crackle noise in left channel at power off

RS Steve

RS Steve

Tube Junkie
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I just restored a really rough Fisher X-202 integrated amp and have been playing it daily for a few weeks. I noticed at the point I shut it off, it lets out a noise on the left channel only. It is a short crackle sound, no other issues with the amp that I can tell. I have restored quite a few Fisher tube units over the years and never had a similar issue, I'm hoping someone can help me trouble shoot this.

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My 800c use to do that but then it stopped, I am not sure if it depended on the speaker I was using at the time? Mine has those russian caps in it
 
Thanks for the replies, I will give all pins and sockets another good cleaning and start from there.
 
Reverse or change out the tubes and see if it follows.

I had a tube with a weak internal weld that did the opposite. Once it warmed up it buzzed
 
I have tried swapping all tubes out and the issue still persists, it seems like the noise is a capacitor discharging. I'm not sure what else I should look at, sure is odd.
 
You could warm it up on the bench with the bottom plate off. Then if you're fast enough try testing cap voltages between sides.

Get a plan together before you start, eg test points, and give it a try. Have you performed the highly technical chop stick test? While you have the bottom off try it on that channel. Probably a bad pin or failing solder joint.
 
I know you probably have to turn the volume down to nothing to turn it off with the usual volume control power switch right? What happens if you turn it off externally? Is the crackling volume dependent or always the same no matter what volume level?

You might be able to localize the noise with a scope. I'm thinking a noisy resistor or coupling cap near the left output tubes or PI, but it could be the power supply caps/resistors too.
 
Tim D said:
I know you probably have to turn the volume down to nothing to turn it off with the usual volume control power switch right? What happens if you turn it off externally? Is the crackling volume dependent or always the same no matter what volume level?

You might be able to localize the noise with a scope. I'm thinking a noisy resistor or coupling cap near the left output tubes or PI, but it could be the power supply caps/resistors too.
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Thanks for all the help everyone, it's heading back to the bench and I will open her up and trouble shoot some more.
 
I am second guessing myself.... If it was power supply I would think you would hear it in both channels.
 
Thinking more about it (because I have also had this problem), I agree with Gadget about how it could also be a resistor. Honestly, I have never heard a capacitor make noise when discharging. Its always been something else.......semi conductor, resistor, tube, connections.

My initial post was my experience with my Bogen amplifier where within the last year or so it would also do that sometimes after shut off so I would have to wiggle the 6EU7 and 7247 preamp tubes and then it would clear up. Its only done it several times in the last few years but still doing fine so far.
 
if it helps any, since the volume control is at minimum when the switch goes to off, that should narrow it down to something within the power amp section. If I had to guess at it, plate resistor on the voltage amp, or a plate or cathode resistor on the phase inverter would be my first guesses. See if any of them measure off value, it might be a clue as to which might be the problem.
 
After going through and testing about every connection, all major resistors, and all tubes, I found the15 ohm 5 watt iffy, and one fancy EH 12at7 very weak on one side. I also reflowed some solder joints. The results are very good! The ole X-202 mutt is singing sweet once more. Talk about relieved!

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Nothing like pissin off the neighbors with a little 20 watt tube amp and some Lascala's. I have had this sweet sounding mutt playing at a high level most of the day out in the garage, amazing sounding amp! Still my favorite sounding Fisher. I'm glad I got the bugs out of it, sure sounds great. Thanks go out to all that gave suggestions. :thumbsup:

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