Problem - muffled sound

[Paully]

These are the voltages I was able to get:

Pins
1    0
2  3.5
3    0
4  192
5    0
6  194
7  Can't get to
8  6.6
9    0

Terminals
16  495
17     0
18     0
19  192
20     0

I am of course looking for questionable solder joints and loose connections.  I will go back and search again.  And I will start getting into the actual wires as suggested if nothing obvious jumps out.

 

[Paul Joppa]

This one is a puzzler.

When LED D2 does not light, but D1 does, that usually means there is no load on the C4S - which would indicate that the driver section of the 5670 is not connected to the board OB. But the voltages on pins 2, 3, and 4 indicate that the driver is drawing all the available current, limited only by the other C4S (A side) which feeds the shunt reg. That would at least explain why the shunt reg is not regulating (pin 6 should be 300v, regulated) - there's no current left to shunt, the driver side is hogging it all.

So the B side C4S is getting bias power (D1 lights up) but is dumping too much current, meaning the 215 ohm current set resistor is not doing its job. My best guess right now is that Q1 (2N2907) has been damaged, possibly by an intermittent connection to D2. If D2 opens up, you could easily apply too much voltage to Q1. I would at the least replace D1, D2, Q1, and Q2 on the B side of the board.

However, I can't make sense of seeing only 6.6v on the cathode of the shunt regulator, pin 8. I would expect it to be higher, if in fact it is not regulating at all. So there may be something wrong with one of the 149K resistors, or the B side 431 chip - or something else.

You can try replacing parts, but at this point I recommend that you simply get a new board and components, build it up carefully, and install it. The old board can become an object lesson on display, if you want to keep it out of the landfill (it's only other application).  :^)

 

[Paully]

I replaced Q1 and it is playing now without incident for a little while.  But we have been there before.  So I will look into replacing the board.  If one of the things you suspect fails, will the failure be catastrophic, i.e. is it going to take something expensive with it so I shouldn't play my amplifier until I replace the board?

 

[Paul Joppa]

While I can't predict EVERY possible failure mode, it's the solid state parts that are easy to damage. Tubes are tough!

A failure of everything could drive the 45 grid pretty high, and might damage that tube. Don't use the pricey ones until you are confident that everything is running well.

 

[Paully]

I was more worried about having to replace a (Magnequest) transformer or some such.  Anyway, I am going to see how it goes for a little bit then.  I suppose I should actually check the voltages now that it is running.  Thanks for the help as always!

Paul

 

[Grainger49]

Let's remember that rewetting solder has helped 3 times before for only hours then a failure.  We can't declare it fixed till Paul gets through the weekend and it works for a week.  I'm thinking there is something we are overlooking.  I hope I'm wrong.

While I can't predict EVERY possible failure mode, it's the solid state parts that are easy to damage. Tubes are tough!

A failure of everything could drive the 45 grid pretty high, and might damage that tube. Don't use the pricey ones until you are confident that everything is running well.

 

[Paully]

The Queen is getting a board together for me so I think I am just going to heed the Man's advice and replace the one that is in there.  We have spent so much on other tweaks that it is probably the wise thing to do so we know we have an amp that is operating properly.

 

[Paully]

Ok, so I went in and replaced some parts on the driver board two months ago and at the same time ordered a new board just in case.  The amp decided to work perfectly for the next two months until yesterday wen the problem resurfaced.  Sound is fine for a few seconds then goes bad.  Flipping it over and watching I can see that two sets of LEDS light up instantly, the last two never do.  So I went ahead and put in the new board and all new parts.  Didn't fix the problem.  I suppose I can replace all of the wires on and into the board in case one of them is bad.  Any other thoughts?  Very frustrating.

 

[Doc B]

I wouldn't necessarily pull all those wires, but it might be worth touching up the connections at both ends of each wire and checking their continuity. If the board itself is good then the problem must lie in one of the connections to it. You might try monitoring the plate voltage of each half of the 5670 as the amp goes thru the "Sound is fine for a few seconds then goes bad.  Flipping it over and watching I can see that two sets of LEDS light up instantly, the last two never do" cycle. That might help to understand what is going on.

 

[Paully]

I did just check continuity, that was fine.  But it wouldn't hurt to touch up the solder joints to be sure.  Will do that tonight and see what I can figure out on the 5670 while I am at it.  Thanks Doc!

 

[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?