Re: Where to start on Missile Command PCB repair

5th. Installed new ram backwards <- yup

backwards I killed my Atari 400 by putting the ram card and CPU card
backwards!!!
and the 6502 pod wont test say's addess line stuck???

On 5/11/2013 9:11 a.m., Grant Thienemann wrote:
> 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
> <mailto: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 <mailto: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 23:48:26 2013