Another Left Channel intermittent post

[Doc B]

Do you have the feet on the base, to allow some air flow under the chassis?

 

[Bryon]

The base is on sorbothane feet and then elevated a further 3/4" on anti-vibration pads so there should be lots of ventilation.  Earlier in the process, I elevated them another inch or so to make sure there was enough air flow but it did nothing to solve the problem.  The Beepree also has room on each side and the top for air flow.  The only thing I can think of is that maybe I overheated one of the components on the board when I was soldering it, not enough to kill it dead, but just enough to injure it to the point of exhaustion after an hour


Bryon

 

[Paul Birkeland]

Given that after the board was exchanged, the problem disappeared, the problem seems to have been a component on the board so probably not an issue on the lug board. 

The connections on the terminal strip will get nudged when swapping out boards, so this is still a real possibility.

Are any of the other components on the board likely to be heat sensitive?  Dan mentioned a possible cap issue - so maybe the .22uf?

The 0.22uF cap is in the high voltage regulator circuit, so no issues there.

My line voltage is high - 121V.

Nah, that's perfect.

  I did notice a change in Pos out and Pos in.  Voltages were Pos In - initially 140V settling to 137V and 9.98 on the Pos Out rather than the 8.6 that I had measured before.

Pos In should be 10-15V.  The voltage that's approximately 150V will develop on the other half of the board that holds the TL431.  Having 8.6V on Pos Out would indicate that something in the low voltage circuit needs some help.

 

[fullheadofnothing]

I seem to recall the prototype needed to have its 33µF and 150µF caps changed after a coupla thousand running hours. You could start there...

 

[Bryon]

The bad news - the left channel has disappeared again after 3 hours play time today.

PB - I missed seeing a decimal point on the Pos In - I just remeasured and it is actually 14V on both channels.  The Pos Out is now 9.975 or thereabouts on each side- 
You may be right about the connections on the terminal strip - I'll try resoldering them and see.

Josh - thanks for the suggestion to change the 33uf and 150uf caps -it's worth a try

Bryon

 

[fullheadofnothing]

The 10,000µF 10V were also replaced at some point. Might be more useful since it's now showing to not be the board.

 

[Doc B]

Something does not add up. You say the tubes stay lit but the filament supply is dropping out.

 

[Bryon]

It doesn't make sense to me either - maybe the fact that the channel didn't drop out had nothing to do with changing the filament board and was just coincidental?  Would the tubes stay lit if it was the sockets?  maybe we're back there.

Bryon

 

[Paul Birkeland]

Ha, yeah, those teflon sockets...  It's really impossible to tell if they are the issue without yanking them out and replacing them, and I'll certainly feel bad if they aren't the cause of the issue!

9.975V and 14V are dead nuts on.

If the tube socket connection gets flaky, you should lose the 5V reading across the paralleled 10W cathode resistors.  I would be probing there next.

 

[Bryon]

I resoldered the connections to the lug board under the C4S board and the both sides worked for about 5 hours last night so I'm hoping you were right PB and this will be a permanent recovery.  I'm gone for a few days, so I'll update when I get a chance to play it for a few days.

thanks again for all your help

Bryon