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Sonic Frontiers SFP-1 Sudden Noise on MC Input
Sonic Frontiers SFP-1 Sudden Noise on MC Input
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Old 6th March 2017, 05:30 PM   #1
ungie is online now ungie  Canada
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Default Sonic Frontiers SFP-1 Sudden Noise on MC Input

I'm starting a new thread here in the Analogue Source forum as I don't seem to be getting any responses in the original thread which can be found here:

Sonic Frontiers SFP-1 Phono Stage

I have had an SFP-1 in my main system for nearly 15 years now and it has been rock solid, until a couple of weeks ago. I have always used the MC input and have never switched it to MM. Recently I have been hearing a random popping and crackling in the left channel only. My first thought was a bad tube as the ones in the unit have been there for over a decade. I tried re-seating all of the tubes, no change in the noise. I then swapped the left and right channel tubes, but the noise still remained in the left channel. Strange. I then cleaned the MC/MM internal switches on both channels, cleaned the tube sockets and pins, but there is still noise in the left channel. Now I'm thinking it's not likely a tube issue. To confirm this I switch both channels to MM mode and there is no noise. OK, on to the MC FET stage for the left channel. I first replaced the B+ supply resistor that drops to the B+ to ~10V, no change. I then swapped out the J310 FET with a new replacement, still no change. Now I'm confused as to what the issue could be. I touched up all of the solder joints in that section and it did not help. Now I'm down to replacing all of the resistors in that stage, but before I do I'd like some guidance as to what might be the cause. Is it in any way possible it could still be a tube issue, even though they are quiet in MM mode? The MC signal goes through the same stages, so I'm not sure how that could be.

Thanks!

P.S. Schematics can be found in the linked thread.
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Old 7th March 2017, 04:54 PM   #2
ungie is online now ungie  Canada
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One other thing I just noticed while going over the schematic that I had previously missed: In addition to the MM/MC switch routing the signal path, a different cathode resistor is switched in for each setting. Is it possible that the 2.74k cathode resistor for the MC setting has become noisy? I guess it's easy enough to swap it out and see.
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Old 7th March 2017, 05:24 PM   #3
ticknpop is offline ticknpop  Canada
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Sonic Frontiers SFP-1 Sudden Noise on MC Input
Any Holco resistors - sometimes the ends went intermittent.
(other brands too)

Last edited by ticknpop; 7th March 2017 at 05:37 PM.
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Old 7th March 2017, 06:05 PM   #4
Bare is offline Bare  Canada
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There is the unspoken aspect :
It ..is..15 yrs old and it ..is.. a Sonic Frontiers.
Both are working against you.
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Old 7th March 2017, 06:32 PM   #5
ungie is online now ungie  Canada
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Quote:
Originally Posted by ticknpop View Post
Any Holco resistors - sometimes the ends went intermittent.
(other brands too)
Hmmm...the majority of the resistors in this unit ARE Holco. I guess I will go ahead and replace all the MC stage resistors and the cathode resistor with new Dale RN60 parts.
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Old 2nd November 2018, 09:39 PM   #6
ungie is online now ungie  Canada
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Getting back to this after some time as I have been using an SUT with the SFP-1 in MM mode with no issues. I would like to get the MC input repaired. What I have done so far is swapped tubes between channels (no change), replaced all four tubes (no change), swapped in another J310 FET on the noisy channel (no change), changed the 20k supply rail dropping resistors for the FET to brand new Dale RS-5 parts (no change), swapped out the feedback resistor with a cheap metal film of the same value (no change). I guess all that is left is to change out all the Holco resistors on the offending channel with Dale parts and see if the noise goes away. At this point all of the active devices have been changed with no improvement, so it must be either a solder joint gone bad (maybe a re-flow is prudent on the entire board?) or a bad resistor.
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Old 2nd November 2018, 10:32 PM   #7
ticknpop is offline ticknpop  Canada
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Sonic Frontiers SFP-1 Sudden Noise on MC Input
Sounds like a resistor end cap gone bad.
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Old 3rd November 2018, 01:09 AM   #8
ralphfcooke is offline ralphfcooke  United Kingdom
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If there are any caps in the signal path I would swap them out as well.
Is there a schematic available?

Last edited by ralphfcooke; 3rd November 2018 at 01:15 AM.
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Old 3rd November 2018, 02:14 PM   #9
ungie is online now ungie  Canada
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Quote:
Originally Posted by ralphfcooke View Post
If there are any caps in the signal path I would swap them out as well.
Is there a schematic available?
The schematic can be found in the first post of this thread:

Sonic Frontiers SFP-1 Phono Stage

There are no capacitors in the MC portion of the signal path. Remember that in MM mode there is no difference in the noise floor between the left and right channels. So there are only a handful of resistors that could be to blame.
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Old 3rd November 2018, 02:36 PM   #10
ungie is online now ungie  Canada
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Also, since once I repair the MC front end this unit will not be totally stock anymore (the original Holco resistors are NLA) does anyone have any suggestions for improving the MC JFET front end of this design without significant changes to the PCB? I know the J310 is not the lowest-noise option out there. I'd appreciate any suggestions!
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