[Techtoolslist] And another weird test result [Solved]

[John Robertson]

On 2021/05/28 10:36 a.m., William Stillwell wrote:
> Check the clock circuit, that board has a clock slow down function for 
> accessing ram, and I have seen it cause issues elsewhere, i thought i 
> had a video of it up on my youtube, but i don't ( its for an hercules 
> board ) , bu it was put on a forum post ( 
> http://villagebbs.com/forum/index.php/topic,24850.msg205478.html#msg205478 
> <http://villagebbs.com/forum/index.php/topic,24850.msg205478.html#msg205478> 
> ) -- a fault 74160 was causing all kinds of random glitches.
>
> William Stillwell
>
It turned out to be my old nemesis,  Signetics 74LS244 date code 7910! I 
have noticed more board weirdness with these buffers than any other 
cause. From now on I'll just replace them.

In this case it was just loading the data buss enough that the EPROM's 
latency was enough to register the contents of the last byte. I played 
with the scope, synced up to the select line for the highest EPROM and 
then watched the data bus (while doing a continuous Read at 1800h) which 
was where I found the very tiny lift on the lines that had lows for data 
55. The EPROM was indeed disabled at that point, but the 74LS244 was not 
letting it go high enough. Adding a 10K pullup resistor fixed that data 
line so it could go high enough, which led me back to the buffer. 
Replacing the LS244 (date code 7910!) fixed all the reluctant data lines.

And the flicker has vanished...

My Fluke 9010 probe tried to clue me in, I had the probe synced to Data 
and the Low light was not lighting on the Low data line. I should have 
realized it was in the illegal state and looked for the reason (LS244 
Date Code 7910)...should have trusted the Fluke gear more!

John :-#)#

>
> Board Member - Inspiration labs, Inc. a 501c3 organization - Tampa's 
> Premier Maker Space.
>
> Board Member & Co-Founder - Byte Amusement Group // Free Play Florida 
> Arcade & Pinball Show a 501c3 organization
>
> Member of MakerFX - a 501c3 organization - Orlando's Premier Maker Space
>
> Volunteer for The Phoenix - a 501c3 Organization - Providing outreach 
> and events for those who are living a sober lifestyle.
>
> Former Board Member - West Central Florida Group - a 501c3 
> Organization. - Providing Emergency Communications across Central Florida.
>
>
>
> On Fri, May 28, 2021 at 1:16 PM John Robertson <jrr at flippers.com 
> <mailto:jrr at flippers.com>> wrote:
>
>     I don't know if you folks are interested in my headaches, but here
>     is my
>     latest one.
>
>     Working on an Atari Superman  pinball board. ROM, RAM tests are good,
>     board boots, issue is the feature lamps flicker when turned off.
>
>     However I am getting something odd on address 1800-18FFh, it displays
>     what appears to be the last data bit in the EPROM located at
>     3800-3FFFh.
>     I plugged a different EPROM in and got its last data bit too. What is
>     odd is the EPROM, when reading 1800h shows the Address lines all
>     as low,
>     but there is the tiniest pulse on /CE(pin 20). I've tried
>     stretching it
>     out, but it appears to be the smallest resolution of my Tek
>     100mhz, 2230
>     storage scope - which I am still learning how to use.
>
>     At this point I figure it is a timing issue so will dig further,
>     but I
>     just thought is was most curious that it didn't matter which address
>     location 1800-18FFh was chosen, I got what appears to be the last
>     byte
>     in the EPROM! I have no explanation on how that can happen. I will
>     now
>     test that by burning a couple of  2716 with only the last byte
>     set, will
>     try AA and 55 and see what happens...
>
>     John :-#)#
>
>     -- 
>                       John's Jukes Ltd.
>     7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
>     Call (604)872-5757 (Pinballs, Jukes, Video Games)
>     flippers.com <http://flippers.com>
>       "Old pinballers never die, they just flip out"
>
>
>     _______________________________________________
>     Techtoolslist mailing list
>     Techtoolslist at flippers.com <mailto:Techtoolslist at flippers.com>
>     https://pairlist7.pair.net/mailman/listinfo/techtoolslist
>     <https://pairlist7.pair.net/mailman/listinfo/techtoolslist>
>     FTP site is: ftp://ttl.arcadetech.org/TTL/Test_Equipment
>     <ftp://ttl.arcadetech.org/TTL/Test_Equipment>
>     Archive site: http://seven.pairlist.net/pipermail/techtoolslist/
>     <http://seven.pairlist.net/pipermail/techtoolslist/>
>

-- 
                  John's Jukes Ltd.
7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
Call (604)872-5757 (Pinballs, Jukes, Video Games)
                  flippers.com
  "Old pinballers never die, they just flip out"