Need a bit more help with Mainline [resolved]

[Larpy]

Turns out I was a bit hasty thinking my Mainline was entirely up to speed at the end of my previous thread, so I'm starting a new one.

Here's the lingering issue.  The B side LEDs on the left channel 4CS board ("C" side of the chassis) don't seem to light up.  I measured the voltage going across them and it's 146v, lower than the 220v across those LEDs on the right channel.

All of my measurements are OK except for the Breg on the B side of the regulator board; it's 146v instead of 220v.  I checked and rechecked my work (reflowed solder connections and rebuilt the B tube socket), and I can't find the problem.

Both tubes are biased at 145v.  Amp sounds fine except center vocals are shifted a bit to the right channel (that's what got me thinking there was still a problem).

Those of you who understand the circuit well, any thoughts on what the culprit might be?

 

[Paul Birkeland]

What's IB and OA on the offending channel?

 

[Larpy]

IB = 147v
OA = 148v

 

[Doc B]

On the regulator board - are the black jumper wires that are connected to IB connected to the correct pads at the other end? Should be one going from IB to -reg on the B side, and another going from IB to IA. If that second one is mistakenly going to OA rather than IA, that could create the situation you have.

 

[Larpy]

Thanks, Doc.  I checked and the jumpers are wired correctly.

 

[Doc B]

Good, because I was wrong with my evaluation. I was confusing the reg board and the C4S board. On the C4S board the red wire should indeed connect OA and IB. If you have a spare, a simple test might be to try a different 12AU7 in the center socket and see if the IB voltage changes at all.

 

[Paul Birkeland]

Here are some things to check:

1.  Are the R1 resistors on the high current C4S board on the offending channel swapped by chance?
2.  If you pull out the 12AU7, does the IB/OA voltage rise up to around 270?
3.  The Kreg voltage on the side of the center board where you measured the low Breg voltage should also be off, what voltage do you see there with all the tube installed?

-PB

 

[Larpy]

Thanks for the advice, guys.  Swapping in a different 12AU7 makes no difference. 

The R1 resistors are in the right spots.

Removing the 12AU7 does not change the voltage:  it stays at 148v.

On the regulator board, the A side Kreg (good channel) measures 11.8v.  B side (bad channel) measures 10.7v.

 

[Paul Birkeland]

I hate to say it, but there may be a damage transistor on that high current C4S board that was caused by the lack of a solder joint (I know because I made the same mistake on an unrelated project a few days ago).

I would go ahead and replace the MJE5731A and PN2907 on the half of the board where the open joint was, and I bet your issues will resolve themselves.  (The big, blue spring loaded solder sucker makes this job pretty easy, solder braid or the solder iron with the squeeze ball will make it a real nightmare)

 

[Larpy]

Delivery from Mouser this morning--so I fired up my solderign station, took out the bad side C4S transistors and put in the replacements and . . . no difference(!).  That wasn't it.

At this point, assuming that the problem must be somewhere in either the regulator board or the left channel C4S board, I think I'll just contact Eileen and order replacement all the parts I need to start fresh on those two boards.

It can't have anything to do with the PS board, right?  I'm getting 275v on both channels from there.  The problem doesn't manifest itself until the regulator board.  And it can't be any of the stoppers, right?  And I've ruled out the tubes.  And I've reflowed solder on all of the ground connections.  So all that's left are the boards.  Could it be specifically the A side 2N2222A transistor or LM431A regulator on the regulator board?  Could either of them been damaged by the unsoldered join on the C4S board?

Thanks again for the help.  I'm chalking this experience up to learning a lesson about patience.

 

[Paul Birkeland]

Pull the 12AU7 and remeasure the IA, OA, and OB voltages on the offending channel.

-PB

 

[Larpy]

IA = 275v
OA = 145v
OB = 142v

 

[Paul Birkeland]

IA = 275v

Good.

OA = 145v
OB = 142v

These are pretty telling.  When you pull the 12AU7, you take the hybrid shunt regulator out of the circuit (to double check, you can measure the Kreg that feeds that channel).  Your OA voltage should pop up to something like 270V when you do this (you can check against the working channel since it will still operate without the 12AU7)

Does the bias voltage on the center board move when you adjust the trim?  I still believe this is some variety of unhappy transistor on that C4S board.

 

[Larpy]

Yes, the bias voltage moves on both channels.

I've now replaced all 4 transistors on the bad C4S board (I replaced the ones on the "bad" side of the board yesterday and the ones on the "good" side today).  Still no change.

Perhaps I'm being punished for bad behavior in a previous life.

 

[Paul Birkeland]

Swap the high current C4S boards from left to right.  That will conclusively tell you where the actual issue is.

It's odd to have adequate voltage at Kreg but low regulated B+ voltage.  Usually that happens when there isn't enough current for the regulator to operate properly (this will also cause the voltage at Kreg on that channel to go high).  I suppose this could also be a PN2907 and LM431 swapped, though your voltages aren't quite in line with that issue either.

 

[Larpy]

Great idea (why didn't I think of it?!).

Turns out that the problem follows the board, so that's conclusive:  it's that particular C4S board that is the problem.

I removed the board and tried to unsolder all the parts so I could reinstall them, but I mangled the LEDs and a few of the resistors in the process, and I've now soldered and unsoldered and resoldered the IA, OB, and bB pads that I'd like to start over from scratch.  So I just e-mailed Eileen to order a replacement C4S board and components.

Once they arrive and I build and install it, I'll report back.  In the meantime, thank you so much for your assistance.  When I couldn't figure out what the problem was on my own, I felt helpless.

 

[Paul Birkeland]

I'd recommend sending an e-mail to the replacementparts(at)bottlehead(dot)com e-mail.  Pasting a link to this thread into your email can be helpful as well.

 

[Larpy]

Replacement C4S board and components arrived and I built and installed it last night.  The amp works perfectly now.  LEDs all aglow and all voltages spot on.  I was hoping there would be an "aha!" moment replacing the old board where I spotted the problem, but nope--some unseen gremlin was the culprit.

In any case, right now I'm listening to the Mainline through Audeze LCD2s and it sounds quite nice.  Neutral-sounding and quite resolving--a much better match for the Audezes than my homemade Wheatfield HA2 (an OTL Pete Millett circuit).  Bass is significantly tighter.

Thank you, PB, for guiding me through troubleshooting. A frustrating couple of weeks, but well worth it now that the amp is finally happy.