Nama wrote:
> On Oct 12, 4:56 am, m...{at}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  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 {at} 14J to the single video pin.
> Crystal clear image. so I'll need to look into that further. Thanks
> for your suggestions on this.

Sounds like a broken PC board trace.

> 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.

Good call.  It may not take a low impedance short to cause this,
depending on the sensitivity of the encoder.  Sometimes just the
slightly conductive residue of a spill will produce this symptom.

> 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.

Don't overlook the possibility of hairline cracks in the PC board.
Keyboard PC boards get a lot of flexing during use and can develop
cracks that are all but invisible.

Shorting the keyswitch further back along the traces can reveal this
problem.

In a matrix keyboard, the exact keys affected can also be used to
isolate the faulty trace.

When (if?) you find it, cleaning the trace on either side of the break
and bridging it with a small piece of wire and solder will repair it.

-michael

Parallel computing for Apple II computers!
Home page: http://members.aol.com/mjmahon

"The wastebasket is our most important design
tool--and it's seriously underused."
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