AR-3 Crossover Capacitor Values - Help Please

[Al W]

Hello,

My first post.

I've been enjoying the forum and wealth of information while planning the rebuild of AR-3s which were my fathers. I hope to give them to my son if I'm successful at the rebuild (S/Ns C66552/C66553).

I though I had all the information I needed already posted or included in the well written AR-3a rebuild article. However, on opening the units I find the crossover network with a capacitor value which is different than in the AR-3 schematic AR3-0305-2. This is the added parallel cap for the mid range which is given as 6 mfd. A 50 mfd cap is installed.  So what's installed is a 50 mfd in parallel with the 24 mfd cap from the dual capacitor. Furthermore, the AR-3 crossover reference (from library) indicates the added parallel cap was introduced starting with S/N C70229, after my S/Ns. I understand that sometimes during builds units are substituted based on availability and wonder if the mid range driver is different such that a different cap value is needed. Can anyone shed light on this?

My rebuild plan is to clean the pots (replace if needed), install new capacitors (extensive forum discussion on selection to keep my confused ;-] ), and hope the Hi and Mid drivers are still OK. I'll save the woofer issue for some other time.

Thanks all and happy listening.

Al

 

[JKent]

Welcome Al W

A very worthy rebuild and I know you want to do it right. I've never seen a 3 crossover like that, but then I've only worked on one 3. Some members more experienced with the 3 will undoubtedly chime in. Patience will be rewarded.

Kent

[Al W]

Thanks JKent for your reply. 

I've now opened the second speaker and find the expected configuration for a AR-3 consistent with the S/N (crossover C from the reference). And I've now payed attention to the MID driver appearance and find the Mid driver on the second unit looks to be the 2" phenolic-dome driver used on AR-3s. The first speaker (one with questionable parallel cap) has a MID driver that has the appearance of a later one (AR-1-1/2 treated cloth dome, A-13 or -14 from the AR-3a rebuild guide). I suspect a replaced Mid driver at some point with the added cap and coil.  I guess i need to accept the mystery.

Now need to decide if the existing configuration of the two units is what I want to restore of if I should try and update to be common.

Photos from second speaker attached.

Thanks and happy listening.

Al W

[RoyC]

56 minutes ago, Al W said:

Thanks JKent for your reply. 

I've now opened the second speaker and find the expected configuration for a AR-3 consistent with the S/N (crossover C from the reference). And I've now payed attention to the MID driver appearance and find the Mid driver on the second unit looks to be the 2" phenolic-dome driver used on AR-3s. The first speaker (one with questionable parallel cap) has a MID driver that has the appearance of a later one (AR-1-1/2 treated cloth dome, A-13 or -14 from the AR-3a rebuild guide). I suspect a replaced Mid driver at some point with the added cap and coil.  I guess i need to accept the mystery.

Al W

I agree with your assumption. The replacement mid, added capacitor, coil, and black glue all date to late 74 or 75. The cabinet stuffing was also replaced with the later AR type. There is no doubt this was a "factory authorized" repair involving the replacement of an original AR-3 midrange with a late AR-3a midrange.

Assuming the added coil is .4mh, the mystery is why the parallel 50uf capacitor was added to the mix and not a 6uf cap as shown in the schematic posted in the Library. The 50uf cap in parallel with the existing 24uf value results in a total of 74uf. Alternatively, if the 6uf cap was added it would be 30uf. Big difference....which I have never seen before. The response of the later 3a type mid is not different enough from the early version of the 3a mid to warrant such a difference in capacitor value.

Are your woofers the original AR-3 version with cloth surrounds or the later foam surround type?

Roy

[Al W]

Roy,

I agree that the unusual configuration for the one speaker looks factory so I'm hopping there was a reason for the cap values. One point to consider is the MID XOver network for the typical AR-3a has a 50 uf along with two inductors (#1 & #4) and a .5 ohm resistor in the circuit (fig 3.5 from AR-3a rebuild guide). The configuration on my speaker for the MID only has the one inductor along with the parallel caps (50 uf & 14 uf).  Again, I may need to accept the mystery.  BTW - I checked the resistance of the drivers (out of circuit) and both HI drivers match at 1.8 ohms and the correct MID measures 2.6 ohms vs the replaced MID at 4.0 ohms.  I researched the low readings on the 3 original AR-3 drivers and find reports that they're correct although seemingly low for a "4 ohms" driver.

My plan is to replace the caps with new and put back together with the cleaned pots. Then hear how they sound.

On the woofers, the sad story is that after I received the speakers many years ago the woofers where shot and I replaced them with other 12" drivers. The original woofers sat around for the longest time and were finally tossed out (likely due to spousal pressure..  ;-] ). I didn't know their value and reasonable ability to rebuild them.  I'm now considering obtaining the correct AR woofers or updating to a later AR woofer with related XOver circuit mod.

Thanks for the help.

Al W

 

[RoyC]

5 hours ago, Al W said:

Roy,

I agree that the unusual configuration for the one speaker looks factory so I'm hopping there was a reason for the cap values. One point to consider is the MID XOver network for the typical AR-3a has a 50 uf along with two inductors (#1 & #4) and a .5 ohm resistor in the circuit (fig 3.5 from AR-3a rebuild guide). The configuration on my speaker for the MID only has the one inductor along with the parallel caps (50 uf & 14 uf).  Again, I may need to accept the mystery.  BTW - I checked the resistance of the drivers (out of circuit) and both HI drivers match at 1.8 ohms and the correct MID measures 2.6 ohms vs the replaced MID at 4.0 ohms.  I researched the low readings on the 3 original AR-3 drivers and find reports that they're correct although seemingly low for a "4 ohms" driver.

My plan is to replace the caps with new and put back together with the cleaned pots. Then hear how they sound.

On the woofers, the sad story is that after I received the speakers many years ago the woofers where shot and I replaced them with other 12" drivers. The original woofers sat around for the longest time and were finally tossed out (likely due to spousal pressure..  ;-] ). I didn't know their value and reasonable ability to rebuild them.  I'm now considering obtaining the correct AR woofers or updating to a later AR woofer with related XOver circuit mod.

Thanks for the help.

Al W

 

Al W

You have a long way to go if you are using non-AR woofers. They will not sound like AR-3's regardless of the repairs you have planned....and even if you were to use original AR woofers, the two speakers of your pair are not very likely to sound the same with the different mids (regardless of the questionable crossover changes).

As far as the unusual crossover changes go, I have not seen these in 40+ years of repair and restoration of AR speakers. The crossover with the 3a type mid was already established, so it is unlikely they were done for this reason. I'm thinking the changes were made so late in the game that it is possible the woofers were replaced with the later ferrite magnet/foam surround version at that time as well. Based on my impressions of the AR-3 equipped with later AR woofers, those changes could make some sense. Later AR woofers are not the best replacement for the AR-3 using original crossover values.

Roy

[Al W]

Roy,

I think your thought is correct on the woofers.  It's been a long while but I do recall when I replaced the woofers one was of the AR-3 configuration with the cloth surround and the other was the AR-3a type with the foam surround. So I'll look for one of each for the rebuild. BTW - When the repair was done (I'll say by AR) the grill was also changed to the cloth grill with velcro attachment. Residue of the original hot glue was still there. The other (AR-3 configuration) still has the open weave mesh grill.  Looks like I inherited one AR-3 and one AR-3/3a hybrid.

Thanks again for you help and insight.

Al W