Thanks so much for the tips and suggestions!
I am not accustomed to Ohm's law not being helpful with these kinds of problems, but then I am not exactly "seasoned" and have only made trivial circuits up before this, my largest project to date.
The boards are professionally made up, and designed by me. All ICs are socketed. I debated about socketing the DRAMs, but since I wasn't 100% confident the board was going to be my "final" version I still went for sockets for these relatively cheap parts.
I think my strategy will be to measure the voltage across as many ICs as possible, moving along the power rails. Would it make sense to pull all the ICs and replace them one at a time, measuring for drops as I go? That might be iluminating. I can work on the main computer board first (MPU, EEPROM, SRAM, DUART, CPLD) because it shows the problem as well, albeit not bad enough to prevent the computer running.
Here is a picture of the main board PCB to give some context:
Attachment:
6809computercpld.png [ 252.83 KiB | Viewed 1493 times ]
The round part south of the Molex at the top left is a vertical fuse holder, incase anyone was wondering.
The design constraints (10mil DRC, only 45degree runs) might be a bit strange and I might not make the next board to the same rules, but I was/am fairly pleased with the use of the space etc. Anyway, you can see the power connectors clearly: A harddisk-style Molex and USB (odd choice, perhaps). Vcc and ground run around the edge. You can see also that there are 3 points used to bridge Vcc and ground onto the "top" IO board (bottom left on the main header, bottom and bottom right). I deliberately wanted to distribute the power rail into multiple places on the IO board.
Some inline replies:
barrym95838 wrote:
One possible strategy would be to run external temporary shunt wires to key areas while monitoring the voltage drop, and see where that takes you.
I think this is an excellent idea. Since I can clearly measure 5V on the Molex, it should be possible to hook this onto the IO board directly at a point where I can see a drop.
GARTHWILSON wrote:
In the 1970's everything took a lot of power yet we did not have multilayer boards, so I don't think that's the problem here.
Yes, this is a source of much comfort.
I'm inclined to hunt out some of those old PCB designs and see how they did it.
BDD wrote:
One other thing: You didn't mention if all of the chips are socketed. You may simply be a victim of junky sockets. I've certainly run into enough of those over the years.
One thing I don't understand, and please forgive my ignorance, is how a bad socket would cause problems with voltage drops? Could it cause problems on other pins? I am measuring the "elbow" of the pin, and not the voltage at the pad, so I guess it could be that? Hmm!!
Klaus2m5 wrote:
Do you have a filter capacitor (usually 100nF) as close as possible to each ICs power inputs?
I have 100nF caps on all ICs, as close as I could get them. There are only 4 on the main CPLD, which I know isn't /quite/ enough. The board is also decoupled as a whole with 100nF and 100uF caps.
Dr Jefyll wrote:
Incidentally, the wire itself will produce a voltage drop, which may be excessive if the wire to the PSU is too thin and/or too long. I made that mistake myself once back in the 80's, and had to switch to a heavier gauge. NMOS. Yeesh!!
It's a standard (working well, I believe) ATX powersupply going directly onto a Molex connector. Yes, NMOSs are a hog, but then those CPLDs draw the current, heat up, etc pretty good as well...
Thanks everyone for the tips and advice! I'll be sure to resume my fault-finding efforts this evening...