FPIII Bad Voltage Readings

[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.

[Doc B.]

Yeah, if the LEDs light up with the meter they are OK. It's very unlikely that the heater filaments blew. I suspect a bad solder joint somewhere in the heater supply wiring or possibly a blown diode in the heater power supply. Check the voltage at the tube sockets. If there is not about 6VDC from 4/5 to 9 then start your way back from the sockets to the heater power supply and see if you can find an open connection. Then check across the pins that the blue 10,000 uF capacitor is soldered to pins H1 ( black test lead) and H2 (red test lead) and see if you get about 6VDC there. If not, it could be a bad rectifier diode. Let us know how that goes and if necessary we will investigate the diodes.

[Frank Breech]

Ok, great.  This will be good to get me started tomorrow morning.  The only thing I'm not so sure about is when you (Doc) say: "start your way back from the sockets to the heater power supply and see if you can find an open connection"
Does this mean keep one test lead on tube socket 9 and test backwards?  If so,(or even if not), I'm not sure which points to touch when working my way backwards.  I don't want to short something out.  Can you please confirm which terminals trace the route back along the heater supply - or otherwise set me straight?

thanks
Mike

[Paul Joppa]

Follow the wire - it's shielded twisted pair - visually. IT goes from teh A socket to the B socket, so test the voltage B9 to B4/5. From their it goes to the heater supply; check the voltage H1 to H2.

[Frank Breech]

Ok.  Sounds simple.  Thanks for the confirmation.

Also want to be sure, would I be tracing this path from tube socket B9?  That is, should one lead stay on 9, then as I back track, I'll see where the voltage stops coming through?

Forgive me & thank you

[Grainger49]

You will read from tube pins 4/5 to 9 on both tube sockets.  Moving the leads from socket to socket.  Then move the leads back to the power supply at the back of the chassis where the STP (Shielded Twisted Pair) comes from.  If you get the meter leads swapped nothing bad will happen you just get negative voltage read on the meter. 

[Frank Breech]

Ok, fellas...

Socket B:
pin 9 - 4/5     1.8vdc

Socket A:
pin 9 - 4/5     1.8vdc

Heater Supply:
H1 -  H2      Zero vdc

I feel like this doesn't even make sense. Is the heater supply upstream of the tubes?  If it has zero vdc, how can the tube heaters have anything?  I did go back and double check these readings.

What do you think?  Is there music in my future this weekend?

Mike

[Grainger49]

Bingo ! ! !

We have found the problem. 

Measure between the transformer terminals 4 and 5.  It should be somewhere around 6.3V AC. 

Verify that the black stripe on the 10,000uF cap is toward the RCA jacks.

Go to page 28/29 in the manual and verify that the diodes are in the right direction.

Verify that the wires connecting transformer terminals 4 and 5 to the terminal strip H1 H2 make good contact.

[Frank Breech]

Measure between the transformer terminals 4 and 5.  It should be somewhere around 6.3V AC.  
6.1 VAC

Verify that the black stripe on the 10,000uF cap is toward the RCA jacks.
This is NOT toward the RCA jacks.  However, the lead closest to the black stripe is definitely connected to terminal H1.

Go to page 28/29 in the manual and verify that the diodes are in the right direction.
Check.

Verify that the wires connecting transformer terminals 4 and 5 to the terminal strip H1 H2 make good contact.
Check, I rewet these.

[Doc B.]

If you have 6.3V AC at the power transformer and nothing after the rectifiers its probably a bad solder joint around the rectifier bridge, or equally likely you have a blown rectifier. With the amp unplugged take a resistance reading across each 1N5820 rectifier with the red on the striped end and the black on the non striped end and record the numbers. Then do the same with black on stripe, red on non striped.

[Frank Breech]

Red on Striped Side of Rectifier:                  Black on Striped Side of Recitfier:
#1  408 ohms ...........................................60 ohms
#2  380 ohms............................................60 ohms
#3  470 ohms............................................60 ohms
#4  450 ohms............................................60 ohms

I should mention that while the 60 ohms readings are very steady the other readings are fairly variable.  They settled to each of the above listed #s, but I did the readings a few times and often got different results.  These readings are the most common ones I got.

Also, My MM instructinos say that using the continuity setting, I can perform a diode test.  I tried this and can give you those numbers if it would help. I did not hear the continuity beep when testing the rectifiers in either direction.