Hey guys:
Long time listener first time caller!
I've been playing around with arcade boards for awhile now and have had a few wins under my belt. Essentially I've been probing boards with a logic probe testing signal activity measuring VCC voltages, verifying roms and testing suspect TTL's with various eprom programmers and to date that's got me a few working boards.
Ive been working on a board for awhile- nearly there but am a little stumped. I've checked for broken traces and continuity across the board, done all the basic stuff - 5volts at VCC verified roms, there are no pins which are dead and or floating. I cleaned out sockets with de-oxidit ran the toothbrush all over this badboy and it looks like it just came off the shelf from 1988.
All inputs/outputs, clocks and control pins are doing their thing as per expected I've got a graphical glitch on the board associated with moving objects- (not fixed in one particular spot). Ive checked all 161 counters which are the last point of output before the signals rolls to the CPU (the CPU checks out got a spare as a test unit) and they all have activity.
I started piggybacking ttl's over one another and may of found a glitchy chip, however I don't really want to be piggy backing as a means to identify faulty chips. I want to learn what causes the glitches. My question is what is the best method to fault find ttl logic in circuit.
I have a little 20mhz hantek usb scope and have tried triggering some I/O's but am really green with regards to understanding what it is I'm looking for on a faulty chip, and cant seem to get a stable single shot image as reference both in auto and or normal modes. Reading various things online some argue a digital scope is necessary others an analogue which tbh I'm leaning towards looks a little simpler to operate and others argue a probe, pulser and a comparator are all you need to get the job done. I'm across truth tables and boolean so thinking I can isolate the video board and power up independently, If im to pulse a few inputs could I then just measure the output voltage and if that doesn't marry up, would that tell me, I have faulty IC?.
What causes a faulty ttl?, None of these guys seem to be running overly hot the fault appears straightaway and as mentioned I've got sub 5 volts to each IC.
The stuff I work on is namely 80-'s 90's arcade boards 7400 series TTL gear all running either z80,68k or 6502 processors so sub 7- 10mhz clk signals. Id like to get my head around using a scope a little more thoroughly but not sure what conditions would the chip in question need to met to determine if its good or bad. Do I just compare an input via an output or do I need to probe the respective clock or o/e pins. What time base do I measure ttl signals in and finally is it ultimately just poor voltage that provide a "dirty" square wave, (if so my pulser theory would do the trick). Most vids I've seen are showing a scope testing ttl's are on a breadboard with just a single value held at the input not a chip oscillating at 6mhz
Ideally if somebody could chime in with with thoughts on techniques and possibly a how to guide on reading and using a scope if needs be that would be ace. Hope I've thrown enough info above that makes sense!
Cheers in advance
Y