26 Aug
2014
26 Aug
'14
8:33 a.m.
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, 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.
On Mon, Aug 25, 2014 at 11:55 PM, Andrew Hoerter <amh at pobox.com> wrote:
On 8/25/14, Chuck Guzis <cclist at sydex.com>
wrote:
Sometimes, you don't even need additional components. For example,
Traco's TSR-1 is a completely standalone unit (I think Recom is
another common brand, these are probably familiar to most here), and
as you say, intended as a dropin replacement for linear regs. Albeit
at a significant multiple of the cost...
Nods on
the 7805s running hot on this machine. Might be a nice place to
install a modern part with higher current handling, and a bit more heat
sink as well.
You can't beat a switching regulator for this kind of thing--only
requires a couple of (small) external components--and I believe that you
can get pre-assembled modules that are a pin-for-pin substitute for a
78xx. Why heat things up?