Stereomour II - Issue with Shunt Regulator?

[Paul Birkeland]

Now try just the A3/D3 jumper, then just the 10/11 jumper.

If the amp is OK with the A3/D3 jumper but not ok wit the 10/11 jumper, then you have a driver issue (9 pin socket) on that channel.  Typically when your DC voltages are OK but the sound is not,, that's a soldering problem or a broken wire that's just barely making contact where it's broken.  Sometimes these issues can be revealed by playing music through the amp and poking around with a wooden chopstick to see if pressing on any particular area dramatically changes the sound or causes pops.  This can help focus your efforts quite a bit.

[lskiii]

So here's what I found:

Jumper on 10/11 only - L and R sound the same
Jumper on 3/3 pins - L is weaker than R

Sounds like that narrows down the situation a bit?  I've visually inspected the L channel wiring many times and reflowed anything that looks fishy to me.  I'm open to any suggestions.

Thanks!

[lskiii]

Not sure is this helps, but I also just tried it with no jumper at all. 

No jumpers, Left is substantially weaker than Right.  Right sounds great.
3 - 3 jumper, Left is weaker than Right but not as extreme.  Also, Right sounds very bass heavy.

I tried reflowing a bunch of connections on the left channel just now.  Still no change. 

I guess my last hope is to tap on every connection on the left channel while the system is playing?

Thanks

[Paul Birkeland]

These results now seem to contradict what you were experiencing before.  If just a jumper between 10 and 11 appears to solve your problem, then you have a wiring/solder problem between the RCA jacks and the amp itself.  This could be something like a loose resistor in your attenuator if you have that upgrade or a bad solder joint on one of the pots.  Jumpering 10 and 11 combines L+R outputs from the input selector and attenuator. 

[lskiii]

I figured it made sense to start over a bit, so I did a full resistance and DC test and I think I might have found something.

Every test checked out except for 14.  I was reading 260.  I looked around at what that's connected to and it's connected to the small board.  On the small board there are 2 heat sinks.  The heat sink that goes to 02B was completely cold to the touch, while the other was warm.

Would this offer any clue as to where to look?  I'll try re-soldering the small board right now just in case.

Thanks

[Paul Birkeland]

That feeds terminal 14 then down to the 9 pin socket on that side to pin 1.  If you have the 10/11 jumper, does that DC voltage change?

[lskiii]

10/11 jumper - no change, still reading 260 at 14.

Just for the heck of it, did the A3 / D3 jumper and it was still 260.

[Paul Birkeland]

For you to get 260 there, here are some possibilities:

1.  Bad solder joint or missing jumper between pins 4 and 5 on the 9 pin socket.
2.  Loose cathode resistor going to pin 3 on the 9 pin socket.
3.  Missing ground reference from attenuator issues (ruled out with the 10/11 jumper).
4.  Bad solder joint on the small PC board itself, especially the center joint of the MJE5731A.
5.  Broken wire getting from the "O" terminal to the 9 pin tube socket itself.

What DC voltage do you see at pin 3 on that 9 pin socket?

[lskiii]

I measured 2.53V on pin 3.  Same reading with or without the jumpers.

[lskiii]

1.  Bad solder joint or missing jumper between pins 4 and 5 on the 9 pin socket.

I just did a resistor test and both pins 4 and 5 go to ground, and the solder looks really solid.

2.  Loose cathode resistor going to pin 3 on the 9 pin socket.

Is this the resistor from the center of the socket to pin 3?  If so, I did a resistance test and it's .366

3.  Missing ground reference from attenuator issues (ruled out with the 10/11 jumper).

4.  Bad solder joint on the small PC board itself, especially the center joint of the MJE5731A.

I reflowed the solder, but it looks good to me.  Is there a way to test the MJE5731A? 

5.  Broken wire getting from the "O" terminal to the 9 pin tube socket itself.

Did a resistance test between the O terminal on the small board and pin 1 and it's a good connection.

[Paul Birkeland]

You can flip the small PC board 180 degrees and reconnect it to see if the problem is on the board or off the board.

[lskiii]

I think we're getting there -
I flipped the board, and now 14 (OA) measures 218 and 7 measures 263.

7 is now going to OB and 14 to OA.  I flipped IA and IB as well w/ 9 and 12.  They measure the same - 297.

Seems like the issue followed the A side of the small board?  Would replacing the small board be the thing to do?

Thanks!

[Paul Birkeland]

I would reflow the joints on the transistors.  When you get high voltage out that is very frequently a cold joint on the center leg of the MJE5731A.  As a last resort you could ask replacement parts about getting a new board, LEDs, transistors, and resistors.  Just replacing the transistors on that side would also likely do the job.

[lskiii]

I just tried reflowing and that didn't do the trick.

As a last step, I just checked the sound and the weak channel is now on the right side.  So, clearly the issue is somewhere on that small board.

I'm going to just buy the parts and swap it out. 

Thank you for your help and patience.