Rt Channel worked then very low volume

[Mike class A]

Just finished building stereomour for buddy who is moving to hawaii.
I did soldering and he did the mechanical stuffing...  Told him to read directions but he did a christmas
day dad build(look at Pics and forget to read instructions)... So all the resistance readings were fine.  Next did voltage and the first 5 I took were off so I checked schematic and saw filament was only on 12at7 and none on the 2a3's.  Looked around and saw that he had put sockets 180 Deg. out ... fixed that and tried it again.  All is good.  Then I tried that last step on hum adj.... no variance from end to end and constant 1.1mV.  Fired it up and heard sound out of both channels with spl @88 8 ohm was maxed out to fill 16 x 20 room so turned off amp and switched the Impedance boards from 8 to 4 ohm and played only out of left side and very faint in right so went back to the 8 ohm setting and still no sound in RT channel to speak of.  I have reflowed the impedance board switched the 2a3 side to side and changed inputs and switched inputs as well. Soldered all a side of socket and the pins 1-5 and the 1.6k resistor area... (sorta played 5 seconds) still no sound.  Could this be a failed 12at7 tube?  Thinking about lifting the .1uf caps and running wires to swap sides to isolate the pre amp from amp to see it that is issue???  Also might take impedance board out and wire to 8 ohms??  But before I do all this is there any suggestions I am over looking?

 

[Paul Birkeland]

Start with a voltage check.

-PB

 

[Mike class A]

Here is both measurements:
      Resistance              Voltage
1.  468k                        376V
2.  3.39m                      178.2V
3.  0                              0
4.  248.2k                      0
5.  1.398k                      59.3V
6.  0                              0
7.  1.6ohm-101.9k          0
8.  0                              0
9.  1.8ohm-86.2k            0
10. 0                              0
11. 1.392k                      60.6V
12. 248.2k                      0
13. 0                              0
14. 3.39M                      179V
15. 470k                        374.7V
16. 1.178k                      59.6V
17. 502k                        405.6V
18. 0                              0
19. 502k                        405.1V
20. 1.173k                      60.6V
A1 1.191k                      60.3V
A2 500k                          375.3V
A3 249k                          0
A4 1.191k                      60.3V
C1 1.186k                      61.3V
C2 502k                          374.7V
C3 248.1k                      0
C4 1.186k                      61.4V
IA                                  405.5V
IB                                  405.5V
OA                                  179.7V
OB                                  179.5V
Kreg  both measurements 2.497V 2.492V
-reg  both measurements 0-0
C1-C4                              2.486V
A1-A4                              2.479V

Nothing jumps out except the volume pot measurements.

 

[Paul Birkeland]

Both your 2A3's and your 12AT7 are working properly, which is good.

You can download a 60Hz tone to your phone or to a computer, then feed that into your Steremour.

Crank the volume up all the way, then use your volt meter on AC volts to trace the voltage through the amp.  I'd start at the jacks, then work through the volume pot.

At the output of the 12AT7, I would measure the AC voltage where the 0.1uF capacitors and the 249K resistors are connected together.

If you have relatively similar levels of AC at these points, then yes, I would pop out the impedance switching board. 

If the impedance board is causing your problem, you would also not hear any hum with your speakers (or that light scratchy sound when you move the hum pot).

Let us know how it goes!

-PB

 

[Mike class A]

I did the 60hz using my iphone using a youtube video.  The output on the "A side on 2a3 was 1/3 the voltage.  So I fired it up and started moving parts around and came to the OT-2 and pins 6 7 8 when moved would work.  Resoldered and the lugs had cold solder joints on windings side.  Ran 3x anywhere from 30 mins to 1 hour and then weak sound again.  Soldered 5x but those pins are damaged the rest are fine.  I hope I can get the parts in before he leaves Monday.  If I had mine that I ordered at same time would have those parts yo use.  Oh well thanks for the help and will see if I can get that part out and hope this reading helps another Class A builder.

Michael

 

[Paul Birkeland]

I did the 60hz using my iphone using a youtube video.  The output on the "A side on 2a3 was 1/3 the voltage.  So I fired it up and started moving parts around and came to the OT-2 and pins 6 7 8 when moved would work. 

Hello Michael,

When you said "A side on 2A3 was 1/3 the voltage", does that mean that the voltage passed equally from channel-to-channel through the selector switch and volume control?

Is this at the input of the 2A3 or the output of the 2A3?

-PB

 

[Mike B]

The tabs on the transformer are damaged?

 

[Mike class A]

The voltage I was comparing to was: side vs side and the output was 1/3 the voltage of the good side.

The solder tabs flexed on pins 6 7 8 which was a problem for me since I installed the impedance matching
board. And yes I soldered the add-on and ohm checked it and soldered the feed thru holes and was about to run pass thru wire thru the open holes since I have seen this alot be issues in logic boards but saw the flexing fix the issue.  Pushing on the plastic lightly really exposed the bad windings issue.

This sure sounds nice the few hours that I have heard when test!

 

[Paul Birkeland]

The voltage I was comparing to was: side vs side and the output was 1/3 the voltage of the good side.

Again, where are you measuring this voltage? (at the output)

 

[Mike class A]

Yes the output.  That's what made me start looking at the OT-2 area.

 

[Paul Birkeland]

Measure the 60Hz signal at terminals 4 and 12, that is the output of the driver stage.

If those are equal, try at 5 and 15.

-PB