B-side C4S board issue at voltage check [resolved]

[Loquah]

I'm finally getting around to testing voltages after the most disjointed build process I have ever encountered (because 2-year-old transitioning from cot to bed)

When I powered up the device and started measuring, the B-Side High Current C4S board LEDs are staying dark and OA is reading just over 180V instead of 150V which seems a bit high. I stopped measuring pretty quickly when I realised that the outboard heatsink on the B-side High Current C4S board was getting really hot.

Any tips on where to begin trouble-shooting without powering-up the amp again and risking cooking the transistor on the B-side C4S?

[Paul Birkeland]

You can run the amps with no 6AQ5s but with the 12AU7 and the 6080.  This will cause the OA voltages on the high current boards to rise and Kreg voltages on the center board to be 0, but everything else can be evaluated under these conditions.  This is where I would start. 

If a B side heatsink is getting warm, it may help to post some build photos and we may be able to spot a miswire.

[Loquah]

You can run the amps with no 6AQ5s but with the 12AU7 and the 6080.  This will cause the OA voltages on the high current boards to rise and Kreg voltages on the center board to be 0, but everything else can be evaluated under these conditions.  This is where I would start. 

If a B side heatsink is getting warm, it may help to post some build photos and we may be able to spot a miswire.

Thanks Paul. I'll see if I can quickly fly through the measurements and share them here along with some photos. I also remembered late last night that the B-side is where I had the faulty socket so my written numbering isn't perfectly aligned and I may have miswired the B tube socket. I'll investigate that as the first likely issue

[Loquah]

I went hunting for mis-wiring before taking any measurements and I've managed to very carefully and consistently wire all of the LEDs on the B-side C4S board in exactly the wrong orientation. Before I begin the task of removing and resoldering, I wanted to ask two questions please:
1) Will the LEDs be OK or have I cooked them?
2) Would this error result in the hot transistor or am I likely needing to find another fault as well?

[Paul Birkeland]

You will probably want to replace those LEDs.  They will only really come out willingly if you have a very quick desoldering vacuum.

[Loquah]

You will probably want to replace those LEDs.  They will only really come out willingly if you have a very quick desoldering vacuum.

Yep, right as usual, Paul. After ordering the new LEDs and having a vacation, I've now finally rebuilt the board with the LEDs correctly positioned and I'm much closer, but we still have an issue. During the voltage checks, here are the issues:

Low Current C4S
IB = 173 (should be 150)
Kreg B = 1.94 (3-6)
Bregb 172.7 (150)

On the right side (looking at the underside with IEC socket at the back) High Current C4S
OA = 173 (150)
bB = 173 (150)

Any suggestions on where I should start looking for my next fatigue-induced wiring errors?

[Paul Birkeland]

What do you get at OB on the high current C4S board on that side?  What are OA and OB on the center C4S board?

The 1.94V of Kreg voltage and the 172V of regulated B+ means the HV regulator isn't working, but that could happen if the 6080 isn't drawing any current on that side.

[Loquah]

Sorry, Paul, I should have posted a full set of measurements originally and I made some wrong assumptions in not measuring the B socket C4S - my apologies. Full test results below:

Low Current C4S
IA 149.4 (150)
OA 74.5 (60-90)
KregA 3.4 (3-6)
bRegA 149.4 (150)
IB 172 (150)
OB 73.7 (60-90)
KregB 1.9 (3-6)
bRegB 172.4 (150)

High Current B
IA 195.8 (190)
OA 149.5 (150)
bA 0 (0)

IB 0 (0)
OB 96.1 (90-110)
bB 149.4 (150)

High Current D
IA 196 (190)
OA 173 (150)
bA 0 (0)

IB 0 (0)
OB 116.9 (90-110)
bB 173.2 (150)

Jack
Tip 18-19mV (0)
Ring 18-19 mV (0)

[Paul Birkeland]

I would double check that you don't have any solder bridges or misplaced jumpers on that D side high current C4S board.  You can also pop that board out and trade it with the one on the A side to see if the issue travels with the board or stays on the same side of the amp.

[Bourney]

I had voltages similar to those (I'm in Australia with 240v mains, which was running at about 242v when I did my tests).

My KregB was also at 1.9v as result of having the LEDs on the B side the wrong way around.

I failed to read the fine print in the manual and was mislead by the printing on the circuit board. I'm still a bit salty about that.

It was a pain to remove those 2 LED and reverse, but that fixed the problem straight away and I ended up at 5.5v.

Just FYI as you're on 240v like me some of my other voltages (now that its working correctly) were:

Low Current C4S
IA 149
OA 63
KregA 5.6
bRegA 148
IB 148
OB 66
KregB 5.5
bRegB 149

High Current C4S
IA 199
OA 149
OB 89

[Loquah]

Thanks for the info, Mark!

Sadly, I think my issue is being tired during assembly and probably not enough light. I'm pretty sure I've used the wrong resistors in a couple of spots (basically swapped a couple with each other). I've ordered fresh resistors and should be back in action soon I hope. At that time, your voltage readings might come in very handy, Mark. Thanks again!

[Loquah]

Hi all, after a long delay I've done some major troubleshooting with no success Here's what I've done:

• Swapped the B & D socket C4S boards, but the problem didn't move with the board
• Replaced resistors that I was unsure that I'd correctly selected - I think it was unnecessary, but wanted to be certain
• Worked through every step in the manual to check that each connection was correctly made and it was - I really hoped I'd find a stupid mis-wiring and solve this, but no such luck

As it stands, the obvious symptoms of the problem are a lack of lit LEDs on the D-socket C4S B-side (I know the LEDs aren't the concern - just a visible indicator to me that the issue didn't follow the C4S board when swapped) and also the B-socket C4S board seems to be pumping out some heat while the D-socket C4S board isn't. Not sure if that's relevant or helpful.

Here are the fresh voltage readings, but they're not much different from before:

Low Current C4S
IA  149.5  (150)
OA  74.6  (60-90)
KregA  3  (3-6)
bRegA  149.4  (150)
IB  172  (150)
OB  73.7  (60-90)
KregB  1.9  (3-6)
bRegB  173  (150)

High Current B
IA  191  (190)
OA  149.4 (150)
bA  0  (0)

IB  0  (0)
OB  96.3  (90-110)
bB  149.3  (150)

High Current D
IA  190.6  (190)
OA  173.3  (150)
bA  0  (0)

IB  0  (0)
OB  117.6  (90-110)
bB  173.3  (150)

Jack
Tip <10mV (0)
Ring <10mV (0)

I'm at a complete loss at this point. Could there be a faulty component?

[Paul Birkeland]

If you rotate the center C4S board 180 degrees, does the low Kreg voltage stay put or switch sides?

If the low Kreg voltage switches sides, then I would replace the 431 regulator that's showing a low voltage.

Your OA voltage on the side that's not proper is high though, so that to me suggests more of a C4S issue where the extra current needed to operate the high voltage regulator isn't present, or that there's some wiring issue on the 7 pin socket on that side that's causing problems.

[Loquah]

I'll check that out. Thanks Paul!

[Loquah]

I've rotated the middle board and the issue switched sides in the sense that the high voltage reading changed from IB to IA.

Next I checked and reflowed every connection on the 7-pin D socket, but still no joy.

Given that the issue is not following any of the boards (the problem stays always with the D-socket), but the wiring all appears correct based on my triple checking, I'm now totally lost for ideas. Thoughts?