FP 2 one channel out

[1108]

Yes I'm still using my FP 2 I built in 2004.
Up to this point it has been working great, no issues.
When I went to use it yesterday one channel wasn't working. If I put my ear right up to the speaker, I occasionally hear a slight static buzz. It doesn't change if I increase the volume, and no other hum, buzz, ect.

So far I have:
Verified it's the FP
Checked the RCA plug
Checked the selector switch
Checked the volume pot
Checked the tube

There's no obvious signs of a failed component or broken connection.

Any suggestions on what to do next?

Thanks
Richard

 

[Grainger49]

I had the same problem a few years back, my dear old FP 2 totally dead!  I didn't have any voltage out of the transformer so I posted and got a part number for a transformer from PB.  Ordered it but then discovered my solder on the power switch had fallen off.

In those days they had no holes in the lugs on the power switch.  I just "stuck it on" with solder.  I drilled a hole and resoldered it.  No problems since.

Since I told you that story, try the voltage checks.  If you don't have them I can give you guidance.

It is such a sweet sounding simple circuit we can knock it out today.

 

[1108]

Thanks Grainger.
It is a sweet sounding. After all these years I haven't found a reason to get something else.
I'll take a closer look at the connections. I should have the manual around somewhere. If I don't I'll get back to you for the info.
Thanks again.

 

[Paul Birkeland]

You'll really need to measure voltages to know what might be going on.

-PB

 

[1108]

working on it. looking for the manual.

 

[1108]

A8 reads 0 volts. Everything else checks out fine.

 

[Grainger49]

What is the resistance reading from A8 to the ground bus?

 

[1108]

I get 30K.

 

[Paul Birkeland]

Do both halves of the 12AU7 light?  Can you swap tubes to see if the 0V reading switches sides?

 

[1108]

I did swap tubes and tried a couple of others, same channel didn't work.

 

[Paul Birkeland]

I'd check to see if both halves of the 12AU7 glow on that side.

-PB

 

[1108]

Looks like only one half of the tube on the bad side glows. I switched tubes just to confirm.

 

[Grainger49]

Then check the connection at tube pins 4 and 5.  That is what jumps the two halves together.

 

[1108]

Well, 4&5 are connected and I'm getting the proper voltage. Is it possible the tube socket is bad?

 

[Grainger49]

It is not very likely that the socket is bad.  But not impossible.  I would check voltage from tube pin 4 to 9 then pin 5 to 9 from the top of the socket. 

It is also possible that the tube socket is a little dirty.  Simply remove the tube and insert it a dozen times.  It is worth a try.

 

[Paul Birkeland]

I'd reheat the joint between 4 and 5 to be sure.  A flaky joint there will produce the issue you're describing.

 

[Grainger49]

He is right.  That is quicker fix than checking for the voltages.  If indeed that is the problem.

 

[1108]

Thanks guys. I'll give that a try a little later today.

 

[1108]

Finally had the time to check the connection. Good news. The tube glows and I get music out of that channel. Grainger and PB, thanks for you help.

 

[Grainger49]

Awesome!  A local tube buddy loaned me a different line stage yesterday.  I doubt it will outshine the FP 2.