ARC SP-6A Tube Preamp

Atmasphere said:

If you have the schematic and feel comfortable working on the equipment (which is to say you have the sense and knowledge to not zap yourself) then the thing to do is find out why one channel is out first. Obviously poking around the mode switch would be high on the list of things to check. Of course its worth cleaning the switches with a small bit of DeOxit (in the green and black can). Usually if switches are a problem they will let you know with static when you operate them.

Only after you have it running would I consider replacing any hookup wire inside. IIRC, IMO the wire they were using back then was nothing special so the wiring installed by that someone might actually be better. Its not work speculating though; get it working first.

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trekpilot said:

Have you looked at the ARC database?

https://www.arcdb.ws/model/SP6

There is a photo not sure if it is the view you need.

Scott

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DasguteOhr said:

The wiring is very complex, I hope it helps you. If not from the list, print out the circuit diagram and go to a TV hifi repair shop near you, they will definitely do it for you

Index of /2-SCHEMATICS/BY-BRAND/Audio-research

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DasguteOhr said:

No. if you have the gray wires in, check the continuity with an ohmmeter. Do you have all inputs aux, phono checked all the same behavior? if yes, check main output cable measure marked cable. If don't clean all the switches and then operate them very often, see if the error has gone away as Ralph recommended.View attachment 125790

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maddmaxxdrummer said:

Hi, now all good. I didn’t have the grey wires. The previous owner had rewired it all with solid core 18Awg copper wire. Was a mess. Had he done it neat and all pairs twisted nicely, I would I have left it. Mind u he had some stuff wired in reverse.(left was right on some channels. I rewired the unit with multi conductor shielded cable as I was told by ARC. the original was 18Awg. I put in military spec wire. Teflon jacket. Silver plated copper. I followed the same layout as the original. Much neater now. And connected to the right inputs/outputs..
recapped all film caps with Mundorf paper in oil and Audyn MKP. the electrolytics we’re changed 10yrs ago so I left those. Got rid of all the carbon comp resistors and put in Vishay, Ohmite & KOA power resistors for the PSU. Caddock, Dale, PRP, Vishay Beyshlag , Riken & Kiwame everywhere else. I left the original 1% resistors that were there as these were all Dale military spec.
The original switches were tired and corroded. All 4 changed to Alco long shaft like the original C&K. (30$/ toggle, ouch)

The original RCA connectors were either corroded or broken inside. Swapped all 16 with Neutrik gold plated.

All pots and switches were cleaned with deoxit.

One thing i can’t figure out is the rated fuse is 0.5A 250V, but there was a 1A 250V inside. Not good. I put in the Rated fuse and it blew. This was before I did all the mods. The original owner that recapped the electrolytics didn’t use 1 for 1 values. There’s extra capacitance each bank. Example 6x 200uF was the original. He put in 6x 220uF. I’m assuming the extra capacitance is causing a current surge on start up. I installed a 60ohm 0.8A thermistor on the hot ac wire to help mitigate this. But the 0.5A still blows. But now better than before because a 0.75A was also blowing before the thermistor. now it’s not. Maybe I need to try a 0.6A thermistor to get the 0.5A not to blow on start up. I measured how high the surge is with my multi meter and it’s surpasses 1A then comes down to 360mA steady state. Will need to experiment with this. If some one has any knowledge of this. Please advise.

i might also change the tired power cord. The outer jacket is frayed in a few spots.
well anyway, it’s looking and sounding incredible.

thx everyone for your help.

Some before and after pics.

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Norman Tracy said:

Well done maddmaxxdrummer !! The SP-6 is certainly worth the hours and care and your craftsmanship is so much better than the previous chaps. I have recapped SP-6 twice. Both times the same unit for my best audiophile friend. One of those friends who started as a HiFi buddy and over the decades became a true friend. This friend loves his ARC. First time I worked on his SP-6 it was blowing the solid state parts in the voltage regulators. ARC advised replace the electrolytic caps along with the fried BJTs and Rs. The aging out caps increased output impedance was causing the regulator to oscillate and cause a cascade failure. When the SP-6 was in production then available high voltage electrolytic caps had a much shorter life span compared to 21st century parts. That first repair held for about 25 years than it was time to do it again. This time no fireworks, we got to it in time when it was sounding a bit noisy and just not right. New caps and it sounds great, I especially love the phono stage on the SP-6s.

One challenge restoring SP-6s is with all the versions the as built circuit on your bench may have some variations compared to the schematic. I made notes of the parts' values coming out and correlated vs. schematic to decide what to buy for the restoration. Also the 1980's circuit board does not mark parts' locations as clearly as modern PCBs. I suggest leaving the old parts in place until new parts arrive and replace one by one to avoid confusion and errors as to where a part's wire lands.

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DonH50 said:

A bit of technical babbling, got long, sorry, but may be helpful for some.

Many DIY mod folk do not realize adding a lot of decoupling (power supply) capacitance also increases the current and places stress on the power supply. In addition to blowing fuses, regulators (SS or tube), wiring and PCB traces, switches, and such all see higher current. This can cause failures both fast (short-term, e.g. blown fuse) and long term (e.g. PCB traces gradually deteriorating, transformers failing).

Older capacitors lose capacitance for several reasons. Their electrolyte, the film inside the capacitor that separates the metal plates (foil, film), can dry out and become a worse dielectric. This both decreases capacitance and increases likelihood of the voltage causing brief shorts that usually burn holes in the conducting film, pin-holes. The holes decrease capacitance, and "junk" around the holes like metal film shards and "goo" provides conduction paths that increase leakage current. That in turn heats up the capacitors, which causes more failures, and so on. Old capacitors thus lose capacitance (the ability to store charge) and increase leakage (causing greater current draw to ground instead of going on to the circuit being powered). Note new capacitors, especially electrolytic types commonly used in power supplies, usually measure about 20% high in capacitance value so they still meet spec (which is often 50% of their rated value) at end of life (varies by capacitor, may be 10,000 hours or more of use).

Another consideration is the internal resistance of the capacitor, equivalent series resistance, ESR. New capacitors may have lower ESR than old capacitors since the conducting film is new, may be thicker by design (to lower the ESR), and that will also increase the inrush current since lower ESR means more current can flow more quickly. Sometimes moderate ESR capacitors are specified to both the limit current and reduce "ringing" on the power rails due to capacitors interacting with inductors like transformers. Choosing the absolutely lowest ESR capacitor may not always be the right choice when replacing capacitors.

The inrush current, the current surge at turn-on, for an ideal capacitor is i = C* dV/dt, the capacitance C times the change in voltage dV divided by the time for the change dt. The change in voltage is usually from 0 V (off) to the full supply voltage when on. Increasing the capacitance thus directly increases the current, since the final supply voltage is unchanged. Adding a resistor (thermistor) increases the time to charge, dt, and depending upon the load also reduces the voltage (until the thermistor heats up so its resistance become near 0 ohms), thus reducing the current. ARC and other manufacturers would sometimes use a series resistor to reduce inrush current. The resistor was often in the incoming AC (wall power) circuit, and a relay would short the resistor after a short pause to allow the capacitors to charge more slowly (thus reducing inrush current).

Determining the peak inrush current usually requires a broadband current sensor, for example a special oscilloscope probe, to capture the peak and slew rate. A broadband current meter may also be used, though these are not usually your average household meters. Since current sensors are not common (and expensive), I'd guess more often a small resistor (say 1 ohm or less) is added to the line and the voltage across it measured by the 'scope to determine the peak current as i = V/R where V is the voltage across the resistor R (Ohm's Law).

To determine the fuse's value, you add some margin on top of that peak current, as running up to the current rating every time you turn on the device will eventually blow the fuse. You (the designer) must also ensure the rest of the supply, wiring/traces, transformer, regulator devices, etc., can handle the surge current without damage. This all means giving a simple answer is not really possible without a lot more information, some of which likely only ARC knows. You can look up ratings of the series-pass regulator devices (derating of 20~50 percent is common) as a start, but you might want to just ask ARC and see what they say. In the past they were pretty helpful when I was repairing gear, but I have not done that sort of work in years and have no idea their policy now.

Given the circumstances, and knowing the fuse is there to blow in the event of catastrophic failure, I'd probably throw in a 0.8 or 1 A fuse and hope it lasts the life of the preamp. That is a pretty loose guess, and no doubt others (like me) would want to dig a lot deeper into why the fuse is blowing, and either fix the reason or more thoroughly understand why the higher rating is needed along with ensuring the rest of the supply is OK with the higher surge. I have no idea what equipment you have and your knowledge base to analyze this.

FWIWFM - Don

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Atmasphere said:

220uf is the value you're going to find these days and should be no worries.

But they need to 'forum up' (they draw a bit more current at startup otherwise; so this will take about 200 hours for them to settle in) and you also should be using a slow blow fuse. If fast blow it will likely vaporize as soon as you try to turn it on.

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