LST Conundrum

[Guest pbda]

A puzzle for the AR cognoscenti on the forum.

My LSTs have developed an odd issue that seems to be centered on the Spectral Balance switch, and by extension, the autotransformers...or at least something inside the box. Here's the situation: Normally, as one switches the Spectral Balance switch up from 1 to 6, the speaker puts out more bass, and treble is attenuated. By the same token, the closer to position 1, the more treble you get.

However on my pair, the closer you get to 1, the more bass response I get. And the closer to 6, the more treble response. On top of this, in positions 1, 2 and 3, the four tweeters cut out entirely. Likewise, in positions 4, 5 and 6, the woofer cuts out entirely. The midranges work regardless of the switch position.

The final oddity is that this occurs on both speakers (they are a consecutive pair, serial numbers 1015 and 1016).

I'm driving these with a Marantz 2600 receiver (300 wpc into 8 ohms, 400 wpc into 4 ohms). The receiver is working properly, and has no problem driving my 3as correctly.

Any thoughts about what may be amiss here, and what I might do to resolve the situation? Having to choose between life without treble and life without bass is no choice at all!

[dynaco_dan]

A puzzle for the AR cognoscenti on the forum.

My LSTs have developed an odd issue that seems to be centered on the Spectral Balance switch, and by extension, the autotransformers...or at least something inside the box. Here's the situation: Normally, as one switches the Spectral Balance switch up from 1 to 6, the speaker puts out more bass, and treble is attenuated. By the same token, the closer to position 1, the more treble you get.

However on my pair, the closer you get to 1, the more bass response I get. And the closer to 6, the more treble response. On top of this, in positions 1, 2 and 3, the four tweeters cut out entirely. Likewise, in positions 4, 5 and 6, the woofer cuts out entirely. The midranges work regardless of the switch position.

The final oddity is that this occurs on both speakers (they are a consecutive pair, serial numbers 1015 and 1016).

I'm driving these with a Marantz 2600 receiver (300 wpc into 8 ohms, 400 wpc into 4 ohms). The receiver is working properly, and has no problem driving my 3as correctly.

Any thoughts about what may be amiss here, and what I might do to resolve the situation? Having to choose between life without treble and life without bass is no choice at all!

Hi there;

Which version of AR-LST's do you have?

Rear photo of both if possible, please, closeup of the crossover area.

The earliest version had the crossovers mounted and bolted into the recess on the rear of the enclosure.

The later version had the crossover mounted through the woofer opening and bolted from within.

This won't get them working but it is a step in troubleshooting.

How long have you owned them?

How long after you bought them did you find this anomoly?

I would wait for futher suggestions before opening up the boxes for surgery.

My first guess would be that the contacts on the switches are heavily oxidized and need a DeOxit coating.

[roundhome]

Hi there;

Which version of AR-LST's do you have?

Rear photo of both if possible, please, closeup of the crossover area.

The earliest version had the crossovers mounted and bolted into the recess on the rear of the enclosure.

The later version had the crossover mounted through the woofer opening and bolted from within.

This won't get them working but it is a step in troubleshooting.

How long have you owned them?

How long after you bought them did you find this anomoly?

I would wait for futher suggestions before opening up the boxes for surgery.

My first guess would be that the contacts on the switches are heavily oxidized and need a DeOxit coating.

just a strange idea but try pulling out the fuse and see if it still does the same thing.

[dynaco_dan]

just a strange idea but try pulling out the fuse and see if it still does the same thing.

Hi there;

Strange, absolutely not, an excellent idea, doesn' t cost anything either.

Now why didn't I think of that? LOL LOL

Testing the fuse for continuity is a definite option and certainly within the capabilites of almost everyone here.

Another option is to jumper across a fuse, at reasonable sound levels, of course.

Even with a blown fuse, the sound level is something like 40 dB down, I believe, not dead at least.

Certainly gives everyone something to think about.

Another sidenote.

Many years ago a semi-trailor truck got itself wedged under a railway crossing bridge.

The truck driver, police, fire department and engineers didn't know what to do to remove the truck.

A small boy suggested letting some air out of the tires.

Sometime there's really a simple solution to a sometimes not so obviously simple problem.

Vern

[Guest pbda]

just a strange idea but try pulling out the fuse and see if it still does the same thing.

Roundhome: Brilliant!

I pulled the fuses and in one speaker that did the trick: It's back to its normal self.

With the other speaker the problem persists; after Christmas I plan to fiddle around with the fuse connection a bit more and see what happens.

Now I need to find some replacements for these gigantic dual-element Buss fuses, which appear to have been in situ since the early seventies...

[dynaco_dan]

Roundhome: Brilliant!

I pulled the fuses and in one speaker that did the trick: It's back to its normal self.

With the other speaker the problem persists; after Christmas I plan to fiddle around with the fuse connection a bit more and see what happens.

Now I need to find some replacements for these gigantic dual-element Buss fuses, which appear to have been in situ since the early seventies...

Hi there;

If you are going to use the, Buss FNM 2 amp slow blow fuse, go to the, "other forum" and look at the topic, "fast blow fuses".

If you follow the trail, I listed a website that has the FNM 2 fuses at $1.50 each, while supplies last.

Here is the link to it.

http://www.electronicsurplus.com/ccp69391-...setron-6693.htm

The fuses are usually upwards of $6.00 - $7.00 each elsewhere, unless you luck out.

Contact Nancy for excellent customer service, without a doubt.

I have nothing to do with that site other than being a repeat customer.

They do not have any other FNM's at a lower amperage rating unfortunately at this time.

If you are unable to find any 2 amp FNM fuses, use 1 3/4 or 1 1/2 amp FNM for a slightly more forgiving fuse size for our old clunkers.

I just went to the site, so the link does work. Hooray!

They do not have as many fuses left as they list.

[roundhome]

Hi there;

If you are going to use the, Buss FNM 2 amp slow blow fuse, go to the, "other forum" and look at the topic, "fast blow fuses".

If you follow the trail, I listed a website that has the FNM 2 fuses at $1.50 each, while supplies last.

Here is the link to it.

http://www.electronicsurplus.com/ccp69391-...setron-6693.htm

The fuses are usually upwards of $6.00 - $7.00 each elsewhere, unless you luck out.

Contact Nancy for excellent customer service, without a doubt.

I have nothing to do with that site other than being a repeat customer.

They do not have any other FNM's at a lower amperage rating unfortunately at this time.

If you are unable to find any 2 amp FNM fuses, use 1 3/4 or 1 1/2 amp FNM for a slightly more forgiving fuse size for our old clunkers.

I just went to the site, so the link does work. Hooray!

They do not have as many fuses left as they list.

I just picked up 20 from them they have vary fast service

Home depo has them also

[dynaco_dan]

I just picked up 20 from them they have vary fast service

Home depo has them also

Hi there;

Do you remember the price and sizes of the FNM fuses stocked at your USA Home Depot?

If not, could you please find out and post that information here.

I checked out our local, Rona, comparable to the Home Depot.

They do not carry the FNM series fuses.

I will return and note the model and amperage sizes they do carry and prices.

For all intents and purpose this style fuse is about 50 years past it's life, as they are only used in very specific applications.

Usually 15 - 30 amperes and perhaps commercial or industrial applications.

I am certain someone here has come across these in the field, somewhere.

A side note:

The most common much older house fuse, here at least is the, glass screw-in fuse, which is, or at least was available.

A special time delay screw-in fuse with a much smaller, non-standard thread, was sold here for years.

These were colour coded as to their amperage rating, blue, being our most common 15 ampere.

A small threaded barbed bushing, with different internal threads, was also needed and had to be bought extra and mated with that colour of fuse.

A standard glass fuse could be replaced with a 15 - 30 amp fuse without a problem prior to this.

With the bushing screwed onto the new fuse, only that fuse and brand can ever be used in that socket again.

You screwed in the new fuse and bushing and if you tried to remove the fuse, the bushing remained, the barbs held it there, limiting the fuse size accepted in the future.

Not bullet-proof but pretty close.

These particular delay 15 amp fuses were great for furnace motors starting and not blowing, unless there was a fault.

[Guest pbda]

Vern, many thanks for your suggestion about the fuses. I didn't see your post, but did manage to order 10 of the FNM-2 fuses from an online outfit for about US2.00 apiece.

Replacing the old fuses was one important part of solving the problem, but the other part was the realization that the spade terminals I was using on the LSTs were too large, and were causing a short circuit (lots of exposed metal on the back of these speakers). Looking more closely at the binding posts, I realized that they have very small holes in them into which you insert the speaker cable. I removed the spade terminals, cut back my 12 gauge speaker cable so it would fit in the holes, and Bob's your uncle, as the Brits say: Perfect functioning, and golly but they sound superb.

Thanks to all for weighing in.

Peter