Another failed voltage check

[SamJ]

Same as figby, after a good resistance check and lots of comparing picture to result, I have some wayward DC voltages. Of note the two LEDs for the right channel aren't lighting up.  I reflowed all the connections, including at the terminal strips.  All tubes glow normally and I haven't had any pops or buzzing, smoke, etc.  I'm sure I screwed something obvious up. 

here are my wayward voltages:

14. 1.856 (170-230)
15. 0.302 (58)
17.  1.007 (0)
19.  2.996 (395)
20.  2.848 (365)
34.  3.014 (392)
36.  .305 (58)
37.  .302 (58)
39.  .302 (58)
40. .299  (58)

Sam

 

[Paul Birkeland]

Do any of the 130 ohm 2W resistors measure open when you check them with your meter (with the amp off)?  If so, you likely plugged in a 2A3 without paying attention to the fat vs. skinny pins, which will destroy one of those resistors and leave you with dead voltages on one side (or both if you are unlucky and force both 2A3 tubes into the sockets the wrong way). 

As always, it's also a good idea to post build photos along with stuff like this. 

-PB

 

[SamJ]

PB,

I'll check that and get some photos up as well.  I don't THINK i had the 2A3s installed incorrectly, but there's alway a chance there. 

Thanks for the quick response!

Sam

 

[SamJ]

The 130 ohm 2w resistor between 31 and 34 is reading either open or sometimes something really low like 1.9 (tough to get the leads in there), so looks to be bad.

 

[Paul Birkeland]

I don't see anything obvious.  I'd snag some replacement 130 ohm 2W resistors from us or online, then attempt to fire up the amp again once that open resistor is replaced.

 

[SamJ]

On it!  Will report back. 

 

[SamJ]

Swapped out the bad resistor, everything checks as per the instructions EXCEPT 14 is reading 391 (vs. 170-230) and the LEDs for the right channel are still not illuminated.  I reflowed all the connections on the PC board for that channel, and I THINK they all look good (I watched Doc's video on the subject).  The board is looking kinda rough with all the work I did on it, maybe I just start over with a new board?  Or re-re-reflow connections and try again? 

Sam

 

[SamJ]

Forgot pics...

 

[Paul Birkeland]

Can you see both 12AT7 tubes glowing?  The only joint that often needs reflowing on the C4S board is the center leg of the MJE5731A, and it can be a bad idea to reflow the solder joints for the LEDs.

-PB

 

[SamJ]

PB,

Replaced the LEDs, and the MJE5731A on the right channel just to be sure.  All 4 LEDs now illuminate, both 12AT7s, but 14 is still reading high (349vs the previous 391).  I checked the MJE5731 joints, they're little silver mounds on both sides. 

 

[Paul Birkeland]

I can see on your PC board that you haven't heated the joints well enough, as the solder hasn't sunk into the board into the through holes.  Please reheat the MJE5731A joints, especially the center one.  Hold the iron on the joint for a good long 10 seconds to be sure the solder flows into it. 

Definitely do not shotgun random parts at something like this, as you can make new problems.

 

[SamJ]

Ok, fair enough.  I'll revist the board and report back. 

 

[SamJ]

So i've reflowed, tried diff. temp settings, a smaller tip on my soldering iron, everything I can think of.  Joints are shiny little mounds on both sides, readings are now 162 on the right channel, 234 on the left channel.  Within 10% on both sides, so should I be concerned about the split at all? 

 

[SamJ]

...I was struggling with one joint (the outboard leg) that simply flowed through and down the lead leaving a gaping hole in the opposite side.  Had to clean a bit of solder up off the leg.  I believe the leg wasn't touching either side of the hole, but was able to put a little pressure on it while i heated it up and make solid contact.

 

[SamJ]

I went ahead and re-did the center leg, seems like it worked b/c voltages are now 208 on 14 and 220 on 7, so well within limits.  Appreciate the help with this, I was pulling my hair out.  I did a complete voltage check and everything is within a few percent.  Going to hook it up and give it a listen later this morning. 

Sam