Posted By
weidi
on 2021-12-17
11:07:26
|  Re: Cartridge port Not working correctly
Hi again,
Looking at the Plus/4 schematics I'm little skeptical about Datasette "test".
Power for it is taken not directly from +5V but from separate +9V unregulated dropped down to ~6V thru TIP29 (D313) transistor.
The main +5V "digital" is little affected by Datasette.
Another circuit in main +5V bus is power switch. it happened to me that contact was not perfect and the switch itself become "resistor" dropping voltage below +4.7 and less depend on the load (attached cart increase the load and voltage can drop below +4.5V).
Would be good to measure with multi-meter main voltage with and without cartridge.
Also: U20 (74LS139) could be the problem.
Please keep us updated as it is interesting case...
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