[Coco] So is this a G board?

[Gene Heskett]

On Friday 28 June 2013 12:32:29 Luis Antoniosi (CoCoDemus) did opine:

> I noticed my "fix" over the mc1372:
> 
> https://twitter.com/RetroCanada76/status/350464954366836736
> 
> is different from this:
> 
> http://www.doki-doki.net/~lamune/computers/coco/26-3003b/P2223732.html
> 
> Are they equivalent ?

Thanks to the yellowed spaghetti, I can't tell.

WRT the discussed problem. I think the scope is going to be what it takes.

In my 60 years, I have seen several chips fail in a manner that had to be a 
poor bonding, or a half failure of the output pins pull down ability.

When the scope is DC coupled, and its worthless for this if its not, set 
the ground trace position on a graticle line near the bottom of the screen, 
and a sensitivity of about .5 volts per large division.  Any signal that 
doesn't exceed 3.0 volts when high is suspect, but you'll need to visually 
ignore the tri-state times where the signal can be seen to take a curved 
ramp upward.  Conversely, and this is what I would look even more carefully 
at, is a low signal whose low time is not well below .3 volts, thats a bad 
one.  Most ttl outputs can pull down to .1 volts, so you have a good "noise 
margin".

When a logic 1 is to be sent, the ttl input threshold to guarantee that it 
is a logic 1 is usually said to be anything above 2.4 volts, and to get 
that same noise margin, the signal itself needs to be at 3.0 volts or more.

Flat, level for a half microsecond signals indicate that an active output 
is pulling it up or down as the case may be.  Those signals that meet that 
timing criteria, but are not well below .4 volts, or above 3.0 volts, 
should be traced on down to the chip sourcing that signal, and the chip 
pulled, a socket and a new chip installed.  If that doesn't fix it to have 
the proper voltage swings, then trace toward the load pins its driving, 
comparing them all as I have found board traces that go through a through 
hole that didn't solder well, and I have found quite a few of those by 
comparing the source signal to the destination point and could see the loss 
of swing at the other end of the trace run.  It may also indicate a blown 
input to that gate in the chip, in which case pull, add socket and replace 
with a fresher chip.

This is all made much much easier to do if you have a hot air rework 
station.  I do, and I know Mark has more than 1, but they aren't that 
common, and are not sold at the shack.

Cheers, Gene
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