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Part of the tweeter mystery solved!

109.156.17.187

Davy

In Reply to: RE: Tweeter works with no connection in the attenuator socket posted by neolith on August 25, 2010 at 08:23:33

Hi Neo,
Well looked behind the panel, - you were dead right, - found a big 1 Ohm 10W wirewound resistor wired in parallel with the terminals of the tweeter attenuation socket! So that partly exaplains why these speakers have a duller treble than I remember a 3 series having. As for the tweeter imbalance, - am not sure yet if its due to one actual tweeter being bad, - not tested them in isolation yet with my NAD power amp, but I suppose its possible there is a bad solder joint there or one of the resistors has a higher resistance than the other (guess this is not that likely though, - they have 3% tolereance). In any case, I am no expert but I think someones gone over a lot of the solder joints behind the panel, - there are brown/orangely marks on the solder, - I think this is signs of the joints being overheated. So I will have to go over them, - do you think I should remove all the old solder and reapply new, or just heat up the current joints and put in a bit of fresh solder?
The fuse sockets (both of them) are not bypassed.
The question is, why did someone put these resistors in? Was it to attenuate the tweeters from a personal preference point of view or it is becuase these speakers are fitted with the earlier 2.3 ohm ribbon which require a 1 ohm resistor in series (the crossover does not have it on this newer speaker obviously)? I forgot to check but it seems my easiest why to check is to measure the gap on the back of the tweeter (according to what Andy said), - the old tweeter is about 1/2 and inch and the newer on has a gap of about 3/8 inch. Also the older is 60.6 inches long and the newer type is 60 inches but thats obviously hard to measure! But I think measuring (if I can tell) is easier (but perhpas less conclusive) to finding out which tweeter they have than measuring the DC resistance of them.

So some progress made, - will report how I get on!
Cheers,
Colin

So I am hoping removiing the resistors and

Follow Ups:

On the road to recovery - neolith 06:58:32 08/26/10 (2)
The brownish stuff on the solder joints is flux. With proper soldering the flux evaporates and the joint looks shiny and smooth. Cold joints often are dull and appear speckled or rough but not always.
If you pry off the Magnepan labels at the ends of the ribbons (you can re-attach later with silicon glue), you can access the ribbon connectors. Just detach one end (make sure you are removing the wire from the connector and not the aluminum ribbon) and then you can measure the resistance across the ribbons without having to remove them.
�

I plan to procrastinate my demise for as long as possible. In the meantime, I practice by putting off all the little stuff.
- Making further progress! - Davy 17:09:04 08/26/10 (0)
  Hi Neo,
  Well I removed those nasty resistors and went over some solder joints. The joints did look browner on the speaker with the duller tweeter. Things do seem better now, - perhaps the tweeters are not 100% balanced but there are closer and better than befor, - I might be able to fine tune with a 0.5 ohm resistor in the louder one or something. Its certainly not worth buying a new tweeter from Magnepan, - it will set me back about $700 with the shipping at least, - its not worth the money.
  I didn't measure the resistance of the tweeters yet yet but the tweeter gap at the back is about 4.5 mm so I am pretty sure they are the later 3 ohms tweeters with the smaller gap. I remember the tweeters on my Dads MGIIIs looking wider in that gap than these do.
  
  Am sorting out my crossover and things are progressing nicely. The main hurdle to come is sorting out the banana peel on the bass panel of these speakers!, - not looking forward to that. Hopefully the DAP contact cement I ordered from the USA will arrive so so I can get on with this pain of a job!
  
  All the best,
  Colin
- RE: On the road to recovery - Davy 08:04:41 08/26/10 (0)
  Thanks a lot Neo,
  Yes these solder joints in the panel all look dodgy I think, - brown stuff eveywhere. I will do a photo tonight and of the resistor someone put in there before trying to fix it. Do you think I should remove all the old solder first or just heat up and add some new?
  
  thanks for letting me know how to get to the ribbon connectors, - that great will do that too and measure the reisstance. After I've resoldered the stuff in the panel, removed the resistors, and measured the ribbons resistances I'll see if the tweeters still sound unbalanced. If they do I'll hook they directly up to my NAD 208 power amp thus bypassing the internal crossover, - so I can hear one of the ribbons is at fault or perhpas something in the crossover.
  
  Thanks again,
  Colin

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