Tweeter/Mid Pots Intermittent

[oldarnut]

I've owned AR speakers since the early '60's. I now have a pair of AR2a's, 2 pr. AR3's, AR3a's, ARLST-2's, AR9LSi's, and a single AR2. I've spent a lot of time through the years cleaning the tweeter and midrange pots with various fixes including fine emery cloth and Scotchbrite. I've used TV tuner cleaner spray liquid, WD40, and other pot cleaning liquids.

In every case, a few months later I notice the tweeter and/or midrange has stopped working and I need to fiddle with the pots. This never happens on my Infinity's. The AR9LSi's have no controls, the ARLST-2's step switches never fail, and the pair of AR2a's I have in my garage have had the pots bypassed for years with no trouble.

I've often wondered why those on this board so often discourage bypassing the pots. On the AR3a's for example, the tweeter level controls are always turned all the way up and the midrange at or near minimum to achieve tonal balance.

Does anyone have a better method of cleaning than I've tried? Or, are there some decent pots available that are reliable?

[JKent]

Hi Nut, and a belated welcome to CSP

Others with more experience may chime in here, but here are some thoughts:

I assume you've read "Restoring the AR 3a", pinned to the top of this section. Pages 13-16 cover pot restoration and replacement well. Also see page 21. If you have rock wool stuffing it will destroy the pots.

I would not use WD-40. You can clean the pots with a small brass wire brush in a Dremel. Badly corroded pots can be soaked home-made concoctions based on lemon juice and salt (try googling "cleaning brass lemon juice"). Make sure the wiper and the coil are bright & shiny, then before reassembly coat them with dielectric grease. Check the pot with an ohmmeter before reinstalling. Sometimes they look clean but still have some crud.

RoyC and others recommend L-Pads and I have installed those in my AR-4x and AR-2ax speakers. They work fine and don't corrode.

If you're a purist and don't want L-Pads, the Ohmite pots mentioned in "Restoring the AR 3a" are very nice, industrial grade and should last forever. Two drawbacks: They have open backs so you'll have to fabricate some sort of shield or enclosure to keep the fiberglass out (I used a small plastic food container), and they are hard to find. I have a few so PM me if interested. Here's the FS post:

http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=5549&st=0&p=80685&hl=ohmite&fromsearch=1entry80685

Hope this is helpful.

Kent

[oldarnut]

Thanks for the prompt feedback, Kent. I have never had a reason to change the interior damping material, so I assume it's fiberglass. I've never tried dielectric grease on the wiper, either, so this is something I'll try on the AR3a's I'm now using for rear surround speakers in my home theater. Finally, maybe the tweeters and mids will work reliably!! As it is now, they usually sound like subwoofers only.

[RoyC]

I've often wondered why those on this board so often discourage bypassing the pots. On the AR3a's for example, the tweeter level controls are always turned all the way up and the midrange at or near minimum to achieve tonal balance.

The white dot on the back terminal panel represents the recommended or "normal" control level. For the AR-3a, this level is near maximum for the tweeter, and approximately 1/4 turn off maximum for the midrange (btw, not "near minimum"). The appropriate levels and crossover points are achieved by variable series and parallel resistance arrangements provided by the pots. Consequently, by-passing the pots will change the character of the original sound/tonal balance...so if that is something of importance to you, it is not a good solution.

If upon disconnecting the pot, a 15 ohm parallel resistor is installed, or the parallel leg of the old pot is retained, the tweeter or mid will be at its original maximum setting. This is the only way to "by-pass" the pots and achieve anything close to the original sound. This is usually not a bad approach with the original tweeters, but does not work well with the midrange drivers, as they usually require more attenuation to sound right.

Other than refurbishing the original pots, a number of methods can be used to maintain the influence of the original controls, and maintain the original sound character:

-Instead of "by-passing" the pots, fixed series and parallel resistors can be installed to emulate the pots at desired settings.

-If common 8 ohm L-pads are used to replace the pots, 25 ohm resistors can be used in parallel with the AR tweeters and mids to emulate the effect of the original pots at typically used settings.

-Even without the addition of a 25 ohm resistor, an ordinary inexpensive 8 ohm l-pad replacement is much better than by-passing, with the added bonus of being more reliable than a troublesome old pot.

-Installation of new 15 or 16 ohm potentiometers, such as the Ohmites mentioned by Kent.

Roy