Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Crack-a-two-a => Topic started by: OutFishing2 on February 07, 2024, 11:06:43 AM
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Hi, I have assembled my c2a and really enjoyed the build. I have hit a stumbling block on the voltage checks with a high result on KregA and various high voltages on all the boards. Please can I seek some help?
I will post voltage results and if it helps I can list the steps I've taken to chase down the error.
I should add I'm in UK so on 240v.
Thank you in advance!
David
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HP: T = 0.6mv steady whilst R cycles between 20mv and 50mv
LowVoltageBoard
IA = 197 (!)
OA = 66
KregA = 12.5 (!)
BregA = 172 (!)
IB = 172 (!)
OB = 72
KregB = 6.8
BregB = 196 (!)
High VoltageBoards
RCA side <> PSU side
IA 198 194
OA 196 (!) 170 (!)
bA 0 0
IB 0 0
OB 108 103
bB 196 (!) 170 (!)
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Are +reg and Breg jumpered on the center board as shown in the manual on page 58 at the very bottom?
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Hi Paul,
Yes bridges look correctly done. Photo of my board attached for ease.
Thank you for the quick reply!
David
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What's the DC voltage on pin 1 of each 6AQ5 tube? The Kreg voltages are plenty high to allow the regulator to work, but the absence of properly regulated voltages on both sides would point to some kind of repeated mistake on each side.
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I get 0v on pin1, consistent across both sides.
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Could you post a picture of the wiring around each 7 pin socket?
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How's this?
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It should not matter, but it might be worth double checking that you also see 0V at pin 7 on each 7 pin socket.
I'd also be interested in the DC voltage present at the terminals marked X on the center C4S board, as well as a bottom shot of that center C4S board.
I've also sometimes encountered oddball problems like this when the power supply feed going into IA is accidentally wired into OA, or the wire that's supposed to leave OA and feed the rest of the amp is instead fed from IA, though these issues are not necessarily indicated by the voltages you are reporting.
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Hi Paul,
So pin7 on the B & D sockets are each 0v.
X at the cap legs are 3.1v, descending slowly on A side and 3.7v ascending slowly on B.
I will double check all the OA / IA leads now.
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Power routing on the build looks as follows:
Terminal 24 to HighCurrent IA on RCA side
RCA side HighCurrent OA to terminal 6
Terminal 6 splits three ways to C2, LowCurrent IA and B5
B5 resistored to B6
LowCurrent OA to terminal 2
Terminal 2 splits two ways to A6 and C1
LowCurrent OB to A1 and C4 via terminal 4
LowCurrent IB to terminal 16
Terminal 16 splits three ways to C5, D5 and HighCurrentBoard PSU side OA
D5 resistored to D6
HighCurrentBoard PSU side IA to terminal 30
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Board underneath...
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Looking over the power routing reminded me of one anomoly I noticed in the build process. I don't think it's relevant but you may know better. On p61 there's an instruction to connect a short black wire between 1U and -reg and in the photo given, there's no sign of a longer black wire which was previously attached between 1U and high current board IB. I've built it with both wires in place.
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I would double check that the 431 regulators are in the correct spot, then resolder their connections if they are where they should be. The 3V at X tells me that the 431 regulators aren't doing anything. Also be extra certain that all those black jumpers on that center PC board are well soldered.
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I've resoldered the 431 regulators. They were in the right spots. No change to X volt readings and IA and KregA are still the same as posted yesterday.
Is it worth slotting in new 431s?
David
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I would double check that the -reg pads on the center C4S board are all reading 0V. Typically a failed 431 will show 1.8-1.9V at the X terminal and Kreg, so what you're observing is more like a 431 that's partially disconnected (hence my initial inquiry about that jumper that's easily left off).
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Thanks Paul.
The -reg pads on the centre C4S board are at 0v effectively, <4mV for both A & B.
I resoldered all the pins on the board for good measure and replaced the black and red wires under the board. Also fitted new wire bridges across +reg. Voltage readings are unchanged.
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I'm still leaning towards a solder problem. The common problems we find that prevent solid joints are insufficient heat and insufficient lead in the solder. You want 900F/500C or so of iron temp, and 60/40 leaded solder is also extremely helpful. Missing one of those elements will make soldering a lot more difficult than it should be. Along with the sufficient heat, you also want enough time for the solder to flow through the PC board holes to really grab onto the component leads. Looking at your board, you can see how this has happened with the LEDs (they have low thermal mass) but not the 431 regulators (slightly higher thermal mass).
More info can be found here:
https://www.youtube.com/watch?v=EN16Pi7pcfk (https://www.youtube.com/watch?v=EN16Pi7pcfk)
I've been building this stuff for 25 years and I still have problematic joints here and there.
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Yes, I appreciate that. My soldering skills have certainly got better whilst building the kit. The troubleshooting phase has given me chance to go back over the wiring around the transformer and power and it's all looking neater now.
I followed your advice and reflowed all the points on the board in the last hour except the LEDs. My iron only goes to 480'C but fairly close to what you recommend.
I had done most of the kit with the iron set at 380'C and I can see now how the extra heat creates much shinier joints. Solder is 60.40 0.7mm.
I'm still 12v on KregA, 3.xV on the X terminal.
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It's also worth mentioning that having the wires that go to each Kreg terminal swapped can give funny consequences like this. Same with the red wires connected to IA/IB on the center C4S board. If one pair of those wires is swapped, the regulators will not function properly.
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Thanks Paul, really appreciate your patience working this through with me.
The wires are definitely in the right places. It's a funny one. Is there any chance it could be the vregs, or the 6aq5s or dudes at Q1 and Q2?
I'm sure it's not the 6aq5s since switching them over makes no difference. Would switching the Q's from A to B reveal anything?
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You have higher regulated voltages than we would expect with adequate Kreg voltages at each 431 regulator to allow them to do the regulating they would normally do, but yet nothing is happening with them. Having this happen on both sides would suggest a build issue rather than a component issue. Were any parts substituted by chance? You could certainly try replacing the 431 regulators, but they aren't behaving like failed 431s.
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Alternative 431s arrive on Wednesday so I'll have a go with them.
6AQ5s arrived today and made no difference to voltages but that was expected. Fascination anticipated with future tube rolling.
I used the BH parts except in two areas.
The 0.1uF caps on the low current board are mundorf. I substituted the stock dayton's in/out during this troubleshooting. It made no difference to voltages.
The four tube sockets are from a local firm. Ceramic gold plated. Described as B9A, B7G and octal sockets. I've since read a thread here where you advise against substituting sockets as they can give trouble so I'm readying to switch those over to stock.
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If you swapped in ceramic sockets that are about the same as what we provided, just leave them in there. The teflon ones and some of the other exotic types can cause some real problems though.
If you do completely remove and replace all the sockets, I would expect more problems to pop up, so definitely don't do that.
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Great news. The new 431s are in and the voltages immediately went to spec. 4.6v on both kreg and 149v on the IA and IBs.
Quite how you narrowed the fault down to the two vregs from the other side of the Atlantic is something I will take a long time to understand. Thank you!
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something I will take a long time to understand.
You and me both! I'm glad you got it nailed down.
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Thank you. The C2A really is a superb amp. I am extremely impressed at how well it sounds as a preamp as well as with headphones. Rock solid performance. The stepped attenuator went in today and it's all working beautifully. Thanks again.