Re: Where to start on Missile Command PCB repair

UPDATE

So I have a stack of these PCBs to fix, this first Missile Command PCB I
was fixing was out of my upright. Turns out these were issues:

1st. 6502 and not 6502A
2nd. 2 bad EPROMs
3rd. Bad sockets for Pokey, 6502 and EPROMs
4th. 3 of the 8 4116 ram chips where bad
5th. Installed new ram backwards <- yup

I installed sockets for the new ram so that helped significantly with
changing them out. I have the voltage set right at 5.02 volts, though I
would like to get the negative voltage down below -5.2 volts, not sure how
to go about fixing that on the ARII pcb.

The game now works fine. I got lucky on this one honestly. For my second
PCB I went ahead a put new sockets on the PCB for the ROMs, 6502A, Ram and
Pokey. I installed new 4116 ram last night as well, just didn't get around
to testing it, I will try the PCB out this evening.

I'll keep ya'll updated! Thank you so much everyone for you help!

-Grant

On Fri, Nov 1, 2013 at 7:54 PM, <solarfox@triluminary.net> wrote:

> On Fri, 1 Nov 2013 08:08:42 -0500, you wrote:
>
> >On Nov 1, 2013 7:52 AM, "Grant Thienemann" <grant.thienemann@gmail.com>
> >wrote:
>
> >> Ok not to keep getting off topic, but as this is related to the Missile
> >> Command repair, I'm going to ask. When I look for 74191 I can't find
> >> anything on Digikey, etc, but when I search for 74HC191 it comes up, is
> >> that the same thing?
>
> >Close. HCT is closer. Either should work.
>
> Not always, though. 74HC is "high-speed CMOS"; its switching thresholds
> for what it considers "low" and "high" logic levels are the usual <33% of
> Vcc = Low, >66% of Vcc = High. In a 5Vdc circuit, this works out to <1.65V
> = Low, >3.3V = High.
>
> However -- TTL gate inputs consider anything <0.8V to be "low", and >2V to
> be "high". On the output side, 0 - 0.5V is considered an acceptable "low"
> output signal level, while 2.7V - 5V is an acceptable "high."
>
> So -- mixing 74HC with TTL *may* work. It will *probably* work. It would
> take a very unlucky coincidence of worst-case scenarios for the input and
> output drive voltages of the two different chips to fall within the
> "mismatch zone"... but it *can* happen.
>
> 74HCT, on the other hand, is "high-speed CMOS, TTL-compatible"; it's
> specifically designed so that its input and output signal levels fall
> within the same ranges as "normal" TTL.
>
> -----------------------------------------------------------------------------
> A: Because it reverses the natural flow of conversation.
>
> Q: Why is top-posting bad?
>
> -----------------------------------------------------------------------------
>
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Received on Mon Nov 4 15:11:15 2013