Mundorf aluminum oil mcap for b&w

[Deepinton]

I've put in a new driver and it reads 3.3 ohms. Here's a picture of the board with one of the caps removed and the 2 power resistors.

 

[Deepinton]

I've put in a new driver and it reads 3.3 ohms. Here's a picture of the board with one of the caps removed and the 2 power resistors.

And here's the other side.

 

[sam_adams]

Well, just to be sure, I checked the resistors and they both show open.

Desolder one of the leads from the board on each of the resistors and measure them again. Properly heatsinked, those resistors are probably rated at 30W and it's doubtful that they are open.

 

[Deepinton]

Desolder one of the leads from the board on each of the resistors and measure them again. Properly heatsinked, those resistors are probably rated at 30W and it's doubtful that they are open.

Desoldered both resistors and they are both open. I guess the dielectric degraded over time until they failed completely and sent a surge through the mid-range circuit, killing the resistors and the driver. Perfect storm. And a $1600 repair bill (not counting my labor).

 

[Extreme_Boky]

Those resistors going open circuit, and an 800V (!) capacitor's dielectric overheating & drying up, means that you have either a mid-driver coil shorting, or (more likely) an amplifier with constant oscillations you can't hear.... especially if you do not constantly listen at obscene power levels for long periods of time.

Check an amp for oscillations, but place the dummy load at the end of the speaker cables you use with your speakers; i.e. do not connect the dummy load to the binding posts.

Edit: I could not really see any obvious damage (bulging/leaking electrolyte, or overheating signs) in the photos you posted.

Wire-wound resistors can go open circuit... The fact that both failed the same way would make me look at the amplifier oscillations, as mentioned above.

Does the mid-driver coil smell like burnt lacquer?

Good luck.

 

[sam_adams]

Desoldered both resistors and they are both open.

Possible replacements for at least the one that the value could be read from your pix:

3.0 Ω, thirty watt, TO-220 style thick film resistors at Mouser.

3.0 Ω, thirty watt, TO-220 style thick film resistors at Digi-Key.

 

[Deepinton]

Possible replacements for at least the one that the value could be read from your pix:

3.0 Ω, thirty watt, TO-220 style thick film resistors at Mouser.

3.0 Ω, thirty watt, TO-220 style thick film resistors at Digi-Key.

Thanks! I got those TO-220 style ones from Mouser. For some reason digikey wanted $49 for shipping!

 

[Deepinton]

Those resistors going open circuit, and an 800V (!) capacitor's dielectric overheating & drying up, means that you have either a mid-driver coil shorting, or (more likely) an amplifier with constant oscillations you can't hear.... especially if you do not constantly listen at obscene power levels for long periods of time.

Check an amp for oscillations, but place the dummy load at the end of the speaker cables you use with your speakers; i.e. do not connect the dummy load to the binding posts.

Edit: I could not really see any obvious damage (bulging/leaking electrolyte, or overheating signs) in the photos you posted.

Wire-wound resistors can go open circuit... The fact that both failed the same way would make me look at the amplifier oscillations, as mentioned above.

Does the mid-driver coil smell like burnt lacquer?

Good luck.

There is a faint dried trace from the top of the cap running down the side. Difficult to image due shiny cap film. I believe these are thin film on ceramic resistors, not wire wound. And I can't smell burnt driver, but then all my sensory strength is probably concentrated in my hearing so I can't smell much anyways.

 

[Deepinton]

Those resistors going open circuit, and an 800V (!) capacitor's dielectric overheating & drying up, means that you have either a mid-driver coil shorting, or (more likely) an amplifier with constant oscillations you can't hear.... especially if you do not constantly listen at obscene power levels for long periods of time.

Check an amp for oscillations, but place the dummy load at the end of the speaker cables you use with your speakers; i.e. do not connect the dummy load to the binding posts.

Edit: I could not really see any obvious damage (bulging/leaking electrolyte, or overheating signs) in the photos you posted.

Wire-wound resistors can go open circuit... The fact that both failed the same way would make me look at the amplifier oscillations, as mentioned above.

Does the mid-driver coil smell like burnt lacquer?

Good luck.

I have to read up amplifier oscillations! So much to learn....

 

[Maxim-ICOM]

Desoldered both resistors and they are both open. I guess the dielectric degraded over time until they failed completely and sent a surge through the mid-range circuit, killing the resistors and the driver. Perfect storm. And a $1600 repair bill (not counting my labor).

All looks quite suspicions. Those resistors are parallel to each other and in tweeter path - you can trace it from the back of PCB, one end is on "HF+"
before you buy any part you need to diagnose it first.
Make a photo of the back of your PCB, mark all the wires and draw there all the elements like this:

 

[Deepinton]

All looks quite suspicions. Those resistors are parallel to each other and in tweeter path - you can trace it from the back of PCB, one end is on "HF+"
before you buy any part you need to diagnose it first.
Make a photo of the back of your PCB, mark all the wires and draw there all the elements like this:

. I'm on it.
Could a hot plug event involving a mc1.25kw amp cause this? If an amplified transient spike was severe enough, it could hit the midrange crossover board with enough force to momentarily exceed the 800V threshold of the 22uF capacitor. The spike punched a physical hole straight through the capacitor's internal dielectric, creating a dead short. The remainder of that massive transient energy then bypassed the shorted capacitor, flooded the parallel 3-ohm resistors (vaporizing them instantly), and hit the midrange driver with enough force to destroy it.
Just sleuthing around.

 

[LTig]

Could a hot plug event involving a mc1.25kw amp cause this?

A power amp able to output 1.25 kW into 8 Ohm cannot output 800V. However ...

If an amplified transient spike was severe enough, it could hit the midrange crossover board with enough force to momentarily exceed the 800V threshold of the 22uF capacitor. The spike punched a physical hole straight through the capacitor's internal dielectric, creating a dead short. The remainder of that massive transient energy then bypassed the shorted capacitor, flooded the parallel 3-ohm resistors (vaporizing them instantly), and hit the midrange driver with enough force to destroy it.

... if this power amp uses an output transformer (like McIntosh for example) and there is a lot of current flowing during a hot plug event a high voltage spike might be possible. In my understanding that would happen on the amp side, not on the speaker side, but maybe a spark got through to the speaker.

 

[Deepinton]

Possible replacements for at least the one that the value could be read from your pix:

3.0 Ω, thirty watt, TO-220 style thick film resistors at Mouser.

3.0 Ω, thirty watt, TO-220 style thick film resistors at Digi-Key.

Those new resistors look puny compared to the originals (which of course you can't source in a reasonable time at a reasonable price). I wonder if the reduced contact surface area will have any heat transfer related issues.

 

[sam_adams]

Those new resistors look puny compared to the originals (which of course you can't source in a reasonable time at a reasonable price). I wonder if the reduced contact surface area will have any heat transfer related issues.

Depending on the age of the speakers, the originals might be a discontinued product from Bourns, Vishay, or some other manufacturer. The part number didn't reveal much in a search on it. Check for isolation from the leads to the case—use an insulator if necessary—and use a good thermal compound for maximum heat transfer when mounting them.

 

[Maxim-ICOM]

I wonder if the reduced contact surface area will have any heat transfer related issues.

In some B&W manuals those blue resistors marked as Powertron 100W. Yours blacks are 30W and in parallel it will be 60W with proper heatsink.

Here how you schematics look like: not sure about driver polarity and coils values are just estimate:

if mid driver is 4Ohm, then power dispassion for R1+R2 parallel (1.5Ohm) resistor look close to 60W, means each should be 30W.
For pure sine - max power 62W at around 600hz - the graph is with connected Satori MW16TX-4 impedance curve and some estimated values for coils.

If resistors really burned, then suppose they worked we like a fuse and capacitors may work fine - double check those 22uF caps, they may be OK

 

[Deepinton]

In some B&W manuals those blue resistors marked as Powertron 100W. Yours blacks are 30W and in parallel it will be 60W with proper heatsink.

Here how you schematics look like: not sure about driver polarity and coils values are just estimate:
View attachment 535886

if mid driver is 4Ohm, then power dispassion for R1+R2 parallel (1.5Ohm) resistor look close to 60W, means each should be 30W.
For pure sine - max power 62W at around 600hz - the graph is with connected Satori MW16TX-4 impedance curve and some estimated values for coils.
If resistors really burned, then suppose they worked we like a fuse and capacitors may work fine - double check those 22uF caps, they may be OK

I really appreciate the deep dive. I believe those are 3 ohm 30 watt resistors. I got the generic version. For now I just replaced the crossover as id rather be listening than sleuthing. But I plan to work on that old board eventually and get to the bottom of it.

 

[sam_adams]

I really appreciate the deep dive. I believe those are 3 ohm 30 watt resistors. I got the generic version. For now I just replaced the crossover as id rather be listening than sleuthing. But I plan to work on that old board eventually and get to the bottom of it.

In the future, if you want to go BIGGER wattage-wise:

100 watt, 3 Ω, TO-247 power resistors at Mouser.

 

[voodooless]

60W for a midrange is already insane.