Akai SW-156 speaker repair advice needed

[Bert Beukema]

Hi guys,

I was gifted these speakers, they are in pristine condition and only need a slight cabinet overhaul. Some new cloth, a bit of veneer repair and removing wood staining and they will be beautiful.

There's just one issue: one of these speakers is not working. These speakers have four 8 ohm drivers: 1) a huge bass driver, 2) a driver for the mids, 3) a tweeter and 4) a super tweeter. The sound, unrestored, from the working speaker is really nice (attached to an Akai 930). On the speaker that is not working it seems that only the mid driver is working; all other drivers don't produce any sound. Doesn't matter what type of wire I try or what amplifier; problem remains.

Now i'll gladly admit to being a complete novice to speaker repairs. I've obviously done some restoration work on newer speakers but the circuit on these is very rudimentary.

Before I do anything i'd like to have some advice on how to approach the speaker. What is common sense? How can I determine whether the drivers are dead? In the case of the big bass driver, it is based on a heavy rubber band (no foam), perhaps the rubber has become too hard?

Any advice is welcome, it would be a shame if these speakers don't get a chance to shine again!

Regards,

Bert

 

[Markoneswift]

Easy - test all your drivers first for continuity. If they are good, drench the level adjustment pots with deoxit and work them a zillion times. After that, consider replacing replacing the electolytic capacitors which are likely dried out by now.

Good luck, let's see some pics

 

[Bert Beukema]

Okay check double check. It seems odd that someone would be able to destroy three out of four drivers while the other speaker is still intact. The receiver it was connected to shows no signs of deterioration at the outputs (15Mv, some small drift) and other than it being in dire need of service and cleaning is as good as they get. I'll post some pics; they are nice looking vintage speakers. Can't compare their sound to my main speakers but for that time period they sound much better than i'm accustomed to. Kabuki these are not.

Regards,

Bert

 

[Markoneswift]

I just acquired these HPM-60s for free. The level adjusters needed a major clean to restore full sound.....

 

[NAD80]

Wow HPM-60's for free ! NICE!! Hope all the drivers work.

 

[slimecity]

I just acquired these HPM-60s for free. The level adjusters needed a major clean to restore full sound.....

Wow - Im obviously living in the wrong city.

I NEVER find anything decent in bloody Wellington!

 

[Markoneswift]

Wow - Im obviously living in the wrong city.

I NEVER find anything decent in bloody Wellington!

I won't tell you about the free Pioneer SA-7800 and Technics SL-5210 that came free with them then - both in their original boxes...

Sorry for the thread hijack OP.

 

[Markoneswift]

Wow HPM-60's for free ! NICE!! Hope all the drivers work.

Yes working perfectly after a deoxit bath of the pots

 

[Bert Beukema]

There are always those lucky bastards I guess ;-). Drivers all test fine and their ohms are also rated as they should; I did not bother to unsolder the drivers. As I recall, continuity alone doesn't necessarily mean that the drivers are good. Just that the primary windings are still intact. But I'm unsure about how to test other parts of the drivers?

There's nothing much of a crossover; just three caps and two coils. I remember some article, it might be i'm misremembering, that spoke about re-activating drivers by throwing a series of waveforms through the circuit. Anyone know about this? Perhaps the drivers are simply stuck, not sure if I am talking boo-boo here.

Some white crud is visible on the bass driver and on the bottom of the speaker floor; it also has a bit of corrosion. Otherwise all drivers look brand new. I've attached some images for posterity.

Regards,

Bert

 

[Bert Beukema]

Oh, the crossover circuit pots are not really dirty (open and clean them anyway). They also don't connect with the bass driver, so if that driver is good is should produce a signal independent of the two pots (for mids and highs). Could it be that the rubber has become fixed? The bass drivers have large rubber surrounds that don't feel exactly pliable anymore, really stiff. That said; on the unit that does work the rubber has the same feel.

Bert

 

[Markoneswift]

Does the woofer cone travel freely in and out when pressed gently ? If it doesn't then the voice coil is cooked.

 

[Markoneswift]

When you say the pots are not dirty, you can actually see the tracks can you ? Oxidisation won't be readily visible.

 

[Bert Beukema]

Does the woofer cone travel freely in and out when pressed gently ? If it doesn't then the voice coil is cooked.

Nope. It feels like the cone has somehow hardened; it feels completely stiff and it's very hard to do anything with it. On the good speaker, the woofer cone can very easily be pressed and is very flexible. In the bad one it almost feels like it is 'glued' or 'fixed' somehow, as if the cone is glued down or something. In my inexperience I put it down to the rubber perhaps having solidified or something.

 

[Bert Beukema]

When you say the pots are not dirty, you can actually see the tracks can you ? Oxidisation won't be readily visible.

Correct. I've cleaned many pots and these are no different. Some crud in there but i've cleaned them in the ultrasonic and then added a minute amount of D5 for a little bit of lube. I've seen pots that were essentially racing tracks, these seem to be hardly used (which is logical seeing that they are at the back of a speaker).

 

[Bert Beukema]

I've removed the bass driver from the circuit; it still tests fine on continuity and the ohms reading is also just fine and within tolerances. I've taken two extra photo's of the backside (magnet) side; the corrosion here worries me though I don't know exactly why.

The way the woofer cone feels is as if it is somehow 'glued' on the bottom (the center). I think the rubber itself is flexible enough; it does not feel significantly different than the other bass driver. I've compared both drivers out of the cabinet and circuit; they both test the same. I've applied a 9v battery to the broken bass driver; it appears it wants to do 'something' but can't move the cone; only the slightest of movements. On the correct woofer the 9v (very short burst) gives a very different result; you can visibly see the cone expand and relax.

So what the hell would 'glue' a cone?

Bert

 

[savatage1973]

So what the hell would 'glue' a cone?

Most voice coils are coated with a varnish or epoxy resin that can melt under high heat (speaker being over-driven) and melt. Once it cools down, it becomes solid again. It basically acts like "hot glue" out of a glue gun. The VC has literally glued itself in place in the gap.

EDIT/ADDITION--this would also explain the dead tweeter and super-tweeter. I'll bet that if you take them out of circuit, they are probably damaged as well. It takes a fair amount of power to over-drive/overheat a woofer, and tweeters are far more sensitive to this kind of abuse.

 

[Bert Beukema]

Hmm, okay colour me confused. I was under the impression that, if the continuity and ohm readings were good that the driver was salvageable. I agree that it is far easier to damage the tweeters, but the mid driver is still fine. That's odd.

The question now is, what can I do? Is there a way to 'unglue' the voice coil? It seems like a waste to just throw the drivers, especially the huge woofer, in the trash. I could take a hot air gun and try to 'melt' the varnish but anything besides refoaming and speaker electronics is alien territory for me ;-).

Thanks!

Bert

 

[savatage1973]

I would take the tweeter and super-tweeter out of circuit and test--they may read open that way or some unusual impedance. The mid is more likely to survive overdriving than the tweeters. The woofer may have taken more abuse because the lower frequencies take the bulk of the power from the amp.

You could try heating the magnet structure to free the woofer VC from its gap, but you will have to disassemble the entire speaker and remove the VC from the gap, clean the gap and resurface the VC (if it is not warped out of round). Pretty much any driver can be rebuilt, but the question is whether it is worth the time and expense.

 

[Markoneswift]

Sounds like the woofer has been cooked due to over excursion to me.

As said above it depends how much effort you want to put into rescuing these speakers. From what I understand, Akai were not really nored for their speaker designs so maybe cosnider walking away from them.

 

[Bert Beukema]

Hmmm, allright that's not a good sign. The reason I wanted to put some time in them is because i'm putting together a second audio room. The vintage speakers would fit nicely in that room; look & feel and all. And they don't sound bad at all (the one that's working). But I agree that they are not worth dropping a lot of cash into them. Time is another thing though, I can spare some in between other projects. But again, I'm not waiting on doing some herculean job ;-). Perhaps I can salvage two woofers from somewhere; replacing the tweeters isn't going to be a big issue. Those enormous woofers though.....

Bert