Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: jjvornov on February 13, 2018, 04:27:21 AM
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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
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Reheat all the solder joints on each MJE5731A.
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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
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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!
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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
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That is progress of sorts. Pull the 300B on that side and recheck C3 and 11.
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11 comes up to 219V and c3 is still 1.3V
James
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I would recommend replacing the 431 regulator on that channel.
-PB
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I’ll give it a try.
James
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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
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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.
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Paul
It is always a loose wire, isn't it? One of the wires had come loose. Resoldered and all good.
James
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Yes, this is pretty common, regardless of who is doing the building. Sorry to waste your time with the 431s.
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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.
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Yes, you can just squeeze the socket contacts a little bit.
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And so this problem continues. After beautiful music for a week, this morning the noise came back. I retightened the socket contacts even more and checked connectivity of every connection going to the tube socket, just in case my playing with the tube in the socket was moving some connection on the socket contacts.
Putting the BeePre back in the system, it continues to be clear that its the position of the 300B in the socket the causes the noise. After playing with the tube sitting in the socket it now is fine again, but I'm pretty sure that at some point the noise will come back. And I borrowed another pair of 300Bs just to make sure it wasn't the tube type. The socket seems to be a very simple set of contacts, but is it possible that the socket itself should be replaced? No obvious cracks or other problems.
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It might help to replace the socket, just to knock it off the list of possible issues.