At least you didn't make any fireworks out of that PCB.
I read somewhere that people think fireworks should be  banned because they could be dangerous, I guess that just leaves us firing guns in the air instead.
Now where can I get an AK47 from cheap......

Marshall 12w Bass

If you have full voltage in parallel mode then the fault would seem to be in the control circuit itself for the slave side.
It appears that the 6303A has analog meters and the 6303D has digital meters, hopefully there is not much else different between the models.
Please note that there are some errors on the schematics so don't trust it completely, it is also drawn in a confusing way and really needs to be re-drawn to make it easier to read.

My brain is not engaging in gear properly lately so I have to wait for another time to have a try at it, in the mean time have a look to see if you can see what changes from parallel mode to the other modes of control.
I think the other people trying to help are smarter then me anyway so you are in good hands here.
Cheers
Mick

EDIT:::::  I had another look at the schematic I posted and it shows the digital meter circuit on page 4 but on the Utube clip at 12min and 23 seconds you can clearly see the analog voltage and current meters like your 6303A meter
Here is a Utube vid worth looking at for you, it might help some :
https://www.youtube.com/watch?v=_dBj0VNB73Q

Quote from: blackcorvo on June 30, 2025, 02:09:48 AMA couple weeks ago, I was gifted a Frahm PS200 (8 ohms, 30W, 4" + tweeter, studio speaker), and installed a TDA8932B inside. It sounds great with my selection of headphone amps through it, both with bass and guitar, but I wanted to have a built-in preamp I could use and, potentially, turn it into a battery powered portable amp at some point.

I've been trying to find something simple, maybe that could get some dirt off of, if I wanted to. Gain-Tone-Volume is plenty good for me. Maybe even Gain-Bass-Treble-Volume. I just don't want a dozen knobs.

I've been looking around but haven't really seen anything that clicked, so I'm asking here in case anyone else can direct me to something interesting.

Thanks y'all.

If you want something interesting then I would build a C2CE pedal to drive that powered speaker you made, you might not be after something with a tube in it but all the C2CE pedals are made to be run on a normal 9V-12V pedal power supply so your 12V would be ideal for these pedals.
The Nobelium is a clone of the Noble Bass preamp pedal so that would work great, the BassDude is a Fender Bassman preamp and the Wrecking Ball is a Mesa/Boogie dual Rectifier preamp in a pedal.
You will not find anything more interesting then a real Tube pedal running the tubes at proper high voltage, and they are fun to build as well.

If you just want to buy a pedal then look at Sushi Box FX pedals, Nathan from Sushi Box is also the designer for the C2CE pedals since he owns both.
Nathan is away on holiday in Mongolia right now but he will be back in 3 weeks.
The Tube pedals do use a bit of power so might not be the best for a battery powered setup though.

I would look at any other solid state pre-amp pedal as well if I was you, if the pedal is a copy of the pre-amp of a real amp then it should be able to drive the TDA IC ok.
Any Sunn BETA preamp pedal should be able to do clean and crunch and would be one of my favorite pedals to build
It all depend on what sort of music you are into, what exactly do you want to play ?

https://c2celectronics.com/
https://www.sushiboxfx.com/
https://aionfx.com/project/beta-preamp/

Try this one

You cannot view this attachment.

Came across this site today, some nice ones in there.

https://www.pedalboard.org/archiv/vintage-amps

It sure looks like this one in the other repair thread
https://www.ssguitar.com/index.php?topic=5359.0

I used to use carbon paper to transfer the tracks onto the PCB's and then "borrow" some nail polish from mum to paint the tracks onto the copper.
It felt like watching paint dry when you waited for the acid to etch the boards, then a little "borrowed" acetone to clean the nail polish off with.
Some of my early PCB's were a bit rough but I don't remember any not working.

Possibly these

X and Y will show you the voltage one way and a time scale the other way, you have to change the time scale if you measure 50Hz and then change to a 5000Hz signal
Nearly all will show volts up and down and time scale left to right.

I think you are learning things faster then I am forgetting things, 5 years from now I will be asking you for help.

g1 has it 110% correct, you should never apply a higher voltage to a transformer then what it is rated at, the only exception is if you have the full specs and  it states what class the insulation is on the winding. ( max volts it can handle )
Often the primary windings will have a different class of insulation then the secondary windings, they do that to save money as the higher the voltage the more or better quality the insulation has to be.

That solid copper wire is varnished with an insulating varnish, you can tell by the orange colour it has for starters.

Varnished copper wire is used to wind transformers and electric motors among other things, if it was not insulated that way a transformer or motor would just short out in the windings.
If it was insulated with a plastic sheath it would become too thick to wind.

It is also common practice to dip a wound transformer into more varnish to prevent them vibrating and making a 50 or 60Hz hum.

Just look at your variac and imagine the shorted out windings if there was no varnish on the wrie, usually the area on a variac where the viper brushes over for the mid output has been sanded down so the viper can touch a bare area of the windings.

I would try using the 240v as the primary tap and then measure the 24V and see what you have.
It should be safe to put 110V on the 240V primary winding, you will just get a lot less on the secondary side.

Transformers work on a ratio so drop the primary voltage by half you should get about half the secondary out.
Just don't put a higher voltage on a transformer then what it is rated at.

The other way is to use the variac to supply the transformer so you could regulate it that way
Cheers
Mick

Is that the first time you pick up the 50Hz signal ?
What about on the input jacks, both sides of the 10k resistors and also both sides of the 100nF capacitor.
Also check both sides of that 470k resistor to Ch2, I like to measure on the components leg if possible since the signal could be picked up in a component or a track.

If you can't isolate it that way then probe with a wire to 0V ( ground ) on these areas and see what happens with the signal.

Components sometimes work like a great antenna for the magnetic field around that transformer.

I would like to add that some heat transfer paste is conductive and will cause shorts if used with heatsinks and mica washers.
The conductive heat transfer paste is often used in computers for the main processor heatsink, it makes no difference in there since there are no electrical terminals close by.

Make sure you only get non conductive heatsink paste if you get some, a small tube of really good expensive paste will last forever and ever so don't get anything cheap and nasty.