The left side connection board. Looks slightly chaotic at first glance, but is (almost dissapointingly) simple, because the close 2 couples of leads are only links: One of the  blacks +  the white are just sent on to the other cup. Yellow is the common ground (shared by one stator in each driver).Silver in the middle is bias voltage for this cup, and green the other stator in this cup.

The boards are interchangeable between left and right cups, which means the you can sometimes  put togheter a decent ESP-9 from units partly out of order. I belive that all boards from ESP-7 to ESP-9B are interchangeable.

You were very lucky!
The main difference between the ESP9 and the ESP950:

The vintage model(s) have a much cleaner sound but are biased towards the mids, and are crying for EQ.  (Then they can compete with anything out there IMO, but EQ-ing will put limits to max volume).

The ESP950 is more balanced and a good, pleasant all-rounder, but more resonant/less clean. They are night and day when it comes to weight & comfort..

Hi @chetlanin,
i have just received an original Koss ESP 9 for few days (from ebay). First time listening it was very distortion, but after I removed old foams the distortion gone (I didn't insert new foam, just clean). Its sound is amazing. Yesterday while I'm listening with AC mode, suddenly the sound was too low like using SE mode. I turned off the ampli and disconnect the energizing from AC. After few minutes I listened again, the headphone's sound was good like before. But today I can only hear the right clearly, it's OK, no buzz or hum. But the left is all of buzz follow music (if I stop playing music, both right and left have no hum or buzz).
Could you help me how to find the reasons and fix it?
This is my energizing, if it help


Thank you.

LOL (hackus supremus). And/but thanks for the generous adding of points, Anax!

I may be KHS, but cannot give a definite answer to the problem, die_for_rock_vn.
Except perhaps mention that the weakest components in the old energizers seem to be the two small "bias" transformers - those on the "second floor" in your picture. I have experienced several E9 units having broken down after a few hours of work. Probably- before being sold on ebay- these units had been in storage for many years. The treads in the coils of the 2
transformers are thin like human hairs, and if the copper in a few places had been
exposed to air, they may simply have corroded into a shadow of their old thickness,
unable to stand vibration or current for more than a short time. (The larger signal transformers are OTOH much more reliable, and the signal circuit rather robust).
You seem to have a rather old version of the E9 energizer (but not the most original).
If the problem endures, the simplest solution may be to try to find a newer and better working energizer. They got improved over the years, with less hum and better reliability. An E9b - the latest version- would be particularly well worth looking after, I think.
Good luck, anyway.


P.S.
In the beginning, when my first energizer unit broke down. I managed to build an
external device supplying the needed 200V AC instead of the broken down transformers.
(this voltage goes into the voltage multiplier section on the component board, where it
is turned into the high bias voltage). A rather clean AC signal (of say 50-60Hz)  is
needed, because it is not isolated from the signal circuit, so any noticeable amount of
higher harmonics will be heard through the phones. OTOH current demand is very small.
Perhaps one can buy some suitable small signal generators for this use, I don't know.

I did the DIY version in a hurry and the unit became rather clumsy, but worked well until I
could find a good E9 energizer. It consisted of a low voltage DIY AC sinus
generator based on a couple of transistors and capacitors, a complete miniature
amplifier board from ebay costing next to nothing, and a couple of quality transformers
stolen from an old Stax SRD unit laying about. Using the volume control of the small amp
allowed me to get exactly the right voltage out from the secondary of the transformers.

One could build the simpler Stax type bias supply instead, I suppose, but for use with the peculiar Koss arrangement one would need two of them.