Leaderboard

Well the other board had the little main clock daughterboard on it and although I'd thought I'd strapped out all the corroded tracks (the main clock was working) I'd not checked the secondary clock (75kHz). Lo and behold a track was broken on the 74LS92 chip. I could have sworn I tap'd them all out but must have missed this one. Once patched out the 75kHz pulse was received at the SIO chip and it passed the motor sequence and fully booted.

Hi folks, this will be the last workshop thread this year and it's a real humdinger. Are you sitting comfortably? ..... Must be a good 6 months or more ago now I was given a PC 92 board that was in a bad state and probably more fit as a donator board. As the owner had done me a few good turns I thought I'd have a go at trying to repair it. There was a fair bit of corrosion around the battery area (as per normal) and some of the large multiplex power transistors had been ripped out along with their vias and associated track then put back with tack wires! The reg transistor and associated heatsink were missing and the smoothing cap had been ripped out along with one of it's vias. These are larger than I stock, to take the cap terminals, and the only place I could get them from was Digikey in the US! Unfortunately I didn't take a pic of the board pre repair but this is how it ended up... So, first thing was to remove all the bad corroded components and give the board a good clean around the battery area. Next step was to try and work out how many tracks were corroded around the battery area. Anyone that's worked on these knows what a pain this is as there's so many connections going all over the board passing through this area. It's a nightmare! MPU4 is childs play in comparison. Fit the new rivet for the main cap and re-fit a heatsink. Fit a total of 14 rivets to renew all the multiplex transistor broken vias and battery terminals and replace all the broken tracks. Then replace 11 of the transistors, 24 resistors and 7 diodes. With these boards I always fire them up first time without the 53v connected as contaminated boards leak through to the 12v and 5v lines and anywhere else that's in the vicinity. Once powered up I check to see what voltage if any I see on the 53v line. If it's any more than a couple of volts it needs investigating and the 53v supply should not be connected until resolved. In this case it was about 12v and the 53v track which runs under the battery was leaking across to other tracks. I usually cut the 53v track at both ends either side of the battery area and run a patch wire as even deep cleaning doesn't appear to effect a cure. Andrew mentioned this a long time ago in one of his repair threads about the boards getting contaminated and conductive so all credit to him. So, with the 53v still not connected I switch on and .....nothing! Well it comes out of RESET so that's a start I suppose. Everything appears to be running ie activity on the ADD and DATA lines. Hmm hold on though there's nothing on A11 on the CPU? From here things start to get a bit hazy and as we're looking back quite a few months it's difficult to remember exactly, but I digress. Removing the ROM showed all ADD lines pulsing OK so it was initialising to some extent then crashing. Nothing was making any sense until one day I switched on, it bleeped and the 7 seg came on. The only thing was it didn't go any further as there were no motor drives? I was coming around to the point of having to remove all the 40 pin chips and test them. So 7x 40 way sockets later, all chips tested OK and I'm back to square one. The 7 seg wouldn't always come on and sometimes would just flash for a second and then nothing. Once the board had been on for while (warmed up) it would then start but only to the same point of no motor drives. It was then I discovered that connecting the 53v supply it would boot first time and every time hot, cold but still only as far as the motor drives? The months are passing and I'm really getting nowhere with this. One day I switched on and was connecting and disconnecting the 53v supply and all of a sudden it booted completely!! This made absolutely no sense whatsoever but it did prove that the board was a goer and all I had to do was to find why the 53v supply was affecting it. This went on and on for a while getting nowhere as usual until one day I had it running and out of interest operated one of the DIL switches (3) which crashed the board?? Out came the DIL switch which was contaminated and leaking voltages all over the place. Whilst on this track I then checked the board under all the diodes in the top left hand corner and there was bad tracking all over the PCB so all the diodes had to be lifted and the board deep cleaned. This cleared all the leakage but I still had the original fault that it would only boot half way with the 53v connected sometimes not at all with it not connected and sometimes boot completely?? This was driving me insane! OK, we're nearly at the end of this monumental saga. This morning some 6 months later I was trying a different fault technique as I was trying to find out why the 53v supply was getting in to places it shouldn't be. Using an oscillator connected to the 53v supply with the PSU unplugged I searched round with the scope looking to see if I could pick up any leakage. Nothing was really showing until I got to the SIO chip then touching pins 13+14 showed almost the same voltage I was applying! Further investigation showed that this connection goes to IC U4 (74LS92) which is part of the main clock circuit, it also runs right across the battery area and under the heatsink. It had a break in it somewhere so it was out with the patch wire. By this point I was getting a bit excited and checking on the diagram showed that this feeds a 75kHz clock to the SIO chip. On switch on it all sprung in to life first time, with or without the 53v connected. Bingo I'd sussed it after a good 6 months. This must rate as the worst board I've ever done but it wasn't going to beat me. I do hope Santa reads this. Funny thing is I have another board that won't get passed the motor drives and it could be the same fault of the 75kHz pulse not getting to the SIO chip. That would really make my Xmas if it was. Merry Xmas to all.

BWB - Gold Touch Tech: MPS2 Do you have a Gold Touch? If not try this early Lo-Tech offering from BWB. Keyboard shortcuts in the notes. Thanks to: Wizard (RIP) for MFME @Amot for the roms & images used. Gold Touch.zip