AR-5 Restoration Newbie / Questions

[TJet54]

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!

Peace, 

Steve

 

[Aadams]

9 hours ago, TJet54 said:

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 $)

I was a pot holdout for years for the sake of originality.  My recommendation would be to replace all of the pots with Lpads.  The LPads are trouble free and give finer control.  The current thinking is the resistor should be on the mid range LPad but only on the tweeter LPad  if the tweeter has been rebuilt.     There are some good looking and well reviewed after market pots available that some members here have used. 

 

9 hours ago, TJet54 said:

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?

This relates to the above question and is why the current advice is to NOT use a resistor if your tweeter has NOT been rebuilt.  Your tweeter may look perfect and operate but the chance is 99% that it is no where close to original performance.  Not using the resistor allows the moribund tweeter output to be boosted somewhat.  If it breaks you get it repaired and then put resistors on the circuit. I have my tweeter lpads with resistors turned up full (bypassed) on all my rebuilt tweeters.  It won't kill them.  You can do more harm with a tone control IMO.

 

9 hours ago, TJet54 said:

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.

This is a ticking bomb.  MY two cents: I replace original with original which means I use NPE s in these old speakers.   And it is not about the money.  I want to remain original. 

Wait for responses from others.  I can be eccentric.

Adams

[genek]

If any of your caps are aluminum cans that say "Sprague Compulytic" on them, just leave them be. 

 

[Aadams]

 

2 hours ago, genek said:

If any of your caps are aluminum cans that say "Sprague Compulytic" on them, just leave them be. 

 

Good point can't  believe I forgot that.  Here is an image from the interior of an AR5 like yours

[TJet54]

As a matter of fact, my 24uF orange caps are just like those.  I took them to an industrial electrical service company here in town, and they measured 28.4 and 28.6.  Since that is greater than 10%, I thought they weren't up to snuff.  Go, or no go?  Thanks  

[RoyC]

2 hours ago, TJet54 said:

As a matter of fact, my 24uF orange caps are just like those.  I took them to an industrial electrical service company here in town, and they measured 28.4 and 28.6.  Since that is greater than 10%, I thought they weren't up to snuff.  Go, or no go?  Thanks  

It somewhat depends at what frequency the capacitance measurements were done. Did you have the 72uf and 4uf caps tested as well? Was ESR measured?

[TJet54]

Yes, I did have them  check (they used a Fluke hand held), and the 4 & 72 Industrials all measured +18% or greater.  I'm sensing the 24uF Sprague Compulytic's must be important in maintaining the integrity of the restoration.  The company that tested my caps specializes in control systems and power distribution.  I'm not sure if they are into frequency/ESR stuff, but I'll ask.

On 9/13/2019 at 8:16 AM, Aadams said:

The current thinking is the resistor should be on the mid range LPad but only on the tweeter LPad  if the tweeter has been rebuilt.     

If i send my tweeters to chris1this1 who would replace the foam dampener behind the dome, build a replacement former and voice coil and resuspend and seal the assembly, I would need to add the resistor, correct?

On 9/13/2019 at 8:16 AM, Aadams said:

There are some good looking and well reviewed after market pots available that some members here have used. 

 Other than the Parts Express 15W, 8 ohm L-pads/10W, 25 ohm resistor, what other options are there?

Thanks

[Aadams]

Have you read this thread?  This is part of Carl's great legacy to us.  About halfway down the first page is his opinion on old Sprague Compulytics.  It seems odd to me that all of your Compulytics of same value measure out out of tolerance by same amount, in light of Carl's recommendation and experience with these specific old caps.   I would not have touched them but truthfully I think at this point you could go new or old NPE and never hear the difference. It is not worth the effort to experiment as there will be no big payoff either way unless one of the Spragues is truly off.  You might replace the tweeter caps with new as a compromise.

  For Pot replacements look at this thread at the bottom of the first page.

 

[genek]

20 minutes ago, Aadams said:

 It seems odd to me that all of your Compulytics of same value measure out out of tolerance by same amount, in light of Carl's recommendation and experience with these specific old caps.  

This suggests to me that they were all that way when they were new.

[TJet54]

Is this photo correct, using the Parts Express L-Pad?   If my tweeters have not been rebuilt, I do not add the 25 Ohm 10W resistor, correct?  Thank you.

 

Edited September 20, 2019 by TJet54
added a 2nd photo

[Aadams]

 

Unqualified Yes  to the first question and recommended Yes to the second.

Adams

[TJet54]

Are the lugs on the L-Pad for a .110, or a .187 female quick connect?

[Aadams]

23 hours ago, TJet54 said:

Are the lugs on the L-Pad for a .110, or a .187 female quick connect?

 I thought someone who knows the answer to your question would have replied by now.   It seems most folks solder the wires onto the Lpads after setting the shaft in the hole. You can also solder pigtails onto the LPads before installing and then make the crossover connection with wire nuts. 

[TJet54]

I heard it through the grapevine the PE Lpad lugs are .120.  Not 100% sure, but a tough size to find for quick connects.  I guess i'll pre solder leads as needed since the lugs will be close to the wood.  I'm planning on using insulated M&F quick connects to/from the capacitors to ease possible future cap changes.  Decided to go small @ .110.