TechTools Archives: RE: Fixing low TTL signals

From: Andy Welburn <warlords_at_punkass.com>
Date: Sat Apr 02 2005 - 12:52:05 EST

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I have seen another IC piggybacked on top
and soldered in place before.. if
it works, then heh, who cares? J<o:p></o:p>

<o:p> </o:p>

The other method I found was to put a 1k
resistor to the leg in question, then the other end to either +5v or GND, thus pulling
the line high or low, depending on what you needed. With an oscilloscope, you
can often see the problem and experiment adding components to help weak gates.
When you can see the right signals, but their thresholds are just too low, then
yu can help them along.<o:p></o:p>

<o:p> </o:p>

This is a dirty fix method, and some would
say that it would die eventually, but I disagree, often multiple chips driven
off of one signal can get more power hungry over time, so rather than replace
up to 6 IC’s being driven from one signal, I tie high or low with a
resistor and havn’t had any problems.<o:p></o:p>

<o:p> </o:p>

Sometimes instead of a resistor, I will
try a 1pf ceramic capacitor, but I usually do this if a signal has too much ‘noise’
on it, or there’s a timing is<st1:PersonName>sue</st1:PersonName>.<o:p></o:p>

<o:p> </o:p>

This doesn’t often happen tho, it’s
a sort of ‘tried pretty much everything else and I’m shit out of
options’ scenario. It has got me out of a several scrapes in the past,
but in general you can find the problem elsewhere and fix it properly. But I understand
there are certain times when its necessary J<o:p></o:p>

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Andy Welburn 
www.andys-arcade.com <o:p></o:p>

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-----Original Message----- 
From:
techtoolslist-bounces@flippers.com [mailto:techtoolslist-bounces@flippers.com] On Behalf Of <st1:PersonName>Robert</st1:PersonName> C. Bullock 
Sent: 02 April 2005 17:01 
To: techtoolslist@flippers.com 
Subject: [Techtoolslist] Fixing
low TTL signals

<o:p> </o:p>

I have a Pac with a low 6M* line, when I piggyback a 74LS86 (IIRC,
whatever 
chip generates that signal) on it, it's fine, but taking it off, the game 
'bogs down', sometimes, but plays with no glitches but somtimes the music 
slows down speeds up, etc. Looks like low TTL levels measured at Pin 1 on 
the z80 sync buss card. When I say piggyback, I put my logic comparator on 
it and it raises the levels by about .4V 
 
Do folks just slap some HC chips in the circuit or is there a better classic 
way to fix this? Course I could just solder a duplicate chip on top I guess. 
I suppose something else tied to that line may be pulling it down but I 
haven't gotten that far yet. Replacing the crappy sync buss single wipe 
socket may help of course.<o:p></o:p>

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Received on Sat Apr 02 09:55:47 2005

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