FPIII Bad Voltage Readings

[Frank Breech]

Hi,
I had my newly built FPIII running for a couple of days and was really enjoying it.  I had some noisiness on the right channel. I thought it was in the right SW pot, but then I realized it was whenever the chassis got jostled at all, especially between the SW and the right 12AU7 socket.  Then, I noticed that moving the tube or at least the tube socket on the right channel made this noise much much worse.  In wiggling the tube around, eventually the sound went away.  The LEDs on both 12AU7 sockets do not light up.  So, just re-checked resistances.  All were ok.
Then just re-checked voltages.  The two voltages that are off are Terminal 15 and Terminal 31.  Both should be around 75VDC according to the manual.  They are now both at ZERO. 

Please let me know how to proceed.  I feel like I need two new 2.2uF 250V capacitors.  All help is appreciated as I'm new to this.

Incidentally, I found a bad solder connection at 12AU7 socket terminal 6, which was probably the cause of my original problem.

Thanks in advance

Mike

 

[Doc B]

Start by going back and reheating every solder joint, and then trying the voltage check again

 

[Frank Breech]

Ok, will do.  What do those two bad reading terminals have in common? Is there a single joint or series I can rewet and /or check first?

 

[Grainger49]

Both those terminals are attached to the  right and left Cathode Follower Cathodes.  With zero volts there the CF stage is not working.  Both 12AU7 tube pins 8 should read 22.1k ohms to ground. 

The associated plates are tube pins 6 on the same tubes.  Are those the right voltage?

 

[Frank Breech]

Grainger,
I will check the 12AU7 from pin 8 to ground.

To test the voltage on tube pins 6, where/how should I measure?  What should the correct voltage be? 

I will try to test this tomorrow.  Also, will go for reheating every solder connection but I want to try to understand this thing at least a little bit, so thanks for these troubleshooting tips.

I'll post my results as soon as I can get to do the work.

Mike

 

[Laudanum]

Mike,  I was going to suggest looking at and reheating the joints starting around that socket.  For some reason that other thread got locked.  Glad you re-posted.  Im sure Grainger and Doc have you on the right track. They are much smarter guys than myself when it comes to this stuff.

 

[Grainger49]

Mike,

I keep forgetting, the FP III takes voltage readings from the terminals rather than the tube pins.  It is safer as close together as the tube pins are.  Read to ground, that is the power supply common.  The two plates should be connected to the OD3 regulated voltage, that is terminal 21.  The plates are attached to terminals 12 and 31.  All three should be 150V or there about.

 

[Frank Breech]

Grainger,
Ok, got it, thanks for the clarification.

Those readings are fine, that is: in line with what the directions call for.  I had only listed my problem voltages. (also, you said "terminal 31" in your last message but I think it was a typo, should be terminal 32, right? Please confirm.

To be sure:

Term:     My voltage reading:
12                  146VDC
32                   146VDC
21                  146VDC

Also, to be sure:
both 12AU7 tube pins 8 do read 22.2Kohms to ground.

All resistances checked out, then all voltages checked out except to terminals 15 & 31 (as in the original post).  Need to get a few minutes free to reheat all connections. I 'll you know what happens.

Laudanum,
Thanks for the support.  Don't be so hard on yourself  

-Mike

 

[Grainger49]

Right, 32.  See, all those are good.  If the cathodes, terminals 15 and 31, are functioning you get some voltage there.  First check the resistance from terminals 15 and 31, it should be about 22.1k. 

If it reads right go back and touch all the solder joints within the two cathode circuits.  Touch up both 12AU7 tube pins 8, terminals 15, 19, 31 and 39.  The two red wires and two resistors comprise the cathode circuits.

 

[Frank Breech]

Hi, Just did all of this that you mentioned above.  Must have been doing it while you were writing.  I retouched all solder connections from terminals:

-11 thru 40 (both H & L)
-All 12AU7 Pins that are connected.  (except i'm not sure how well I got the center pin, it was a tight fit with my iron tip)
-All Sweetest Whispers Channel A (right channel) since this is where I thought I was hearing the noise originally.

Then Rechecked voltage at 15 & 31, but still getting 0VDC there.  Also, the LEDs on the 12AU7 sockets do not light when I power up.

Mike

 

[Doc B]

Try measuring the voltage at pin 1 and pin 6 of each of the 12AU7 sockets

 

[Grainger49]

So both channels are dead.  Booger!  If it were one you could swap tubes.  

You have a properly regulated voltage at the plates, well at the terminals that feed the plates.  Let's try this another way.

Put one meter lead on the Octal socket pin 5.  That is the output voltage of the OD3.  Measure resistance to tube pins A6 and B6.  Each of these measurements should be 22.1k ohms.  This would say that the ~150V of the OD3 is getting to the tube plate.  

You verified that the other 2 - 22.1k resistors were attached to the respective cathodes and ground.  So that sounds good.  Checking the plates back to the source where you do have the right voltage checks the other part of the circuit.  

And now you tell me that the HLMP 6000s are not lighting.  So nothing is conducting.   Hmmmmmm.  

Stick with us, we can figure it out.

 

[Frank Breech]

Doc, Measured Voltage at the 12AU7 pins you suggested:

1A 147vdc
6A 147vdc

1B 147vdc
6B 147vdc

Grainger, Measured the resistances you suggested:
Octal Socket 5 to A6 0Ohms
Octal Socket 5 to B6 0Ohms

however Octal Socket 5 to A7 and B7 are 22.2K  (not sure if this info is helpful)

Also, those LEDs have not been lighting since the very beginning of my trouble.  When I first lost sound.  I wiggled the right tube (socket B) and the LED went on and off.  After power cycling once, the LEDs were dim and audio was VERY LOW.  Power cycled a second time and neither LED came on and, no sound.

-Mike

 

[Grainger49]

  .  .  .   Grainger, Measured the resistances you suggested:
Octal Socket 5 to A6 0Ohms
Octal Socket 5 to B6 0Ohms

however Octal Socket 5 to A7 and B7 are 22.2K

  .  .  .  

My mistake when I said A6/B6 I should have said A1/B1.  A7/B7 is almost like A6/B6 so your 22.2k is good.  Matter of fact spot on to what you should read.  With no LEDs lighting in the VA side and the cathodes at ground, passing no current, neither side of the tube is conducting.

I'm convinced it is something we have overlooked, simple but catastrophic to both sides of the tubes.

Something is wrong here and it is both channels and both triodes for each channel. 

 

[Frank Breech]

Is it possible to ring out my tubes with the mm?

 

[Grainger49]

The tubes should be fine.  They should be new EH tubes.  The only connection is between tube pins 4, 5 & 9.  That is the heaters.  All others are isolated by the vacuum in the tube.

Are you asking about ringing out the tube sockets?

 

[Doc B]

I'm sorry that I don't understand your last question, Mike.

If you have 146 V on pins 1 and 6 the high voltage is getting from the high voltage supply to the tubes. So either the LEDs are shot - not that likely - or more likely the 12AU7 heaters are not getting the correct voltage. Try measuring the heater supply voltage at the 12AU7 socket pins. That would be using the black test lead on pin 9 and the red test lead on either pin 4 or 5, as they are connected together. You should see about 6.0 to 6.3VDC.

 

[Grainger49]

And Dan came up with something very simple that would keep both triodes in each 12AU7 from working.

Plus, if they get warm that indicates some voltage on the heaters.  Not necessarily the right voltage but some.

 

[Frank Breech]

Hi, Guys,
Thanks for the notes.  

I will try measuring the heater supply voltage as you describe.

I will get back to you tommorrow morning.  I have the unit in my office.  Actually, my boss is on vacation tomorrow so I can devote as much time as need be to this.  I'd like to listen to Tom Waits's Heart of Saturday Night this weekend, if possible.

RE the LEDs: While I'm not sure how to tell if they have gone south, I can tell you this ... When I use the continuity tester through them, they do indeed light up.

RE my question about "ringing out the tubes with my multimeter":  Maybe that question just plain doesn't make sense.  I was thinking maybe the heater filaments (is that the right word?) were blown or something, kind of like a lightbulb, and I could test that with continuity or resistance meter from pin to pin.  Is something like this possible?

Thansk again
Mike

 

[Grainger49]

Mike,

You should pull a tube and put your meter between tube pins 4 and 5.  Whatever you get there you should read about half that between 4 and 9 and the other half between 5 and 9.

Give that a try.  I bet they are fine.