Rebuild thread AR-LST

[Lars]

Hi all,

 

New on this board.

Just picked up a pair of my teenage dreams, AR-LST from the northern part of Sweden. Not many pairs were probably sold here in Sweden.

Now I really need your advice on how to proceed from here!

The previous owner replaced all the tweeters to new ones and the woofers need refoaming.(But one woofer is airtight enough to get a decent impedance-reading, I hope. will be back on that)

I list a few things here:

• Do I really need a 2500uF cap on the input (And a 10 ohms resistor in paralell)?
• Are the other caps due for replacement?
• Do I need a fuse on the input? And must it be a FNM2?
• The replaced tweeters are #200013-2 and #1200013-2 (from what I can see now) Any good?

Any additional advice is very much appreciated.

 

Regards//Lars

Stockholm, Sweden

[Giorgio AR]

Hi Lars, dear Swedish friend, congratulations on your purchase of the AR LST pair, you were right to follow your instincts...and many kilometers traveled by car to take them to their new home.
From the photos, nothing serious: the woofer on the left seems to have an incorrect but efficient foam (this speaker can therefore be tested for functioning), the one on the right presents to my view an original foam that has now deteriorated (well, it means that this will be the first refoaming it receives).
The tweeters seem to be replacements made years ago, probably by an AR laboratory: the one in the photo (and I suppose the others too has n° 200013-2 and should be original late spare parts at 8 ohms, adapted with the resistance and further capacitor to operate at 4 ohms of the AR LST.
You don't have the wax and paper capacitors inside the cabinets that always fail, so if these AR LSTs were mine with only the change of the foams, check of the contacts only and seal tightening of all 18 speakers, and following listening session ... then if something doesn't satisfy you (but I doubt it!) you can do a job after the crossover by replacing any capacitors out of tolerance.

I wish you good listening, you will also be able to make a report on the behavior of the tweeters, thanks to the "spectral balance" you can intervene on the emission of the tw and mid and adjust the medium/high range according to the environment and personal taste.
You will see that Frankmarsi will also emerge who has a lot of experience with AR LST replacement and replacement tweeters, and it will also be interesting to know the pair sound of AR LST with 8 tweeters of the 200013 series!

Attached a photo of my AR LST just arrived home... all good, but the woofers had a lot of glue and other material on top of the foam, masonite and cone, it was hard work, but the final effect is perfect, then I replaced only the paper and wax capacitors in my pair's cabinets... now I've been enjoying them for almost two years, perfect!

[Lars]

Hi Giorgio and thanks for the reply

Today, I am making impedance measurements on the speaker with the better surrounds, just as a sanity check.

Oddly, it seems that the resistor/cap appliance on the back of the tweeters are not connected. Is the tweeter 8 ohms in itself and the extra components ment to be connected to compensate down to 4 ohms? Is this procedure described somewhere?

Is it really your advice to not replace the caps? they are marked 1974.

And the big 2500uF cap in series with the whole speaker? Can I take that one out of the system?

And the fuses. That type is expensive! Can  ordinary 2A fuses be used instead?

As a "LST-rookie", I have a lot of questions. Sorry for that!!

Best regards//Lars

STockholm, SWeden

[Aadams]

You are getting good advice from Giorgio.   The tweeters appear to be later versions that do not blend at the crossover point without the capacitor and resistor.   They may be the 8ohm version but I have lost access to my reference guide and cannot check the number. The 2500 cap is there to protect your amplifier from extreme low impedance.  All of the caps except the 2500 are Sprague Compulytics which have proven to be durable and seldom need replacing.  The 2500 appears to be an ICC Royalitic which was also decent and unlikely to be out of tolerance.

The fuses are critical and can create audible output issues as they deteriorate with age.  You should use new fuses of the same type.

[Lars]

Hi all

The fastest and safest measurement in this situation is a impedancesweep. I have made sweeps on the speaker with the best (least bad) surrounds on all six positions of the switch on the front and I think that the measurements show some kind of trouble.

 

Can anyone help with diagnostics? (See the attached Powerpoint-presentation)

 

Regards//Lars

Stockholm, Sweden

 

 

Summary measurements 221205.pptx

[Aadams]

Page 5 of this link contains the published AR impedance curves

http://www.classicspeakerpages.net/library/acoustic_research/original_models_1954-1974/original_models_manuals/ar-lst_manual/

 

 

[Lars]

1 hour ago, Aadams said:

Page 5 of this link contains the published AR impedance curves

http://www.classicspeakerpages.net/library/acoustic_research/original_models_1954-1974/original_models_manuals/ar-lst_manual/

 

 

Hi,

Yes I was hoping that my measurements would be like the published ones from AR, but obviously, something is wrong.

If you "scroll" through the six positions in the PP-presentation, you see that the impedance peak "evolves " towards higher frequencies.

 

I wonder if anyone has experienced something like this before??

[Aadams]

I am reading this on a device that does not have PP.  If you want to share widely post each PP page as a small jpg image.

Are you checking impedance through the fuses?  You might need new ones or you could bypass for your impedance sweep.

17 minutes ago, Lars said:

I wonder if anyone has experienced something like this before??

Probably but you would need to search for relevant LST posts or wait for one of the ancient ones with deep LST wisdom to surface..........which could be a while. 

You could trust the caps are good enough for now, install new FMN2 fuses, check the woofer assemblies for integrity and put a low volume signal through each system to check for sound in every driver. 

 

[Lars]

Hi again,

Sorry, my fault! I forgot to mention that My units weren´t delivered with fuses. And my measurements are made without the 2500uF cap

I try to attach the six screenshots from measurements of the six positions för the knob.

And I realize that this line of questioning is a bit much to throw at you from a rookie on the board but hopefully, there is someone who realize what is going on....

 

Best regards//Lars

Stockholm, Sweden

[Aadams]

The tweeter you are showing is an 8 ohm replacement per Giorgio.

It

If all of your tweeters have three little blobs of material around the dome they are the proper era tweeter. If all have the same part number they have been adapted for a 4ohm application.  It appears they have been painted black to resemble an AR11 tweeter. The correct part is 200013-1.

Maybe the 8 ohm tweeters are the reason you are seeing higher impedance at higher frequencies.

 

[Lars]

I read somwhere on this board that 200013-1 are 4 ohms tweeters and 200013-2 are 8 ohm ones.

I plan, among other things tomorrow, measure the combined impedance behaviour of the respective tweeter and mid "clusters"

Is the added cap/resistor-combo meant to be a correction to 4 ohms operation? (They are not connected on the tweeters today, as far as I can see)

Will be back! (I never give up!)

Regards//Lars

[Aadams]

35 minutes ago, Lars said:

Is the added cap/resistor-combo meant to be a correction to 4 ohms operation? (They are not connected on the tweeters today, as far as I can see)

IIRC the cap should not be necessary for that tweeter but the added resistor should  lower the DCR of the tweeter to between 3 and 3.5 ohms to act as a 4ohm 200013-1.  A Cap would be used on the later AR11 type tweeters to make it blend at the crossover frequency. 

You may be aware that your tweeters are so old they have a reduced output that is a product of aging, crumbling suspensions (the foam blobs that should be orange).  They can be rebuilt as 4 or 8 ohm.

Sleep tight.

Adams

 

[Aadams]

Lars
 

7 hours ago, Lars said:

Is the added cap/resistor-combo meant to be a correction to 4 ohms operation

Disregard what I said  about adding the resistor in the previous post.  That would work if adapting 4ohm tweeters for 8 ohm use.  If all your tweeters are 200013-2 then you  have tweeters that will not work correctly. 

Your best option is to have all your existing tweeters rebuilt as 4 ohm. There are new 4 ohm AR11 reproduction tweeters that will fit the hole but would need additional filter work and could be more difficult to adapt to front wiring.  The cost is about the same for either path but the rebuild is the better choice

Somehow I got it reversed and only realized the mistake in the middle of the night.

[Lars]

Hi all,

 

If you have f****d up and made a fool of yourself, the best way to deal with it is to admit, take the blame and move on………

 I have shown to myself (and to all of you) that even a schematic as simple as the one on AR-LST present difficulties beyond my capacity to deal with!

 I have now made the correct impedance sweeps and it is obvious that the XO is intact and that I have 8 ohm tweeters.. I attach pictures from one of the six positions with/without the 2500uF/10 ohm series components and yes, the effect is below 20 Hz. My amp, H/K 870 can deal with loads down to 2 ohms so I think that I could remove the combo.

 Next thing, after reading your comments (that is if you think that I am worthy of that) will be to refoam the woofers and then measure the SPLs

 Best regards//Lars

Stockholm, Sweden

[KlausDK]

Hi Lars

I have restored a couple of AR-LST's, and on this Danish vintage hifi site, I have a thread on the model and upgrade of the x-over. I guess that you can read Danish

https://vintagehifi.dk/forum/viewtopic.php?f=557&t=12513

The models I've worked with, are from the beginning of the models lifetime, X-over looks a little different and it can be accessed from behind. The early models didn't have the 10 Ohm resistor across the big cap, that came later. The big capacitor is important, in order to protect your amplifier, don't try driving the speakers without it. I allways bypass it with a smaller capacitor (1-5 uF) to improva "passage" of the higher fequencies. I would also advice to replace the resistors to MOX versions instead of wire-wound, MOX sounds better in my ears.

BRgds Klaus

[Lars]

Hi and thanks for the reply.

Other posters have adviced against replacing the capacitors. I haven´t tested them but the impedance plots seem to indicate that the XOs are OK.

What is your opinion on that? (My reaction to abt 50 years old caps is to replace them...)

It turned up that my tweeters are 8 ohms. I will listen and measure SPL and see how well they blend in, in spite of the wrong impedance. It could work or one could modify the 6 uF-cap to something a bit different.

 

Best regards//Lars

Stockholm, Sweden

 

[KlausDK]

Hi Lars

I would as a minimum change the capacitors for tweeters and midrange speakers, not neccessary because I suspect that the old spragues are faulty, but because I think modern film capacitors sound better, i.e. the 6 uF and the 40 uF, and I would replace the 6uF with a 4uF, to comply with your tweeters being 8 Ohm versions. Furthermore, I would, as mentioned above, replace the 10 Ohm resistor for a better type, and bybass the big 2500 uF cap. with a film capacitor of 1-5 uF.

BRgds

[Lars]

Hi and thanks for your input.

 

Yes, I suppose that it´s a good way to go to replace all the caps and resistors to better ones. (and younger ones, too)

That is: 6uF, 40uF, 150uF (Bipolar), 0.5ohms (Mid) and 10ohms (//2500uF)

But where are you suggesting to put the 1-5uF-cap? On the woofer-shunt?

 

Regards//Lars

[Lars]

Ah, now I get it.

 

The 1-5 uF is // with 2500uF and //10 ohms.

This morning, I have replaced the old 10 ohms resistors with MOX and also modified a thing that perhaps not all of you agree on:

I have replaced the fuseholders on the back with ordinary "banana-plug-terminals". I plan to use ordinary 2A fuses hanging on the back of the speakers.

 

Now I wait for the woofers to be ready with new surrounds and at the same time, I will buy (22+18)uF caps and 0.5 ohm MOX´s. 55 USD, but it will be worth it!

Regards//Lars

Stockholm, Sweden

 

[giovanni56]

Il 5/12/2022 alle 16:33, Aadams ha dichiarato:

La pagina 5 di questo collegamento contiene le curve di impedenza AR pubblicate

http://www.classicspeakerpages.net/library/acoustic_research/original_models_1954-1974/original_models_manuals/ar-lst_manual/

 

 

The filter schematic published on CSP is different from that of our Lars, which has the input capacitor is 2500 nF and has a 10-ohm by-pass R in parallel. The published schematic has only a 5000 nF capacitor group without the 10 ohm R.

[giovanni56]

Il 7/12/2022 alle 11:11, Lars ha detto:

Ciao e grazie per la risposta.

Altri poster hanno sconsigliato di sostituire i condensatori. Non li ho testati ma i grafici dell'impedenza indicano che gli XO sono OK.

Qual è la tua opinione in merito? (La mia reazione a tappi vecchi di circa 50 anni è di sostituirli...)

Si è scoperto che i miei tweeter sono da 8 ohm. Ascolterò e misurerò l'SPL e vedrò come si integrano bene, nonostante l'impedenza sbagliata. Potrebbe funzionare o si potrebbe modificare il cappuccio da 6 uF in qualcosa di leggermente diverso.

 

Cordiali saluti//Lars

Stoccolma, Svezia

 

Vorrei cambiare i condensatori da 6 nF in condensatori da 4 nF, ma elettrolitici.

[Lars]

1 minute ago, giovanni56 said:

Vorrei cambiare i condensatori da 6 nF in condensatori da 4 nF, ma elettrolitici.

Hi, I will try with 4.7 uF but why electrolytic capacitor?

Do you think that all the caps in this system woluld be electrolytic ones?

 

Best regards//Lars

[Lars]

By the way, the woofers are back from the company in Stockholm that mount new surrounds.

Would you be interested in the DATS-measurements after repair?

[giovanni56]

27 minuti fa, Lars ha detto:

Ciao, proverò con 4,7 uF ma perché condensatore elettrolitico?

Pensi che tutti i condensatori di questo sistema sarebbero elettrolitici?

 

Cordiali saluti//Lars

Si, sono tutti elettrolitici quindi sarebbe giusto, se proprio si volesse sostituirli, farlo con altri condensatori aventi lo stesso dielettrico, quei condensatori andrebbero controllati, comunque si potrebbe verificare che al 99%, sono ancora efficiente.
Se devi provare 4.7 è meglio provare 3.9 nF.
Resistori MOX potresti anche non inserirli, soldi buttati. I fusibili che potresti eliminare. 

[DavidR]

17 minutes ago, Lars said:

Do you think that all the caps in this system woluld be electrolytic ones?

If they are original then YES