Hello, fairly new forum member here, and first time poster.
First a little back story:
I'm a retired H.S. shop teacher (not in electricity/electronics, but i can read a schematic and solder well). My hobbies are acoustic guitar repair, HO slot car racing and the endless search for the perfect hot sauce. My amplifier is a '74 Dynaco SCA-80Q I built in college, and has recently been modernized using the UpdateMyDynaco kits. This wonderful "new" amp powers my original A-35's to the fronts and A-25's to the rears. Kudos to Dan at UMDynaco. I don't remember my system sounding this good.
Last spring, I came across a beautiful pair of 1976 AR-5's from the original owner. He worked at a Stereo Discounter's in Baltimore, and the preserved receipt (along with other literature) shows he paid $154 each with his employee discount. The serial numbers are 42961 & 42964. The cabinets are 99% perfect, the velcro attached grill cloths have never been off and the clear protective plastic is still on the brass badges, they're that nice. I bought them knowing they deserve some internal restoration, for sure.
The woofers are back from Millersound with new surrounds. They look awesome, and Bill went above and beyond his stated service. The mid-range drivers and tweeters seem fine (by the 1.5v test), but it nice to know Chris1this1 is waiting, if the tweeters need attention.
One pot in each cabinet (I can't remember if they were the mid or hi), shed a lot of what seemed like porcelain dust. Upon opening them i realized this white powder was solid at one time, and held the bottom half of the resistance coil in place. I also noticed these same two pots silver contacts were more pitted than the cleaner two. Just for fun, i tried this: 1) I carefully lifted the coil an put a small bead of clear silicone caulk in the well, and pressed the coil back in position. After the caulk cured, the coil seemed just as secure as the two good ones. 2) I pooled a little solder to the pitted areas of the contact plate, then sanded them smooth. Once i got over my accomplishment, i came to realize these two pots degraded for a reason, probably heat, and I shouldn't risk using them.
So my first questions are these: Where do you think the degraded pots came from, the mid or the hi spot? Could I return the good pots to their original position and replace the bad ones with the Parts Express 15W, 8 ohm L-pads/10W, 25 ohm resistor, or should i just replace them all? (it's not about the $)
My next question stems from an excerpt from the "Restoring the AR-3" file. "At maximum output, the parallel leg of an L-pad is disconnected from the wiper and can't provide the 16 ohm parallel resistance of the pot. The lack of parallel resistance can add load the hi-range driver when playing high levels at maximum clockwise shaft rotation; this is not so important with Ferrofluid-cooled hi-range as with the original AR driver." Can someone tell me what this means? If I don't play at high levels should I be concerned about this?
And now, on to the 4uf, 24uF and 72uF capacitors..........I've read a lot about this on the forum, and my head is spinning. I keep coming back to this combination. Please offer your opinion: 4uf - use Dayton DMPC 4uF, 250v, 5%, PE#027-255. 24uF - use (2)Dayton DMPC 12uF, 250v, 5%, PE#027-430. 72uF - use (1)Dayton 22uF, 100V Electrolytic Non-Polarized PE#027-348 plus (1)Dayton 50uF, 100V Electrolytic Non-Polarized PE#027-354. (again, it's not about the $, I would go with 60/12uF DMPC's if justifiable). Please advise.
My last question also comes from the "Restoring the AR-3" file. "If you install PP capacitors, you may wish to add a small amount of series resistance to compensate for their reduced ESR and maintain the authenticity of the AR crossover" [It then gives recommended values for the AR-3] Should i do this? If so, what values are appropriate for the 4uF, 24uF, 72uF AR-5 capacitors?
Thank you so much for reading this, and hopefully taking some time to reply with comments or suggestions!