Back in time, I mused about the NAB / IEC playback equalization circuits in the 1500 series machines and opined that they all seemed identical based on the schematics available to me at the time. Well I was half wrong. I'm visiting the beautiful Newburyport home of a Walter Clay, fellow tape enthusiast and Tape Project member. Walter happens to have both a 1500 (without switchable IEC/NAB) and a 1520 (with switchable IEC/NAB) along with the service manuals for both so I spent some time this beautiful New England winter morning comparing schematics and PC board layouts and here's what I came up with.
The Playback PC board, called the "Main Amp" circuit board" have identical layouts for both machines. The 1520 differs in that there is some additional circuitry on its "Sub Amp" board which switches an IEC-related time constant at the output of the playback preamp. This "stuff" is difficult to follow as it is not completely shown on the schematics and can only be traced by comparing both them (pages 65 and 67), their parts layouts and the wiring diagram on page 71. To try and simplify things, it appears that during IEC playback, an additional time constant consisting of a series-connected 5.6K ohm resistor and 22 nanofard capacitor is switched in between the "top" side of the (PC board-mounted) playback level controls on the Main Amp board (VR106/206) and ground. Thats it!.
If I've got this right and please understand that no testing has been done, it should be relatively easy to add a a DPST switch and the four IEC EQ components. I removed the bottom panel of the 1500 and there is the bottom of the Main Amp board sitting there - with the connections to VR106/206 readily exposed. Don't know where you'd put the EQ switch - that's your problem. Have a ball!!!!