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Manny11 25th November 2012 10:03 AM

Vintage capacitor - help identify
 
Hello, fellow audio enthusiasts.

I have a pair of 30 years old Wharfedale Laser 90B speakers (pictures: 1, 2). I remember them having a very clear, warm tone. Lately the tweeter's have started to sound grainy, cutting in and out so I want to change the capacitors in the crossover.

The board is simple, three coils and 4 capacitors. Two of them are large enough to have their values clearly written, 450 and 4.7 uF. The smaller two have only this marking:

Quote:

ALCAP

93-8330
Measuring them is not an option since their value is probably completely off.

Last option would be to try dissecting the crossover, but I don't know the tweeter's frequency response... Quite mysterious.

Any ideas are most welcome.

Mooly 25th November 2012 10:31 AM

Maybe post a good picture of the caps !

Manny11 25th November 2012 11:08 AM

I had no batteries for my good camera, but I do have some taken quickly with my phone.

They're out of focus :(, but maybe the shape and color tells something.
You can barely figure out "99-8330".


http://deviantpics.com/images/GxPgX.jpg

http://deviantpics.com/images/SAnXw.jpg

http://deviantpics.com/images/PhDt1.jpg

Mooly 25th November 2012 11:18 AM

They are "bipolar" caps which means they are not polarity concious. Can't help with the values though (the ALCAP 93-8330)

Is there any more writing on the lower half of the cap, the bit we can't see ?

If the tweeter is cutting out completely the caps wouldn't be the first suspects tbh. Other than poor connections and poor joints it could well be a problem with the tweeters themselves.

Elvee 25th November 2012 11:57 AM

They are indeed bipolars. In doubt, replace them with mylar or even PP substitutes, they are not expensive anymore, and they will settle that part of the equation once and for all.

Which doesn't mean there are no other problems, but that's a simple and worthwhile fix for present and foreseeable problems related to capacitors

davidsrsb 25th November 2012 01:49 PM

19mm tweeters have very limited power handling, so I would suspect them first. Try the tweeters direct to the amplifier with a 100 Ohm series resistor to avoid frying them

DF96 25th November 2012 02:43 PM

There should be a law about mounting caps with their value hidden underneath. If necessary, carefully desolder them to have a good look. They will probably be standard bipolar electrolytics, which were common for crossovers in those days especially for the cheaper brands.

tvi 25th November 2012 02:54 PM

1 Attachment(s)
Heres a pic of the "90" crossover
Kolumny Wharfedale Laser 90 - graj?, a impedancja ∞ can't quite make out the values though

These caps are still available at falcon acoustics, maybe the dimensions can help narrow down the value?

rgds
James

sofaspud 25th November 2012 03:19 PM

In that last pic they look very much like 6 MFD 50V NP 10%

Manny11 26th November 2012 01:33 PM

OK, so, we looked carefully underneath the mystery capacitor, there's definitely no other markings. Also, one is turned in a way so you can see the ALCAP 93-8330 repeating.

A quick check on the solder side of the board, the joints were nice and shiny so I suppose a bad connection is not the case. I'm a bit scared connecting the tweeter directly to the amplifier, maybe if I only played a sinewave over a few kHz to see if it breaks up then... But I rather wouldn't, this is only about a 10 euro investment, if we manage to figure the value out. :)

Nice find on the polish discussion and crossover picture, tvi. I did a little detective work and tried to match the capacitors in size with my blurry picture.
Animated
I first overlapped the biggest cap so that "ALCAP" writing would be the same size, and then rearranged the others. Looks promising. Still, because of the bad quality, I can't tell exactly the value, could be 4, 6, 8 uF... What makes you think 6, sofaspud?

The other thing is, the middle-sized capacitor in the Laser 90 crossover is a lot smaller than the one in 90B's, even though the specs for Laser 90 and 90B are the same, except the 90's are 3 liters bigger in volume.
Maybe it has something to do with 90B's middle capacitor being 10 low loss. Don't know if that makes them bigger.


I'm going to try contacting Wharfedale directly (too optimistic? :D), also, maybe Falcon Acoustics knows something about those weird old markings (could it mean they were custom made?) and open the speaker again to get exact measurements of width and length of the little bugger. ;)


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