Resistances

[nick]

I've a bit of a story, so strap in and I'll try tell it to the best of my memory...

Did the resistance checks and everything checked out.
22 was 208k so I figure that's approximately equal to the * in the PDF.

Powered on and all 3 tubes lit up. All voltages passed within +- 10% except Kreg which from memory was 2.5V(small chance it was was 4.5V) and only had 2 out of 4 LEDs on the A side C4S board light up. The ones on the top, B side didn't light up.

Went back to the PDF and realised I forgot the R1 jumpers on the bias regulator board so I added those in.

Redid the resistance checks and:
1 is 16.45k. <-- this was ok before soldering the R1 jumpers
19 is 208k
22 is 208k
Everything else is correct

Resistance checks have now failed so I'm not going to power on till they're correct.

I'll attach some photos and apologies in advance, I installed the LEDs right way up after quadruple checking polarity so I'm 99% sure they're correct, subtracting a 10% variance for building it till 1am so I'm 90% sure. Also full disclosure, one resistor on the volume selector is unsoldered, I'll sort that on the next solder run and did press down on it to make sure it was connected when I rechecked the resistances.

[nick]

Photos

[nick]

Photos(it won't let me post more than 2 at a time).

[Paul Birkeland]

Q1 on the B side of your center PC board may have a solder bridge on it.  Since you have already turned it on, you'll just have to do that again and report on the voltages. 

With LEDs that aren't lit, it is extremely helpful to install the LEDs as we show in the manual.  This allows us to be able to look at your PC board and tell that all the LEDs are installed in the correct direction, and the bit of added length on the leads of the parts helps them not absorb as much heat during soldering.  If the issues persist, you may end up needing to remove all the LEDs on the PC board on the side that isn't working and replace them with inverted LEDs just to rule that out, as it's such a common mistake.  It is also somewhat possible that your meter may light them up in the diode check setting, though that may only work for half of them.

[nick]

Resoldered Q1 on the centre PCB as you said and all C4S LEDs are lit up and all voltages are good.

I've managed to adjust the B side pot till I get 145V but the A side is hovering around 75V. Adjusting the A side pot only changes the voltage +- 5V. Resoldering didn't change the previous bad resistance reading so perhaps that's related?

[nick]

Just playing around with the volume knobs(and due to all the volume control resistors being hooked up to that spot) I'm guessing the resistance measurement for that lug must be volume based. Resistance jumps pretty close to the expected value when I turn the fine knob 1 click past 0dB so I'll double check my volume pot wiring.

[Paul Birkeland]

I wouldn't worry tremendously about that.  The fine switch adds resistors in parallel with the coarse control, so even if none of the resistors on the fine control are properly connected, you should still get sound out and the coarse control should work.  I would suspect that your meter may not be super happy reading such high resistances, though it's advertised as being about to read 40M.

[nick]

Any ideas on the 75V tho? I’m guessing I should get that sorted before pumping music through it.

[Paul Birkeland]

Either OA or OB on the center PC board will feed the 6C45P where you're getting 75V.  Can you report the OA and OB voltages, as well as which one goes to the side where you're having the problem?

[nick]

OA is reporting 1.94V and OB is 2.14V.

OA is the side with the issue.

[Paul Birkeland]

What are your Kreg voltages? 

[nick]

8.9V on side A and 1.1V on side B.

[Paul Birkeland]

Be extra sure that the 431 regulators and PN2907 transistors didn't get swapped.  Do you have proper OA/IB voltages on each high current C4S board?

[nick]

I've checked that the 431 regulators and PN2907 transistors are in the correct place.

Here are the measurements(I've also measured the other points just in case they come in helpful):

Left C4S board:
IB 216.2V
OB 144.6V
OA 216.1V
IA 270.4v

Right C4S board:
IB 267.8B
OB 76.3V
OA 268.0V
IA 268.0V

[nick]

It seems that through all the diagnosis, the B/right side Kreg wire broke. I've fixed that and am now getting

215V for both IB and OA.

OB is still ~75V tho.