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August 12, 2022, 09:26:18 PM

Author Topic: B-side C4S board issue at voltage check  (Read 92 times)

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Offline Loquah

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B-side C4S board issue at voltage check
« on: July 23, 2022, 09:17:40 PM »
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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Offline Paul Birkeland

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Re: B-side C4S board issue at voltage check
« Reply #1 on: July 24, 2022, 04:00:13 AM »
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.
Paul "PB" Birkeland

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Offline Loquah

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Re: B-side C4S board issue at voltage check
« Reply #2 on: July 24, 2022, 01:38:07 PM »
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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Offline Loquah

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Re: B-side C4S board issue at voltage check
« Reply #3 on: July 26, 2022, 01:11:20 AM »
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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Offline Paul Birkeland

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Re: B-side C4S board issue at voltage check
« Reply #4 on: July 26, 2022, 04:29:15 AM »
You will probably want to replace those LEDs.  They will only really come out willingly if you have a very quick desoldering vacuum.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man