire across the points your 2 probes connect to.  Hopefully the meter fuse will save the piece of gear you are working on.