Mordaunt Short MS20 Crossover

[MarkPUK]

Hello from a new member
I am trying to ressurect my old Mordaunt Short MS20 speakers and am getting a nasty distortion from one of them, the other working perfectly. I have swapped the speakers and it is definitely the speakers and not the amplifier.
I have removed the crossover board (labelled MS20a) and cannot see any obvious problem. My first instinct is to replace the components. The capacitors (Alcap) are marked at 10 and 5 mF but the inductors are not marked. Does anyone know the value of the inductors in this circuit. A circuit diagram would be a bonus. Alternatively can you recommend a replacement circuit? I understand from a review that the crossover frequency for this speaker is 3500.

Mark

 

[Mooly]

First things first 🙂

Are you sure the distortion isn't coming from the actual drive unit itself ? If you gently press on the bass driver cone does it move smoothly or can you feel or hear it rub ? Is the distortion from the bass driver or the tweeter.

You can couple the bass driver with no crossover to test it by connecting direct across the amp output. Do not do that for the tweeter though.

 

[MarkPUK]

Many thanks for the speedy reply. I think you might have hit it straight off. There is definitly some rubbing when I press in the woofer I do get a gentle grating sensation which I do not get with the working speaker (although that one is still in the cabinet) and there is distortion when I connect the input leads directly to it.

Anything to try or do I need another speaker?
Mark

 

[Mooly]

I'm no speaker guru but I suspect its not good news.

I believe some speakers suffer from the weight of the magnet distorting the frame over many years, and I've heard of rotating the speaker by 180 degrees can help in those cases. Those more knowledgeable than I in these matters will know for sure.

Another possible cause is a fragment of metal caught in the pole piece of the magnet such that it causes the coil to rub and distort. Very unlikely but worth a look with a torch where the coil enters the magnet.

I would say (and wait for other replies) that if all else fails you could try physically correcting any frame distortion by brute force.

Also the coil could have overheated and gone out of shape causing rubbing... to many loud parties 😀

 

[alan-1-b]

Procedure

Mark ,

I have heard the original MS20s often.

I doubt the woofers will still be available , so look on ebay.
A Pair of worn looking MS20 at low price on ebay may have damage to one or more of the drivers ,
but you only need 1 woofer , and perhaps 1 tweeter ,
so ask the seller for a description , then take a chance if need to.

Next I would test the capacitors in that crossover.
The Inductors will likely be OK as it is difficult to damage those ,
but Alcaps do deteriorate with age ,
so I would use the crossover from whichever sounds better of an ebay MS20.

I can advise about better replacement capacitors , but first get another working MS20.

M.S.'s "Carnival" and "Festival" models used the same woofer , or at least one of them did ,
because MS20 was a plainer finished version of one of those models.

Try Mooly's suggestions to see if you can get the woofer to work.

 

[MarkPUK]

Thanks again Mooly. I will try leaving it inverted for a few days, otherwise it is Ebay. There is no access to the coil where it enters the magnet to try blowing it out and no visible frame distortion.. The drivers are Mordaunt Short DB 208. These speakers have never been over-driven but have been stored in a loft and I suppose corrosion is one possibility.
Mark

 

[Toaster]

Good speakers the MS 20s- a bit of a budget speaker classic IMHO. The midbass unit is the Mordaunt-Short DSB 208, which as Alan says was common to many M-S models at the time. Long out of production, but you see them on ebay fairly regularly. Upgrade with SB neo dimple-dome tweeter maybe?

 

[phivates]

Voice coil disintegration happens. I would not vote for replacing the tweeter since HF drivers get a lot of blame for breakup effects in the bass-mid on these lower middle class systems. Hope you find a replacement, and then drive it with moderation.

 

[Inductor]

MS20

"stock MS bass/mid driver, the DSB208, with an off-the-shelf Audax TW74 tweeter" by Phil Ward 🙂

Mordaunt Short MS20 eh?

That takes me back. I worked at MS from '80 to '87. The original MS20 was a cost-reduced version of a product called the Carnival 3 and its success probably saved MS from an early grave. It incorporated the very well regarded stock MS bass/mid driver, the DSB208, with an off-the-shelf Audax TW74 tweeter. The TW74 was an interesting design with a self-supporting 12mm voice-coil and precious little power handling. The MS20 was a closed box system with a relatively high LF Q.

A "pro" version of the MS20 was developed (by me) with a different tweeter but thanks to MS having no viable pro distribution never saw the light of day. The MS20 is a good speaker and would make a perfectly satisfactory nearfield in small rooms but play it loud and the tweeters will surely die. Finding a good pair will be tough. The crossover incorporates reversible electrolytic capacitors which will probably have gone leaky by now. The spring connection terminals were pretty low-rent even at the time too.

Later MS20s were designed by a completely different bunch of people following purchase of the company by Goodmans and were all reflex loaded.

Phil
Forum - Main Forums : Recording Techniques

 

[Toaster]

Very nice to see something on these from the designer- thanks Inductor! A purchase or two from the original Sound Organisation near London Bridge stalled, many years ago, when I preferred the MS20 they demoed with a Naim Nait, to their 'next speaker up' the Linn Kan. I expect they thought I was deaf! Perhaps I am, but I thought it was no contest really, even though the Kan's tweeters were a bit more civilised.