Another AR-4x restoration

[Mike2D]

Mine are original owner speakers, stored in ideal condition for several years. Pots are in need of replacement or repair. Speaker serial numbers are FX-250128 and FX-246425. Strange, because they were purchased at the same time, of course.

So questions. The cloth surrounds look OK but the "press test" doesn't seem to indicate a slow return of the diaphragm on the woofer. Does this test apply to the 4x's? On the other hand, RoyC seems to advise not to reseal them on some models. How about the 4x? I'm tempted to leave them alone.

The inductor coil is mounted on top of the wax capacitor box and I can't read the numbers, but apparently I need a 20mfd capacitor to replace that, which I have ordered. I've not seen a photo of the inductor coil strapped to the top of the capacitor block.

Rather than try to clean the original pots, can I assume that the use of the same L-pad with an 8 ohm resistor works as well on the 4x?

I've read everything I can get my hands on including the excellent AR-3a manual. I do have a pair of 3a's that are next in line when the 4x's are finished.

 

[ra.ra]

Welcome to AK, Mike.

Having non-consecutive serial numbers is typical - - these speakers were never consciously sold as a matching "pair". The inductor sitting atop the wax cap block is also very normal - - - a new 20uF cap can be installed and fitted without actually removing the original cap - - this option re: removal is your choice.

On the woofers, you should observe a dust cap that is semi-transparent - - this allows some air to pass through which means that the 4x woofer does not need to be completely air-tight. Hence, the "press test" does not fully apply to this speaker model.

Replacing the pots with new L-pads is a very reasonable approach - - you'll want the 8-ohm, 15-watt version with short (3/8") stem. These can be installed with or without some added resistance - - experiment and see what you think. Part of this decision will depend on how well your old tweeters are still delivering high frequencies.

 

[Mike2D]

Welcome to AK, Mike.

Having non-consecutive serial numbers is typical - - these speakers were never consciously sold as a matching "pair". The inductor sitting atop the wax cap block is also very normal - - - a new 20uF cap can be installed and fitted without actually removing the original cap - - this option re: removal is your choice.

On the woofers, you should observe a dust cap that is semi-transparent - - this allows some air to pass through which means that the 4x woofer does not need to be completely air-tight. Hence, the "press test" does not fully apply to this speaker model.

Replacing the pots with new L-pads is a very reasonable approach - - you'll want the 8-ohm, 15-watt version with short (3/8") stem. These can be installed with or without some added resistance - - experiment and see what you think. Part of this decision will depend on how well your old tweeters are still delivering high frequencies.

Thanks. I have duplicated this build report on another forum, so I'll try to keep up with both of them as I progress. I assume the resistor would go between 1 and 2 terminals on the L-pad, and what value? Sorry if that's a simple question of ignorance. 8-ohm is used on the 3a's and some have used none on the 4x's. There is also another forum with a "hybrid" 4-4x which is additionally confusing to a novice. Adding the resistor would "brighten" the highs or attenuate them?

 

[ra.ra]

Hi Mike, I just saw your other thread on the CSP forum where I am also active, but I'll try not to duplicate my remarks in two places. Your question about the resistor is indeed a very valid one, and the decision will depend on the quality of your (old and) original tweeters as well as your personal listening tastes. Adding any resistor will definitely not "brighten" the tweeter performance, but there are threads on CSP that fully describe this process. I'd suggest not to confuse yourself with whatever 4-4x hybrid you're reading about - - most likely, this does not apply to your situation.

 

[Mike2D]

Hi Mike, I just saw your other thread on the CSP forum where I am also active, but I'll try not to duplicate my remarks in two places. Your question about the resistor is indeed a very valid one, and the decision will depend on the quality of your (old and) original tweeters as well as your personal listening tastes. Adding any resistor will definitely not "brighten" the tweeter performance, but there are threads on CSP that fully describe this process. I'd suggest not to confuse yourself with whatever 4-4x hybrid you're reading about - - most likely, this does not apply to your situation.

While waiting for the cloth surround dope to come in along with the 20 mfd capacitors, I think I will try the 8-ohm 15W L-pads without a resistor. I think the original pots were 16 ohms, were they not? And they are the same as the 3a's? Middle of next week before I get an answer on how that works. Thanks again @ra.ra.

 

[WaynerN]

The pots are easy to take apart as they are held together with a wire bail that "snaps" into place. You will find the wire coil inside has turned green and the wiper contact has also tarnished. a little Deoxit 5 and a toothbrush will bring it back to life, then you don't have to fool with a different L pad. L-pads put resistance in series with the tweeter and put resistance in parallel with the voice coil to maintain the impedance of the driver regardless of level position. Since the capacitor and its cross-over point are based on the tweeter's impedance, it is important that that value is maintained.

 

[Mike2D]

The pots are easy to take apart as they are held together with a wire bail that "snaps" into place. You will find the wire coil inside has turned green and the wiper contact has also tarnished. a little Deoxit 5 and a toothbrush will bring it back to life, then you don't have to fool with a different L pad. L-pads put resistance in series with the tweeter and put resistance in parallel with the voice coil to maintain the impedance of the driver regardless of level position. Since the capacitor and its cross-over point are based on the tweeter's impedance, it is important that that value is maintained.

I attempted to clean contacts on these pots a couple of years ago and although they appear to be in good shape (seldom if ever rotated during the life of the speakers) the cleaning was not very successful. I may give it another shot but I'm not optimistic. Thanks.

 

[WaynerN]

Did you use Deoxit 5?

 

[Mike2D]

Did you use Deoxit 5?

No, I tried to use an electrical contact cleaner but did not remove the pot from the board, so it was awkward. AR's pots are notorious for corrosion so I simply assumed that if I did clean them, it wouldn't last very long. The AR-3a's pots are almost universally replaced with 8-ohm 15W L-pads with a resistor. Not as many 4x's are being rebuilt so they may differ although I don't think the pots are any different. The only difference is that many advise to use the L-pads "as is" on the 4x's but with the resistors on the 3a's and 2ax's.

 

[WaynerN]

Deoxit cleans down to the base metal, then leaves a slight protective film. I have cleaned mine several years ago with no issues since.

 

[Mike2D]

I hate to have to take it back down after trying it, but it might be worth a shot. I don't have Deoxit so I'll have to run that down. I would certainly rather have original pots than L-pads, especially to maintain original circuit values. The caps are going to be replaced, regardless. I appreciate your comments.

 

[Mike2D]

Well, half-successful restoration. One speaker is working nicely. No tweeter response on the other so I ordered a replacement.

 

[Mike2D]

Final post here. Replacement tweeter received and installed. I noted a much larger magnet on the replacement speaker, but the pair seems properly balanced and both are working nicely. I did install L-pads in place of the original pots which did not look all that bad. I just didn't want to take a chance on having to pull them back down in a year or two to re-clean them, and I wasn't really excited about trying to unsolder those connections to get them out and re-solder them back in. I considered using the 8-ohm resister on the pots, but one of the gurus suggested it was not absolutely necessary unless you are trying to duplicate the factory control range. For my purposes, they are perfect.

So thanks to everyone here who chipped in with advice. Now to refinish the walnut cabinets .....

 

[ra.ra]

Replacement tweeter received and installed. I noted a much larger magnet on the replacement...

This doesn't sound right - - - can you post pics and more info on the two tweeters? All original 4x tweeters I have encountered have the exact same magnet, which is about 2-1/8" diameter.

 

[Mike2D]

Speakers are back together. I can take a shot at the front of the tweeter if that means anything. Not the same as the remaining original which is OK. Source was simply-speakers and the replacement tweeter was the 4.25" AR-T-135.

 

[ra.ra]

Oh....OK, now I understand. The replacement you are using is one of the phenolic ring tweeters (PRT) that perfectly fit the cabinet cut-out, but I was unaware of the difference in magnet diameter. These are purported to be reasonable replacement drivers for the 4x, but I don't know if any adjustment to the value of the tweeter capacitor is recommended.