RoyC
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Posts posted by RoyC
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8 hours ago, ReliaBill Engineer said:
True. But it’s difficult to address distortion from mechanical degradation issues when the VC windings are bad, reading discontinuity from being fried or severed. Mechanical degradation becomes secondary to a bad voice coil.
This mid driver read “open circuit”:
Obviously....but many posters and readers of this forum do not have meters (of any type). Simply testing for functionality is the first step.
Roy
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6 hours ago, jviss said:
Cool. I'm curious as to the defects you find and how you address them.
Some tweeters and mids have degraded suspension and under-dome damping material due to issues with the old foam, and some require voice coil replacement (and associated suspension foam). A number of woofer parts can be replaced as well. Much of my work along these lines is done for "Vintage_AR" and forum members.
4x tweeters are not being rebuilt.
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5 hours ago, jviss said:
That makes sense.
Has anyone ever done a mechanical restoration of an AR driver?
At least a few of us are rebuilding various tweeters, mids, and woofers.
Roy
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16 hours ago, ReliaBill Engineer said:
I usually remove the drivers from the circuit, then test the drivers using a DMM set to 200 ohms. This tells me if the VC has continuity and at what DC resistance. The DC resistance tells me what impedence range the driver is in, 2/4/6/8 ohms nominal. All necessary to repair or replace the driver.
It is not unusual for the DC resistance of AR drivers to measure appropriately, yet subjectively exhibit distortion due to mechanical degradation.
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5 hours ago, jviss said:
Oh, wow, is that so? You mean, used originals that have been pulled form cabinets? I have at least two functional ones, maybe four, depending on whether the tweeters or crossovers are at fault in two cabs.
Yes, and yes. You most likely have 4 functional tweeters. The pots/level controls are the usual culprits.
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On 12/22/2023 at 4:12 PM, genek said:
Signal generator apps are readily availalble for both iphone and android. The advantage over music is that you are sending the actual tone you want to know whether the driver is outputting without the distraction of every other tone in a bit of music. Especially important once you get into the range above 10kHz.
Agreed...and certainly extremely helpful when identifying possible response issues.
Imo, however, determining whether or not the drivers are even functional separate from the typically compromised level controls is the first item to check with the old beasts. We conducted some measurements of AR high frequency drivers some years ago and found them to vary despite not exhibiting obvious subjective anomalies. To complicate matters, it is not easy to find perfect used original tweeters. And in the case of 4x tweeters it is very hard to find any functional used originals at all.
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4 hours ago, DavidR said:
I use my stereo receiver and put a 4uF (or even a 3uF) capacitor on the plus side wire that would go to the appropriate driver terminal. It should filter out any low frequencies.
The use of a cap is very prudent advice. Assuming jviss is referring to his 4x project discussed in his other threads, a 20uf cap could even safely be used. The crossover cap for the 4x tweeter is 20uf.
Disconnecting the tweeters from those nasty old pots to test their functionality should be one of the first things on the agenda.
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15 hours ago, jviss said:
I wish I had access to my engineering school's anechoic chamber, I could measure the effects of the various grill cloths! Wouldn't that be cool?
This type of equipment would work fine to measure relative differences in any listening space.
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The easiest way to test them is to simply play some music at low level through each driver with the level controls/crossover disconnected. The woofers will be fine, but be cautious with the tweeters. Be especially careful if using signal generator.
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Hmmm...I've never seen AR cloth or Wichelt Lambswool "linen" referred to as "aida" cloth. When researching the AR material for the 3a restoration guide posted in the Library, we had access to microscopic examination through a university laboratory (thanks to forum member John O'Hanlon). 18ct Wichelt linen was a precise match to the linen most often found on AR speakers. By the mid 70's AR was using a thinner linen with a somewhat higher thread count.
Ha, for most if not all of us reading this thread, any staple will have a lifetime guarantee against rust issues. :) Only some of the very thin staples used by AR in the 60's show some rust, and it was only an aesthetic issue for the very earliest front-stapled frames.
Roy
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3 hours ago, jviss said:
Thanks. Being the OCD engineer that I am, I'm going to keep looking into this. I happen to like the look of the 18 count Aida linen, and it's actually closer to the original in thickness and weight than the lambswool recommendation.
I might just set up my binocular microscope to count threads on the original cloth, and pull some of it apart to measure thread thickness.
I appreciate all the input and attention!
The now extinct 18ct Wichelt lambswool was the best match to the original cloth, but some forum members have reported very good results with this linen:
https://ulsterlinen.com/shop/linen-use/scrim-linen/medium-weight-natural-linen-fabric-y0400nt036/
I recommend an electric staple gun. The staples do not need to be stainless steel. Also, when spritzing the cloth be careful around the frame. It will warp if it gets too wet.
Measurements of the effects of various types of cloth material were previously conducted. Tight weave and heavier material was shown to reduce high frequency by up to 3%.
Roy
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8 hours ago, jviss said:
O.K., I get that. But other than the schematic, component material types, and component values, what could there be to the crossover network?
I recognize that my Ohmite rheostats might not be original equipment, but they're probably indiscernible from a rheostat or pot they are replacing. Perhaps not so much if replacing an L-pad, I imagine.
Your 15 ohm Ohmites are an exact electrical match to the originals, and will work just fine. You just have to cover the open backs.
I was only responding to your statements below while agreeing with Kent's comment regarding the replacement of non-original components.
"It would be interesting to see a crossover plot at various pot settings for this network."
This has actually been discussed a number of times over the years in the forum. In summary, the variable series and parallel resistance provided by the original 15 ohm pots influences the overall response of the speaker, as the behavior of the capacitors is affected by this resistance. The pot's "white dot" setting on the back of the speaker is what AR considered to be the "normal" tweeter-attenuation setting for most rooms. If the pot is completely eliminated, a 15 ohm resistor placed in parallel with the tweeter will fix the tweeter's output at the former pot's maximum setting by eliminating the pot's corresponding series resistance. In other words, the pot cannot simply be removed if the goal is to achieve the original characteristics of the speaker.
Also, I wonder if folks don't just punt on restoring these networks and just install a crossover board form, for example, Parts Express, which can be had for around $12. No adjustment, but then, so what? (I paid $8 for just the caps!)"
You purchased nice film capacitors for $8. Does Parts Express sell a crossover board with the necessary components (including the inductor) in accordance with the schematic? If so, it would likely be more difficult to install. Of course, this point is moot given the level controls (or equivalent components) are necessary for the appropriate response of the speaker system. As Kent mentioned, people have replaced/omitted a variety of original items, resulting in no longer owning the AR speaker under discussion.
Roy
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I just found some photos of some later AR design and response work...
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The original crossover is easily as important as the individual drivers. It doesn't matter if an expensive off-the-shelf replacement driver is used or a generic crossover. There is much more to achieving a successful speaker "system", much less the replication of a particular design. The original design work is often overlooked.
Roy
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5 hours ago, jviss said:
Ah, thanks. I don't suppose they're still available?
Not for about 40+ years.
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jviss,
-The original controls/"pots" were notorious for forming aggressive corrosion, and are almost always the reason for the silence of high frequency drivers. Ohmite controls were never used as original equipment.
-This is the link to the Parts Express gasket products: https://www.parts-express.com/speaker-components/cabinet-hardware-speaker-grill-cloth/Speaker-Gasketing-Tape-Caulk The 3/8 inch version of the gasket tape works very well, but is back-ordered at the moment. If using putty, the black rope "caulk" sold by PE is much better than "duct seal".
-I agree with Kent regarding your choice of caps. They will outlast the rest of the components of your speakers...and us.
-I also agree with Kent regarding the aida cloth. It is much too tight.
-make sure to place the cabinet stuffing in separate bags so you put the same amount back in each cabinet.
Roy
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Vintage_AR is selling the Parts Express/Dayton tweeter. At my suggestion he brushes some of my surround sealant on the cone to damp its response a bit. The effect is marginal, but seems to be somewhat helpful. AR actually treated the original 4x tweeter in a similar way.
Roy
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4 hours ago, jviss said:
I have a couple of new rheostats on hand if needed. I'm assuming I'll be replacing the caps.
Wish me luck!
Corroded level controls are the most likely reason for tweeter output issues, though it is always prudent to replace the capacitors when you are doing that work.
Also, take a look at the perimeter of the tweeter cone. It is not unusual to find it to be separating from the basket due to degraded adhesive.
Roy
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That sounds like a good approach. In this case it is more about the most acceptable compromise.
Your situation is not uncommon. The harshness becomes more apparent as the volume is increased. It can be annoying, especially when compared to other vintage speakers (including Bose).
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On 12/9/2023 at 1:30 AM, Anthonyv62 said:
I used phenolic ring replacement tweeters from PE in my AR-4X restoration...
My question is this: If I used a 25 ohm 10 watt resistor across the phenolic replacement tweeter terminals what would the audible changes be to the tweeters (when adjusting the knob from "decrease, normal, and increase on the L-pad) compared to not using resistors? The main reason I'm asking is because the phenolic tweeters can sometimes sound a little bright depending on the recording. If possible, I'd like to tame the highs a little more than what the L-pads with no resistor allows for.
Anthony,
The various new replacement tweeters being sold by PE and others for the AR-4x have a different response than the original tweeter. Regardless of the level control used there is an upper midrange peak, which can be harsh sounding at higher volume levels and/or with certain recordings. Additionally, the original tweeter is actually stronger and smoother through the lower midrange frequencies, so a simple change in attenuators cannot correct for all the response differences. Unfortunately, original tweeters are getting very hard to find, and there is nothing closer than these drop-in replacements.
Given your general satisfaction with the new tweeter, the short answer to your question is that the 25 ohm resistor would tame the tweeter's response a bit. In this case, however, the usual value (25 ohm) resistor is not necessarily pertinent...though it should be the first one to try. If it does the job satisfactorily, your controls will then be ready for original tweeters, should you ever find a pair. Just be aware that any parallel resistance will raise the crossover point and reduce the new tweeter's contribution to the lower mid frequencies. You won't know if it provides the desired effect for you unless you try it.
There is no simple answer to this issue. There was a forum member (or two) working on a crossover change, but I don't recall the outcome.
Roy
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On 12/3/2023 at 5:33 PM, J8944 said:
I hope someone with more technical expertise than I have can help me in understanding...
Your controls are incorrectly wired, including the placement of the resistors. It appears you labeled the L-pad terminals 1,2,3 (from left to right in your photo). If you look at the numbers printed on the terminals, you will see they are actually 3,2,1. The L-pad's terminal numbering is different than the 1,B,2 of the original controls.
The resistors are also supposed to be connected between 2 (center terminal) and 1 (far right terminal) of the L-pad.
The L-pad without a parallel resistor will provide greater output than connecting a resistor (of any value). The 25 ohm resistor will provide greater output than a 16 ohm resistor, and will result in a close match to the behavior of the original controls. A parallel 16 ohm resistor in this application is not at all appropriate.
Roy
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The 1.5" kit is the one to go with.
The only thing I do differently is to remove the spider, not the fiberboard mounting ring. Re-glue the ring in place before proceeding.
Roy
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2 hours ago, gahamby said:
RoyC, Thanks for the reply.
What diameter should I be looking for? I put my dividers onto the pole and measured 1-1/2'" (38mm.) Is this a slight difference or something quite obvious?
Millersound has not responded to my email and their phone number answered with a constant busy signal to multiple attempts.
GH, The earliest version has a 1.3" voice coil. The 2nd and 3rd iterations have a 1.5" voice coil. After looking at your photo again, it appears your woofer is the 2nd one with alnico magnet and foam surround, so the kit should work. Sorry for any confusion.
Post a photo of the whole woofer when you get a chance.
Roy
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GH, The 1.5" rebuild kit shown in the link will not work for your woofer basket. Your woofer is the earlier version with a smaller voice coil diameter.
Millersound may be the best bet for this one at the moment. Most of my repair work for this woofer has been re-gluing the cone and spider replacement. Up until now there have been enough used ones available to keep demand for this repair relatively low.
Roy
AR-4X wiring question
in Acoustic Research
Posted
I provided the photo to Kent (and others), but it shows the wires of the AR-3a mid, not the 4x tweeter. The numbering of the terminals is the important part. Disregard the wire colors. It is meant to illustrate the corresponding numbers of the original pot relative to a modern L-pad. Simply connect your 4x wire formerly soldered to #1 pot terminal to #3 L-pad terminal, etc.
Roy