FPIII Bad Voltage Readings

[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.

 

[Doc B]

OK clip the black lead to H1. Now take a VDC reading with the red lead at each end of the white resistor, H2 and H5.

 

[Frank Breech]

H1 to H2 --  1.6VDC (see next post)
H1 to H5 --  1.8VDC (see next post)

Interesting.  Earlier today, when I did H1(black lead) to H2 (red lead) I got a reading of zero.  see post Today at 05:20:20 AM Posted by: Frank Breech

 

[Frank Breech]

Ok, found a loose connection at the heater.

My I am editing the above readings, the new readings are:

H1 to H2  6.75 VDC
H1 to H5 6.75  VDC

still no light at the LEDs at the tube sockets.

 

[Doc B]

So you must now go back and see if that voltage is actually at the tube sockets too - 4/5 and 9.

 

[Frank Breech]

The voltage is at the tube sockets.  The LEDs are lit.  The 12Au7s are lit.

I feel that everything should be ok now.

That bad solder at H1 must have been the culprit. 

I will bring the unit home and post the result.

Thank you for all the guidance.

Mike

 

[Doc B]

Now you see why my first post in a troubleshooting thread is most often -

http://www.bottlehead.com/smf/index.php/topic,3284.msg28329.html#msg28329  

Carefully reheating every solder joint and visually inspecting it with a magnifier to make sure everything is properly coated with solder and secure in the joint is the best way to start to sort out a "not working" issue, particularly one where the circuit was working initially. It can save a lot of other testing and probing.

 

[Frank Breech]

Please refer anyone to me who might doubt this.

I will let you know what happens this evening when I get it hooked up.  I'm hoping I'm not back to square one with the noisy right channel.  I did rewet all sweetest whispers connections and cleaned the tube sockets with Grainger's method (dental brush and contact cleaner).

I might put on Tom Waits's Heart of Saturday Night on a Friday.  Unheard of, but possible.

I did also enjoy the troubleshooting as it got me realizing what was what better that before - even if just a little bit.

Thanks Again
Mike

 

[Grainger49]

I should have described the black stripe on the capacitor differently.  I was looking at the first picture on page 28 of the manual.  I see now that the input RCA jacks are to the right, the output RCA jacks are to the left.  The black stripe should be to the left.

Two things you have measured point to the diode string.  You got 0V H1-H2, which is where the cap is, and you got 6.1V AC on the transformer.  The diodes are all that connect those two points.  Of course the diodes are what make DC out of AC and smoothing it gives you a little more than the RMS AC value.  

You are right on top of it now.  This has got to be it.

Edit: I now see all the back and fourth that I missed where you and Dan figured out the missing/loose connection at H1.

 

[Frank Breech]

Grainger,
Thanks for the follow-up.

I'm at home, the amp is sounding very nice.  No more noise on the right channel when touching that side of the chassis.  I'm having just one issue (which might even be a hold over, partly, from my original issue):
one channel is softer than the other.  In this case it follows the tube.  Whichever socket I put the suspect tube into, that channel is about 2 sweetest whispers clicks below the other side.

Any ideas?  

I did not check the pins of my 12AU7 tubes as you described to me in an earlier post.  I can do that.  I'm burnt out for tonight ... just glad to be listening, even with the SW pots at different settings.

Thanks Again,
Mike

 

[Frank Breech]

It could be fatigue, but I 'think' the channels are evening out, level-wise.  Is is possible that it could take a while for one channel to get warmed up or something to output the same as the other channel?

They are still a bit far off, but better than before.  When I attenuate each all the way, but the CD player is still going, I can hear the music through one channel (the louder one) a bit, but cannot hear it at all through the other (softer) channel.

Mike