Sony SA-H7900 electrostatic tweeter repair

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First of all I'd like to say that I have no experience restoring audio, but I am pretty handy and up for a challenge. With a good explanation I'll be ready to go. I posted this question in another thread but there was no response, so I thought a new thread might get more attention. Also this is basically my first post and I might not know all the lingo. Anyway, here we go.

I have a Sony MHC-7900 system with Sony SA-H7900 speakers. The system has been collecting dust for the past twenty-something years. I'd like to revive them since it is supposed to be a great system, but the tweeters (Sony 1-504-388-11) in the SA-H7900s are very quiet. I already checked if it was an equalizer problem and it was not. With a flat equalizer there is hardly any sound coming from the tweeters. This problem is also shortly discussed in this thread, but that hasn't been of much help.

One tweeter was slightly quieter than the other. I took it apart and cleaned it following this youtube video. I first cleaned it with dish soap, which made a small improvement. Then I did it again with cleaning spirit, which made no noticable difference.

As far as I understand either the electrostatic foil is not conductive anymore or the high voltage circuit doesn't work. The fact that both speakers have the same problem makes me think that the foil is the culprit, not the circuit. For those that are interested, I found a service manual of this loudspeaker.

Does anyone know what to do next? Are there still replacement parts available? I'd love to restore this system in it's old glory. Thanks in advance!
I don't have direct experience with your speakers, but most ESL's are similar in their basics. Unfortunately, there's some non-standard test equipment that most ESL tinkerers have. Without a high voltage probe, a meter able to measure high resistance on diaphragms, etc., things are kind of difficult. In your case, I think you also have active electronics in the speaker that could be faulty.

While a new diaphragm or a new coating certainly looks feasible from the youtube video, if you can get one known good tweeter it would help you diagnose the problem more easily and rapidly. The post below is a few years old now, but I would start with a call to Sony to see if tweeters are still available.

If you haven't made sure you have good audio levels from the onboard amps, that's something else worth checking before you try to recoat a diaphragm. That was also discussed in the thread. The posts by bolserst and the responses to them are worth reading carefully.

electrostatic tweeter repair Sony SA-EX100

The post about tweeter availability:
Guys! the tweeter are still available from Sony Japan as spare part, they range from $80.00 to 120 USD, depending of the model

you need to contact your nearest sony spare parts supplier or a service center(sometimes these guys are too busy so have the spare part handy so they can check on the system quickly)

how i know this, I am a former sony engineer ;)

how ever sometimes the STK IC is faulty and not the esl tweeter

80% stk ic failure


The posting on the forum has been kind of light with the holidays, so more people may chime in in a few days.

And, since you seem to be new to these, be careful when probing around in electrostatics. Their hazards are different than typical electronics.
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Have replaced my tweeters with a 3 inch full range driver from Aiyama via Aliexpress. Main problem I found was that the back waves caused phase problems with the sound quality. Have installed a full width mounting panel and added damping behind the tweeter which is open backed normally. I would imagine that the poor reviews done on this particular speaker was probably due to this fact not any engineering factor as it all looks well made/engineered but poor in this part of design. With a bit of attenuation on the new conventional driver as the old electrostatics must have been fairly inefficient (old ones were dead anyway) using a 15 ohm 10 w resistor in series with the new tweeter in order to get to a listenable level with the bass driver it sound great...focused and detailed.Pity the original tweeters were dead as might have been possible to do this adaptation to the original design...Bit of damping on the infilled panels aside the tweeters also reduces reflections...Bass driver really goes low on these....sound much bigger speaker then it actually is...
SONY SA-H7900 - Service Manual Immediate Download

Looks like the bias is something like 6000V
Could some of the diods is out function (diodes ERA34-10 and Capacitors of 4700p 1kV) 7 cascades (?)

I can't read electronics and understand the cascade method
but the parallel capacitor to it is 2200pF 6kV

normal voltmeter cant read so high voltage

looks like the regular audio signals pass through 1:35 transformer
(reminded me a regular ESH amp)

*Still understand nothing in electronic, so afraid to mislead you
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