Altec 1567 Mod
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EQ and Channels 3&4 lift switches added to rear panel:
Switches are added to the back panel for EQ bypass and to lift Channels 3 & 4 off the mono summing bus. Locking switches are used to prevent accidental toggle.
This modification is fairly simple, requiring only judicious bypassing of EQ circuit and lifting of Channles 3&4 off the mono summing bus.