Monitor 5b issue- known good tweeter is silent. Might be crossover?

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Hi folks, I picked up a pair of M5Bs from a thrift store the other day as a refurb project. Thus far, one works great, while the other one's tweeter doesn't seem to work. I've taken the tweeter and plugged it into the other enclosure and the tweeter runs just fine. I've replaced the wires going directly to the tweeter to no avail. All of this suggests an issue with the crossover of some kind, but my knowledge of these devices ends there. At a glance, the capacitors look ok, and sound does come out of the other active speaker, so this is something specific to the signal path going only to the tweeter.

Does anyone have any troubleshooting suggestions for what to try?

Here's some pictures:

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One other thing- this speaker is a model with a fuse, but the fuse appears to be intact.

dkfreebird

Try cleaning the contacts where the fuse goes in the holder or simply swap fuses to check if it is the fuse if you hadn't already tried that.

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dkfreebird wrote: »

Try cleaning the contacts where the fuse goes in the holder or simply swap fuses to check if it is the fuse if you hadn't already tried that.

For some reason it never occurred to me to do more than look at the fuses to make sure they weren't blown. Sure enough, pulling the fuse on the "good" speaker killed the tweeter. I literally just poked the fuse on the bad speaker a few times and BOOM, the tweeter kicked in. I hit it with some contact cleaner and it's all better. Thank you!

dkfreebird

A free fix is a good fix. Welcome to Club Polk!

pitdogg2

Well now you know what the crossover looks like... lol
Welcome to club Polk! Great folks here with a wicked sense of humor

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Question for the folks that are more knowledgeable than me: I hear from a lot of people swapping out the old capacitors for something new. For now, I'm satisfied with what I've got, and I don't want any more malfunctions. But, out of curiosity, how do you actually access those solder points? The back panel is attached to the crossover via some plastic pieces that I can't really envision removing without damaging.

pitdogg2

It's really easy. Those have a flexible part you just squeeze together with needle nose pliers. Squeeze and pull up one at a time. Desolder the big inductor wire and the the whole board is free.

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Sweet, sounds easy enough. I will have to do some research on what capacitors to swap with and so on, as well as if that inductor should be replaced. Thanks!

pitdogg2

I'd just go with new caps and resistors. Inductors do not wear out can't see much gain replacing

delkal

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Question for the folks that are more knowledgeable than me: I hear from a lot of people swapping out the old capacitors for something new. For now, I'm satisfied with what I've got, and I don't want any more malfunctions. But, out of curiosity, how do you actually access those solder points? The back panel is attached to the crossover via some plastic pieces that I can't really envision removing without damaging.

That is a simple crossover with plenty of room to get things to fit. I would be very tempted to just take the crossover out, have someone hold it, cut the old caps off leaving about 1/4 inch of the leads, then solder on new caps. Should take you longer to take the crossover out.

I know this will not be approved by many here but its fast and should make your speaker sound better (even with cheaper caps).

pitdogg2

Right cold solder joints would be so much better. You are correct it is simple so why not do it correctly.

delkal

pitdogg2 wrote: »

Right cold solder joints would be so much better. You are correct it is simple so why not do it correctly.

Not sure what a cold solder joint is. Do you think soldering the new cap to the old leads will effect the sound?

Yes, it is always better to do things "correctly"..........but with this crossover you could be finished in a couple minutes without having to know anything about schematics and no risk of hooking things up wrong.

pitdogg2

delkal wrote: »

pitdogg2 wrote: »

Right cold solder joints would be so much better. You are correct it is simple so why not do it correctly.

Not sure what a cold solder joint is. Do you think soldering the new cap to the old leads will effect the sound?

Then you should not be offering advice.

delkal

pitdogg2 wrote: »

delkal wrote: »

pitdogg2 wrote: »

Right cold solder joints would be so much better. You are correct it is simple so why not do it correctly.

Not sure what a cold solder joint is. Do you think soldering the new cap to the old leads will effect the sound?

Then you should not be offering advice.

I have been playing with electronics for years. I know what a good solder connection is too.

I guess I always thought cold solder joint was a bad thing to do. Why are you saying you want one? Did they change the terminology?

F1nut

I'll be blunt, only a hack would snip the leads then solder new caps to those.

pitdogg2

Wrong I'm saying that is a good way to get one. That big copper pad the original lead is soldered to will act as a large heat sink which could cause a cold solder joint. So now this OP will be back asking what is wrong now...

delkal

pitdogg2 wrote: »

Wrong I'm saying that is a good way to get one. That big copper pad the original lead is soldered to will act as a large heat sink which could cause a cold solder joint. So now this OP will be back asking what is wrong now...

You are correct and it could be a risk if you are not careful. I misunderstood your first post. Thanks

F1nut wrote: »

I'll be blunt, only a hack would snip the leads then solder new caps to those.

Hack or thinking outside the box?...........Nevermind, I guess I am a hack!

F1nut

I didn't call you hack, I said only a hack would do that. Thinking outside the box....Ummmm, no.

nspindel

The Monitor 5 crossover can get very tight when you start installing modern film caps. A good solution is a riser board to provide the extra space. Here's a thread documenting my Monitor 5 rehab, I hope it can assist you.

http://forum.polkaudio.com/discussion/167490/my-newly-refurbished-monitor-5s

pkquat

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Sweet, sounds easy enough. I will have to do some research on what capacitors to swap with and so on, as well as if that inductor should be replaced. Thanks!

Back to your original question. The preferred favorites here are Sonicap and Mills resistors. Next is Clarity ESA caps. Mundorf resistors are fine too. For some extra savings Solen, or lower grade Clarity caps have been used on the low pass filter (the larger values). Daytons are an improvement over stock, but not many people are a fan of them. It depends on your budget.

If you can solder and have a desoldering tool, removing the old caps and such is easy. I cut the caps and resistors off, and then desoldered the nubs. This made removing some of them easier since some wires were twisted together. I left the inductors soldered to the board unless they shared a hole with another component.

With new larger caps, height is your friend. Do not let them over hang the boards too much. This is where people run into trouble trying to fit them back in the round hole. A hot glue gun is great to secure things, as wells as zip ties if needed.

nspindel

Another highly informative Monitor 5 thread:

http://forum.polkaudio.com/discussion/166855/is-there-a-schematic-for-the-monitor-5a/p1

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I appreciate all the advice on cap swaps folks. I put in some solens the other day, as I simply want the thing to run, I'm not an uber audiophile by any means.

Unfortunately, I'm afraid that the dreaded tweeter silence has returned! I'm fairly use that this is unrelated to the capacitor change, given the crossover's history and the fact that the issue appears to have reemerged prior to the swap. Like last time, the woofer is still putting out sound, only the tweeter is dead. Earlier, simply fiddling with the fuse was enough to get it going again. Now, not so much. Does anyone have any suggestions for what to try? Here's what I've done:

1) All caps and resistors are replaced, solder points look good

2) Wires going from the fuse connections were resoldered to the board

3) All contacts were hit with deoxidizer

4) Tweeter is known good, swapped with the other speaker's tweeter to verify.

I really don't want to fiddle with the actual fuse apparatus unless I need to, as there is a crap load of glue that I'd need to work through before I can reach the bolts. That also makes me wonder if that is the issue, since it should be sealed in the goop.

dkfreebird

You may need clean the holders with some fine steel wool or fine sand paper lightly and then deoxit.They could be slightly pitted or rusted where the fuse makes contact. If you have a volt meter put the fuse in and check continuity between the holders,that will tell you if the problem is at the holder or internally.

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I'll give the contacts a bit of a sanding, I think I might have found a potential culprit. The fuse itself doesn't appear to pass the continuity test (though that doesn't stop the other speaker from working just fine with it). Might be worth a shot to replace it, do you know what kind of fuses these things require? It looks like a 1a 250v slow blow?

Continuity is detected when no fuse is present ( just touching the contacts), as well as the fuse is in place.

dkfreebird

Yes on the 1 amp fuse.Are you checking from holder to holder for continuity?

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I am touching the two sides of the holder when I check for continuity. I've convinced myself that it is unlikely to be the fuse itself. The fuse works in the other speaker, and the old tin foil fuse bypass trick didn't seem to do anything.

dkfreebird

You shouldn't get continuity without the fuse in place.

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Hmm. So that implies a short somewhere around the fuse. I checked the entire board for continuity, as well as the components and it appears that there's a signal running the red coiled wire all the way down to the bottom.

I'm not sure how this kind of short could have happened, though, because the whole thing is covered in glue. Here's a picture:

dkfreebird

Are you checking the holder like this?

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I am putting the black side of the multimeter on the left end, and the right side on the right end. To be honest, I'm not sure how I'd test the holder like that without essentially touching the probes together.