mainline help request - 2 LED's dark [SOLVED!!]

[dubiousmike]

[EDIT - for those reading this thread in connection with their own trouble shooting, my ultimate issue, as described at the end, turned out to be a bad joint on R3 on the reg board, the 2.49k resister that, combined with R4, determines the variable output of the LM431 (labeled Kreg, which sets the cathode voltage on the 12au7).  If Kreg is too low, the regulated voltage going into OA/IB on your C4S board will be too low to light up your B-side LED's!]
 


Just completed the initial stab at my mainline, and apparently I took a misstep somewhere along the way.  The two LED's on the B side of the C4S board above my A socket aren't lighting up.

All resistances and most voltages check out, except for the Breg and Kreg measurements associated with that board.

       Bad Side    Good Side
Breg    163             220
Kreg    1.8              10.8

My tubes are glowing, and I just finished double checking the orientations of all the components on the problem board.  Are there any other measurements or steps I might take to help isolate the problem?  Thanks in advance for your help!

-Mike

[Paul Birkeland]

Check that the resistors in each R1 position on your faulty sides are the correct values.  Swapping those can throw things way out of whack.

[dubiousmike]

Thanks for the quick response.  Just checked the resisters (R1) on the C4S boards, and they are as the should be.  37.4 ohm on the A side of the board and 49.9 on the B side. 

Any other thoughts as to what is in series with the two LEDs that I might check, measure or resolder?

Here's a link to a photo of the problem board if that would help diagnose the issue: http://cdn.head-fi.org/a/a7/a7e8c330_20131102_160013.jpeg

[Paul Birkeland]

What do you have at IA on the funky board?

How about OB/OA on the center PC board?  (Does turning the trim pot affect these voltages?)

[vetmed]

I'm not familiar with the circuit but looking at your photo there appears to be a difference in the # of wires connected to the bA and bB pads. Maybe something is not connected that should be?

Robert Lees

[Paul Birkeland]

Oh, also, the TL431 and PN2907 look pretty similar, they are easy to swap by accident.

-PB

[dubiousmike]

Thanks for the further suggestions.  PB - I double checked and do have the tl431's and pn2907's in their proper places.  Robert - The A and B sides of the C4S board are supposed to be wired asymmetrically, but I plan to go back over that section of the manual tomorrow with fresh eyes. 

Is it possible to measure one or more of the pins of the 2907's or main transistors to see if I might have a dud (or I suppose I could have damaged one while installing the heat sinks)?  Do you guys have the circuit in splice or some software that would allow you to tell me the target voltage at some other points on the C4S board? 

[Doc B.]

Somewhere there is probably a sticky about checking transistors. Basically a transistor is a set of diode junctions. So the way to test one is to do a "diode check" (resistance) between each possible combination of leads. They should all read about, say, 1K ohm. A transistor is dead if any pair has a very low resistance reading, like around 50 ohms.

[Paul Birkeland]

Can you remove the screws from the PCB and roll it over, then shoot another pic?

Also, did you swap 6C45's side-to-side?  Do both 6C45's glow?  The problems you describe could all be attributed to the 6C45 on your offending side not being able to draw any current.  This could occur if you miss the step labelled in red when building the center PC board, not having the heater feed going to tube socket pins 4/5 connected properly (which would mean no glow), or having a miswire at the outer sockets themselves. 

[dubiousmike]

Thanks for the further help Doc and PB!  First, let me address the below because I forgot to respond to it before:

What do you have at IA on the funky board?

How about OB/OA on the center PC board?  (Does turning the trim pot affect these voltages?)

IA on both boards is 278v.  OA appears to be where things get interesting on the problem C4S board (with the unlit LED's).  This would seem to make sense, because if OA isn't delivering the right voltage to IB, then perhaps the LED's on the B side wouldn't light? 

Breaking it down - on the bad board, OA reads 163 - whereas it is 220 on the good C4S board.  This same voltage (163) is at terminal 22, on +reg of the B side of the regulator board, and on pin 6 of the B socket.  Does this mean my real issue is on the regulator board?

In answer to your other questions - OB and OA on the regulator board both read around 2.0, and the trim pots do affect these voltages on both sides.  Also, I double checked the red instruction (bending the trim pot pins), and it looks like I did that correctly.  All tubes glow, and I have tried swapping them with no effect.  I have also tried a different 12au7 that I know is a good tube.

For further comparison purposes between the C4S boards:
      Bad Board     Good Board
IA      278               278
OA     163               220
IB      163               220
OB     161               174

Thanks again for your ongoing help!   

[Doc B.]

I would be inclined to check the A side transistors on that bad board.

[Paul Birkeland]

You can also yank the 12AU7 and run the amp without it, though I doubt that will fix your issue.

I would focus on comparing the wiring between the 9 pin sockets. (There are some differences, as some pins are tied together internally).

I'd also reheat the joints on the jumper that goes from bA to bB on the offending C4S board.

-PB

[dubiousmike]

Thanks again so much for the further help!

I did discover an apparent error: my problem C4S, A-side MJE5731A was installed in the inner set of three holes, instead of the outer (as depicted in the manual).  Sadly, moving it did not fix the issue.  In fact, as I only pieced together afterwards - the two sets of holes appear to be shorted to one another.

On the upside, when I went to measure this - I think I pretty much confirmed doc's theory  that that particular transistor is the problem.  The middle/collector pin on the A-side MJE5731A is evidently the first spot in the circuit where my undesirable 163v shows up.  By contrast, the collector pin on my A-side MJE5731A on the good C4S board reflects the proper 220v.

Does it make sense for me to reach out to Josh about a replacement MJE5731A at this point?

PB - I forgot to try pulling the 12au7 when I had her fired up, but I did go back over all the wiring of all three sockets and the wiring from them to the various boards, and it all looks right.  I also metered bA to bB and it has continuity and 0 resistance - so that shouldn't be the issue.
 

[Paul Birkeland]

Yeah, a backwards MJE5731A will cause this problem.

We can send you a replacement if you like, they are not all that easy to obtain locally.  I'd also recommend replacing the PN2907 that works in conjunction with it, just as a precaution.

-PB

[dubiousmike]

Thanks PB - that would be great!  I just sent an email to [email protected] confirming my request for those two transistors, my address info, and including a link to this thread.

Any chance you could steer me towards some reference on the best way to desolder a PN2907 if it turns out to be necessary?  I assume picking up a solder sucker might be a prerequisite?