Got it. Thank you for all your help!

To take this a step 🪜 further 

Make the transition to real ice hockey 🏑🏒 then the DC is ice

So if we need to fix the DC and get it in order then we need the Zamboni 

Got a problem with your amp ?

Summon the GREAT ZAMBONI 

I would connect the power cord ground wire directly to the chassis.

I think that did it! I'm getting 13.95v at the red wires now when running it from the wall. The spec sheet for the two TDA2030A poweramps quotes 14W@14V so that seems pretty spot on for a 28W amp.

What are your thoughts on the two ground pins T1/T2? Connect one from a new power cord and one to the chassis?

I got to thinking that maybe they are like air hockey
Like DC is the air
You have to have it for the puck to ride on
And the signal is the puck

Or maybe not   

I have taken snips of the wiring for other JC amp transformers.
Unfortunately, there are no wiring colors.
On your setup:
Primary
T3 AC In (Blk)
T4 AC In (Wh)

T5 Common (Wh)
T6  ____         (Blu)
T7 _____         (Ylw)

Secondary
T8 (Blk)
T9 (Red)
T10 (Red)

It appears that T5 & T6 /T7 is the clue.
If you can, disconnect T5,6 & 7.
Either T6 or T7 is going to connect to AC Hot, the fuse.
If so, it may be as simple as swapping T6 & T7 on the board.

I believe the white/yellow/blue is the primary as the are connected near the power cable and the red/red/black are the secondary as they are connected near the bridge diode and filter capacitors.

Heres how they are connected in case you cannot see in the pictures.

T1 GND - Not connected
T2 GND - Not connected
T3 AC IN - Black wire from power cable
T4 AC IN - White wire from power cable
T5 COM - White wire from transformer
T6 PRI - Blue wire from transformer
T7 NC - Yellow wire from transformer
T8 BLK - Black wire to transformer
T9 RED - Red wire to transformer
T10 RED - Red wire to transformer

The english version of that message says CAUTION for continued protection against fire hazard replace only same type fuse(s)

What does it say under the Ylw & Blu wires?
Attention something.....

Can you show where the red and black wires from transformer are going?
If it were not adjustable I would only expect 2 wires on the transformer primary.
Or maybe the blue/white/yellow is the primary side.  Show where those go on the board as well.

Here you go!