Re: Apple IIpluse motherboard test results

[mjmahon@aol.com (Michael J. Mahon)]

Vince Briel asked:

>> I fno beep: Connect a keyboard and press RESET (or Ctrl-Reset). Use a
>> voltmeter or bicyle lamp to check the voltage on the /RESET line. It
>> should go low immediately after power-on or when RESET is pressed, and
>> high a few miliseconds afterwards. If not, one of the 555s or its
>> inverter transistor is dead. Or the electrolytic cap connected to it.
>> 
>> 
>> Patrick
>
>
>Ok, I tested 5V on pin 8 of 6502 and only show 3V with a known working
>P/S.  The reset pin 40 pulls low 0V when reset is pressed, but also only
>shows 3V.
>
>Why only 3V to the cpu?  I have used this p/s on other ][plus' no
>problem.  I tested p/s voltages and get all the proper +5V, +12V, -5V,
>-12V, so I think its in the voltage regulation?
>
>Where should I check next?

If the power supply is OK, you can't pull its 5v output down to 3v
without overloading it and causing it to shut down periodically (tick).

Do you have +5v everywhere else but on the 6502?  If so, then there
is a poor solder joint or connection between the +5 and the 6502.

If +5 is only running +3v everywhere on the board, then the problem
is the power supply or possibly its connector.

Testing a power supply without a load does not guarantee that it
will work under load.

-michael

 Email:  mjmahon@aol.com
 Home page:  http://members.aol.com/MJMahon/