A couple LST questions

[SUNNBOBB]

Hi gang,

After combing the site and reaching the end of the internet, I'm left with a couple questions regarding restoration of a couple sets of LST's.

Over the past month I have restored both a set of 2ax and 3a's. Now I'm onto 2 sets of LST's and a single set of LST-2's. After opening them up and checking them out, I have a few questions before I dive in.

Set 1 of the LST's have a serial number of 139 and 140. The set has all the original tweeters and midranges, and have TGN 1210003-2A replacement woofers. I plan to fully recap them. As you can see in the picture, they have the old 5000 60V capacitor. I've read that AR changed over to 2500 caps and the reasons why. My question is simple.. would you guys recommend going ahead with replacing the 5000 with a 2500? Or, go with the OEM 5000 set up?

Set 2 of the LST's is more of a historical cabinet question.  The previously mentioned Set 1 is set up with a removable back panel, allowing easy access to the crossover. Set two has no such panel. Having no serial number on set two (back papers missing...grrrrr) I am curious if anyone can shed light on the cabinet change?

As always, I appreciate you guys and your knowledge. 

Richard

[jnolan5784]

I have LST 2705 and 2708 and they do not have a removable backs for the crossover. I haven't started working on my set and have many of the same questions about recapping. I was planning on having all of the tweeters rebuilt. When I restored my 3a's (recap and potentiometer replacement), the difference after having the tweeters rebuilt was substantial. I thought the 3a's sounded good before tweeter rebuild, but was really surprised how much it helped. The LST's are worth the expense, and really only want to have them apart once. 

[SUNNBOBB]

Good on ya Jnolan! We'll be on this journey together. I've looked at the various schematics and LST discussions, but Really didn't get a strong sense of whether to replace that cap with a 2500 (and accompanying cap add on). I've been in touch with Roy, who has been very helpful about the tweeters. I figured the best approach was to get the crossover right, then go from there.

[SUNNBOBB]

I haven't been able to find a schematic that replicates the existing crossover in my LST. I have attached 2 photos. The first shows the existing crossover with the capacitors labeled. The second is an old diagram (which shows a 2500MF setup). The question is, the old AR LST Drawing is missing two capacitors present in my existing crossover. A 4 and a 6.  In addition, the schematic shows a 40mfd, which I presume is the 30mfd in my existing crossover. Thoughts?

[JeffS]

I am not familiar enough with the LST to offer specific advice. One thought, is the 30 mfd cap, and the dual 6 and 4 mfd cap, wired in parallel to make a 40 mfd?

[Aadams]

Asumming all components are present and correct, the most audible improvements will be to restore the woofer, rebuild the tweeters, install new fuses and replace the wax capacitors.  The can caps appear to be spragues which almost never cause problems.  

[SUNNBOBB]

Thanks JeffS, you might be on to something there. Aadams, are you recommending to leave the 5000 cap?

[RoyC]

7 hours ago, JeffS said:

I am not familiar enough with the LST to offer specific advice. One thought, is the 30 mfd cap, and the dual 6 and 4 mfd cap, wired in parallel to make a 40 mfd?

Correct, Jeff...I've seen this is a number of times in the LST. Replacing the parallel arrangement of the old caps with a single 40uf cap is the way to go. AR was probably just using up inventory of old capacitors.

I would keep the large 5000uf caps.

As Aadams mentioned, replace the fuses. Their construction is such that they can be partially functional, resulting in diminished sound.

Roy

[Aadams]

56 minutes ago, SUNNBOBB said:

. Aadams, are you recommending to leave the 5000 cap?

I am saying in the absence of any evidence to the contrary, I would assume they are ok for now, and not spend time and money replacing the Spragues.  That single 5000mf is also a sign of provenance.  It was replaced by a single 2500mf to make the LST friendlier to a wider range of power amps in the 1970s.  Not everyone had a DC300 or MC2300.   

[SUNNBOBB]

You guys are great. 

Just a small update. 

Replaced old FNM-10 (previous owner?)with new FNM-2

New woofer surrounds are drying as we speak

Cabinets are sanded and ready for finish

Testing capacitors later today

Ordering replacements for the old wax caps and any other caps that show out of spec.

Once that is all installed, I'll be testing the tweeters to determine if they need to be rebuilt.

I'll post up some pics of the progress.

 

Again, thanks guys!

[Aadams]

15 minutes ago, SUNNBOBB said:

nce that is all installed, I'll be testing the tweeters to determine if they need to be rebuilt.

The betting line on your tweeters performing anywhere close to original spec would be a 1,000,000 to 1 even if they are perfect on a volt/ohm meter. All of us who have had our tweeters rebuilt know this a near universal pragmatic truth.

[SUNNBOBB]

14 minutes ago, Aadams said:

The betting line on your tweeters performing anywhere close to original spec would be a 1,000,000 to 1 even if they are perfect on a volt/ohm meter. All of us who have had our tweeters rebuilt know this a near universal pragmatic truth.

After my abysmal NCAAM bracket failure, I'm not betting on anything LOL. I'm going on Roy's advice to get the crossover taken care of first. I'm crossing my fingers, as having tweeters rebuilt on 2 sets of LST's will make a serious dent in my lunch money!  I did luck out on the AR3's, the tweeters were OK.