B&W P4 - no sound from mid driver but driver itself tested OK

[mcbrucer]

Hello there

I'm in need of some advice to sort out a speaker problem.

I have a pair of old B&W P4 floorstanders and just recently I noticed that only the tweeter was firing on one of the pair, the mid driver was silent.

So, I pulled out the mid driver and tried it on the other speaker - works ok so the driver itself appears to be fine. Next I tried the driver from the good speaker on the bad one, no sound. So at this point I'm assuming the crossover is the root cause.

I pulled out the crossover - at least as far as I could and notice there are 2 caps on there and not much else.

https://photos.app.goo.gl/pBYbWCVWY58ta7Ds8

So, I'm assuming that one or both of these caps needs replacing but I'm totally new to this kind of thing so I've no idea how to properly identity these caps or source replacements. Alternatively, I could keep an eye on ebay for a entire crossover - though I assume the failure is age related and so any of these units will be liable to fail at any point given the age of them (25 years-ish).

Any help / guidance would be very much appreciated.

Thanks!

 

[Mooly]

If you are trying to attach images then it's done like this 🙂

How to attach images to your posts.

 

[Galu]

Below, is a B&W P4 crossover. Is it the same as yours?

The capacitors are marked 10uF and 7uF, both rated at 70V.

I would replace with new bipolar electrolytic capacitors of standard values 10uF and 6.8uF like the ones included here: Mundorf Electrolytic ECap Capacitors

 

[Galu]

Of course, there could simply be a bad solder joint on the circuit board.

Do you have a multimeter with which to check the continuity of all the connections between the components on the board and of the wiring going to the midrange speaker?

 

[mcbrucer]

Yes, it looks just like that. Hopefully I've attached the image properly this time.

Thanks Galu! Your help is very much appreciated.
I think I'll order 2 sets of both caps now and just replace them all while I'm testing for continuity. Once I've figured out how to remove the crossover board from it's housing - one of the nuts holding it together is obscured by one of the coils...

Cheers!

 

[giralfino]

I posted it here before, but it seems nobody search the simple things. B&W has online the service manuals for almost if not all speakers:
B&W Group North America Service & Support - Service Manuals

Looking for a P4 crossover diagram, it doesn't exist, but there is a design of the PCB with parts and wires. From that it is pretty evident that the sole component for the LP filter for the bass is the biggest coil, while all other parts are for the tweeter HP filter. So replacing caps has nothing to do with the original problem, but it can be a sensible thing to do for an old speaker. Galu gave the correct suggestion on what to choose.

Going back to the problem, the only component in series with the bass driver is the bigger coil, and there are only 6 points on the PCB that potentially are without electrical continuity: the nut that provides + IN, the two wires from the coil, the wire that go to bass +, the nut that provides the - IN, and the wire that go to bass -. The first place to look is for a bad contact between the coil and the PCB, because the coil is heavy and can disconnect from the PCB. In that case simply resolder the bad joint.
Another point to look is the bi-wire terminal. If you are not bi-wiring, check if the input parts are fastened correctly.

Ralf

 

[Galu]

I posted it here before...

I probably stole the image of the crossover from your post, Ralf! 🙂

Your excellent troubleshooting information will help mcbrucer find the break in the circuit to the mid/bass driver (which he incorrectly referred to as a "mid driver").

For future reference, I've also stolen this description of the crossover, this time from Steve (System7):

It's a 1.75mH bass coil, with third order tweeter filter, 1.5R and 7uF series, 0.15mH shunt and 10uF series.

 

[Galu]

Ralf, I think your final suggestion is usefully the first place to look, and is worthy of emphasis.

If the terminal links (which can be removed to allow bi-wiring) are fitted to the input terminal panel, the first thing to do is make sure they are tightly fitted.