Trouble with ARC SP-6

[jprossi]

Hi all,

I have a late 70s Audio Research SP-6c preamp that I acquired in non-working condition from a family member several months ago. I took it in to my local tech, and $300 later I had it home and was loving it. Now I'm having issues again. When I power the unit on, I get a high pitched whine coming from inside the preamp. It usually starts out steadily rising for a few seconds and then starts to modulate in pitch like a siren. It's not loud, but it is noticeable and was not always there. When I un-mute the unit, i get a LOUD screeching from the speakers. This is happens regardless of which input I have selected. I can hear music (if I have music playing) through the screeching, and when I turn the gain up the music gets louder, but the screeching remains constant. The screeching is also present with the gain at 0. I also notice that I can hear music whether or not I have my power amp powered up. Perhaps this was the case before and I never noticed - the SP-6 does have some pretty significant gain and I have reasonably sensitive ADS L-570 speakers.

All tubes light up. While I am not comfortable messing around too much inside the unit, I am comfortable changing tubes or tube position. I believe all the tubes are 12ax7s. I did lightly tap each tube with a pencil, but through the loud and obnoxious screeching, I didn't notice anything. My power is supplied by an ADCOM GFA-555. Is there anything I can troubleshoot, or should my next stop be back to the tech? Thank you for your help!

-Jon

 

[arts]

Jon-It sounds to me like something has gone wrong in the power supply,to the point where it is going into serious oscillation.The power supply in the 6C (and this entire preamp in general) is really rather sophisticated,and many components are critical as to maintaining stability.At a cost of $300,that unit should be going back to the tech real quick,although quite frankly,some ''techs'' just can't handle Audio Research equipment. This is one ARC model that is NOT for anyone who is not extremely competent and experienced.

Be real careful about operating the 6C into your GFA-555 until it is operating beyond reproach; that amplifier will smoke most high powered speakers with the greatest of ease,and your L-570s won't stand a chance.

Good luck,

Art

 

[cademan]

Sometimes a dirty tube pin or socket will cause this. Gently rock the tubes in their sockets. this will help for better contact and rule out other possibilities.

 

[airtime]

That is a very complex piece of audio gear. Try contacting AR and see if they have anyone capable of "really" servicing that preamp. PS are very tricky and very hard to fix if you're not up to the task.

 

[arts]

Audio Research can and will service (within certain condition-based limits) any piece of equipment ever produced in their product line.They can perform complete electrical restoration if and as necessary. The price for this work will be exorbitant,as will be the shipping costs.If you have no other options,and can afford it,this is a great value.

While I cannot speak of their ''authorized'' dealers and service centers in general, I can say that I am seriously unimpressed and very diappointed with their local service center,as I am presently adressing a preposterous ''upgrade'' performed by their ''technician''.

 

[airtime]

Audio Research can and will service (within certain condition-based limits) any piece of equipment ever produced in their product line.They can perform complete electrical restoration if and as necessary. The price for this work will be exorbitant,as will be the shipping costs.If you have no other options,and can afford it,this is a great value.

While I cannot speak of their ''authorized'' dealers and service centers in general, I can say that I am seriously unimpressed and very diappointed with their local service center,as I am presently adressing a preposterous ''upgrade'' performed by their ''technician''.

This seems like a case of - how much are you willing to spend repairing and restoring vs buying a new piece. I agree that letting anyone else but AR touch it is NOT a good idea.

It comes down to how much does the OP like this preamp. The restoration will last the OP another 15-20 years of service. Besides trying to replace a preamp of this level will cost many thousands of dollars.

 

[Bob]

since you paid $300 to the tech, get a detailed blow-by-blow on what he did, what tests were made,
what parts were replaced (you got those back? - like auto repair laws, they're yours), any soldering
done. you're looking for the analysis/diagnosis/screening - directly for what he fixed and indirectly
for anything he may have touched or disturbed. be real careful about resistors since most modern
ones are usually only rated for 300 volts and that's below most tube voltages - and look
for any soldering/reflowing/retouching.

use a toothcleaner pick/brush to get into every tube socket pin. use IPA (the alcohol not the beer).

exercise the switches, and if you have de-Oxit/FaderLube use it in moderation (do not empty
entire can inside) and then exercise the switches.

sounds like a bad connection (either switch or solder joint) - these are 40 years old and need
an electronic colonoscopy.

enjoy this music - it is a legendary unit and worth every penny (or every Benjamin) you put into
this and when you sell, you may find you made money.

 

[Tubologic]

Actually the H.V power supply section of the SP-6C is not so complex as it looks: nothing more than a basic series feedback regulator followed by a post-regulator for the low level stages. This can be easily de-bugged/fixed by any competent tech. 3 transistors, a couple of zener diodes and an opamp is not rocket science. You should first check if the raw DC (unregulated) input voltage and the (regulated) B+1 and B+2 voltages are within specs (+530, +423, + 410 VDC respectively) and not oscillating (A scope with a probe rated at least for 600VDC is needed for this latter test). If your power supply is defective or unstable it is VERY UNLIKELY that any cleaning, socket re-tensioning, re-soldering or tube rolling (all standard procedures) will cure the trouble. I would first check the main input electrolytic filter caps (C14, C15) which are series connected, I had a few SP-6's where one of these two caps was open, completely upsetting the working of the circuit. Then the B+1 output regulator filter cap (C21) should also be checked. Replacing C14,C15,C21, C13 and C17 elcaps should be considered as a standard preventive maintenance procedure on any SP-6C. Was it done on yours ? Also check the +24VDC heater supply which is also the source of supply for the HV comparator opamp and reference voltage: a defective 24VDC supply will also impact the working of the H.V regulator.
Of course, if you don't feel confident enough to proceed yourself with these (sometimes dangerous !) procedures it is much wiser to go back to your tech and ask him to re-check his work. At the price you've paid you should have some form of warranty, even if it was only a small fraction of what you could have been possibly charged at an ARC official (factory) service center . A last word of caution: If you're unsure about what you're doing DON'T TAKE RISKS AND STOP IMMEDIATELY.

 

[jprossi]

Thank you everyone for the advice. Here is the breakdown of work performed when I had the unit in in the spring:

Standard Service Performed:
-Interior Cleaning
-Deox Controls
-Deox/Tighten RCA
-Lamp Check
-Fuse Check
-Tighten Sockets
-Capacitor Leak/Value Check
-Power Supply/Voltage/Ripple Check
-Minimize Noise
-Ground Hum-Safety Check
-In-Circuit Tube Test
-Music/Euphonic Check

Additional Work Performed:
-Dismantle
-air brush
-locate and replace failed components in B+ distribution and regulator circuits
-resolder regulators and tube sockets
-complete test

Technical Notes (parts, I assume)
-chemicals and misc
-poly cap
-fast recover rectifier
-special fuse

exercise the switches, and if you have de-Oxit/FaderLube use it in moderation (do not empty
entire can inside) and then exercise the switches.

I don't have deoxit, but I do have CRC QD Component Cleaner. Are they comparable products? Seems to me the shop would have done this anyway.

use a toothcleaner pick/brush to get into every tube socket pin. use IPA (the alcohol not the beer).

Good to know - I'll save the Heady Topper for personal use. I'm much better at brewing (and consuming) beer than I am fixing electronics anyway. I'll give the connections a cleaning and if that doesn't fix the problem (and I have the sinking feeling it probably won't), then I'll let the pros handle the rest. What is the reason for using isopropyl instead of deoxit/contact cleaner on the tube connections?

I should note that my original reason for bringing the unit in is that it would instantly blow a fuse every time I would try to power it up. I should also add that I did notice a couple of crackle/pops on one occasion when I powered up. This was after the other issues. I am interested in getting this thing up and running, but I think I'll give the local shop a chance to at least let me know what they think is wrong with it. Perhaps they'll work with me since I've already put some good coin into it. I know it's hard to say, but anyone have a ballpark idea of what sending something like this in to ARC might cost?

 

[Tubologic]

" Thank you everyone for the advice. Here is the breakdown of work performed when I had the unit in in the spring:
Standard Service Performed:
-Interior Cleaning
-Deox Controls
-Deox/Tighten RCA
-Lamp Check
-Fuse Check
-Tighten Sockets
-Capacitor Leak/Value Check
-Power Supply/Voltage/Ripple Check
-Minimize Noise
-Ground Hum-Safety Check
-In-Circuit Tube Test
-Music/Euphonic Check

This is all basic maintenance work allmost anyone could do. NO parts replaced. I especially like the "Euphonic Check"(?)
Additional Work Performed:
-Dismantle
-air brush
-locate and replace failed components in B+ distribution and regulator circuits

This is the only "real" technical work I can see, and the most important one. Did you received the old "failed" components ? FYI, the main electrolytic filter caps are quite big parts.

-resolder regulators and tube sockets
-complete test
Technical Notes (parts, I assume)
-chemicals and misc
-poly cap
-fast recover rectifier
-special fuse

Too vague description, again you should have received the old (replaced) parts.

 

[Bob]

so, it's either fluff or open heart surgery. Suggest you bring it back and do a murphy (do over),

there's some "resolder", parts swap, etc. I'd look there. and would not rule out power supply.

good luck.

 

[savatage1973]

Tubes were tested "in-circuit"--not the way to test and rate tubes--get a full report on their actual "health".

 

[Tubologic]

Thank you everyone for the advice.

I should note that my original reason for bringing the unit in is that it would instantly blow a fuse every time I would try to power it up. I should also add that I did notice a couple of crackle/pops on one occasion when I powered up. This was after the other issues. I am interested in getting this thing up and running, but I think I'll give the local shop a chance to at least let me know what they think is wrong with it. Perhaps they'll work with me since I've already put some good coin into it. I know it's hard to say, but anyone have a ballpark idea of what sending something like this in to ARC might cost?

No idea but you should first ask them for a repair quote, and also HOW MUCH they'll charge you for the quote If you decide not to go ahead. Again, this is the last option you should consider if you really can't find anyone in your area competent enough to fix your preamp. Again, the SP-6C is not really hard to repair, the most difficult part is doing a clean work without butchering the 2-sided PCB, and this is the area where most techs fails...