Interesting.
April 1975 changes notes say L and R headphone “drivers” have been updated for better sound.
The difference is
the 2 small holes in the foil on the newer models.
Additional black foam on the inside of the inner grill.
seen in 1975 https://user.xmission.com/~alhaz/img_1022-crop.jpg
The diaphragm is exposed to the ear side just behind mesh, so maybe it’s talking about the lack pressure from the foam when the covers are unscrewed. The translation from original Japanese maybe a bit off in the service manual.
I suspected that finger oils and acids would eat away at them over time so didn’t touch the foil. There is a previous post somewhere that did similar and didn’t report any issues. I guess you could measure the capacitance before and after to see if they have degraded.
I have only wiped the foil with 99.95% ethanol via a small ball of tissues wrapped in another tissue, so under as little pressure as possible.
Had to make sure nothing is growing on it since they are a little stale smelling. The spiders couldn’t get foil only into the outer area so that was good.
I’ve had them refitted with foam and cover in place most of the time while sitting waiting for parts to arrive or when not cleaning or drying the outer parts. Mainly as was worried if knocked or dropped the foil would get damaged.
The covers needed a deep soak and dry so they have had maybe 4 hours without the foam clamped. They have some material between the plastic and outer metal mesh trim so needed a very good dry before
The foam was still sitting on them for all but 2 hours where the foam was in the sun to make sure it wasn’t mouldy. Not that I can see anything.
I even gave up on the idea of removing the driver to soak and clean the inner grills. The glue is rock hard after 49 years so didn’t want to risk breaking them. I suspect this is possibly the exposed part of the manual, as it shows pulling the foil assemble out.
Had to soak a folded stack of tissue in ethanol and let the grill sit on that to get the crud out. Then Sellotape to get the rest out. I’d say 95% cleaner overall but far less risk.
They sounded fine before tear down if you could get the cable to work and wrapped paper towels around the headband and earpads to stop the flaking.
They were very flat but clear, can’t say the detail was there like HD800s but it was very pleasant sound. They may have needed more power and some better sealing pads.
Will find out in a few weeks when everything will hopefully be fixed.
I’m also building 2 of the junction boxes j21 or whatever it was and will run them from a little 40 watt class AB amp. The old parts look to be rated to 10 watts max input, So about 7.4% with its 8ohm and 100ohm resistors.
Modern components without speaker switch should be about usd$40 including small case.
Thread in DIY section with details and progress
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Did you plan to change it to numbers instead of emoji?
