Voltage issue with High Current C4S

[Toobuzz]

Hello,

I built the Crackatwoa and listened for a day. It was outstanding and great fun to build, of course. I also changed out the caps and listened some more. Today I installed the Twoquiet along with some bigger caps. Once I completed the Twoquiet upgrade I ran the resistance checks and all passed (still do). I then registered the following voltages:

Low current C4S
IA 107
OA 73
KregA 1.1
bRegA 107

IB 148
OB 70
KregB 3.25
bRegB 148

High Current C4S (B tube socket)
IA 195
OA 107
bA 0
IB 0
OB 78
bB 107

The other higher current board measures perfectly. After going through the manual twice, I focused on the bad high current board. I removed the jumpers and all excess solder/flux. I took my time and reflowed all pads and ran new jumpers. I also inspected, checked for continuity/resistance all connections on the B tube socket as well as solder lugs 6-10.

Here are the new voltages:

Low current
IA 49
OA 33
KregA .36
bRegA 49

IB 148
OB 70
KregB 5.5
bRegB 148

High current
IA 198
OA 49
bA 0
IB 0
OB 30
bB 49

The B side LEDs of this HC board also went out after reflowing and reattaching the board. All 4 LEDs on the low current board have not illuminated since attempting this upgrade.

Was I correct to focus on this HC board?  Is there something that these numbers suggest I focus on next? 

I may do the same rework on the low current board but I’m just guessing at this point. She was working great before doing the Twoquiet and that upgrade didn’t really touch any of the areas that seem to be giving me issues. Any help would be greatly appreciated!



Sam

[Toobuzz]

I reflowed the A side of the low current board and ensured all connections were clean around terminals 1-5 and 11-15.  I even reattached the white wire on pin 2 of the 7-pin socket under the offending board. I swapped the 7-pin tubes and am still getting these second set of voltages from my OP. Is there some way to rule out a bad transistor?  I suppose I could swap the high current boards, and if the issue doesn’t follow the board then I must be missing a mis-wire or short.

[Paul Birkeland]

Can you tell me about the soldering products you're using?  I would very strongly recommend not adding flux to your build, as there is already flux in your solder.  If you pull both 6AQ5 tubes and run the amp, does the offending voltage pop back up? (It should go up closer to 200V)

[Toobuzz]

I use a Hakko FX-888D and Cardas Quad Eutectic.  I’ve never used flux on any of my builds, although I sometimes wish this solder was a smaller gauge. After pulling the 6AQ5 tubes I only see a small difference in voltages.

LC
IA 60
OA 40
KregA 0
bRegA 60
IB 205
OB 70
KregB 1.25
bRegB 205

Would swapping the high current boards help isolate the issue?

[Paul Birkeland]

Which flux does your Cardas solder have?

Are you 100% certain this build was working prior to the modifications? 

Next I would pull the wire out of the IA pad on the low current board that goes to 6U, then see if the voltage at OA on the high current board pops back up.

[Toobuzz]

I am using Cardas Audio Quad Eutectic With Rosin Core.  I have a .25 lb spool.  I pulled the red wire from IA on the LC board (other end still attached to 6U) and the OA on the HC board is only up to 64V.

I am 100% sure that is was working, both in stock form and with the Mundorf SO's on the LC board.  The EVO 100uf are new as well as the TwoQuiet (as they kinda needed to go in at the same time), but those mods don't seem  related at all to this issue.

Thanks for helping me.  While a bit stressful, these challenges actually maximize the enjoyment later on.  I was happy to have built the BeePre2 w/o any help, and then I was proud to have troubleshot an issue where I lost the right channel about 10 days in.  I'm still pretty new to the hobby but I feel like I can read a schematic and troubleshoot these things on my own, but this one is stumping me.

[Paul Birkeland]

My concern regarding the solder comes from your photos. There's light colored flux gooped all over everything.  That's not the color I would expect from rosin core flux and seeing it just all over the place is inconsistent with the flux flowing out form inside the solder. 

On the high current C4S board, disconnect the wire leaving OA and going down to the 5 lug terminal strip below it, then see if OA on the high current C4S board pops up into range.

[Toobuzz]

Well, I desoldered the wire leaving OA on the HC board by gently pulling down with tweezers, and wouldn't you know, the short wire fell to the chassis as it broke off from the 5-lug terminal strip.  This seems likely to be the root cause, if this wire has become compromised!  It passed my basic continuity checks but it was obviously extremely week.  I'll rework that connection and post my results.

Thank you for leading me directly down the (or a)correct path!

BTW, that flux certainly looks disastrous on the A tube base, but that was the only photo I used a flashlight to assist with lighting.  That clear/blue haze is a weird reflection from the flashlight and really just a shiny bulb of solder.  The yellow/gold splatter is all me using too much solder I suppose.

[Toobuzz]

I repaired the wire, connected all boards, and put all tubes in.  I am still getting the same voltages. 

I did get a 200V reading on OA of the high current board when there were no 6AQ5s plugged in, IA on the low current board was detached and OA on the high current board was detached.

I'm still getting those same voltages where IA on the low current and OA on the high current are only showing 50V.  The other changes I can report are that the LEDs on the low current board are all lighting (dimly), but one of the 2 LEDs in the 9pin socket is not lit. This is new as both were lit this morning.

[Toobuzz]

After dinner I’ll swap the high current boards. If the problem follows the board then that obviously narrows down the location the issue.

While she was running yesterday I noticed that the chassis plate was much warmer than the Mainline  or BeePre2. Not alarmingly so, but I though it worth mentioning.  I figured it was due to tight fitting ceramic tube sockets.

[Paul Birkeland]

I did get a 200V reading on OA of the high current board when there were no 6AQ5s plugged in, IA on the low current board was detached and OA on the high current board was detached.

Now reconnect OA on the HC board to the terminal strip but leave IA on the LC board detached.  I would not swap any boards at this time.  We still don't have things narrowed down enough for that to be helpful.

[Toobuzz]

Ok, we’re getting somewhere. After reconnecting OA to the high current board (from terminal strip), without 6AQ5s, and without OA attached on the low current board I am getting 200V on both IA and OA of the high current board.

Adding the tube makes OA drop to 50V.

[Paul Birkeland]

That would tend to point to a shorted TL431 on the center low current C4S board.

[Toobuzz]

Ok, excellent!  What can I do with this?  This doesn't look shorted to me, or am I missing something?

[Paul Birkeland]

I would recommend removing it and replacing it.  You can't just look at it and say with certainty that it's shorted.