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Audio Research SP15 : faulty phono stage

Hi everybody,
I have a SP15 defective in the phono stage: hard noises are heard in the speakers.
Sometimes the unit operates properly, but when I change the position of the input selector from line to phono, it oscillates strongly at a very low frequency.
I've disconnected the output of the phono amplifier (R30) from the line amplifier stage, and the fault disappears.
I tested the output line of the phono with the scope and I can see a big negative dc offset when I connect then disconnect the output line from the earth (like the input selector does).
When it oscillates, R30 begins to heat and burns.
The same fault is with the two channels.
Thank you very much for any help

:cop: ARC Schematic removed by moderation. This is a clear violation of Forum rules and ARC copyright.
 
There is a negative voltage on the node R29, C1 and R42 when the output of the pream is disconnected from the earth. When the SP15 is hot, it's very difficult to make it oscillate (only a "clock" when I turn the input selector from the line input to the phono input).
I tried to let the SP15 outside (0°C) about one hour and tested it after. The fault came immediately, and I heard a fast "tic tic tic" from the speakers. More the preamp is cold, more the fault is present.
But the fault is on the two channels. I do not think they became faulty together.
 
I tried again to let the SP15 outside.
This is what happens (the scope is connected to the line out).
I hear a very big "poc" on the left speaker, one second later the same on the right speaker. Then there's a silence of about 4 seconds, and the same noise comes again.


 
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Hav you contacted AR with your issues?
This unit wasn,t the most praised as far as reviews went.... And maybe they have updated (as normal business practice for them) it since then?
Have any caps been replaced in the seperate PS or main chasiss by you or any one else that you know of?
Make sure the connecting cables between chassis are not an issue. This has been an issue on the sp10,11
They get moved around a lot over time.. And its been a long time for this one compared to a solid single chassis design.
 
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The ext power supply is right. I've two SP15, two PS and two cables, so I've tested quickly and know they're not faulty.
This morning the SP15 was very cold, and you can see what happens when power is on in this condition. The scope is connected to the left side of C11. The resistance R30 burns quickly ! 18 V peak to peak !

I think I've found the guilty, I've used an hair-dryer to warm up Q1 and the fault disappears !
Note you see only one Q1 on the schematic, but in reality two transistors are used, they are soldered in parallel, one above and the other below the print.
You can see too they're protected by a little plastic tube (against what ?).


You can see the schematic here : http://elektrotanya.com/audio_research_sp15_sch.pdf/download.html
 
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It worked in this configuration. When T1 are hot, no oscillation !
I will replace the transistors, but I don't know the original type (colour coded)
Any idea for an easy to find field effect transistor silicon P channel junction type low noise audio to replace them ?
 
The transistors are effectively marked by ARC, and it's very difficult to obtain them. And I can't see any other mark on them (Dremel).
Do you think the two psu can be defective ? I tested the supplies with the scope and they're very clean.
What does tx stand for ?

Tx is transformer, it's very strange two PSU defectives, have you tested all resistors & capacitors near Q1? all your voltages are OK as per schematic?
 
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To resume, I got two SP15 and their own cable and PS, and the two have the same symptoms.
All the incoming voltages are ok. I tested the capacitors and resistors, nothing defect.
When the preamp is hot (after one hour) there's no more problem. Only a little "cloc" when the selector is moved to phono input.
I will try to freeze the Q1 and see what's happening.
I think too there are some voluntary mistakes on the schematic.... when I test it, Q1 is a N JFET and not a P JFET !