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B & O S-75 Crossover Rebuild

The erse caps have a length of 2 1/4 inches and a diameter of 1 1/2 inches. They will crowd things up on the board pretty quickly. The board is mounted on a strip of wood that runs across the back of the speaker box. The board is mounted to this which allows room for the solder connections on the back side of the board.

After removing the board I put the woofer in the speaker so I do not know the distance between the back of the speaker box and the board. However, I was considering mounting the caps and resistors both on the top and underside of the board. Thoughts on this idea? If I need more length on a lead from a cap or resistor is there something I can use? Also the wiring in the cabinet is a small gauge wire. Not sure what it is but wanted to make sure I used the same gauge. And the best way to splice the wires back into the drivers.

Thanks
 
Mounting front and back, and extending the leads is fine. I wouldn't worry about the wire gauge too much; just so long as it's at least as large as the original wires. Reattach the drivers the same as were originally attached, I guess. Do they have those little slide-on lugs? I sold mine so long ago I can't recall.
 
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When you have time, I'd love it if you could post a list of all the cap and resistor values, in case I or anyone else needs them later. If I'm back in Maine I could redo my buddy's pair that I sold him.
 
The resistors were what you posted. There were two that were unreadable; however, through a process of elimination I was able to determine the 3.3 ohm resistor is the one that burnt in half and fried the white block 250v capacitor. This left a 1 ohm resistor remaining. I am 99% sure the below listed resistors are the correct one.

1 ohm x 2
2.2 ohm
3.3 ohm
4.7 ohm x 3
8.2 ohm

The following is a list of the Capacitors

47uF 10% 40v (two of them)

33uF 10% 40v

12uF 10% 40v

10uF 10% 40v

3.3uF 10% 40v

2.2uf 10% 250v (burnt bad from the 3.3ohm resistor burning that was laying on it)

A link to what you would replace them with would be appreciated Zonker. I have been playing around with D cell batteries (similar in size to 47uF and 33 uF film capacitors) on the crossover board. It would take some serious reorganizing. Have not counted it out yet. Hope this info helps you out Zonker. Will document the build once I decide on parts.
 
Awesome; thanks. :thmbsp: I will try to give you a simple shopping list, if I have time in the next day or so.
 
I am currently working on a recap project of my S75's. Nothing burned up, but sounds muffled. Hoping the recap can bring them back to their former glory.

Anyway, I hope some of this info can be useful
The values from the caps are as follows:

From Schematic____From printing on caps

c1 50u_____________47MFD 40v
c2 2u2_____________2,2 10%
c3 10u_____________10/40
c4 37u_____________33MFD 40V
c5 50u_____________47MFD 40V
c6 12u_____________12uF 25v Peak Type ECI BiPolar
c7 3u3_____________3,3/40v


Here is the schematic taken from the B&O Service manual:
photo1.jpg
 
Thanks cjonesgo. That will definitely help out. Have you already sourced parts? I am trying to get the same values in each resistor without having to join two in parallel.
 
All my internals seem to be in working order, so I've only investigated caps.

Martin over at Beoworld sells rebuild kits and has been a helpful resource. I was only inquiring about caps, but I got the impression he could make a more complete kit if desired.
 
Burnt resistors are possibly not operating as originally intended, due to cracks in the enamel allowing atmospheric gasses in, causing corrosion. Just a guess.
 
I just noticed the values on the caps. They list two 50uF caps instead of two 47uF caps. The values I provided are right from the caps on my crossover. The 37uF is also different then the 33uF cap. Also, mine are listed at 40v instead of 25v. Interesting.
 
for the jobs that those caps are doing, that variation in uf is not a huge deal.

you can always get the lower value and another small capacitor to parallel it and do some a/b to see if it makes a difference to you.

the original caps sort of look like oldschool wima axial tantalums. modern electrolytics should be a good replacement, films even better. and the voltage rating really should be 100 or better for longevity.

47 and 33 uf are standard values (since the 50's or whenever they decided to make fewer values). a 10% tolerance 47uf may be as much as 51uf and I find that most of the time, when dealing with electrolytics that aren't engineered for super-compact size, they tend to measure larger than the nominal value. similarly a 33uf 10% cap may be as much as 36uf.
 
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C1, C3 and C5 on the schematic are shunt caps that are not directly in the signal path. They are also larger values. For a speaker that's not high end (no offense, middle of the road speakers outnumber high end ones by a huge margin), you can save a lot of $ by using nonpolar electrolytics for high uf values like 20, 30 and 50. If they are in these positions in the circuit it's not critical to have a film cap. PE has NPE caps that I've used.

What were these guys thinking using 40V caps in a crossover? 100V minimum! Someone around here actually made a case to me once that an 8 ohm speaker rated for a decent wattage - say 100W - can easily see voltages over 100V. Ohms Law would give you the numbers. That's probably why the Daytons that PE sells are all 250V. It may be overkill, but not by as much as you might think.
 
toxcrusadr said:
C1, C3 and C5 on the schematic are shunt caps that are not directly in the signal path. They are also larger values. For a speaker that's not high end (no offense, middle of the road speakers outnumber high end ones by a huge margin), you can save a lot of $ by using nonpolar electrolytics for high uf values like 20, 30 and 50. If they are in these positions in the circuit it's not critical to have a film cap. PE has NPE caps that I've used.

What were these guys thinking using 40V caps in a crossover? 100V minimum! Someone around here actually made a case to me once that an 8 ohm speaker rated for a decent wattage - say 100W - can easily see voltages over 100V. Ohms Law would give you the numbers. That's probably why the Daytons that PE sells are all 250V. It may be overkill, but not by as much as you might think.
Click to expand...

Great post! Yeah, 40 volts is nuts. But it looks like the little 3-watt resistors are what gave out first; some actually caught fire!
 
Thanks for all the input. Your right the 3w resistors are suspect. Replacing them with 10 watt resistors.
 
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Zonker92 said:
Great post! Yeah, 40 volts is nuts. But it looks like the little 3-watt resistors are what gave out first; some actually caught fire!
Click to expand...

Yeah, the first time i put together a crossover from scratch that needed resistors, I checked with a friend of mine who has an A/V & PA systems business to ask if they really needed to be high wattage.

he said yes, and related tales of seeing tweeter resistors glow bright red in his roadie days.
 
The average listener recapping the average speaker will be delighted with the improvement in mids and especially highs. Depending upon the ability of the speakers and your ears, further improvements using more expensive caps may be audible. Another way of saying, use the Daytons and I bet you'll like the results. Go fancy if you want but don't feel like you have to. :thmbsp:
 
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