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In Reply to: RE: Audio research SP 11 mk 2 weird issue. posted by SchoolOfRock on September 24, 2017 at 12:39:21
Hi SOR,
It looks like you have blown the input FET. There is a weird feedback op-amp that is some sort of filter. My guess is that the blown input FET has a really high gate leakage current that makes it OK with a 100 ohm load, but that raising it to 49.9k (as shown in the schematic) causes everything to go haywire. It's not a tube problem and I wouldn't let anybody touch it other than Audio Research as they keep the identity of all of their transistors secret.
Hope this helps,
Charles Hansen
Follow Ups:
Thanks Charles. Very kind of you to go to the trouble for this info. Will try cleaning to switch first. You never know.
You say you would not let anyone touch it besides arc. Very expensive proposition. I was thinking of taking it to my arc dealer. They repair older units which were sold at their location. I'm sure arc will send them the part once they identify it. I know they order tubes from Arc to retube their units, hopefully they have access to all parts. Let me know if you've heard otherwise.
Will let you know what happens and thanks again for your analasys. You and the other gentleman have been great
You can try cleaning the switch first but I would go with Charles on this - left channel input FET. It looks like the switch is not in use on the 47K setting (bypassed), and MM cartridge has a higher drive than MC so it works on 47K setting.
The scenario you present is expensive.
Perhaps more than the op paid for the whole unit?
Would one way to IMO test your theory be to add a pair of 47K resistors. (or 49K? 50K added to the cartridge on the phono input, with the switch set to the 100 ohm setting.
SO running the op cartridge with an added 47K through the 100 ohm level of the switch.
Would this be enough to cause the same problem to show itself?
I am all about being cheap.
IMO if the op can use the preamp at 100ohms instead of spending as much or more than he paid.. I certainly would just use it at 100 ohms! The cost to fix it would be at least $700 to send it in.. plus shipping...
I have to assume he bought it used..And the final cost could be much more than that.
(I paid $600 for a SP-10. $1,500 for an Sp-15
The boxes alone would be at least $100 to replace. (ARC never sends stuff back unless it is in the original type boxes. So no original boxes when one sends the items in (two chassis set for SP-10 Sp-11 Sp-15, 40th anniversary), the extra cost is passed on for new boxes. At least $100 like I wrote.
I know Charles Hanson is a smart guy.. I just am hoping he is wrong!
I would still try cleaning the switch..
And if that fails, and the 47ohm added to the 100 ohm fails.. I would just live with the 100 ohm.. unless the op is rich enough to pay the $700 to $1200 a ARC repair would cost?
I will certainly start with the cleaning and see where that goes.
I would love to send it to arc but there is a big arc dealer here in Montreal with their own techs who work on these old units. I would hope they would use the same transistors from Arc but would have to ask them
If you look at the schematic, the 49.9kohm resistor is *soldered* to the PCB. It is the other values that are connected in via the switch.
It is 99.9% chance that the input JFETs have gone bad. As long as the local dealer can get the correct part from Audio Research, this will be the way to go. I would guess this will cost around $200, as most shops are aroun $100 per hour. Good luck!
Very kind of you. Thank you for your help.
Many audio companies do have other facilities for outside the USA.
Same for Bryston. They have a facility in USA. so not cross border duty confusion.
SO hopefully the dealer in Montreal can work on it if the cleaning does not fix it.
Thanks a lot for your input. Will let you know how it goes.
Hi...i am a new user here. As per my knowledge Switching tubes around changing the sound is just normal tube oddness.The failure is in the switch itself. Most likely some sort of fault to ground at the 47K position.Replace the switch and I bet all will be well.
Edits: 11/10/17
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