On Oct 12, 4:56 am, m...@willegal.net wrote:
According to my 1982 RCA SK series replacement guide a C1214
transistor has
an RCA replacement of a Sk3124/289 which is an NPN transistor with
similar specs to
2n3904. If it is wired in correctly, I would think that it would
work. It is only connected
to the color burst signal, so worst case is that the color wouldn't
work.
Check voltages on the inputs the each of the three resistors going to
the video circuit, as well
as the output. It is easier and faster to debug these problems with
an oscilloscope or logic probe than
by working blindly.
Regards,
Mike Willegal
On Oct 11, 12:15 pm, Nama <forums6...@mac.com> wrote:
Hi,
I'm trying to bring a very early (serial # in the 12000's) rev 1 Apple
II back to life. I have been using the swap method to interchange
chips from a known working apple II+ motherboard.
At first it was completely dead, and so far I have now gotten it to
beep, power light comes on, and about 50% of the time it will access
the disk drive and look for a floppy, but no video at all!
If I insert DOS 3.3 disk on the times it is looking for a disk, It
will seem to boot, and I can then do a blind IN#6 and the drive will
boot again, so I'm assuming at least some of the time the machine is
actually functioning at a basic level, However when the drive works,
or does not work seems totally random, and is probably a separate
issue.
I think my first and biggest hurdle is to get the video working. I
have replace all the IC's in the video circuit (and many more) without
change. There is nothing on the screen at all, and It is completely
blank. I'm don't even think there is an onscreen flicker when I turn
the machine on and off.
With further inspection of the video area I noticed that someone has
done a bad repair of replacing the 2n3904 npn with a c1214(?). At this
point this seems like the likely culprit, but before I try to fix the
dodgy repair (some pad damage) I wanted to see if anyone else would
agree that this could be the suspect video part.
As usual, thanks in advance
Philip
Thanks for the replies. I have gotten the apple II up and
running...almost. The sometimes boots from floopy, and sometimes
doesn't was a RAM issue. Ram replaced.
I still not sure exactly what the issue is with the video, but it's
working after I jumpered between 2n3409 @ 14J to the single video pin.
Crystal clear image. so I'll need to look into that further. Thanks
for your suggestions on this.
The only big remaining issue is the early rev 1 keyboard (without the
encoder). I have fixed a couple of these before, but this one has me
stumped. I have already replaced the 5 keyswitch which was smashed,
and now it works well. The keys that don't work are 1, 2, R, and F. K
is also a little intermittent, but thats an easy fix. and finally
pushing either the 4 key or the W key both display 'W4'...maybe there
is a short somewhere between these keys.
Shorting the contacts on the rear of the board, on keys 1 or F makes a
1 or F appear on the screen, so I can assume the problem is just a
keyswitch issue. However shorting the contacts on R and 2 do nothing!
Maybe a dead component elsewhere is causing this.
If anyone has any insights again, I would greatly appreciate your
help.
Philip