From: William Levak (wlevak_at_cyberspace.org)
Date: 2004-11-18 02:40:10
On Thu, 18 Nov 2004, Marko [iso-8859-1] Mäkelä wrote:
> My flat C128 (artwork no. 310381 rev 7) seems to think the play button on
> the cassette port is stuck down. It responds to LOAD by writing SEARCHING
> and blanking the screen, instead of writing PRESS PLAY ON TAPE. The CASS
> READ, CASS WRITE and CASS MOTOR lines appear to work, because loading a
> program with "c2nload" worked.
>
> According to the service manual (schematic diagram on page 73), CASS SENSE
> should be connected via C72 (470 pF) to ground and via RP5-2 (3k3) to +5V.
>
> I measured 3k2 between pin 1 (ground) and pin 6 (CASS SENSE), and 3k06
> between CASS SENSE and +5V. Also the pin 26 of the processor (8502) was
> well connected to CASS SENSE.
>
> Can this be anything else than a broken I/O gate on the processor? If not,
> does anyone have a spare 8502 for me? I could trade it for a 6560-101
> (NTSC-M video chip for the VIC-20).
Did you measure the voltage on the CASS SENSE line? A logic probe would
be ideal. It is normally at +5V, and is pulled low when the play switch
is depressed.
If your 8502 is socketed you can test it by pulling it up and bending out
pin 26. If the problem persists, the 8502 is definitely bad. If it is
not socketed, you will have to cut the trace. You can solder it back
later.
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