Mike Nowlen
November 30, 1997

Some older models exhibit spurious oscillations due to the square wave limiter tube oscillating when the SINE-SQ function switch is in the SINE position.  What happens is the limiter section input is un-terminated and if enough stray coupling is present, it freely oscillates, and the oscillations are coupled into the output.

A modification was made in later versions that grounds the input of the square wave limiter when the switch is in the SINE position.

The modification involves re-wiring the SINE-SQ switch. The Eico 377K manual denotes the pins of the SINE-SQ switch as shown below, looking at the switch from the inside of the chassis with the unit up-side down:

-----Bottom of Chassis------

         1B  RB  2B
        -----------
       | O   O   O |
       |           |
       | O   O   O |
        -----------
         1A  RA  2A
 
------Top of Chassis-------
 

Older units are wired per the OLDER VERSION column in the table below.  To perform the modification, rewire the switch as shown in the LATER VERSION column.
 

     OLDER VERSION      LATER VERSION
PIN   CONNECTS TO        CONNECTS TO
---  ---------------    ---------------
1A   SWITCH PIN 1B      CHASSIS GROUND
RA   C9,R1,R2,R3,R4     C11
2A   C11                SWITCH PIN 1B,C9

1B   SWITCH PIN 1A      SWITCH PIN 2A,R1,R2,R3,R4
RB   R22                R22
2B   C12                C12
 

Copyright 1997