One channel cutting out/distorting

[RKH]

After months of playing beautifully, my FPIII has an issue on one channel.  It cuts out/distorts somewhat randomly. Much more noticeable if I change the volume.  I tried swapping tubes and the problem persisted in that channel.  When I turn out the lights it appears that the filaments ( lower portion of the 12AU7) glows much less brightly.

Does anyone have any suggestions as to where I might look for the problem?

Thanks very much.

Ryan

[JC]

Good, you tried swapping the tubes.  Since the problem stayed in one channel, it's probably not the tubes.

Next thing I would want to make sure of is the source, by swapping the input cables.  If the problem still stays on the same channel, return the input cables to their original positions, and swap the output cables.

If the problem persists in the same channel after those two additional swaps, then you will have localized the trouble in the FP, and the tubes are probably fine.

Then, it's time to see if you can find out what is different between the good channel and the bad channel.  You might want to start with taking resistance readings or Voltage readings as per the check-out in the construction manual.  Or, you may just want to start with a good visual inspection of the bad channel to see if you can spot anything amiss, particularly any suspicious solder joints or something out of place.  One good place to start looking might be around the tube socket of the bad channel to see that the heater connections are good.  Judging how bright a tube glows can be misleading; better to check to see that the heater Voltage is what it is supposed to be.

Intermittent problems can be quite challenging to find sometimes; other times they jump right out when you start hunting.

Oh, if you get to the point of checking resistances and Voltages, you might want to make a little chart of your readings and any discrepancies for reference as your troubleshooting goes forward.

[Steve_in_NV]

Could the volumn pot just have a loose connection or dirty contact?

[JC]

You bet it could.  A dirty tube socket contact could do it, too, but I tend to think that when you swapped the tubes to check them, you would have knocked enough of any crud loose to change the symptom in some noticeable way.

If it is a dirty volume control, just rotating it back and forth a few times through its entire range (amps off, of course!) might be enough to change the symptom, too.  It wouldn't necessarily solve your problem, but it might help you to localize it.

[Grainger49]

Rewet all the solder joints on that tube socket.  Do the same for the volume pot on that channel as well.

[RKH]

All,
Thanks for the great advice.  All of it is/was valuable as it got me thinking.  I tried a different source and it appears that my source was the problem.  I must try a few more cable/source permutations but the FP III appears NOT to be at issue here.  I don't want anyone to take away that the FP-III is not a top notch pre amp.  It is solid and very, very good.

I will post back after I have another listen and try a different DAC.  For now, all appears to be well.

Thanks again for the generous support.

Ryan

[JC]

Thank YOU for posting your conclusions when you reach them.  One of the most valuable aspects of a forum like this, IMO, is the searchable database that is created when these types of issues are discussed.

Say, I just learned something from this thread, too:  The word "searchable" is apparently not in the spelling database for this forum, for some reason!