[Whiskeyrebel]

Hi, this is my first post. I joined in hopes of getting some repair advice.
 
I posted this question on Audiokarma as well. So far I have a recommendation to report the auction listing but no suggestions on the repair.

I picked up a pair of headphones off Ebay. They are Realistic Pro II made by Koss around 1981 or so. The seller listed them as "new old stock". When I got them, I found one side had no sound. It's open circuit through the plug. I told the seller that listing should have been "For parts or repair". The seller claims he checked them before shipping. I sent him pics of the ohmmeter reading but no further reply so far.

I called Koss to ask about sending them for repair. They said if they have the Realistic brand, the warranty would have to come through Radio Shack. When I asked about paying for a repair rather than free under warranty, they said they don't do that. Just not getting anywhere on the phone with Koss.

Continuity through the cable to the driver terminals is OK. The driver has a damper pad on the back held in place by a plastic screw plug. I took those off carefully and can see what looks like the back of the voice coil through four little holes. It doesn't look cooked. The driver is heat staked to the baffle plate, not screwed or bolted, so it would take some cutting to examine the dead driver any further, or to swap it out.

But swap it out with what, anyway? I mean, if I can't get Koss to even check if the parts are still made...?
 
Is there any chance that the VC lead could have detached from the pin? Does that ever happen? Any idea what Koss model might use the same driver? I could post snaps of the drivers if it might help.

 

[obobskivich]

Pictures would probably help. Koss' warranty is top-notch, if your product fits into it, otherwise I've found them to be polite but non-responsive. 
 
Just a thought, if you didn't pay too many bucks for these, you might consider just getting something new. The PRO4/AA is still in production, and would come with the Lifetime Warranty right out of the box. I do understand if you'd just like to say you fixed something though. 

 
As far as the headphones themselves...let's see:
 
- Can you contact the pins on the driver with your meter? If so, what does it do?
 
- Any visible damage?
 
As far as replacement parts; your guess is as good as mine. I can't hardly find anything about these headphones with a quick search - aside from what you've already posted. I've been told that the KTX Pro has some similarities to the Optimus/Realistic rebadges, but I don't know how true that is. KSC75s are popular to rip apart for their drivers, but I don't know if it would be a clean fit. I wouldn't get too "into" these, unless again, this is just a side-project.
 
 

 

[Whiskeyrebel]

The silent driver is open circuit across the pins. Externally I can't see any damage and there is no visible problem on the back side of the driver. I'd have to cut off the heat stakes to see what the diaphragm side looks like. The driver has a closed back shaped like a shallow, translucent plastic cone.
 
I only paid 10 bucks plus shipping. I picked these out because they look structurally rugged. I used a pair of AKG K301s at work but their enclosures wore out. I kept fixing them until there was literally nothing left to scotch tape the rubber bands onto anymore. Then I transplanted their drivers into a cheap pair of Yamahas that fit comfortably but sounded like hippo farts. Now those too are crumbled away. So this time I wanted something that can take a beating.
 
I am not sure if the AKG drivers can be made to work in the Koss cups. The AKGs are open back and the inside of the Koss are just an empty plastic bowl because the Koss driver is self enclosed. I guess I could keep the backwave from ringing in the cup if I packed the cup with open cell foam or batting but what effect would that restriction have on the motion of the diaphragm?

 

[dorino]

You can always bitch to eBay, as well. They're generally very, very customer friendly, often irrationally so.

 

[Whiskeyrebel]

Does this driver look recognizable as one that is shared with an identied Koss model?

 

[Whiskeyrebel]

bump...any repair ideas?

 

[Neighbour]

Please clean the connection.
Measure the impedance of the other driver.
From the picture i would call it a pro4aa in a different housing.
 
Hard facts.
150 Ohm is the HV/1 or 2 driver.
250 Ohm is the pro4aa driver.
 
You can take all pro4aa(at) spare drivers for this driver plate. There is only a difference in generation and holding points.

 

[Whiskeyrebel]

Hmmm...spec sheet says 220 nominal and the good side measures around 230-250 depending on how well I get the probes to contact the cord.

 

[obobskivich]

You're measuring DC, not AC. But we'll go ahead and assume it's the Pro 4 driver. You can buy a new pair of Pro 4's for under $100 and have a lifetime warranty. They're supposed to be able to survive a lot. The Beyerdynamic DT48 is the most durable headphone I'm aware of (I've heard of them taking on cars and winning; I don't suggest trying it) but they're also fairly expensive and have unique purchasing conditions (they're made on-demand by Beyerdynamic in Germany, so you don't get to return them if you're unhappy, at least that's what B&H says - they cost around $500). 
 
You might also look at something smaller - I've found that a decent pair of in-ears actually hold-up better than bulky full-size headphones, simply because they're less likely to get smashed/bent when being transported, as they have less size to get damaged. I like the Denon AH-C ECHs, they're aluminum and they sound good. I don't know what your usage scenario is though, so that might not be practical. If you're just using these indoors, but are extremely hard on your equipment, perhaps re-think how you handle things; might be the easiest solution. If these have to survive traveling (I don't advocate use of headphones while operating any vehicle), I'd probably go with some cheaper buds or ECHs (or even IEMs, if you're on trains or planes often), and plan on replacing them every few years. 
Quote:

Hmmm...spec sheet says 220 nominal and the good side measures around 230-250 depending on how well I get the probes to contact the cord.

 

 

[Ishcabible]

Oh, my Technician VFR's have that same type of mounting scheme. There is absolutely no way to get the driver out of that for replacement, nor is it a good platform to do anything with actually. It's the most infuriating enclosure I've ever dealt with. You're frankly better off getting something new because if there's something up with the voice coil, it'll take irreversable damage to fix it.

You could probably get a pair of Pioneer SE505's for cheaper than the Kosses, but they apparently need a ton of power.

 

[Whiskeyrebel]

Well, I finally dug into the driver. The voice coil was not blown, but the wire was severed between the coil and the pin. There was no way to tease the cut end to unwind a bit further so I could try to resolder it to the pin. The lacquer or adhesive binding it into a coil shape is stronger than the wire itself.
 
On the bright side, the diaphragm itself wasnot glued in. It was just sandwiched in place, exactly like the diaphragm for a PA horn's compression driver. So if Koss would sell me a replacement, I could possibly replace it. And if not, well, I have one side out to transplant in some other transducer.
 
Does anyone sell raw drivers for cans?

 

[Whiskeyrebel]

Koss says due to the age of the model, parts aren't available anymore. I think it was a canned form response.

Until I come up with a heat-free way to reconnect the VC, the original drivers are stashed away.

For the time being I transplanted in the drivers from my old AKG K301s. Those drivers are now on their third enclosure counting the original. I sealed the drivers into the baffle plates with nonhardening plumber's putty, and stuffed polyfill in the cups behind the drivers to damp any backwave. With the seal from the soft pneumatic cushions, they were shaking my jawbone and temporal plates when driven by an Ibanez guitar headphone amp I have.I haven't tried them through my stereo receiver yet.

 

[xiphmont]

I know this is an old thread, but others are bound to find it in search, as this is _the_ common problem in Koss drivers of this era (along with damping foam disintegrating and getting into the magnet gap through the venting holes in the back, sometimes even corroding out the magnet).

The major Koss products of the era used variations on the same basic driver. A nylon housing (often full of radial stress cracks; the cracks don't seem to cause any problem) with a ferrite magnet stamped in, a 2" mylar diaphragm with a 1" formerless copper voice coil, four venting holes directly behind the voice coil (either covered by a foam damping disk and nut or just thin black mesh). The diaphragm appears to be sanwiched in place without any adhesive. Be careful in disassembly, it's more prone to stick to the front cap than the driver housing and you'll rip out the voice coil wires out if you don't notice.

Anyway, the important part is that the voice coil is not soldered to the terminal pins. They're simply threaded through the hole and the pins driven in to make force-contact.

I've seen cases where the contact simply got lost due to a stress crack or overzealous repairman loosening the pin's fit, as well as an example where the voice coil wire failed right at the housing. In the first case, you can just tin the lead and re-fit the connection, possibly also tinning the pin to increase the diameter slightly and get a new, tight fit. In the latter, you can solder a new length of pre-tinned magnet wire to the broken end, and use that as the new tail for the force fit.

Soldering to extend broken voice coil leads works well enough (I'm listening to an example of it right now), but I don't know its long term prognosis. Solder usually isn't great under vibration, but the resulting joint would be much stiffer than the rest of the wire. Given the smaller length available for flexing, I'd expect a new failure to happen at the edge of the joint rather than in the joint itself. Also, if you're going to solder--- use good old-fashioned 60/40 leaded. The mechanical properties are simply far superior.