AR-2ax Speaker - what kind of wood veneer? Pine?

[HamboneF18]

Did you check the impedance of the old caps?

Not impedence. The DC resistance was 1.2 megohms, if that matters. (Yeah, these are caps, so injecting a voltage is of minimal use.) The big thing to me was drift, or value change. I needed to remove the pots, so I took the capacitor(s) out also. At least now I know the condition of everything. And I know the caps I put in are very close to the intended AR design value.

As I’ve played this one speaker, the tweeter and midrange are loosening up. They still sounded “subdued” when I first put a music signal to them.

 

[HamboneF18]

I'm thinking it was a combination of the capacitors and the pots???
As long as everything is working it's all good!

I think you’re right. Time, sitting, not played, pots and caps. About all I can say.

 

[gkimeng]

Not impedence. The DC resistance was 1.2 megohms, if that matters.

I was actually talking about AC impedance, which for a good cap should be zero (DC resistance should be infinite). The fact that your DC reading is measurable is a good indication that the cap is bad for audio use, regardless of how the capacitance measures. An AC impedance of more than zero would be the clincher. And would explain why the sound to the MR and HF drivers is attenuated.

 

[HamboneF18]

I was actually talking about AC impedance, which for a good cap should be zero (DC resistance should be infinite). The fact that your DC reading is measurable is a good indication that the cap is bad for audio use, regardless of how the capacitance measures. An AC impedance of more than zero would be the clincher. And would explain why the sound to the MR and HF drivers is attenuated.

Well, technically, the AC impedance depends on frequency, no? I would expect the impedance to be high at 100 Hz, for either the 4 or 6 uF cap. I don’t think that the condition of the original caps were bad enough to account for the very low outputs of the mid and tweeter drivers. On the second speaker I’ll clean the pots and keep the original caps in, then have a listen. This is exactly why I’m only doing one speaker at a time!


I already changed out the caps in this speaker. Now to fix that tweeter! I made this video to illustrate the tweeter is not putting out full volume. Cymbals are there in the track, but not at the volume they should be. Mid pot is at 50%, tweeter pot is at 100%. After I work on this tweeter, I’ll record the same track. Using the Micro Acoustics MA309 cartridge which I repaired, new cantilever and nude elliptical tip, and repaired the suspension.

 

[gkimeng]

Well, technically, the AC impedance depends on frequency, no?

Yes. As frequency increases impedance should go down. Which is pretty much the opposite of what you appear to be experiencing.

 

[HamboneF18]

I think the pots had just enough corrosion to cause an intermittent connection.

 

[RTally]

Did you check the impedance of the old caps?

Not impedence. The DC resistance was 1.2 megohms, if that matters. (Yeah, these are caps, so injecting a voltage is of minimal use.) The big thing to me was drift, or value change. I needed to remove the pots, so I took the capacitor(s) out also. At least now I know the condition of everything. And I know the caps I put in are very close to the intended AR design value.

As I’ve played this one speaker, the tweeter and midrange are loosening up. They still sounded “subdued” when I first put a music signal to them.

Yeah, checking impedance of caps is not a thing. Neither is checking resistance of capacitors with an ohmmeter.

With capacitors, there are two variables that come into play for speaker crossovers. The variables are capacitance value and equivalent series resistance (ESR).

Generally, as electrolytic capacitors age, the capacitance value slowly changes, typically decreasing in value, but not always. Sometimes they blow up (as the black body & red end caps (the firecracker Temple brand, for example) are known to do). A change in capacitance value will shift the crossover frequency. A higher capacitance value results in a lower crossover frequency and vice versa. For tweeter caps, aging electrolytic caps can create a slight peak or null at the crossover frequency. It is often difficult to hear a change in capacitance value unless the change is extreme, such as doubling or halving in value.

Almost always, as electrolytic capacitors age, the ESR increases. Some brands are worse than others. It is not unusual for old caps to have several ohms of ESR, which attenuates the tweeter. If the tweeter is muffled or sounds like a blanket is thrown over the speaker, the caps have high ESR and definitely need to be replaced. ESR varies based on frequency. Cheap ESR testers test at frequencies above the audio range and often do not provide meaningful information for crossover caps.

If ESR is not being tested properly, you are wasting your time by testing.

Since caps need to be removed from the circuit for testing, it is just as easy to install a new cap instead of reinstalling the old cap.

 

[HamboneF18]

Yeah, checking impedance of caps is not a thing. Neither is checking resistance of capacitors with an ohmmeter.

With capacitors, there are two variables that come into play for speaker crossovers. The variables are capacitance value and equivalent series resistance (ESR).

Generally, as electrolytic capacitors age, the capacitance value slowly changes, typically decreasing in value, but not always. Sometimes they blow up (as the black body & red end caps (the firecracker Temple brand, for example) are known to do). A change in capacitance value will shift the crossover frequency. A higher capacitance value results in a lower crossover frequency and vice versa. For tweeter caps, aging electrolytic caps can create a slight peak or null at the crossover frequency. It is often difficult to hear a change in capacitance value unless the change is extreme, such as doubling or halving in value.

Almost always, as electrolytic capacitors age, the ESR increases. Some brands are worse than others. It is not unusual for old caps to have several ohms of ESR, which attenuates the tweeter. If the tweeter is muffled or sounds like a blanket is thrown over the speaker, the caps have high ESR and definitely need to be replaced. ESR varies based on frequency. Cheap ESR testers test at frequencies above the audio range and often do not provide meaningful information for crossover caps.

If ESR is not being tested properly, you are wasting your time by testing.

Since caps need to be removed from the circuit for testing, it is just as easy to install a new cap instead of reinstalling the old cap.

I already changed out the caps in this speaker.

Failure distribution of electrolytics:


As ESR increases, it presents a relative short circuit. That’s what the failure distribution indicates.

So now, with the pots clean, the caps replaced, I still get low output from the tweeter. Using my phone app, I presented 1kHz-12kHz signals to the mid and tweeter. 5kHz is definitely the XO frequency to the tweeter. 8 kHz is perceptible but low in volume. 10kHz is nonexistent. Injecting the same frequencies into my Polk tweeters, I can hear clean tones out to 13kHz (my hearing limit).

 

[gkimeng]

Do you really care what the ESR of an old cap is if you have no idea what it was when the cap was new? If you're just after a pass/fail binary decision, probably not.

If you've cleaned the pots and changed the caps and are still getting low output, then there's nothing left but drivers.

 

[HamboneF18]

Do you really care what the ESR of an old cap is if you have no idea what it was when the cap was new? If you're just after a pass/fail binary decision, probably not.

If you've cleaned the pots and changed the caps and are still getting low output, then there's nothing left but drivers.

That’s the conclusion I came to last night. The tweeter has abysmal output beyond 8kHz. I attribute that to the failure of the tweeter dome suspension. So I’ll be reworking the 4-point suspension and membrane seal on the dome perimeter.

 

[RTally]

Do you really care what the ESR of an old cap is if you have no idea what it was when the cap was new? If you're just after a pass/fail binary decision, probably not.

If you've cleaned the pots and changed the caps and are still getting low output, then there's nothing left but drivers.

Yes you should care about ESR. A known failure mode of electrolytic caps is increasing ESR, and that high ESR will attenuate the tweeter output. New electrolytic caps, even way back in the 60s and 70s, have an ESR that falls in a range, generally around 0.5 to 1 ohms. Old caps can have ESR that are several ohms, much higher than when they were new.

I agree about changing caps. Caps are inexpensive and easy to replace, they should be the first thing replaced on speakers more than 30 years old, particularly if the tweeter output is low or not audible and the VC is not open.

As ESR increases, it presents a relative short circuit. That’s what the failure distribution indicates.

Um. No. Increasing resistance is the exact opposite of a short circuit, which is defined as zero resistance. The ideal capacitor has zero ESR. In the real world, ESR varies with the type of capacitor and often its age.

Failure distribution of electrolytics:
View attachment 2842398

Also, I question your source material. Most vintage electrolytic caps in speakers are aluminum, not tantalum.The two types of capacitors have different failure modes.

Also, I suspect that the cited chart is taken out of context. Industry data, from manufacturers and not specific to the NASA Space Launch System, indicate that the short circuit failure mode is due primarily to manufacturing defects and open circuit failure modes are primarily due to manufacturing defects and improper handling and installation. Whereas the change of operating parameters, C and ESR, are primarily caused by deterioration over time. Both C and ESR change slowly as electrolytic caps age. When relying on failure mode data, it is important to identify the point where the capacitor is considered failed. As the ESR changes over time, that change is likely audible well before the capacitor is determined to have failed for the data collectors.

 

[HamboneF18]

Also, I suspect that the cited chart is taken out of context. Industry data, from manufacturers and not specific to the NASA Space Launch System, indicate that the short circuit failure mode is due primarily to manufacturing defects and open circuit failure modes are primarily due to manufacturing defects and improper handling and installation. Whereas the change of operating parameters, C and ESR, are primarily caused by deterioration over time. Both C and ESR change slowly as electrolytic caps age. When relying on failure mode data, it is important to identify the point where the capacitor is considered failed. As the ESR changes over time, that change is likely audible well before the capacitor is determined to have failed for the data collectors.

You are correct about ESR. A new cap should have low ESR, meaning it passes an AC signal with very low losses, within its useable frequency range.

But you’re a tad off on the rest here.

That chart is just a sample; it’s for a tantalum electrolytic. The reference contains many types of capacitors, and includes aluminum electrolytics. The chart is based on many sources of data, from commercial, industrial, military, and manufacturers’ data. This particular chart is from RAC, Reliability Analysis Center, The Defense Technical Information Center, Rome Laboratory, Griffiss Air Force Base.

This reference, and others are used for suppliers and customers to calculate Reliability and Maintainability during preliminary design, critical design, and Design Critical review. It is used to form a part’s FMEA and FMECA (Failure Modes and Effects Analysis; Failure Modes and Effects Criticality Analysis). I don’t do the FMEA for avionics; I do the mechanical parts: TPS, umbilicals and explosives/pyrotechnics. Concepts are the same. NASA doesn’t build, they buy. Always been that way. Parts, systems, avionics, software have to be analyzed from concept to prototype in hand. All parts have to be analyzed to predict failure and modes. NASA uses these reference charts to predict failure rates, to look over the shoulder of suppliers who certify every part.

The avionics folks analyze every chip, diode, resistor, capacitor, inductor, discrete semiconductor, wiring board, connector, wiring harness, motherboard, back plane, and compare software response and redundancy.

I do that for every wire, pin, QD, seal, lever arm, solenoid, gas tube, hinge pin, tube connector in all 5 umbilical panels. Also for the initiator, flexible charge guide and shaped charge in the flight termination system and stage separation system/part.

In the real world, the actual failure rates line up well to calculations., in million hours between failures. Also, environment is factored in for failure rates.

The data for aluminum electrolytics looks like this:

 

[doctorolds]

Here is a simple suggestion, put the good tweeter in the cabinet you think the tweeter is bad in, and vice versa. In 15 minutes you will have an answer to where the problem lies.

A capacitor that is shorted will still pass a signal unless one end is connected to ground, but it may not be the signal/frequency desired which could cause damage to a driver.

Buy a cheap $20 LCR meter or better to test components, its wort it.

 

[HamboneF18]

Here is a simple suggestion, put the good tweeter in the cabinet you think the tweeter is bad in, and vice versa. In 15 minutes you will have an answer to where the problem lies.

A capacitor that is shorted will still pass a signal unless one end is connected to ground, but it may not be the signal/frequency desired which could cause damage to a driver.

Buy a cheap $20 LCR meter or better to test components, its wort it.

At this point, there is only 1 tweeter. The other speaker cabinet hasn’t been opened. This is by design. I want to be able to A/B the speakers. Both arrived with identical symptoms: Low mid output, no tweeter output.

Already been there, done that. Actual capacitance value is written to the left of each printed value:



LCR meter:

 

[doctorolds]

Based upon the values listed in the picture you are 20 percent off, not good in my opinion. What is the ESR reading on those?

I understand the A/B testing you want to do, but isn't it possible that both tweeters will sound the same and you may be chasing a problem that doesn't exist???

 

[HamboneF18]

Based upon the values listed in the picture you are 20 percent off, not good in my opinion. What is the ESR reading on those?

I understand the A/B testing you want to do, but isn't it possible that both tweeters will sound the same and you may be chasing a problem that doesn't exist???

Oh, it exists!

I used the test tone app on my phone last night. I was looking for better balance between mid and tweeter. The turnover at 5K seems to be correct. The mid is plenty loud. But at 5K I can hear output from the tweeter, but not nearly as loud as the mid. Above 5K the tweeter descends in loudness. At 9K it’s perceptible. At 10K there is barely any audible output. My hearing goes to 13 kHz.

I already have the new caps. Installed in the one cabinet I opened.

 

[doctorolds]

I said possibly, not that there wasn't a problem. I'm just giving you one idea to try that may lead you to the problem or solution.

What about the 20 percent out of tolerance, or the actual ESR reading(s)?

What position are the pots in during your test, and did you try adjusting during that test.

I have also read that those older AR tweeters need rebuilding, so I'm not saying that you are right or wrong, yet.

 

[HamboneF18]

The original caps are a dead issue. They’ve been replaced in one speaker. After cleaning the pots and replacing the caps, I have very low output from the tweeter. Using a frequency “sweep”, the tweeter output is low. Nothing over 10kHz. Tweeter pot set to maximum. Mid set to 50%.

I have set my amp to mono. The AR is wired to the R channel. My Polk is wired to the L channel. After applying test tones to the AR, I did the same for my Polk. The Polk had very audible output from 1kHz to 13kHz, the latter is my hearing limit. So it’s a problem with the AR tweeter.

 

[RTally]

You are correct about ESR. A new cap should have low ESR, meaning it passes an AC signal with very low losses, within its useable frequency range.

But you’re a tad off on the rest here.

I still think you are off the mark. You are talking about failure modes during component life cycle. You are talking about failures per million hours of operation. That is a population-based failure rate during the period the components are used within their expected life. Open and short circuit failure modes are primarily manufacturing and installation issues. These failure modes manifest themselves during the expected life of the capacitor, mostly during the initial operating period (I am sure you are aware of the U-shaped curve for failure.) Component parameter degradation occurs over time and manifests itself closer to the end of life of the component. These type of failure mode analysis, by being related to the life of the electrolytic capacitor, are not applicable to vintage speaker caps, which are operating past their end-of life.

When talking about electrolytic capacitors used in vintage speakers, we are talking about degradation and failure modes after end-of-life. Individual electrolytic capacitors have an expected operating life of 5,000 to 10,000 hours, typically, and an expected shelf life measured in single digit years, typically.

What I believe most here are interested in is how the capacitors in their speakers fare over time. Every electrolytic capacitor in a vintage speaker is past its expected life, whether considering shelf life or operating life. The question is how has the electrolytic cap degraded and what needs to be done about it.

As for my creds, I am an EE who has participated extensively in aging and failure mode tests for electrical components, primarily in the nuclear industry. A lot of the tests focused on component performance at predicted end-of-life, primarily to determine if the expected life could be extended or needed to be shortened. The lab I was associated with likely contributed to the data you cite and rely upon.

 

[doctorolds]

The original caps are a dead issue...
The AR is wired to the R channel. My Polk is wired to the L channel. After applying test tones to the AR, I did the same for my Polk. The Polk had very audible output from 1kHz to 13kHz, the latter is my hearing limit. So it’s a problem with the AR tweeter.

Okay, why are you so hesitant to pull the tweeter out of the other cabinet to test it and compare it to the current one you are working on???