Coil rub, blown fuses on Sherwood/HK setup

[PE Williams]

Hi all, been searching threads the past couple of days to try to solve my issues, but figured I'd just post my situation as nothing I've found quite matches my setup.

Receiver: Sherwood S-7225—has both a single power fuse and two speaker fuses, one for L (front and back) and one for R (f+b)
Speakers: Harman Kardon HK-40s (10" subs) Just purchased these on Craigslist last week

Issue 1: Coil rub. Annoying distortion from one of subs on the new-to-me HK-40s. Have basically narrowed this issue down to voice coil rub of some kind. The cone "whooshes" and is definitely rubbing when gently pressed. The sub on the other speaker is silent when this is done. Have tried rotating 180° etc. to no avail.

Issue 2: Blown speaker fuses. When trying to diagnose the coil rub issue, I had the sub out of the enclosure with everything powered on, playing a 30Hz tone. All of a sudden, sound cuts out. Check the back, L channel fuse blown. R channel fuse fine. Next day, I pick it up again and switch the speakers to the R channel. That fuse blows.

In my mind, there are two possible causes of this: either the voice coil is so bad that it is shorting, or it was the cables I was using to connect the speakers. The cables to hook 'em up were once a single RCA male-to-male that was cut in half, with the sheathed silver wire going to ground and the copper going to +. (This is bad, I know. I have since ordered 18 gauge bare wire to gold plated RCAs that are now hooked up. Waiting on new fuses to arrive.)

Really open to any thoughts on the situation. Could the coil rub cause the shorting? Could it be just as simple as the cables? Has anyone got a DIY solution for coil rub on HK-40s? The actual sub enclosure is a web of metal, and there appears to be an metal case around the magnet that is glued or welded to the enclosure frame, so it's difficult to get in and do anything yourself.

Anyways, thanks in advance.

 

[roger2]

Yes, coil rub could cause shorting. There is thin wire wrapped around the coil. That wire has a very thin insulation that I believe is lacquer, or some type of clear paint-like substance. Rubbing can remove that insulation. Bare wires can short. Don't use those subs again.

Your Sherwood is a gem worthy of care. Hopefully the fuses prevented damage. When you replace fuses use fast blow types with original amp rating. The correct amp rating should be printed on the back panel of the Sherwood next to the fuse holders. If it is not, post here and I or someone else can look it up for you. It is important to get the fuse rating correct and never go higher.

Welcome to AK

 

[century tek]

Welcome! Not trying to be mean but I would just like to know what your basic level of expertise is because you don't want to tinker around with it and then end up with a door stop!

If you are a novice (I didn't say beginner), take a look at the blown fuse. Is it blackened? If so, then another one is not going to fix it so don't even bother trying. The blackened fuse indicates excess current draw so that means that there is a short somewhere. Usually those of use who know our way around electronics and these things, will most of the time automatically go to the output transistors in that channel with the blown fuse.

Do you want to do it yourself? It's not for the faint! Can we see what you're working on?

Could have been an accidental slip or a bad speaker but either way, Been there, done that!

Tools: Basic meter and dim bulb tester should be able to tell what's going on there.

 

[PE Williams]

Thanks folks. I am a novice with audio equipment and electricity but my degree is in computer science so closed systems are nothing out of the ordinary. Just learning all the rules.

The fuses are not blackened or otherwise discolored, but the wire/coil inside them is split. I think my receiver is in good shape. Would a short in the sub (e.g. bad coil rub) cause a blackened fuse?

I ordered a basic multimeter and plan on testing the subs as soon as it arrives. Sounds like the good sub should read 4 ohms (others have said 8, but really I'm just looking for a difference between the good one and the bad one).

If I can cut the dust cover off and simply put a shim in to re-adjust the voice coil placement, then close the dust cover back up, I would do that myself. Anything more and I'll probably take it to a shop. I'll post pictures tomorrow.

 

[CT Jim]

The shimming of the vc is done when rebuilding the woofer. You can't just shim it and it's fixed.
The hk 40 is a 10" 2-way spks system. Looks like original woofs are rubber surrounds. Are yours?
Are the woofs original and the same?
Perhaps the seller already fried a woof and installed a substitute which has now failed.
Need more info

 

[century tek]

If the fuse is just open, then the receiver is most likely okay. Let's hope so!

 

[roger2]

........

If I can cut the dust cover off and simply put a shim in to re-adjust the voice coil placement, then close the dust cover back up, I would do that myself....

Don't do that. The insulation that should be on the voice coil windings is very likely gone. They were rubbing and you ran a 30Hz tone through them until your amp blew a fuse...twice. Consider it a lesson learned and move on to different speakers. The only way to fix them would be a re-cone including voice coil. They are probably not with the expense and effort to repair.

Protect your Sherwood, assuming it is still healthy. Those don't fall into your lap very often and they are highly regarded by many here on AK. The next time you check out new speakers, check for voice coil rubbing, and also put a DMM on them and measure DC resistance. The measured resistance will not be exactly 4 ohms or 8 ohms. But if it measures way below its nominal ohm rating, do not buy the speaker, or do not connect it to any amp. And if it is rubbing....run away from it.

Don't feel bad. I did it once myself. Brought home "new" speakers that I got for $10. Didn't put the DMM on them. Blew the outputs in an amp that I really liked. THEN I measured the speakers and they were .3 ohms

I eventually fixed the amp, but it took quite a while to get to it and it was a lot of unnecessary effort. I leaned my lesson the hard way. Many of of us in here have. You can learn from us.

 

[bowtie427ss]

Annoying distortion from one of subs

Pretty sure there's no submarines in an HK40.

I had the sub out of the enclosure with everything powered on, playing a 30Hz tone. All of a sudden, sound cuts out.

Assuming you're talking about the woofer, operating it outside of an enclosure that provides the proper acoustic load is inviting destructive over-excursion. Driving it with VLF tones out of the enclosure is multiplying that risk factor.

 

[Dazaa]

The woofer (not sub) that is making noise is knackered and needs to be either reconed or replaced. If the fuses aren’t blackened chances are the amp is fine.

 

[PE Williams]

Thanks all. Attached are some photos. (EDIT: For reference, top two photos are the bad woofer, bottom photo is the good speaker) To answer questions:

Yes, I believe the woofers (thanks for correction...I often make up my own shorthand—as a kid I used to say "Just mike it" for microwaving food, as opposed to the more common "nuke it") are in fact the originals. Not sure if they have been repaired. They are identical, and both have wire coming in through the underside of the cone, down into the dust cover. Is this standard? Wouldn't the added weight of the glue and wire likely lead to coil misalignment? Is this a shitty repair job?

I would love to keep using the one working HK-40 speaker, and maybe just get a new similar 10" woofer in the other one. Sets of the speakers go for $150 and up online. If you think they suck I would love to hear why.

New fuses arriving today, apparently. Will not plug in bad speaker obviously but will test the Sherwood.

Update—Just got new speaker fuses. Sherwood is working great, as is the one good speaker.
The remaining issue, the bad woofer, will have to wait until multimeter arrives to solve. It is either very bad rub causing the short, or, if resistance reading is normal, then it was the wiring and the coil rub issue is not as bad as I suspect. Is this a correct analysis?

 

[PE Williams]

Ohm reading on the bad subwoofer is 2.7. Total junk? Worth a repair?

 

[roger2]

Something...was shorted out to cause two amp fuses to blow. It could be the woofer itself, or it could be somewhere in the wiring.

The woofer may, or may not, be the culprit. If it is a 4 ohm rated woofer, then 2.7 ohms DCR does not sound unreasonable, at least to me. But I am NOT the foremost authority on speaker anomalies. It could be that insulation is rubbed off of only the outside of the voice coil wires, so that the coils wires are not shorting to each other. But could be shorting against the inside of the voice coil.

Good pics...keep plugging at this until you know for sure what happened. The Sherwood is worth protecting

 

[PE Williams]

Thanks. Might just take em to a shop to check out the voice coil as it's not just the fuses but the sound is also rough/unpleasant. Appreciate all of your help! I'll let you know when I get it resolved.

 

[toddalin]

You may try to use the ohmmeter while pushing the cone in/out to see if the reading goes to a short due to rubbing.