7.0 Updates and Corrections 2N4401 Diagram 7 Diagram 6 7.1 New SRAM Supply Circuit • All Revisions. The new SRAM supply circuit (See diagram 7) supplies a solid 5V to the SRAM, preventing input data from being higher in amplitude than the supply voltage, which can cause data corruption. First, remove the original diode (D8), then install the new circuit. Diagram 8 shows the location to install the circuit in Revision A & CA PC boards, while diagram 6 shows the location for revision AQ PC boards. The installation of this circuit is highly recommended, and is absolutely necessary if a 58257 SRAM is in the unit.
This is because '257s are much more sensitive to supply voltage the older '256 SRAMs. 7.2 Glass Zener (D15) • Revisions A & CA. For a short time, glass package 1N5231B zener diodes were used instead of the more familiar metal package. These were found to be unreliable, and should be replaced with metal package zeners. Diagram 8 HR-16/HR16B Service Manual 1.00 9