crackling/static noise in one channel (X-202)

[ssouci]

I've suddenly started hearing a crackling/static noise in one channel of my Fisher X-202 integrated amplifier

Description:
- sound is an uneven "crackling/static" noise, NOT smooth white noise
- in the right channel only
- not too loud, however loud enough to be heard in a quiet room once music stops playing... sometimes is worse, sometimes better
- volume-dependent: hard to hear at "0" volume level (must put ear up to the speaker), but audible from approx. 10ft away with volume set at healthy listening level

- SELECTOR SWITCH:
> choosing different inputs makes no difference

- MONO/STEREO SWITCH:
> "reverse" causes the sound to move over to the left speaker
> "mono" puts sound in both speakers equally
> "A" eliminates sound from both speakers
> "B" puts sound into both speakers equally

Q: what could be causing this, and what is the most efficient way to troubleshoot this?

Note: this amp has been fully recapped and some resistors replaced within the past few years... generally runs perfectly, is very quiet, and sounds great

thank you!

 

[1rebmem]

My first thought is the tube socket pins need cleaning and tensioning/tightening.
I'd suspect V1, V2, or V3 because they are before the mono/stereo switch.
I believe tube socket maintenance should always be done on old tube equipment regardless.
I have had similar noise issues, more than once, due to old tube sockets.

Could be a tube itself but my first thought is tube pins.

Jef

 

[dcgillespie]

Great job of supplying detail information! Because of that, the problem is clearly identified as residing in the V2 stage -- as identified by the Sams schematic -- or the V3 stage, as identified by the Fisher schematic. In either case, the same 12AT7/ECC81 stage is targeted. This tube serves both channels, with one section serving the left channel, and the other section serving the right. The noise must be in this stage, as this stage appears after the selector switch -- whose setting you indicated had no effect on the noise, but immediately before the Mode switch, whose setting you indicated did affect the noise.

One way you can confirm this diagnosis is to use the Tape Monitor switch. That is, when the amplifier is making the noise, turn ON the Tape Monitor switch. If this stage is in fact causing the noise, then the noise should be all but eliminated when the Tape Monitor switch is turned on.

If that is the case, then certainly check the cleanliness of the tube socket and tube pins as Jef mentioned, but if that doesn't give any satisfaction, then try another tube. The 12AT7/ECC81 tube is a wonderful sounding tube, but is well known to generally be a noisy tube unless those used for audio purposes are carefully screened for noise. While the 12AT7 tube is very capable as an audio amplifier, it was primarily designed to be an RF amplifier, and so construction for low noise was never high on the list of requirements for this tube. Therefore, try another tube is cleaning the socket doesn't solve the problem.

If the noise persists even with a known quiet tube installed, then the likely suspects are the plate and cathode resistors, and/or the cathode bypass cap for the stage as well. Assuming that the Fisher schematic is the correct version, the right channel is serviced by section 1 of this tube, or pins 6, 7, and 8. Just to ensure that you're thoroughly confused however, Sams indicates that it is section 2 of this tube that serves the right channel, or pin 1, 2, and 3.

Good luck with it!

Dave

 

[ssouci]

thanks Jef and Dave -- problem SOLVED!

turns out that the noise was coming from the V3 tube ("HIGH LEVEL VOLTAGE AMP" for both channels A & B)... it must be noisy on one side... it was a 12AT7 Raytheon (made in USA)

first, I pulled the V3 tube because that was the most likely culprit (V1 and V2 are "PHONO & TAPE HEAD PREAMP" for channels A & B respectively, and I don't even use Phono), and made sure the pins were clean... sprayed some isopropyl into the tube socket, and also exercised the tube in/out several times... the sockets seemed nice and tight... this had NO effect

next, I swapped in another 12AT7 tubes (stolen from the V4 / V5 position), and when I did that, the noise disappeared!... hence the Raytheon tube itself was the culprit, not dirty connections

I then had to relocate the "bad" Raytheon over to V4, since I don't have any other 12AT7s

QUESTION:

will putting the "bad" tube in V4 cause any degradation in sound?... I haven't noticed anything obvious so far, and the amp sounds great and is super quiet

V4 / V5 are "SCRATCH FILTER & REMOTE CONTROL" for channels A & B respectively, so I assume that if I don't use either of those functions (I don't), then there will be no audible effect???

thanks

 

[dcgillespie]

V4 and V5 (as identified by the Fisher schematic since you've identified V3 by that schematic) are used for all signals passing through the amplifier, regardless of input chosen. The fact that Fisher identifies those stages as serving the Scratch Filter and Remote Control features doesn't mean that the signal does not pass through them when those features are not being used. All signals pass through them at all times, with these stages also providing for the features they are labeled for when they are used.

Also, use of the tube originally in the V3 position now in either the V4 or V5 position will be a good move, because V4 and V5 operate at a higher signal level than the tube in the V3 position does. Therefore, the signal to noise ratio of these stages is much higher than at the V3 position, so whatever noise the tube generates in the output will be greatly reduced in its new location.

I hope this helps!

Dave

 

[ssouci]

thanks Dave, your info makes sense and is hugely appreciated!