[Coco] XPander-Coco lives!

[Gene Heskett]

On Sunday 01 May 2016 18:42:47 RETRO Innovations wrote:

> On 5/1/2016 5:38 PM, Zippster wrote:
> > Hmmm, I would think so, but the MAX811 has a master reset on pin 3. 
> > You can drive this low to activate reset as well. Glancing at the
> > data sheet, consider !MR internally held high by the equivalent of a
> > 10K pull up.
> >
> > Are you wondering if a cart will be able to reset the system with
> > the MAX811 in place?
>
> Yep.  I think the supervisor is a great idea, but since RESET is a
> "wired or" type line, I think the device needs to allow others to pull
> reset low.
>
> Jim

10k and 5 volts is half a milliamp.  Anything that wants to pull the 
reset line down, can and will.  Without damaging a thing.  But not 
mentioned is that there is an electrolytic capacitor on that line 
designed to slow up the reset/recharge so the power supply is stable by 
the time it comes out of reset after you power it up.

That capacitor is now pushing 30 years old, and there is no way in 
anybody's world that it still has the original capacitance, so the 
possibility of the coco coming out of reset before things have settled 
is pretty high.  In my own, I've seen the symptoms of it by needing 
multiple resets with flying confetti monsters all over the screen, until 
it finally works.

Even with a fresh cap in those 2 holes, that 10k will shorten the reset 
pulse, but it should be sufficient for at least another decade, possibly 
much longer if that cap is replaced by the much longer lasting tantalum 
type.

I bought an assortment of tantalum caps a few months back, around a $25 
bill for quite a few.  It paid off like a slot machine a month ago when 
after a power bump, my fawncy Kitchenaid dishwasher came back up brain 
dead.  Long version of the story was that whirlpool wanted $266 for a 
new controller pcb, which I reasoned was likely N.O.S. and stood a 
chance of being just as brain dead after 6 or 7 years on the shelf.  I 
looked it over, checked a few caps, and 3 of them, 10 u-f's at 16 volts, 
measured in the .02 u-f range.  My kit had 3 of that size, so my shaky 
hands changed them.  Runs like a brand new one again.  My cost was under 
a quarter each plus my time, which because it was coated with some sort 
a very high temp tolerant clear coating I had to chisel away to get at 
the solder joints.  I took 266 dollars out of my pocket and pretended to 
pay myself for a new pcb.  Whats not to like about that? :)

Cheers, Gene Heskett
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