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Re: EH Deluxe Memory Man

2/1/2000 7:32 AM
Rich
Re: EH Deluxe Memory Man
The Bias trim of an SAD1024 is set by observing the chip's output (see below) on an oscilloscope. With a maximum-level signal input, adjust the trimmer for minimum output distortion on both positive and negative signal peaks. (Please don't tell anyone this is also how you bias a guitar amp.)  
 
The Gain trims are set to preserve successive output levels from chip to chip.  
 
The SAD1024 is a dual 512-stage BBD. The EH-7550 (Memory Man Deluxe) uses four 1024's, each serially configured. They are IC4 (A&B), IC5 (A&B), IC7 (A&B), and IC8 (A&B). There is a 4558 buffer between IC's 5 and 7.  
 
I don't have a component layout for you but the wiper of each of the four Bias trimmers (10kB) is fed simultaneously to both A and B inputs (pins 2 and 15) of its particular SAD1024. Each is fed through a 100k resistor.  
 
Three of the four Gain trimmers (5kB) are 2-legged, hanging on a 1024's B+B' output (pins 5 and 6 connected together). In the case of IC8, however, the trimmer is hooked between B and B' (between pins 5 and 6), with the wiper feeding the junction of a 16k and a 2.4k resistor. That final-stage wiper should typically read 4.6 V DC.  
 
To set the first (IC4's) Gain trim, match its A output level (pins 11 + 12) to its B output level (pins 5 + 6).  
 
Good luck.