Problem - muffled sound

[Grainger49]

I just talked to Paul, Paully.  He did the chop stick test while I was talking to him.  He tried both ends of all the connections/wires to the soft start board as well as every component on the board.  There was no change in the sound.  I think he rewet all the terminations on both ends also.

What is happening is that after some delay the sound comes on and is beautiful.  Then after 4-5 seconds of music it gets extremely muffled.  It is like the B+ has dropped but it is still playing, just not right.

Is there a chance that the delay circuit is at fault?  I'm guessing that the initial delay is the circuit doing its job.  But then the poor sound comes in.

Paul will check the 5670 high voltage tomorrow and post.

[Doc B.]

Two different PC boards, two different delay circuits, same problem. So I still think the problem is off the board. Paul, I'm guessing you have tried a different 5670?

[Paully]

I am positive that I swapped all of the tubes, wouldn't hurt to try again.  Will do tomorrow.  Thanks!

[Paully]

Just as a reminder, I do have 45's as the output tubes.  Reading at IB and OB on the board, which should give the reading on the 5670 for pins 4 and 6 I get 150V DC.  I went ahead and checked pin 4 just to be sure and got basically the same reading.  Last time, before replacing the board, I got 192 on both pin 4 and 6.  Also swapped the 5670 just to be sure.  Re-wet multiple solder joints leading into the board.  It has to be whatever powers the last two LEDs on the B-side of the board shouldn't it?

[Doc B.]

Did you replace the zener diodes when you replaced the PC board? If not, that may be what is pulling the regulated voltage down. It would be worth checking the diodes with a diode checker or ohmmeter to see if any of them are reading shorted (very low ohms in both directions).

[Paully]

The diodes were the first thing I replaced a few months ago before I put in a new board.  So those are new.  But I will go look at resistance readings on them anyway just to be sure.

Edit: All read 25 to 35 Mega-ohms when measured in the same direction.

[Grainger49]

Remembering this is a 45 output...

Paul is measuring the voltages now.  He reads 476V incoming from the power supply (IA).  The schematic for the SS board says 450V, sounds good.  But at OA, the schematic shows 300V and he is reading 148V.  That is about 1/2 of what it should be.

Any help here?  I'm guessing a connection because it sounded sooo good when we finished last time.  But it is driving both of us to distraction, well me remotely but Paul directly.

Some voltages:

IA       476V
bA       1.6V (remember there is a resistor in parallel with a Zener diode on this connection to ground)
OA/IB/+reg (B)  148V
OB       146V
bB/breg (B)   74V
Kreg (B)/driver tube pin 8   1.9V

[Doc B.]

When the amplifier is first turned on the voltage at OA should be what the zener string pulls it down to. That should be something a little over 300V, like maybe 325. Then as the 5670 heater warms up the voltage at OA should drop to 300V as the 5670/431 shunt regulator begins to draw current. What I would like you to do is attach the voltage probe to OA or IB, whatever is easiest attach to, and monitor that regulated voltage as the amp starts up and goes through the heater warm up period of the 5670. We need to see if the 148V shows up immediately upon startup or if it starts out at something around 325 for a few seconds as the zener string properly regulates it and then drops to 148V as the 5670 begins to conduct.

[Paully]

At IB it started out at 350V or thereabouts.  It seems about the same time as the sound cuts out and it goes muffled it started to plummet to 150V.

[Paul Joppa]

R3(B) should be 2.49K, ans R3(A) should be 4.99K. Are they backwards?

[Paully]

They aren't reversed but the one in R3 (b) appears to be 249K ohms not 2.49.  I will go to work on this and see what happens.  Thanks!

[Paully]

I replaced the 249K resistor incorrectly in R3(B) with a 2.56K resistor I fashioned together.  Close enough I assume.  Amp comes up and IB settles out at 290V and the amp plays and the second set of LEDs light up.  So that appears to have been the problem. Not sure what happened.  I checked all of the resistors before I put them in.

Here is the odd thing.  I went back and checked the board we replaced and the resistor in that board was 2.49K ohms as it should be.  So I replaced a bad board with some problem somewhere that was causing muffled sound and non-lighting LEDs.  I put in a new board and didn't notice I had the wrong resistor which caused the same muffled sound and non-lighting LEDs even though the original board had the proper resistor.  Go figure.

I suppose at this point I will just be thankful for the help and let it play.