[JPK]

Test 4 (low voltage check) -> -15V
Your tube doesn't seem to be heated. Is it glowing in the dark?

[heimito]

No the tube does not glow at all.

[JPK]

No the tube does not glow at all.
This is the issue. What is the DC voltage on U4 pin 1 (against 0V)? Should be around +13VDC.
Check value, direction and solder joints of U6, U7, R51, C26, C28.

[heimito]

black Probe on 0V + red probe on U4 pin 1 = -14,49V
red probe on 0V + black Probe on U4 pin 1 = 14,5 V

U6 - orientation ok, solder joint ok
U7 - orientation ok, solder joint ok
R51 = 7,6 Ohms, solder joint ok
C26 = 10uF - 63V, solder joint ok
C28 = 100uF - 25V, solder joint ok

I resoldered all the joints.
The tube is still not glowing.

[JPK]

black Probe on 0V + red probe on U4 pin 1 = -14,49V
It should be positive.
Can you measure the DC voltages on the 8 pins of U4?

[heimito]

I confirm:
black Probe on 0V + red probe on U4 pin 1 = -14,49V (negative)

DC voltage measured against pin 8 of U4:
2,851V (slowly rising by +0,001V)

[JPK]

Can you measure the DC voltages on the 8 pins of U4?
I mean all the 8 pins.

[heimito]

1 -14,49 V
2  12,78 V
3  12,00 V (1 times sparks, when touching the pin )
4 -15,01 V
5 -5,677 V
6  12,53 V
7  12,55 V
8  2,850 V

[JPK]

1 -14,49 V
7  12,55 V
8  2,850 V

With these values, the outpout pin 1 should be positive. 
If the values are correct then U4 seems to be dead again.

[heimito]

I bought a new 0372

The values have changed a bit, but pin 1 is still negative.
The tube is still not glowing:

1: -14,50 V
2: 12,80 V
3: 12,02 V
4: -15,01 V
5: -5,722 V
6: 12,40 V
7: 12,40 V
8: 2,922 V

[JPK]

remove U7.
Check if U4 pin1 goes up to +12V now.
If not OK U6 is probably dead. Remove U6 and check again U4 pin1. If OK U6 is surely dead.
If OK look for a short circuit between TP8 and V- (tube removed).

[heimito]

I removed U7 --> -14,51V on U4 Pin 1

I removed U6* --> 0,0V on U4 Pin 1

I removed the tube --> 5 MOhms (and falling) between 0V and TP8

*removing U6 was not possible without destroying it.

[JPK]

removing U6 was not possible without destroying it.
To remove these 3 pins devices, simply heat the 3 pins together with a large blob of solder.
You can replace >U6 by this R-7812-0.5 or this PXO7812-500-S

I removed U6* --> 0,0V on U4 Pin 1
Not good. U4 might be dead again.

[heimito]

I doubt it. I just replaced U4 two days ago.
It's the 3rd one now.

[heimito]

I have bought a new U6 and a fourth (!!!) U4

(It is getting pretty expensive by now)

The values have changed significantly on Pin 4 and 8. Everything else remains the same.
The tube does not glow.

1 > -14,5V
2 > 12,15V
3 > 11,68V
4 > 2,86V
5 > 6,02V
6 > 12,22
7 > 12,21
8 > 5,66

Will it ever work?

[JPK]

Can you measure the DC voltages on the 8 pins of U4, without U4 inserted?

Will it ever work?
Sure!
You can always decide to send it to us for a repair.

[heimito]

I removed U4 and measured again:

1 > -14,9V
2 > 13,9V
3 > -15,04V
4 > -15,05V
5 > -5,393V
6 > 13,7V
7 > 13,7V
8 > 0V

[JPK]

With U4 inserted, U4-pin4 = 2.86V
With U4 removed, U4-pin4 = -15V
Weird! U4-pin4 is connected to the V- rail.
Can you check the continuity between U4-pin4 and the V- test pin?

Do the voltage values on V- and V+ remain correct when U4 is in place (-15V, +15V)?

[heimito]

New testing sessions, new results:

U4 Pin 4 with U4 inserted: -14,91V
U4 Pin 4 with U4 removed: -15,05V

Between U4 Pin 4 and V- > 0V

Voltage on V+ 14,99V
Voltage on V- 14,91V

But meanwhile C23 is not looking healthy anymore:

[JPK]

New testing sessions, new results
I am puzzled. I will recommend you again to send it to us.

[heimito]

Seems like a good idea.
Where can I send it?

[JPK]

Contact me here:
https://www.soundskulptor.com/en/content/6-contact