Getting started with probing - Page 1
Forum section: electronics / beginners

author: rthorntn
author_rank: Frequent Contributor
timestamp: October 28, 2018, 08:26:53 pm
content: Hi,I really appreciate all the help this forum has given me.I want to resurrect an old Amiga 2000 board, like this:https://www.bigbookofamigahardware.com/bboah/media/download_photos/a2000rev43mb.jpghttps://www.amigawiki.org/dnl/schematics/A2000_R6.pdfThe board powers on OK but I never see the insert disk logo.The motherboard has some battery leakage and it's corroded some of the 68000 pins, I have cleaned it up, I want to start there.https://www.nxp.com/docs/en/reference-manual/MC68000UM.pdfI haven't really used an oscilloscope before.I have the Rigol DS1102E.The first issue is just the size of the board, any suggestions on how to "mount" a board this size to work on it?The second issue is which settings to use on the scope?Third, where to put my probe?Fourth, what I'm looking for on the probe?Maybe to make it simple we could focus on a pin or two on the 68000 that should be doing something and go through that?Thanks for looking.CheersRichard

author: bson
author_rank: Unknown
timestamp: October 28, 2018, 10:06:04 pm
content: First remove socketed and potentially corrosion damaged parts and use a continuity tester, to make sure traces weren't corroded into open circuit. Look at the schematic, and for each CPU pin find somewhere else on the board it's connected to and make sure there's continuity.Eyeball for bad caps - leaky or skanky looking.Then power it on and check all supply voltages with a volt meter. Probe various locations around the board.Next, time for the scope: probe the oscillator to make sure it produces a clock. With passive probes you will likely only see a signal on the driven side of a crystal. That's alright.Then check the reset - make sure it comes out of reset okay. Scope the reset line while powering on, or hitting a reset button.Then probe EPROM control signals: CE#, OE# to make sure attempts are made to read from it.If that's okay and you still see no signs of life, make sure there is a video signal.If that's present, and it's just blank, then remove the EPROMs and read them to make sure they're okay. If not, get some compatible EEPROMs and replace the originals. A cheap and simple EPROM reader/programmer that will do the job is the TL866.If it's still not working, use a continuity tester to verify all bus signals are correct. If there are buffers scope both sides of the buffer along with a OE# control (you need three channels for this) to make the output tracks the input when it should. A logic analyzer can be used for this, too.If it executes code, all looks fine, then it could be a peripheral is dead, for example it might not prompt you for a floppy if it doesn't detect a floppy drive. (Don't know anything about Amigas, sorry.)

author: rthorntn
author_rank: Frequent Contributor
timestamp: October 29, 2018, 06:35:33 am
content: Thanks bson, just did some basic continuity testing around the CPU, pin 5 was flaky, took the CPU out and scratched away some of the corrosion, now I get the kickstart boot graphic, result!

author: stmdude
author_rank: Frequent Contributor
timestamp: October 29, 2018, 11:28:55 am
content: For some really good videos on Amiga repairs (done right), check out RetroGameModz on YouTube.He's the only retro-repair guy on YouTube that does things the correct way, using the correct tools imho.