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Please help identify Mordaunt Short crossover component

B

bernado

New Member
I've picked up a pair of MS100 speakers (I don't know their history), one of which is extremely quiet and sounds generally very poor.

The drivers measure OK on a meter so hoping the crossover may be at fault I've put a meter on all the components and found what I think is the faulty item as it measures very differently compared to the one in the working crossover.
I thought it was a 45000pf disc capacitor from how it looks it but it measures more like a resistor with no meter swing and with a value of 300 plus ohms, compared to the few ohms of its equivalent from the working speaker's crossover.

I've since become aware of Mordaunt Short's use of Positec driver protection in some older speakers, is that what this is?
If so it makes sense as this component is before the positive leg of the woofer in the circuit which is where I'd expect a fuse type device.

This is it -

7rdpEkq.jpg



And this is the crossover minus the blue component -

eGXCPiu.jpg



Can suitable replacements still be sourced, of whatever it is?

Many thanks
 
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Poly switch? Its probably installed to protect the tweeter. It will go open if driven too hard. Once it cools down it will close. Kind of like a circuit breaker. Bigger ones are installed on heat sinks as thermal protection for an amplifier section. I replaced the poly switches in a pair of speakers I own with resistors. Over time the poly switches that were used degraded and had a 3.4 and 3.7 ohm reading where it should have been .5 ohms.
 
Lavane said:
Poly switch? Its probably installed to protect the tweeter. It will go open if driven too hard. Once it cools down it will close. Kind of like a circuit breaker. Bigger ones are installed on heat sinks as thermal protection for an amplifier section. I replaced the poly switches in a pair of speakers I own with resistors. Over time the poly switches that were used degraded and had a 3.4 and 3.7 ohm reading where it should have been .5 ohms.
Click to expand...

Yes it'll be this by the looks of it then.

There are two on each board, this one goes to directly to the woofer and the other smaller one heads to the tweeter, though with a resistor, capacitor, resistor sequence in between.
The bad one reads 300+ ohms, the other 3 all measure 2 ohms.

I'll leave the smaller tweeter protection ones in place as they seem consistant but replace the woofer based ones with 2 ohm resistors, what type of resistor is suitable for this would you say?

Thanks
 
I use precision wirewound types like Ohmite, etc. You should be able to find them at PartsExpress. They have tighter tolerances than the standard cement bricks.
 
Lavane said:
I use precision wirewound types like Ohmite, etc. You should be able to find them at PartsExpress. They have tighter tolerances than the standard cement bricks.
Click to expand...

Great, thanks.
One final thing if I may, what power rating would be appropriate do you think?
 
Since three of them seem to read the same I would think they are still in spec and you may want leave them. They do protect the drivers. I replaced mine do to the type used in my speakers are known for drifting out of spec or failing. I don't push them to the limit, so I used resistors. It would be better to get the replacement for the one. If you do replace them you will have to be more careful with the volume.
For the tweeter, 5 watt should be sufficient. A ten watt can be used also if a a 5 watt isn't available. These are 25 watt speakers aren't they? And did you test them out of circuit? As for the one reading 300 ohms, I'm not sure what wattage would work there. These are only 25 watt speakers from what I've read. I haven't run across them connected to woofers or mid bass drivers before. It would be nice.to have a schematic to see just how those are wired in circuit. I would first try to contact the manufacturer and ask if they can supply a replacement, or give you a brand and part number for it. If the switch goes open at a sustained wattage around 25 watts, or more, that's going to be a physically large resistor.
 
Thank you @Lavane for your detailed comments. Yes I did measure out of socket, I'll see if I can get anything from Mordaunt Short.

For what it's worth you can pretty much see the circuit layout in the below, the blue line denoting the removed switch.

zAQbtF0.jpg
 
It looks like the woofer is running full range, naturally rolling off. Ive just never seen protection like that on a woofer, other than fuses. You might try using a jumper in place of the thermal device on the speaker that's quiet to see if it gets louder again just as a test.
 
I had to Google "polyswitch" because I wasn't familiar with them, but curious. From what I've read, it's possible for a polyswitch to trip and take a very long time to return to it's original value. Or in some cases never return to its original value. Which seems might be the case here. Any resistance in series with the woofer is a bad idea. And 300ohms is an awfully high value for a speaker crossover.
 
That one is most likely bad. 300 ohm is way too high I think since it's directly between the speaker terminal and the wire going to the woofer. Im wondering if you would need anything at all there. Since the other one reads two ohms like the others for the tweeter, and these are supposed to be 8 ohm speakers, it might drop them from 8 ohm down to 6 ohm, maybe if removed for both woofers? :dunno:
 
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