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Audio Research SP3 Problems

R

rjstedman

New Member
Hi, hope I got this in the right thread.

I have an Audio Research SP3. I sent it to AR 19 years ago and they upgraded it to a SP3C. New power supply etc. I did this because I just couldn't find new parts. Anyway, it's been working great for 19 years. Now however, when I switch it on and then either adjust the bass or contour knob the relay trips and the unit starts up again. I have changed tubes but the problem still persists. Do you think the fault is in the Power supply board or amp board? Any suggestions or help much appreciated. Anyway after nearly two decades it's probably time for a recap. Thanks! Robert
 
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If you don't adjust the bass or contour knob does it work?
Hopefully arts will see this post and chime in.

BillWojo
 
Not familiar with the specifics of the -C but it might be a bad tone section coupling cap. Does the unit work with tone bypassed as that is a different set of coupling caps on the not C units and one did fail on Tom's unit.
 
BillWojo said:
If you don't adjust the bass or contour knob does it work?
Hopefully arts will see this post and chime in.

BillWojo
Click to expand...
Hi Bill, Yes everything works fine until you rotate the knobs. Sometimes If I rotate them slowly the unit doesn't cut out. It's usually when the bass is turned up fast it switched the relay to the off position and the unit restarts. Strange. Not sure how a bass or contour knob could effect the relay on the power supply board? It's probably sending a spike back. Not sure. Any suggestions much appreciated..
 
Blue Shadow said:
Not familiar with the specifics of the -C but it might be a bad tone section coupling cap. Does the unit work with tone bypassed as that is a different set of coupling caps on the not C units and one did fail on Tom's unit.
Click to expand...
Hi the amp works fine with the tone out or off. But then again that switches off the bass and contour knobs.
 
That is an interesting problem. Unlike some other ARC designs that use output sampling to prevent amplifier or speaker damage in the event of malfunction or excessive output levels,the SP-3-C does not.The relay section is used as a power-up/power interruption mute only.

The rotation of the contour or bass controls is likely introducing LF instability due to power supply issues,which is affecting the mute circuit.Also,don't forget,the tone controls are of the feedback type,any failures in this area could have an effect.

Another possibility is that,as the mute circuit shares the same power supply as the filaments of the line stage,heater-cathode shorts or leakage could be involved.Lastly,drifting of the components in the mute timer could be causing it to be more susceptible to issues of this nature.

rjstedman said:
Hi the amp works fine with the tone out or off. But then again that switches off the bass and contour knobs.
Click to expand...

Actually,the tone circuits are always "in circuit" regardless of the selector position.The switch just chooses the pickoff point; before or after the tone shaping.
 
Intermittent problem - hummmm. Sounds like a crapped out connection somewhere in the tone circuit.

I'm not making a judgement here! It also sounds like you use the tone controls a lot. Wear and tear can cause these problems. If it's working well otherwise it could be an easy fix.
 
C26 may be leaking, they use same circuit in some of their other preamps, have seen C26 go leaky and cause problems like that.
 
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