Technical Service Bulletin SN2-004
September 30, 1982
During manufacturing, Signetics chips were substituted in place of Texas Instruments' chips in a limited number of Chromas. Although both chips carry the same designation (74LS74) the respective change in logic levels during transitions recognized by the chips differ. Because of this, Signetics chips may not function reliably in certain circuits of the Chroma.
The chips in question are dual "D" type flip flops, (74LS74) and are located on the I/O P.C. Board. The reference designations are Z10 and Z38. Z10A is used to receive cassette data during load operations. Z10B is used during the auto tune process. Z38A is used to write to the tapper driver and Z38B is used to write to the A to D converter to start a sequence. Problem symptoms in any of these four areas should alert you to first check to see if the chip is a Signetics or a T.I.
Rhodes Chroma National Service Manager