Technical Service Bulletin SN3-007
I/O Strobe Decoder
April 21, 1983
Recent intermittent problems have been traced to the I/O Strobe Decoder. The symptoms are quite similar to those described by Field Change Notice FCN2-004. A program parameter may change with no apparent reason or the tapper might spontaneously start to trigger in rapid succession. The Strobe Decoder on the I/O Board consists of I.C. chips Z39, Z40, and Z41. These chips should be type 74LS138.
During some manufacturing runs type 74S138 was used instead of the specified 74LS138. Typical propagation delay for the "LS" chip is 22 nano seconds but for the "S" chip only 8 nano seconds. The faster 74S138 I.C. may cause timing problems which result in intermittent conditions as described earlier. So, if you have a Chroma with these intermittent problems and all of the update changes such as FCN2-004, (EOC MOD) and FCN3-006 (POWER SUPPLY MOD) have been accomplished; check Z39, Z40, and Z41 on the I/O Board. Replace them, if applicable, with type 74LS138, 3 to 8 line decoders.
Bill Thomas
Rhodes Chroma National Service Manager 