Newbie looking for some diagnostic help (Solved)

[elawyoung]

I just recently attempted to build the Stereomour 2A3.

At the end all resistance checks were good.  My problem is on the DC voltage checks.  Below is a summary of my readings.  Any help with a diagnosis would be greatly appreciated.  All 3 tubes filaments began to glow after a few seconds.  One symptom I noticed was the 5W 4K7 and 10W 1K6 resistors on the A side were getting really hot, so much so that the black lettering turned brown and started to fade.

AC Check with the manual values in ()
A1/A4 2.52 V  (2.5 V)
C1/C4 2.53 V  (2.5 V)

DC Check with the manual values in ()
HV+ A side 372 V  (380 V)
HV+ C side 410 V  (380 V)    Out of Range

Terminal
1  280  (350 V)    Out of Range
2  215  (165-220 V)
3  0  (0 V)
4  0  (0 V)
5  162  (60 V)    Out of Range
6  0  (0 V)
7  0  (0 V)
8  0  (0 V)
9  0  (0 V)
10  0  (0 V)
11  0  (60 V)
12  -.5  (0 V)    Out of Range
13  0  (0 V)
14  198  (165-220 V)   
15  410  (350 V)    Out of Range
16  161  (60 V)    Out of Range
17  372  (380 V)
18  0  (0 V)
19  410  (380 V)    Out of Range
20  0  (60 V)    Out of Range

A1  162 (60 V)      Out of Range
A2  278 (350 V)    Out of Range
A3  ~0 (close to 0)
A4  162 (60 V)    Out of Range
C1  0  (60 V)    Out of Range
C2  410 (350 V)    Out of Range
C3  ~0 (close to 0)   
C4  0  (60 V)    Out of Range

IA  410  (380 V)    Out of Range
IB  372  (380 V)
OA  216  (165-220 V)
OB  198  (165-220 V)
Kreg A side  2.48  (2.5 V)
Kreg C side  2.48  (2.5 V)
-reg A side  0  (0 V)
-reg C side  0  (0 V)

Thanks for all the help.

 

[Paul Birkeland]

Neither of your output sections are drawing proper current.

The C side is particularly disturbing, do any lugs of the hum pot touch the chassis plate?

Can you post some photos of your build? 

Don't run the amp for more than 10-30 seconds when rechecking voltages, you have a short through the high voltage supply that is going to do some damage if left running for very long.

-PB

 

[elawyoung]

I can post some photos later this evening, are there any specific angles or connections that would be most useful to see?

-EY

 

[elawyoung]

I've attached some photos of the build.  I'm a little nervous I may have done some damage.  I powered the amp for about 5 minutes during the initial testing.  I went back over all the connections this evening and found that the wire connection from B9 of the nine pin connector to terminal 8L was not soldered on the B9 side.  I'm not sure what this would have done.  Everything else looked okay to me.  None of the hum pot lugs were touching the chassis.  After correcting B9 I quickly re-tested, but got the same results.  I also noticed that the 5W 4K7 and 10W 1K6 resistors overheat very quickly.  I only powered up once for the quick re-test for about 10 seconds and the resistors were very hot.

 

[elawyoung]

Photo

 

[elawyoung]

Photo

 

[elawyoung]

Photo

 

[elawyoung]

Photo

 

[elawyoung]

Photo

 

[elawyoung]

Photo

 

[Paul Birkeland]

Will you run the amp without the 2A3's, then report the following voltages:

IA/IB on both sides of the PCB
A2 and C2
A1 and C1

-PB

 

[elawyoung]

Results without the 2A3's.

IA 441 V
IB 441 V
A2 441 V
C2 441 V
A1 0 V
C1 0 V

-EY

 

[Paul Birkeland]

Where does the red choke wire on the A side land?  I would wonder if it is somehow shorting to the 1K6 and 4K7 resistors.

-PB

 

[elawyoung]

Choke wire was clean into 17L. So I decided to do one more close inspection, especially underneath the PCB. I noticed that pins B7, B8, and B9 were very close together. I think maybe B8 and B9 solder joints were touching. I got in there and spaced them out... All voltage checks were clean.  I think when I mounted the PCB and positioned the connection from B8 to the PCB I torqued the pin toward B9.

Now I'm ready to make sure they stay clear, and move onto the hum balance and some tunes.

Thanks so much for your help and quick responses.  I can't wait to get on some music.

 

[Paul Birkeland]

Did that clear up your voltages? It wouldn't be impossible, but seems unlikely.

 

[elawyoung]

That cleared up all the voltages.  Got it hooked up and it sounds great.

From the time I first tested the only things I changed were to solder B9 (the wire connection was pretty tight so I don't think there was much of an issue there).  Then this evening, I noticed the spacing of B7, B8, B9 were pretty close.  It may have been B7 and B8 touching or B8/B9 or all three.  But after making sure that there were gaps between all three I checked the voltages and everything was right in range.  I didn't de-solder any connections, but there was some type of short that caused the resistors to overheat and that lettering to fade as you can see in the picture.  I moved through the hum balance and sat back and enjoyed some music.

Thanks again for the support.

 

[elawyoung]

Unlikely indeed. After finishing the base, I hooked it back up and the problem resurfaced. I thought back over anything that changed and I'll have to own up to a rookie mistake. When I first placed the 2A3 tubes I didn't line up the correct connections. When I read in the manual that it might be difficult to insert them the first time I was a little too focused on getting it in the way I had it lined up and forced it. I've noticed now that there is some play in the 4 pin connections and when I position them right, everything works great. But a slight bump will cause the A side to cut out.  I forced open the pin connections and slightly scuffed the tube connections by inserting them incorrectly.  Would this cause the issue?  Do I need to consider replacing just the 4 pin connections or the tubes as well?

 

[Doc B]

Just tighten (bend) the pin contacts in the socket so they make good contact again.

 

[elawyoung]

I was able to pinch the contacts together and make a nice tight connection. Everything looks good and sounds great. Thanks again for the help.