Oh NO! Left channel down! [resolved]

[Ivan303]

OK, tubes light, swap with right channel tubes, still no sound from left channel (thankfully not one of the EML 2A3s which are REALLY expensive).

Where to start?

Near 400V B+ on the 300B.

2 LEDs light up on the B side but no LED lights on the A side of the regulator board.

About 100+ V on each of the heat sinks of the two MJE5731.

This after working perfectly for near to a year.

Sudden, not intermittent.

Maybe not something I did(for a change), as have not messed with these guys for nearly a year.

Guessing one of the solid state parts on the board?

Thoughts?

Thanks.

 

[Paul Birkeland]

Can you post the voltages on all four pins of the 4 pin socket? (which hopefully has a 2A3 plugged in, not a 300B).

Also measure and post:

IA, OA, IB, OB, KregA, KregB on the driver board.

-PB

 

[Ivan303]

Type-o

Tube is a 2A3EH GOLD! 

61.9V
64.3V
382.0V
14.5 mV

 

[Paul Birkeland]

OK, the 4 pin voltages are fine.  How about the ones on the driver board?

 

[Ivan303]

AND...

IA  409V
OA  111V
IB  111V
OB 111V

KReg A  2.71V

KReg B 109V

 

[Paul Birkeland]

Remove the 5670 and recheck those voltages.

 

[Ivan303]

IA  419V
OA  419V
IB 418V
OB 417V
KREGA 0V
KRegB 416V

 

[Paul Birkeland]

There is nothing on the board that should have KregB shooting up that high, and having no tube in the socket rules out a badly shorted tube.  Some adjacent terminals somewhere on the PC board or tube socket are touching that shouldn't be, and this is causing the super high voltage at KregB, or possibly the black wire from bA to -Reg B on the B side is broken and not providing a ground reference for the regulator.

You will need to remedy the issue causing the high voltage at KRegB, then it's 50/50 that the TL431 on the B side of that board will need to be replaced. 

 

[Ivan303]

Keeping in mind that KReg B is 109V with a tube in the socket?

Will try to figure out where there may be a short?

With the tube out of the socket, is it possible for the pins of the socket shorting to each other?

 

[Ivan303]

And, of course, I was reading Reg B at 416V, not KReg B as instructed.

KReg B is also OV with the tube removed.

Sorry.

 

[Ivan303]

And...

KReg B with the tube installed is about 5.6V

Sorry

 

[Paul Birkeland]

Slip the wire out of OB and leave it poking up into the air, then recheck IB and KregB. 

 

[Ivan303]

Wire to OB removed.

Tube out

IB 412
KReg B 0V

Tube in

IB 116
KReg B 6mV

Thanks

 

[Paul Birkeland]

What voltage do you see on the X pad on the B side? It's next to bB. 

If you could post pictures of the top and bottom of the driver board, as well as the 9 pin socket wiring, there may be something we can see. 

 

[Ivan303]

Tube in or tube out?

 

[Paul Birkeland]

5670 in.

 

[Ivan303]

Tube in:

x pad .777V

Pictures attached.

As there are no broken wires that I can see and it's been running perfectly for nearly a year then just up and died,  a failed component on the board maybe?

Thanks

 

[Paul Birkeland]

I would put a new 431 regulator on the B side and see what happens.  Also if you set your meter to beep for continuity, check each pair of legs on each transistor to be sure that none of them are shorted out.  It can be challenging (for me at least) to sort out whether the C4S is allowing the 5670 to draw too much current or if the 431 regulator has failed.  Based on your voltages, a shorted 431 regulator is very plausible, and they do fail from time to time.

 

[Ivan303]

One of these?

TL431

https://www.mouser.com/ProductDetail/SparkFun/COM-11078?qs=WyAARYrbSnbLnDtTAKoHfg%3D%3D&mgh=1

 

[Paul Birkeland]

I don't know who Sparkfun gets to make their parts, so maybe go with this instead:
https://www.mouser.com/ProductDetail/Texas-Instruments/LM431ACZ-NOPB?qs=QbsRYf82W3GYwUIvsPyMPw%3D%3D