Stumped by AR-3a! Advice Needed!

[Guest leopoldstotch]

Hello all! I have just recently taken in my Uncles AR-3a's. The speakers were in need of a pair of original woofers which I happen to have. Upon inspection I noticed that the mids and tweeters were not working so I did what most of would do and I cleaned the pots and tested them and everything is fine with the pots. Now comes the problem the midrange on one pair isn't working at all when it is hooked up to the original pot (which when tested is reading 15 ohms). Figuring the pot may be bad I tried the mid with a brand new 8 ohm pot only to be unsuccessful. I know the mid works because I touched the green lead to the black tweeter lead and the mid worked. I did this very briefly btw.

I am completely baffled by this. I know that I have the pots wired correctly because I used my working pair of AR-3a's as my model to wire my uncle's pair. Has anyone ever heard of this problem and do you have any suggestions? Thanks!

Bruce

[Guest matty g]

Hi -

I would look into the crossover capacitors (probably a good idea to change them anyway). If the mid works when you touch the leads in front of the baffleboard but not when you go through the crossover circuit, and if you know that the pot is ok, there's not much else to it.

Hope this helps.

Matt

[onplane]

Bruce, chances are the problem is still those blasted pots. Just becasue you measure 15 ohms doesn't mean the wiper is making any contact. Indeed, the wiper may have corroded away to nothing.

Try this mod on the mid pot and see whether you can get sound out of the mids:

http://www.classicspeakerpages.net/dc/user_files/2519.jpg

Notice that you cut the speaker lead and solder to the top of the pot. This is identical to setting the pot to max increase 30 some years ago before the corrosion set in.

Hope this helps ...

Regards,

Jerry

[johnieo]

Hello:

Matt is correct, the caps likely need to be replaced as they are quite along in years. Also, as noted, you may have a corroded wiper. The correct pot is a 15-Ohm, 25 W wirewound potentiometer, do not use 8-Ohm L-pads as a direct substitution, they are not the same beast. There are many posts on the forum about the AR-3a potentiometer. It can be taken apart from the inside of the cabinet, and cleaned. Usually the wiper and inner brass ring needs to be polished to remove corrosion. Some pots are so bad that they need to be replaced and there is a fix for that-- an 8-Ohm L-pad with an added resistor. Roy C determined that a resistor soldered across two of its terminals will make it behave like the original potentiometer in the angular rotation around the "flat" setting, or white dot on the cabinet backside.

Please do not short the wiper to the hot side as you loose all ability to balance the tweeters for your room conditions.

Please do not use the Layne Audio schematic shown in a reply to this thread, as it has the polarity of the drivers reversed from that chosen by Roy Allison, its designer. Please retain the original wiring.

Cheers,

[RoyC]

Bruce,

Along with the good advice above regarding the capacitors, you may also have to check out the connections in the midrange section of the crossover by tracing it back to the #1 cabinet input terminal.

One possible area of trouble could be the solder joint or connection of the inductor coil wire to the level control. You probably removed that coil lead to clean the mid pots. Make sure the enamel was completely removed from the wire at that point.

Another possible problem area is the resistor wire in the mid circuit. It can be seen on the right side of the attached photo, connected by two rivets.

Keep us posted,

Roy