Left Channel Out [resolved]

[heavycowboy]

Hey All,

My Eros 1 has been working flawlessly since it was repaired by Bottlehead more than two years ago.  However, yesterday the left channel suddenly went out (though it is slightly audible with the volume maxed). I went ahead and switched and swapped tubes to no avail. So, on to the voltage checks...

A Side                  B Side
Kreg 1.33             Kreg 18.1         
Breg 96.9             Breg 137.9 
OB 95.9               OB 139.2
BA 0                    BA 0
OA 168.8             OA 142.1   
IA 224.4              IA 149.9

I'm guessing I've got a cold solder joint somewhere on the b side that finally gave up the ghost. Any thoughts on where to look and reflow?

Thanks in advance,

Todd

[Paul Birkeland]

I would look for debris around the sockets.  The low voltage at IA indicates that either there is something wrong with the regulator above the 12BH7 socket (you would remove the connection at IA on the C4S board up front where you have 150V, then check OA/OB on the board back by the 12BH7 to see if it pops back up to 220V).

The 18V at Kreg means the servo is trying to pump the brakes really hard to get things back into alignment, which would be consistent with some kind of short somewhere.  The dragging down of the high voltage rail to me indicates that the current that would otherwise be available for the regulator to work is being consumed by whatever fault you have.

[heavycowboy]

Hi Paul,

Thanks for the quick reply. Unfortunately, after inspecting the sockets I couldn't find any loose debris that would be causing a short. I went ahead and removed the connection at IA on the c4s board to confirm that the voltage would pop back up to 220v on the 15bh7 board, and it did.

I'm hoping to give it a fresh set of eyes tomorrow. Do you have any other suggestions on where I should be concentrating my search for a short?

Todd

[Paul Birkeland]

Pictures!

[heavycowboy]

Here you go. Let me know if you see anything suspect. Thanks again.

[Paul Birkeland]

The area around pin B8 looks concerning.

With just the 12AU7 in and no tubes up front, what is IA on the font C4S board on the B side?

[heavycowboy]

With only the 12bh7 tube in, I'm reading 224 on the b side IA.

[Paul Birkeland]

Do you have a different 6922 to try?   

[heavycowboy]

Yes, I have a few.

[Paul Birkeland]

The cathode and plate of the B side of the 6922 are at about the same voltage, and the servo is sensing that and trying to adjust itself to stop that.  It could just be a tube problem, or something touching on that middle socket that shouldn't be, or debris on the socket, etc.

[heavycowboy]

Ok. I've tried a handful of 6922's, all with the same result. I've gone through that middle socket with a jeweler's loupe, deoxit, sewing needles, etc. and still can't seem to find any shorts or debris. How confident are we that the 6922 is the problem socket? Should I be investigating elsewhere or should I start gathering the courage to disassemble and reassemble that socket?

[Paul Birkeland]

What is the DC resistance between pin 8 and pin 6 on the 6922 socket?  Also check 8 to 7, as well as 6 to 7.

[heavycowboy]

8>6 ≈75k
8>7 ≈100k
6>7 ≈ 120k

[Paul Birkeland]

Those resistances and the consistency of voltages points to an issue with the C4S board on the B channel.  You can set your meter to beep when you touch the probes together, then see if any pair of legs on the MJE350 or MPS420 shows a dead short.  You may well just have a shorted transistor. 

[heavycowboy]

Still no luck. Can't find a short on either set of transistors. Anything else I should measure?