Re: 1977 original apple II restoration (continued)...
schmidtd <[email protected]> wrote:
I had a different kind of video problem with a board of mine. But like
you, I saw some real characters. So I figured the video circuitry was
good, so was character ROM, so was at least some memory, and the CPU
was probably ticking too. I also went through the process of swapping
and testing each and every socketed IC. No dice. Mine turned out to
be one of the transistors - the 3904 between A and B, 12 and 13.
I'd replace the transistors on the board. Desolder and solder in new
parts. You can get them from Radio Shack or Jameco.
That is the transistor that drives the /RESET line.
It is the perfect candidate in this case.
CHECK /RESET !
First unplug the keyboard cable connector and then check the voltage
there (in the keyboard connector, on the MLB, with the keyboard cable
unplugged) between pins 8(GND) and 3(/RESET) :
red tester probe here-->3- -14
4- -13 <- KEYBOARD CONNECTOR ON THE MLB
black tester probe here-->8-------9
If there are (close to) +5v, jumper (short) somehow pin 8 with pin 3, to
manually "feed" a reset. If it still does not work, the problem is
likely somewhere else, nor related to /RESET. Well, you should first
check too for continuity between pin 3 on the MLB keyboard connector,
and pin 40 on the processor, just to make sure that the 6502 is getting
the /RESET too.
If there are 0.x volts, then the transistor is most likely bad, or it is
being (badly) driven on by the power up reset circuit, the 555 at A13.
To check the transistor without removing it, first remove the 555 at
A13, then jumper somehow pin 3 to pin 1 on the now empty 555 socket at
A13. Turn it on and check again the voltage in the keyboard connector
(as above), it should be (close to) +5V. If it is 0.x V, the transistor
is most likely bad. I say most likely because there remains the
possibility of a short in the /RESET line, somewhere else. BTW, if
you've got some business cards nearby, insert one in each slot
connector, just to make sure they are not shorting. I've seen more that
one Apple II with the pins bent inside the slot connector, shorting
inside the connector with the pin just in front of it.
If you feel the transistor is bad, remove it by first cutting the legs,
connect everything in place and try to make the Apple II work by
manually feeding a /RESET, this time from the RESET key on the keyboard.
This way, without a reset circuit, it's more like a real original Apple
II, I think.
It's of no use to check any further without making sure first that
/RESET is not stuck low. So let us know... GOOD LUCK !