Arggghhh! These pots are driving me mad!

[TimmyTonga]

Here are a couple of photos of my offending mid-pots. In retrospect I may have been a little optimistic in expecting the one on the right function well, as it has quite a lot of pitting - however, my argument ran, the central section seems pretty much OK, and that's where the wiper will probably be.

 

I am quite interested to try the Korean solution, with two reservations;

• the buying process seems to be not entirely straightforward

• I'm not sure how practical it would be to drill out the existing rivet with a hand-drill.

Tim

[ar_pro]

Yes, a drill press would be optimal.

If you don't have access to one, how about having a local machine shop drill the rivet for you?

[owlsplace]

1 hour ago, TimmyTonga said:

Here are a couple of photos of my offending mid-pots. In retrospect I may have been a little optimistic in expecting the one on the right function well, as it has quite a lot of pitting - however, my argument ran, the central section seems pretty much OK, and that's where the wiper will probably be.

 

I am quite interested to try the Korean solution, with two reservations;

• the buying process seems to be not entirely straightforward

• I'm not sure how practical it would be to drill out the existing rivet with a hand-drill.

Tim

Simply grind the head off and punch it out the back. Unfortunately the wiper tracks on the outside edge of the disc.

I think someone needs to track down the email address of the "AR manic" in Korea

Roger

[JKent]

On 11/28/2016 at 8:03 PM, ra.ra said:

I have no experience with this, but this Korean site seems to offer new wipers and contact discs for old A-P pots.  

http://baenadeul.blogspot.com/2014/11/this-component-is-contact-point-plate.html

Aha!

They were selling on ebay for a while but apparently gave up on that. I'd be interested to know how they work out.

Tim--the buying process looks OK to me. You email the guy with your order, pay using PayPal. I think PP has a buyer protection policy if you're concerned about not receiving the product.

But I still have to ask: Why bother? Don't know about your side of the pond but over here the L-pads are under $4 each and they'll work better and last longer than pots. The Korean replacement parts are $12 per pot + $9 shipping. Then there's the problem of drilling out the old rivet without mucking it up.

-Kent

[genek]

Those discs have a fair amount of thickness to them. If you have a soldering gun that puts out enough heat, you could flood the pits with silver solder and then grind/polish them smooth with a Dremel tool.

[RoyC]

3 hours ago, TimmyTonga said:

Here are a couple of photos of my offending mid-pots. In retrospect I may have been a little optimistic in expecting the one on the right function well, as it has quite a lot of pitting - however, my argument ran, the central section seems pretty much OK, and that's where the wiper will probably be.

 

I am quite interested to try the Korean solution, with two reservations;

• the buying process seems to be not entirely straightforward

• I'm not sure how practical it would be to drill out the existing rivet with a hand-drill.

Tim

Tim,

The one on the right and its wiper are not even close to being satisfactory! Frankly, when they are pitted like that I toss them. The pot on the left needs more work...and one of its wiper contact tips doesn't look good. (Btw, the wiper tips only contact the outer edge of the disk and the resistor coil.)

+1 regarding everything Kent posted above.

Roy

[owlsplace]

1 hour ago, JKent said:

But I still have to ask: Why bother? Don't know about your side of the pond but over here the L-pads are under $4 each and they'll work better and last longer than pots.

Well, L-pads are actually a mod that changes the crossover's operating parameters. Just curious if anyone has actually run the numbers on the original crossover frequency variation as the pots are adjusted? I was always more of a visually oriented-type preferring to use a sweep generator and spectrum analyzer which are not available at the moment; however, I expect the math is fairly simple.

Roger

[TimmyTonga]

4 hours ago, ar_pro said:

Yes, a drill press would be optimal.

If you don't have access to one, how about having a local machine shop drill the rivet for you?

Yes, I do have such a thing, and it would be a good solution if I were to take that route....

[TimmyTonga]

21 minutes ago, owlsplace said:

Well, L-pads are actually a mod that changes the crossover's operating parameters. Just curious if anyone has actually run the numbers on the original crossover frequency variation as the pots are adjusted? I was always more of a visually oriented-type preferring to use a sweep generator and spectrum analyzer which are not available at the moment; however, I expect the math is fairly simple.

Roger

I suppose this is one of the things that bothers me, although I am not coming at the issue as an electronics expert. How does the difference between a pot and an L-pad compare to the difference between, say, the use of film capacitors and NPEs? In terms of their relative effect on the integrity of the crossover as a whole? 

[owlsplace]

I haven't reused any that look much worse than this personally:

You can see the bright ring around the outside edge where the wiper tracks.

Here is what the late version pots look like when they are in good condition:

Roger

[TimmyTonga]

1 hour ago, RoyC said:

Tim,

The one on the right and its wiper are not even close to being satisfactory! Frankly, when they are pitted like that I toss them. The pot on the left needs more work...and one of its wiper contact tips doesn't look good. (Btw, the wiper tips only contact the outer edge of the disk and the resistor coil.)

+1 regarding everything Kent posted above.

Roy

While I grant you the one on the right is a bit marginal, the pot on the left is at least as good as the ones I got from Vintage-AR guaranteed for five years. But it's wiper has a notch in one contact, and that probably isn't helping, I will concede.....

tim

[lARrybody]

I tried to clean up this mid range pot on a 2ax rebuild.

 

I turned out that the center disk had a chunk corroded out of the edge.

Thought about trying to turn the disk so the missing edge was between the two contacts where the wiper would not travel. Alas I just replaced it with another pot out of my parts collection.

 

[RoyC]

4 hours ago, TimmyTonga said:

While I grant you the one on the right is a bit marginal, the pot on the left is at least as good as the ones I got from Vintage-AR guaranteed for five years. But it's wiper has a notch in one contact, and that probably isn't helping, I will concede.....

tim

Tim,

If you were able to compare your pots to Vintage AR's, you already knew what you had to do before starting this thread. Attached is a photo of a pot I've only begun to refurbish for an AR-3a restoration. It represents about 10 minutes of work with some sandpaper. If you are not able to at least reach this point with your pots, it's time to move on.

Roy

[RoyC]

7 hours ago, TimmyTonga said:

I suppose this is one of the things that bothers me, although I am not coming at the issue as an electronics expert. How does the difference between a pot and an L-pad compare to the difference between, say, the use of film capacitors and NPEs? In terms of their relative effect on the integrity of the crossover as a whole? 

Tim and Roger,

As a matter fact, the level control issue has been addressed and measured a number of times in the past. It is best understood by comparing the difference between series and parallel resistance presented by each type of level control at any given setting. For example, when the old pot places 3 ohms of resistance in series with the tweeter, it also provides around 12 ohms in parallel. When the L-pad places 3 ohms in series, it places around 26 ohms in parallel with the tweeter, resulting in a bit more output, and a fractional lowering of the crossover point. The addition of a 25 ohm parallel resistor. however, compensates for this throughout the typically used range of the level control (up to around 2/3rds off maximum) resulting in no change to the crossover point at any usable setting compared to the original pot...But, read on...

When it comes to the old style tweeters found in the AR-3a, 2ax, 5, LST, and LST 2, there are very few on the planet meeting original specifications, due to suspension compliance issues and decomposing foam....and the resultant variation in performance (mostly characterized by lower output and less extension into the upper mid frequencies, ie, raising of the crossover point) easily exceeds any tiny differences between types of level controls or capacitors! The crossover point of these old tweeters is often significantly affected by their mechanical limitations. In any model with these tweeters I personally only use L-pads with the resistor for the more consistent midrange driver to maintain its original crossover point, but I use the L-pad with no added resistor for the tweeter. The reason is the L-pad without the parallel resistor provides a small increase in tweeter output and lowers the crossover point enough to occasionally bring these reticent tweeters somewhat back to life by compensating for age-related deterioration. One could argue to what degree these tweeters vary...which is exactly my point. You guys often wring your hands over inconsequential issues compared to something like this. Whatever type/brand of capacitor or level control you use means nothing to the sonic "authenticity" of these tweeters unless you are measuring response curves.

Btw, Vintage AR has used L-pads without the added resistor for 95% of his restorations and repairs over many years...and has never had a complaint (or a burned out or corroded one) that I'm aware of. He only uses original pots on special request, tosses the bad ones, and saves the best ones to sell to neurotic enthusiasts.  I've encouraged him to use the resistor on the midrange control, but he does not believe the small differences (which he has measured, and is completely aware of) compared to the AR pots are worth the effort.

I don't own stock in L-pads. I just think they do the job very well, are easy to use, and are cost effective.  A well refurbished original pot, a new Ohmmite pot, and the Korean repair kit (if it is as good as it looks) can all make me happy as well...but depending on one's degree of tolerance and persistence, there comes a time to toss a crappy old AR potentiometer.  As an aside, I work on many other brands of old speakers for a local shop, and the award for the worst speaker level control in history appears to go to our beloved pots. I have found them in some Infinity and Cerwin Vega models in typically non-functional condition...and I sent them directly to the scrap bin.

Roy

[owlsplace]

1 hour ago, RoyC said:

As a matter fact, the level control issue has been addressed and measured a number of times in the past.

Haven't found it anywhere in the archives, per se ... at any rate I have come to prefer the 80s-era sound -- just say no to pot, er, drugs-era

Though for someone doing a restoration maintaining the original configuration may still be desirable.

That the original Classic-era AR upper range drivers originally were attenuated probably speaks volumes for their longevity -- at least the ones that survived. Some of the pots did get a little warm though as evidenced by the scorch mark here:

 

 

 

[GD70]

Hey, I'm a neurotic enthusiast!

I'm testing my 3's as I type this, and the pots are working very nicely!

Glenn

[RoyC]

14 minutes ago, GD70 said:

Hey, I'm a neurotic enthusiast!

I'm testing my 3's as I type this, and the pots are working very nicely!

Glenn

Hey Glenn,

That's what Larry calls me when I get into adding the resistor to the L-pad thing, altering cap values to accommodate the HiVi tweeter, polarity issues regarding same, and a few other things that have come up recently. I retaliate by telling him to read the CSP forum for a few hours.

Be careful with those mids!

Roy

[RoyC]

29 minutes ago, owlsplace said:

Haven't found it anywhere in the archives, per se ... at any rate I have come to prefer the 80s-era sound -- just say no to pot, er, drugs-era

Though for someone doing a restoration maintaining the original configuration may still be desirable.

That the original Classic-era AR upper range drivers originally were attenuated probably speaks volumes for their longevity -- at least the ones that survived. Some of the pots did get a little warm though as evidenced by the scorch mark here:

 

 

 

Hi Roger,

Yeah, that would go along with a melted plastic shaft. As the resistance builds up due to corrosion the pot runs hotter and hotter.

The first poorly functioning pots I worked on was in the 70's in an 8 year old pair of AR-2ax's. When AR began installing them with metal shafts, the composition of the wiper was changed. I believe it was all nickel or nickel plated, and much more resistant to the aggressive corrosion and pitting of its predecessors.

Roy

[TimmyTonga]

12 hours ago, JKent said:

Aha!

They were selling on ebay for a while but apparently gave up on that. I'd be interested to know how they work out.

Tim--the buying process looks OK to me. You email the guy with your order, pay using PayPal. I think PP has a buyer protection policy if you're concerned about not receiving the product.

But I still have to ask: Why bother? Don't know about your side of the pond but over here the L-pads are under $4 each and they'll work better and last longer than pots. The Korean replacement parts are $12 per pot + $9 shipping. Then there's the problem of drilling out the old rivet without mucking it up.

-Kent

When you guys can pick up ARs in thrift stores for very little money, the expenditure on Korean refurb kits probably seems disproportionate. Over here where the speakers themselves tend to fetch higher prices, it doesn't seem so bad.

I like the challenge of getting the old stuff to work. My original post was expressing my bemusement at why a pot should ohm out OK on a bench but be intermittent in a cab. I think this is because when testing a pot hand-held there is, possibly subconsciously, more pressure being applied on the spindle. So I shall clean more and bend the wiper more. And probably contact AR mania in Korea to replace units beyond redemption.

Tim

[GD70]

Tim,

Keep us updated on the Korea connection.

Glenn

[TimmyTonga]

I certainly will Glenn. No word yet!

Tim

[Phxjohn]

I bought a pair of restored pots from Vintage AR for my AR 6's a few years ago. So far, so good. My AR 3a's also from Vintage AR, have L pads and Hi Vi Q1R tweeters. After all that I had read over many decades I was surprised at how powerful the dome mids are. I run mine at slightly less than half rotation of the L pads. Same for the tweeters. I cannot imagine 'hot wiring' the mids if they are anywhere as powerful as mine are. 

[newandold]

On ‎11‎/‎27‎/‎2016 at 2:53 PM, TimmyTonga said:

I clean them and test them and the impedance runs smoothly from 0 -16 ohms as I turn. Then I put them in a cabinet and they are immediately intermittent. I mean immediately. Am I doing something stupid? Perhaps I need to bend the wiper more, because if I press gently on the black knob, contact is fine again.....

Any enlightenement out there?

Getting despondent and eyeing up L-pads.....

Tim

Hi Tim,

I don't do a lot of posting here, but your pot dilemma struck a nerve with me.....no doubt with your determination you will have them operating ok.

I had the AR 2ax for 5 or 6 years and got them brand new, and those pots started giving me grief in a very short period of time, If I recall, just a few years after getting them.

Ultimately, I wanted to get away from the pot design and look for another loudspeaker. My dad had the 2a and my brother, the 4ax, all developing the same problems.

I always wondered if Roy Allison never left Acoustic Research, if the slope switch (pictured below) would have shown up in the AR loudspeaker.

The "slope switch" was very robust and "self cleaning" by simply moving it back and forth (many times when they got old!) but they always worked with only occasional cleaning. Mine worked well until I sold them off after 35 years of constant use.

 

Bill

[alkermes]

I've been following along as well.  I am planning on finally refurbing my 3A's over the Christmas break.  I will be using l-pads to replace the old pots, which I think must be truly shot based on feel and performance and beyond repair.  I am glad to have read RoyC's advice on using the resistor on the mid l-pad and leaving it off  the tweeter l-pad.  This looks like an update to the 3A restore guide, the advice is much appreciated.

[TimmyTonga]

3 hours ago, alkermes said:

I've been following along as well.  I am planning on finally refurbing my 3A's over the Christmas break.  I will be using l-pads to replace the old pots, which I think must be truly shot based on feel and performance and beyond repair.  I am glad to have read RoyC's advice on using the resistor on the mid l-pad and leaving it off  the tweeter l-pad.  This looks like an update to the 3A restore guide, the advice is much appreciated.

Personally I'm not a fan of leaving the resistor off the tweeter L-pad. I don't find that extending the range of the tweeter downwards gets you anywhere you want to be. 

Tim