Acoustat 2+2, MK-121-2a interface Bias Circuit update. Restore Treble and Dynamics for $50.

CAUTION! - You must properly discharge the interface before making any repairs! 1-Unplug the interface from AC power. 2-Unplug any speaker wires from the interface. 3-Remove the six screws that mount the interface to the base. 4-Tilt the interface away from the 2+2 panel and lay it on it's back, exposing the three sets of wires that connect the 2+2 panel to the interface. 5-Touching only the Red wires, pull them away from the interface exposing the small metal pin on the end of the wires. 6-Touching only the Red wires, touch the pin on either the Blue or White thumbscrews on either side of the Red connector. 6-Once you have grounded the red wire pin to either thumbscrew the system is discharged and it is safe to remove the White and Blue wires and service the interface.

A friend gave me his old set of Acoustat 2+2 speakers and even though they worked, they did not have very much treble and sounded very 'blah. I received the instructions below from a former Acoustat Engineer and replacing the old bias circuit parts made my 2+2s come alive and sound amazing.

NOTE : Even if your interfaces are working fine, and with no damage
and because they might have had decades with no service at all...
I highly recommend changing all of the bias circuit parts
as I listed below....to ensure best performance of the speakers.

BE SURE TO OBSERVE CORRECT POLARITY
WHEN INSTALLING DIODES.

Order these part numbers from Mouser (dot com) :
R5000F-B HV Diodes --- Qty 5 per interface
(order 10 pieces)
75-564R30GAD33 Cer Disc Capacitors --- Qty 5 per interface
(order10 pieces)
588-SM104035006FE Bias Resistor ---Qty 1 per interface
(order 2 pieces)

and for best protection of the LF audio transformer
...change the externally accessible fuse next to the speaker wire connections to this :

576-0313003.MXP Audio Fuse ---Qty 1 per interface
(2 pieces, order extra spares)

All other parts in the interfaces will last virtually forever.

After installing the new parts.... clean the installation areas
of the PC board with acetone and a brush to remove any
solder rosin residue.
The bias circuit area of the PC board especially, must be clean
to prevent high voltage corona and arcing.