Voltage issue from C4S board... 1st build, 1st post

adastra · 3685

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Offline adastra

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on: December 21, 2011, 06:14:53 AM
Hello all,
Go easy, Im a newbie at this, but I've followed the forum with interest for a while.  So I'm finishing up my Seduction build, with C4S, otherwise stock, and I've run into one issue.  Resistance checks all came out good.  Upon firing it up for the voltage checks, the first thing I noticed is that the bias LED on A3-A9 has failed to light up.  Continuing with voltage checks, everything looks good until Terminals 30 & 40.

30 is correct at 70V, while 40 shows low microvoltage that begins ramping up into the hundreds of millivolts while metered

then...

B1 is correct at 70V, while A1 shows no voltage

then...

B3 is correct at 1.6V, while A3 shows .65V which is less than 1/2 of spec.  

I'll be rechecking solder connections of course, but any insight you guys might have would be much appreciated.  Again, I'm an experienced hand at soldering and electronics builds, but I'm no engineer and I'm new to tubes.  Need some help here, my LP12 anxiously awaits!
Thanks.

Edit: also, working backwards from A1, I determined that there is no voltage at T32 or before that at the center pin of the C4S transistor that connects to T32. I metered the MJE transistors and the other 3 all show 130V on the outer pins, and 70V on the center pin.  The problem one (connected to T32) shows 130V on one outside leg, 120V on the other, and nothing on the center pin.  Bad transistor?
« Last Edit: December 21, 2011, 06:23:42 AM by adastra »



Offline Doc B.

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Reply #1 on: December 21, 2011, 06:40:06 AM
Check to make sure that the 2n2907 metal can is not touching a PC board trace.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline adastra

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Reply #2 on: December 21, 2011, 07:02:51 AM
Thanks Doc, working on it.  I did fix the issue with T40, so that's one less thing... 



Offline adastra

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Reply #3 on: December 21, 2011, 07:27:25 AM
Got it! Bad solder on one post of the 2n2907 can.  Everything checks out, now time to give it a listen!

Thanks very much.



Offline Grainger49

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Reply #4 on: December 21, 2011, 08:06:58 AM
Since transistors like that are prone to being damaged by heat I solder them with long legs.  They end up looking like the alien crafts in War Of The Worlds.  But I haven't burned one up since I adopted that method.

Don't feel rained on!  My Paramour build was botched by a missed solder point on one of the transistors on the C4S boards. 



Offline adastra

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Reply #5 on: December 21, 2011, 12:25:57 PM
Thanks for the encouragement guys.  Progressing. So, now all voltage and resistance checks are good, we are passing audio without much noise, BUT one channel is considerably brighter than the other.  Its a little hard to say which is correct, but I'm thinking its the brighter one.  Swapping inputs doesn't affect it, the extra brightness is linked to one channel's output only, so I know its coming from the Seduction.

Any thoughts on that?  I'm checking and rechecking the connections on the passive eq filter, not seeing any issues there and the values are quite closely matched so far.



Offline Grainger49

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Reply #6 on: December 21, 2011, 02:14:07 PM
First thing is to swap the tubes from right to left.  See if the noise follows the tubes.  Sometimes the act of swapping cleans the tube pins and sockets and it goes away.



Offline adastra

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Reply #7 on: December 21, 2011, 03:00:53 PM
Thanks Grainger, I tried that and the noise stayed in the same channel. Think I may have found the culprit.  The 9.31kOhm resistor on T29-30 is cracked/overheated.  Looks like I need to replace it.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi33.photobucket.com%2Falbums%2Fd88%2Fbrentlind%2Fphoto-5-1-1.jpg&hash=c76c5b5ab902a2c15beca704a047d7c0a95afdd8)

BTW, Grainger, love your stuff, spent a lot of time on your website getting inspired.
Cheers.



Offline Grainger49

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Reply #8 on: December 21, 2011, 04:05:50 PM
Hmmmm... Are we talking about WW Grainger?  No relation, sadly.



Offline adastra

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Reply #9 on: December 21, 2011, 04:12:13 PM
woops, had you mixed up with someone else, but regardless, thanks for the help and for being so active on the forum.  Good to know there is a little assistance out there for the uninitiated.



Offline Doc B.

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Reply #10 on: December 21, 2011, 06:50:52 PM
It doesn't look overheated. I would guess that it got cracked during installation - easy to do. Call Eileen tomorrow and she will mail you a replacement.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.