Intermittent loud channel noise [Resolved]

[jjvornov]

After working fine for months, my BeePre made a loud scratching noise from the L channel. When I put it on the bench the LEDs on the B side of the C4S Board. Reflowing solder joints restored the channel but only for a while. I've had the preamp in out out of the system a dozen times, thinking I've got it fixed. Each time it works fine for a while (hours), then the noise comes back. It seemed like mechanically associated with the 300B tube, but wiggling, tapping etc doesn't help.

Now I'm just able to get the LEDs flickering some times, but generally its not working at all.
Voltages are off around the board:

* 11  146V+/-1%     37 
V
* 12  0V 

* 13  0V 

* 14  0V 

* 15  186V.           225 V
* 16  5V                  5
 V
* 17  90-110V         31
 V
* 18  0V 

* 19  9.85V            9.72
 V
* 20  0V 


Voltages at terminals 27-44 are all fine.

Any ideas of where the fault might be?

James

[Paul Birkeland]

Reheat all the solder joints on each MJE5731A.

[jjvornov]

Paul

No luck in getting it working. However, I think you're right about the site of the fault. I wiggled the heat sink of the MJE5731A on the OB/IB side of the board and got the B side LEDs to flicker on and off. I took the board out and resoldered both sides and checked continuity with the board, so I don't think its a bad solder joint. Possible that the MJE5731A itself is at fault?

James

[Paul Birkeland]

You can certainly replace them.  The middle leg of the MJE5731 is connected to the metal tab of the transistor, which is then mounted to the heatsink.  Consequently, soldering that joint can take a ton of heat!

[jjvornov]

Paul

I"ve made some progress. I've got all the LEDs lit but 11 is now high at 188. and c3 is only 1.1 and the EL84 on that side is not glowing at all. In the system, it sounds okay which I gathered is expected from other threads trouble shooting the voltage regulation.


James

[Paul Birkeland]

That is progress of sorts.  Pull the 300B on that side and recheck C3 and 11.

[jjvornov]

11 comes up to 219V and c3 is still 1.3V

James

[Paul Birkeland]

I would recommend replacing the 431 regulator on that channel.

-PB

[jjvornov]

I’ll give it a try.

James

[jjvornov]

Paul

I've replaced the 431 regulator finally and it's not fixed the problem. I have all 4 LEDs lighting, but the EL84 is not heating and C3 is 1.1V and 11 is 188V

I'm willing to keep trying rather than send it in again. The preamp went in for repair when I had problems with one of the boards originally. So we know everything is wired up correctly and had been working at some point.

James

[Paul Birkeland]

the EL84 is not heating

This is a new piece of information.  The EL84 is heated by the shielded twisted pair of wires that go from the power transformer to pins 4 and 5 on the 9 pin socket.  There's certainly an issue with this, I'd have a close look at both ends. 

[jjvornov]

Paul

It is always a loose wire, isn't it? One of the wires had come loose. Resoldered and all good.

James

[Paul Birkeland]

Yes, this is pretty common, regardless of who is doing the building. Sorry to waste your time with the 431s.

[jjvornov]

My poor BeePre is the problem child of my Bottlehead kits. And that right channel!
The noise started up again in that channel after months of great sound. So when I tried swapping 300B tubes, I noticed that the good channel had lots of friction on inserting the tube into the socket and the problem channel feels loose. I was able to wiggle the the tub on the problem side to recreate the intermittent noise, so I think I've had another problem with the tube making intermittent contact on that side, maybe as it heats up losing contact sometimes.

I think I've seen similar discussed on the forum. Is it as simple as squeezing the contacts on the socket together with pliers to improve contact? I've seen some talk of replacing sockets as well.

[Paul Birkeland]

Yes, you can just squeeze the socket contacts a little bit.