FPIII Left channel crackle and static ... helps needed

[audiblesoundwave]

The crackle and static on my FPIII (non-extended version) left channel started a week ago.  They can be heard from the listening position.  The volume switch does not affect the crackle and static sound; however, they do get a bit louder sometime for a short burst.

I replaced a different set of the 12au7 tubes and swapped the position but no help!
I replaced the two 220 ohm 1/4w carbon composite resistors but no help!
I replaced the output capacitor but the outcome is the same which with the crackle and static on the left channel.
I also checked all voltage and resistance against the manual check list and the results are well within the reference results.

Any other pointers to resolve the crackle and static issues?

There are three more resistors (22.1k 3/4w and 470k 1/4w) and one LED.  Should I go change those?
Is there anything else that I need to replace?  Could the Goldpoint attenuator or the selector  switch cause the crackle and static problem?

Thank you for your help!
Milton

[Doc B.]

Look for any parts that might be discolored from overheating. You might also want to reheat the solder joints on the left channel connections. Do you see the 0D3 flicker when you hear the crackle?

[audiblesoundwave]

Thanks for the quick reply!

I replaced the 0D3 with another new one and the crackle and static are still there.

I will reheat the connections tomorrow and to see that would resolved the issues or not.

Thanks again,
Milton

[Doc B.]

 If you dont see any flicker the crackle is likely to be coming from somewhere near the left channel 12AU7 socket. Since your voltages seem good it's likely to be an intermittent connection. A typical cause is connections on the socket getting loose or broken over time from tube rolling. Also if the left side output capacitor was ever accidentally nicked with a soldering iron and it penetrated through the outer jacket that could create a crackle.

[Grainger49]

It sounds like time for "the chopstick test."  And since you are an "old timer" you don't need me to explain it. 

I have seen a solder joint fall off after 6 years.  I have seen non soldered joints work for 2 years.  I expect there is a solder joint that just needs reheating.

[audiblesoundwave]

Thank you!

I did the "chopstick test" last night but did not seem to locate anything.  However, I will go try to reheat the joints tomorrow.

What is the chance that the 9-pin socket gone bad?  I tried to "rocking" gently the 12au7 tube and there were loud static coming out to the left speaker from the left tube; however, the right side did not!

Thanks again,
Milton

[Doc B.]

Yes it sounds like you have found the issue. The socket is either has corrosion or the contacts are stretched from tube rolling. Take a very tiny screwdriver and gently work it between the ceramic and the metal contact in each hole to very slightly crimp the contact a little smaller to grab the pins better.

[audiblesoundwave]

Thanks Doc!

I performed your suggestion on the socket hole contacts with the tiny screwdriver.  It did seem to help a bit.  The crackle and static seem to subside a bit more during idling, no music playing.  However, the crackle and the static came back when music is going.  I also did reheat all connections on the left channel.

Could it be the 470k ohm resistor?  I am waiting a new one to come in a few days.  I did take the resistor out for measurement and it seem to be ok with 469 ohm.

Thank you,
Milton

[Doc B.]

If it helped a little it's most likely that you found the problem area and it just needs a little more attention. If you have some contact cleaner try pulling the tube out, applying the cleaner to the tube pins and plugging and unplugging the tube a few times. That may remove that last bit of tarnish or dirt that is causing an intermittent connection.

[Paul Birkeland]

Hello Milton,

Try the chopstick test on those 220 Ohm resistors. 

Sometimes when I have to track something like this down, I will use a cheap headphone amp and garbage pair of heaphones on my head, with the volume turned down pretty low.  Then, with the Foreplay driving the headphone amp, I can hear if my chopstick hits something funky (crackle crackle).

It might be that you have a 220 Ohm carbon comp that's given up.  When I get a FP-III in for repair with crackles on one channel, I'll just yank those out and put carbon film resistors in instead. 

-PB

[audiblesoundwave]

Thanks Paul, Doc, and Grainger for all of the pointers.

After many days of chopstick testing, number of times of resistance and voltage readings, number of joints reheating, and  resistors and capacitors swapping, I located the cause finally!

It turned out to be the 22.1k ohm metal film 3/4w resistor that was connected between pin 6 and pin 1 of the left channel.  The resistor's external did not show any problem but it caused the crackling. May be internal issue.

Anyhow, the new resistors I purchased last week arrived today and I clipped the new one on.  The crackling is no longer there.  Back to pure music!

I decided I will replace all of the resistors (220 ohm, 22.1k ohm, and the 470k ohm) for both channels in the next couple of days.

Thanks again for the support and helps!

Have a wonderful week,
Milton

[Doc B.]

My advice is just leave the circuit as-is for a while now that you fixed the problem. That resistor failing is a pretty unusual occurence and changing all the rest out would, for me at least, be about a 50/50 proposition on whether there might just end up being a new sketchy solder joint somewhere in the rework. If it ain't broke don't fix it can be a worthwhile approach some times.

If on the other hand you see any resistors that looks obviously heat discolored they might be worth replacing.

[audiblesoundwave]

Well Doc,  I thought I had fixed the problem but the crackle and static are still there!

I did not hear the crackle and static when I was using two 10" clipper cable to clip the new resistor onto pin 6 and pin 1.  However, the crackle and static are back after I solder the resistor in!

Does this make any sense?  I am back to step 1!

Milton

[Paul Birkeland]

You may simply have needed to clean the tube sockets from the get go.

-PB

[audiblesoundwave]

Thanks Paul.

I have used the DeoxIT to clean the socket a few time in the past week. 

I have also  performed Doc's suggestion that "Take a very tiny screwdriver and gently work it between the ceramic and the metal contact in each hole to very slightly crimp the contact a little smaller to grab the pins better".

I have already replaced the 220 ohm resistors and the 22.1k resistor connecting pin 6 and pin 1.

The only one resistor left is the 22.1k ohm connecting pin 8 and terminal 39.  Can it be the LED gone bad?

Thanks again for help,
Milton