Any Vintage HiFi Geeks Here? Difference between a Pot and an L-Pad

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Fat Bastard

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I'm trying to fix a pair of 40 year old RTR180-D floorstanding speakers, They've got a simple crossover with a couple of capacitors and 3 resistors in each side, and a 16ohm wire wound pot. Which is dead. Well, I reckon it's a pot (a dead one), because it has a single wiper arm. L-Pads have a second arm at 180o .Apparently. This is so it presents a constant impedence, or resistance or something. Apparently, according to Google. I'm not really a "wires" guy.

16ohm pots are kind of hard to find nowadays, but L-Pads are available in all manner of ohmerage. Can I replace the pot with an L-Pad?
The rest of the bits were easy to find and buy.

I've asked exactly the same question over on Audiokarma forums.The silence has been deafening, but at least I didn't get the DASFFS stuff.

Anyway, does anyone have a clue? I've tried opening it up to clean it which is how I know how many wiper arms it has and at what angular spacing they're at.

In abscence of any real knowledge, I'll gladly accept semi humourous smart arsed replies that make me giggle.

Cheers!

FB
 

Fat Bastard

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Anticipating the stampeding herd of people rushing to answer my question, here's a pic of the crossover. Yes it is just a bunch of stuff glued to a bit of masonite. These things cost $550 US in 1976. Made by two brothers in a shed in California. The photography is all mine however.


I've also done a drawing. In MS Paint. I left the work computer at work where it belongs last weekend.
 

Camo6

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Pretty sure a pot wired in series is just a rheostat and a quick google of "16 ohm rheostat" produced a few for sale although the one I saw was a bit pricey.

Are you sure that it's faulty? Have you measured the resistance?
 

TheWiggman

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Impedance and resistance are similar concepts except one is for real current, the other for imaginary. They serve a different purpose and are not interchangeable.
The pot should be for your tweeter attenuation (check the cable to the speakers - it should run to your tweeter +ve). If you can't find a pot I'd check the resistance between those terminals on the other speaker using a multimeter. That is, the resistance when it's adjusted to a pleasing listening level. Buy two resistors equal to that resistance and install them inline in place of BOTH of your pots.
 

sp0rk

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I would first try cleaning your pot, some good contact cleaner should do the job
16 ohm pots are more or less impossible to find new these days, and I think the L Pads would significantly change the sound output
A rheostat would be your best (and most expensive) choice imho, but I would be trying to clean that existing pot first
This thread topic has a great instructional about how a guy made some switched attentuators to replace some 16 ohm pots
http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=1849
(note, read from the bottom up)
 

Ducatiboy stu

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You could wire a number of 2ohm resisters in series at just tap them ( or use a rotary switch ). Would do the same as the pot.

or as spork said give it a clean first
 

Fat Bastard

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Yeah, Ive given it a good clean, contact cleaner, fine wet & dry on the contacts and coil, then more contact cleaner, but she still doesn't work. I just realised I've left the bloody thing on my desk at work so I can't give it a good flogging wth harsher abrasives over the weekend. It feels & looks (internally) 100% better than it did though!

I guess at a last option i'd go for one of those 15-20 ohm rheostats made my Ohmite, but my word they are expensive! I think they actually do a 16 ohm version, but I can't find one online at the moment, but I'm sure ive seen one. I guess spending a few bucks isn't too bad, $550 US in 1976 probably equates to a few thousand in modern money, and these have cost me 30 bucks to buy 'em, and another 28 for the resistors and caps, so i reckon if I can get out of it for under 500, I'll still be in front. From what ive read on the net, they're quite a highly regarded speaker. maybe not as much as the Acoustic Research type stuff, but well regarded none the less.

Just seems to be 3/5ths of FA info about them out there, I think my MS paint diagram i posted to Audiokarma is the only circuit schematic on the net at the moment!

Thanks for the replies, Ive had 400% more here than I got on the HiFi nerd forum!
 

komodo

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Why don't you contact one of the audio nerds that does custom stuff and get a new cross over made up?

Are you using the attenuator anyway?
 

real_beer

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Fat Bastard said:
...... and a 16ohm wire wound pot. Which is dead. Well, I reckon it's a pot (a dead one), because it has a single wiper arm.

In abscence of any real knowledge, I'll gladly accept semi humourous smart arsed replies that make me giggle.

Cheers!

FB
HI you Fat Bastard,

You should be very careful not to burn your fingers & lips smoking reefers wrapped in wire. The tips burn really hot and the wire will heat up quickly. If your worried about a 16mm joint falling apart or dying before you finish it, just use more cigarette papers when you roll one. Wire will hold them together nicely but as I said it'll get way too hot to enjoy the experience.

As for the old pot being dead, it's a lot like packet tobacco, once you've it opened it pays to put in a bit of citrus peel to help keep it fresh then seal it up again asap to stop it drying out. I'd say your pots just been left out in the open too long and all the resins have lost their volatility, or as you say died.

Hope this helps :drinks:
 

Fat Bastard

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Komodo said:
Why don't you contact one of the audio nerds that does custom stuff and get a new cross over made up?

Are you using the attenuator anyway?
Yeah, I'd like to use it. The speaker has 2x10" drivers, one firing downwards and an array of 4 3" tweeters. The good side sounds great with the tweeter array turner down, but the dead pot side only powers the tweeters when it's all the way up and sounds miles too bright with my Cayin valve amp. Eventually is like to cram a mid in there, but for the moment I'd just like to get them working as designed. I've bought new caps and resistors and I reckon I can solder well enough to do the job.

I had another crack at the pot and I reckon it's more like 20-25ohm, but it's still rooted... I really need to open up the good side and measure the working pot properly before I proceed much further with it.

Not brewing this weekend, so I reckon the good speaker will get pulled down too...
 

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