Vintage Preamp caps replacement

[artto]

I have an Audio Research SP-6B (cira 1980). It blew up around 1984 and ARC did the repair work. I haven't used it much recently and I think it's best to replace some capacitors just for peace of mind.

My question is, do all the caps need to be replaced (ie: polyester, mica, tantalum etc) or is it just the electrolytics?

I was told by someone more knowledgable than myself who also uses Luxman MB3045 triode power amps (as I do) that he's never found any significant leakage in the large top mounted power supply caps and suggested leaving them alone, but that I should replace the eletrolytics. I'm wondering if the same is true with the SP-6B.

When the SP-6B has been off for a few months, and then very slowly powered up with a variac, one channel (from phono section only) does produce an intermitant static-like noise which goes away after about 24 hours. The preamp uses (6) 12AX7 for audio but has four electronically regulated solid state power supplies.

Thanks in advance

[Deang]

I was told by someone more knowledgable than myself who also uses Luxman MB3045 triode power amps (as I do) that he's never found any significant leakage in the large top mounted power supply caps and suggested leaving them alone, but that I should replace the eletrolytics.

Hi Artto -- The top mounted power supply caps on those amps are electrolytics. I wonder what he meant by "significant leakage"? Any leakage is bad when it comes to power supplies.

When the SP-6B has been off for a few months, and then very slowly powered up with a variac, one channel (from phono section only) does produce an intermitant static-like noise which goes away after about 24 hours. The preamp uses (6) 12AX7 for audio but has four electronically regulated solid state power supplies.

That sounds more like a tube issue. Have you ever pulled the tubes and cleaned the sockets?

[artto]

Hi Artto -- The top mounted power supply caps on those amps are electrolytics. I wonder what he meant by "significant leakage"? Any leakage is bad when it comes to power supplies.

That sounds more like a tube issue. Have you ever pulled the tubes and cleaned the sockets?

Thanks Dean

This what he said. "Here is my two cents. I have had the opportunity to measure the leakage af all the top mounted caps, in the Luxman. None of them have ever exhibited bad behaviour and the leakage was very low considering their voltage and capacitance ratings. I would leave them alone."

Yes, I've pulled the tubes and cleaned the pins/sockets a number of times, and every time I have to fire it up with a variac. At first I thought it was a tube too but I've never heard this kind of noise from a tube just disappear after 24 hours, so I figured it must be a cap that is getting weak.

[Deang]

I'll email Craig and point him to this thread, let's see what he says.

[Duke Spinner]

bear in mind Audio Research is a complicated product ..

loaded with parts 'n pieces

bad power supply cap's..usually make " Hum ", not static, and are what contribute to spectacular failure, not the Small Signal caps

.... tho they might be the cause of the noise

it is only the Electrolytic's that you need be concerend about, the best indicator of condition is ESR

[NOSValves]

I've never worked on a audio research preamp so my advise is very general. The noise you describe could be leaky caps forming up. Most likely the polyester coupling caps. As a general rule mica, tantalum caps will hold up for ages. I'd be first changing out the polyester film caps. I'd also replace any of the small form factor electrolytics. I'd save the large high voltage cans for last but it really depends on what brand they used and the voltage ratings compared to the actual voltage present. If they used 450V main power supply filter cans and 425V is present they maybe on there last legs also.

Nearly impossible to diagnose something like this VIA the internet when I've never worked on the piece in question .

[artto]

Thanks guys.

I just got off the phone with another local referral who my friend said "can fix any preamp or amp". After speaking with him his best advice was to let Audio Research do the servicing.

I can feel it. Here comes another HUGE service bill. [li]