26 Aug
2014
26 Aug
'14
11:10 a.m.
On 2014-Aug-26, at 8:33 AM, drlegendre . wrote:
So get this.. something didn't ring right when I
replaced that 7490 chip -
I didn't recall that number from the BOM. Turns out I was right - some Mc
Bozo had put a 7490 in the hole where a 7473 should have been.
Not only does it no longer heat up, but the machine suddenly began showing
hints of genuine sanity! I was able to step up & down through memory,
Do you mean to say you have a memory board installed, or just that the address lights are
at times stable and will increment-by-one when examine-next is pressed?
stop / start the CPU, etc. But there are still big
issues, the machine is quite
erratic - and for some reason, it's sanity is sensitive to the position of
the A0-A15 switches. Set just one or two many 'on' and things go totally
wacko.. bunch of blinking lights, no control response, etc.
And the D0-D7 LEDs are still permanently 'on'. It's certainly a result of
the previously-discussed power termination scheme, which I will stress
appears NOWHERE in the original drawings or BOMs. It's almost certain that
it's interfering, at least with the Data LEDs. What good is a data readout
that always says "FF"? (Yeah, maybe it's right twice a day or something,
heh).
Termination has to go. I'll start by disconnecting the power. But if that
doesn't do it, then it's bye-bye for all of it.