Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Crack-a-two-a => Topic started by: Loquah on July 23, 2022, 09:17:40 PM
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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?
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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.
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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
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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?
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You will probably want to replace those LEDs. They will only really come out willingly if you have a very quick desoldering vacuum.
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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?
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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.
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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)
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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.
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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
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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!
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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?
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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.
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I'll check that out. Thanks Paul!
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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?
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Just in case the tube could influence things I switched the 7-pin tubes and it had no impact.
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Did the high voltage switch to the other large PC board? If you rotated the middle board and the high voltage went from IA to IB on the small PC board, then you would swap the large PC boards to see if the offending voltage follows one of the high current C4S boards.
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Did the high voltage switch to the other large PC board? If you rotated the middle board and the high voltage went from IA to IB on the small PC board, then you would swap the large PC boards to see if the offending voltage follows one of the high current C4S boards.
I've swapped the large PCBs already and the voltages remained the same and didn't change sides with the PCB swap (i.e. the large boards aren't the cause of it).
Am I understanding you correctly with this tes?
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OK, so the high OA voltage on the big PC board stayed put when you switched the big boards around. Did this big board OA voltage also stay the same when you flipped the small board 180 degrees?
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OK, so the high OA voltage on the big PC board stayed put when you switched the big boards around. Did this big board OA voltage also stay the same when you flipped the small board 180 degrees?
No. Switching big boards left the high voltage reading identical on small board. Rotating small board made high voltage switch from IB to IA so the issue stayed on the same side of the total circuit regardless of the board's orientation
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I would be interested in seeing the 7 pin socket on the offending side. Does the tube glow in that socket?
-PB
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Here are some photos of the D socket. I've included one to show that the red wire attached to pin 5 isn't shorting with pin 6 because it looks like it from above.
https://photos.app.goo.gl/udjU7uq8tuPffGGp6
https://photos.app.goo.gl/TBWDe7YvHztAb4ws8
https://photos.app.goo.gl/jzS4HsLnNmGgvZGs5
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Does the 6AQ5 glow in that socket?
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I've had issues with a C2A build where it was all related to the 6AQ5's not biassing properly on first power on. Switching them from side to side got the sleeper to fire up.
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Does the 6AQ5 glow in that socket?
Yes. Glow test passed on all tubes (and still does). Swapping tubes also had no impact
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Can you verify that the resistance of the little blue resistor between 5 and 6 on that 7 pin socket still measures appropriately?
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Can you verify that the resistance of the little blue resistor between 5 and 6 on that 7 pin socket still measures appropriately?
236.2 ohms measured in-situ
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This should probably come visit my bench and I'll have a look over it. As a last resort, if you had a broken wire between the high current C4S OA and pin 5 of the 6AQ5 socket, I suspect you'd end up with this issue as well.
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I'll rewire OA and D5 first and then it'll be time for a return trip 🙂
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For anyone following along at home, this kit ended up making the long trip from Australia to the USA over the Christmas break. I'm looking forward to an update from Paul when he has time to look at it (I believe he was quite swamped at the time so I appreciate the offer of assistance).
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I’m following along from home and keen to hear how the mystery is solved.
It will be a well travelled amp by the time it returns to you. :)
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Has the mystery been solved? Does the Crack-a-two-a live?
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I found a large solder blob had migrated through one of the C4S PC boards and up against the body of an LED, which destroyed the LED itself. I replaced the two LEDs on that side of that particular PC board and that was all that was required to repair this C2A.