Some more boards giving me the run around or beyond economic repair.
First one was a real pain, it wouldn't initialise and for the life of me I couldn't see why? Someone had been here before which is always a worry but they work seemed of a good standard.
So the nitty gritty....on switch on there was just a short pulse to the RAM and then a few address lines died and it just sat there. IC9 (74LS138) and IC24 (74LS12) had been changed so I knew all was good in that dept and all the other usual stuff had been done. The first suspect was the CPU which was swapped but no difference. Next one was the 6840 as I've seen this stop boards booting before, but again nope! Not wanting to continue the wild goose chase I thought I'd try and think this one out.
Next step was to tap out all the data and address lines to make sure there were no shorts or open circuits, which there weren't. Then it was the turn of all the other functions eg E, NMI, VMA etc. When I got to the R/W I found there was no connection from the RAM (pin 10) to the R/W line feeding the rest of the board? What a relief I'd actually found something.
On the previous repair the two RAM sockets had been changed but because they were using the RAM daughterboard they had to be the stamped pin type as you can see in the pic. On removal of the socket in IC8 I found this...
The via at pin 10 of IC8 had been ripped apart and consequently lost connection. So a quick rivet insert, a tack wire and a bit of solder mask to finish off and she's as good new.
When all put back together it booted and ran fine.
....and the next one.
This had already been deemed beyond economic repair as you will see from the pic....
The board was toasted right through just above the power diodes and various components further up the board varied in incinerated, well done to medium rare depending whether they caught the flame or not.
As I was head scratching yet another board I thought, why not, and stuck it on the bench. First thing is to give it a clean and try and remove as much carbon as possible. Once done this left a sizeable hole in the board. This has to be done though as any carbon left will just track and start the whole process over again.
I must admit it does look scarey but I've done this quite a few times now so it's pretty much business as usual. The next step is to make up some resin, seal the bottom with cellotape and then fill the hole.
Once this has set the track needs to be re-made, holes drilled, rivets fitted and solder mask applied.
New track cut and fitted and riveted. They do look close together but it's a trick of the light! I still need to drill and fit another rivet for the white power lead but that's tomorrows job. Then a bit of solder mask to finish off.
All that's left is to swap out all the burnt components and the usual bits and pieces. I just hope after all this it doesn't have any obscure faults!!