Old pots or new?

[Guest BC9]

I've got 2 sets of speakers that are next on my list to fix. A pair of 2ax's & a pair of 3a's. They both need new surrounds and the pots either cleaned or replaced. I've done the surrounds and replaced the pots on 2 pairs of AR-5's so I'm not anticipating any problems.

I'm curious as to what the general feelings are on the pots. Clean the old or replace with new?

If clean the old, what's the best way to do it?

Thanks...

[stan461]

I have 2 pair of AR3a's and replaced them both with new pots. No more dropouts! Ever!

I cleaned the pots on one pair but got tired of getting up off my chair while listening to beautiful music to jiggle one of the knobs in the rear.

Honestly, the only drawback would be that the speakers are not truly original.

And the new pots do NOT, I repeat, do NOT alter the sound.

[Guest Allen deSomer]

Hi,

What is the Ohm rating of the replacement pots? I'm a novice but I figure I can tackle the job on my pair of AR2ax's if I know what to buy.

I've had the grills off for two months and can't wait to put the pair back in service.

Thanks In Advance,

Allen

>I have 2 pair of AR3a's and replaced them both with new pots.

>No more dropouts! Ever!

[RoyC]

You may want to do a forum search, as this topic has been discussed extensively in the past, including photos.

The bottom line is that "pots" (short for potentiometers; also known as rheostats) are not the same as overpriced 8 ohm l-pads currently being sold as "replacement pots" by some AR parts sellers.

The 8 ohm l-pad alters the crossover impedance, especially in the 4 ohm rated AR-3a, and does alter the sound of that speaker. Many folks are OK with that, or do not know any better..but it is not the "same" sound. The 8 ohm rated AR-2ax would be least affected by the use of 8 ohm l-pads (less than $3 each from Parts Express).

A true replacement is a 15 or 16 ohm pot/rheostat.

I would clean the old ones unless they are too far gone. If you do it right they will last a very long time.

Roy

[Guest Allen deSomer]

>I would clean the old ones unless they are too far gone. If

>you do it right they will last a very long time.

>

>Roy

Thanks for the advice Roy. I decided to go ahead and clean them since the best replacement parts are hard to find a bit expensive. A cleaned the tweeter control first but when I tried to clean the mid control I broke off the tip of the thin copper sweeper mechanism because it was corroded through in that area only. It does make contact with the spring as it is turned but I wish it was in better shape for the future.

Any creative ideas to keep this control functioning properly?

Thanks

Allen

[RoyC]

Hi Allen,

It sounds like the pot was pretty nasty. The best option would be to find a used AR pot and use it or the wiper from it.

If you have no need to adjust the tweeters, you could use the best pots or wipers for the mids and simply install an external jumper connecting "1" and "B" on the tweeter pots, bypassing the wiper. The 15-16 ohm resistance between "1 and "2" (which is very seldom affected by the corrosion) will maintain the crossover impedance, but will fix the tweeters in the full "increase" position.

Make sure the metal disk inside the circumference of the resistor coil is clean. That is where most drop outs occur. I've attached a photo of a pot that is not cleaned to show a corroded/pitted disk.

I have extra pots and possibly a wiper or two floating around if you want to send me an email through the forum.

Roy

[Guest daveshel]

Couldn't you use a jumper wire or low value resistor to 'bypass' the pot?

[dynaco_dan]

>Couldn't you use a jumper wire or low value resistor to

>'bypass' the pot?

Hi there;

I will suggest you go to the AR library and copy the AR-3A and AR-5 sheet on volume settings.

This is an often talked about issue of eliminate the pots but give that sheet a careful read before attempting that.

I have mentioned this before, Dynaco used their resistor network and rotary switch which made settings repeatable on the A-25, A-35 and A-50.

AR, I believe from day one, used their pots until ?

Many millions were used, that is for sure.

For a startup company, I feel every penny is scarce and a switch, resistors and labour to assemble may have been highter priced than a pot.

Not withstanding the finer adjustments available with a pot.

I am sure other members will now jump in with their, and I do welcome, opinions and comments.

[RoyC]

If the pots are completely by-passed, the resistance they provide in parallel with the tweeter and midrange is eliminated. This raises the circuit impedance which lowers the crossover points. This in turn makes the drivers sound harsh and puts them at risk. Adding series resistance without the parallel resistance formerly provided by the pots lowers the crossover points further, and makes things even worse.

The only way to eliminate the pots and maintain the original sound is to choose your ideal pot settings and install fixed resistors to replicate the settings.

For example, assuming typical 15 ohm AR pots in an AR-3a speaker:

-The normal or "dot" setting for the tweeter would be 1.75 ohms in series and 13.25 ohms in parallel with the tweeter.

-The "dot" setting for the midrange would be 3.25 ohms in series and 11.75 ohms in parallel with the midrange.

-A wide open pot setting would be 15 ohm resistors in parallel across each driver and no resistance in series...the same as leaving the pot in and jumping from "1" to "B", or running the driver lead directly from "1".

Roy

[RoyC]

I was informed by a forum member who was employed in the AR factory that since AR-3a's had adjustable knobs on the pot stems, they were fine tuned at the factory for flat response at the "dot" settings. This would compensate for driver response and pot impedance variations.

For those attempting to reinstall the original pots it should be noted that there is a small stamped groove in the masonite crossover board inside the cabinet for placement of the pot tab (see attached photo of the tab). This positions the pot so that the "dot" setting is close to the original optimal resistance values, and varies with every model. The values in the example above are taken from the forum library and are unique to the AR-3a.

Attached is a photo of typical AR-3a pot knob settings postioned to full "increase". Notice the tweeter control has less distance to travel to achieve its approximate original flat response (the dot), reflecting the lower series resistance required for that driver.

The point of all this is that these old speakers were finely tuned machines in their day. Folks currently acquiring vintage AR's with bad controls who are simply removing or bypassing them, may never know how they were intended to sound.

Roy

[dynaco_dan]

>I was informed by a forum member who was employed in the AR

>factory

Hi Roy;

Would this former AR employee know what type of sealant AR used for their speakers?

Also how it was applied to the cabinets.

I have always assumed that pneumatic screwdrivers would have been used to tighten so many speaker screws on the assembly lines.

[RoyC]

Hi Vern,

I don't know.. I'll ask him. He worked in the electronics (amplifier) area.

By the way, Parts Express sells a black sealant which is virtually identical to the gray electrical putty you mentioned. It works quite well and is relatively inexpensive.

Roy

[dynaco_dan]

>Hi Vern,

>

>I don't know.. I'll ask him. He worked in the electronics

>(amplifier) area.

>

>By the way, Parts Express sells a black sealant which is

>virtually identical to the gray electrical putty you

>mentioned. It works quite well and is relatively inexpensive.

>

>

>Roy

Thank you, Roy.

Another option, great.

When I do my write-ups please remember, that I write as if I am talking to you, one on one.

Or to the chap in Italy, who is very well mannered and asks that we excuse his poor English.

These other worldwide readers need as much or more advice and options than we may need, as we have Home Depot, etc.

At least with all of the discussions that take place overall, the advice I read from yourself and others is very much appreciated.

I have enjoyed reading, even when the topics are way over my head, and getting very technical, I am still learning.

We certainly have a broad membership around the world and growing quite quickly now.

Thank you again, Roy.