Leaderboard

Well the silly season has definitely started and nowhere more apparent than in the workshop! Are you stting comfortably.... This one came in as 'stopped working and popping noises'. Usual suspicion goes to one of the tants being short and low and behold, it was. I always change at least three of the tants on these boards for the +&- 12v supplies as they suffer more ripple than the 5v line due to the caps in the PSU falling in value. I also swap out the TVS diodes if there's any signs of green plague or corrosion as these can (and do) go short! Next it's at least one if not all the 4 buffer chips around the CPU depending if they've got the green rot. Then there's the CPU chip socket which can be welded to the CPU chip if you're unlucky. So, a few other odds and sods needed replacing to get the board back to pristine condition, also removed IC35 (been caught out too many times with that one). Time to switch on and be greeted with a lamp test from the diag software except this time it wouldn't boot. It did come out of reset though which can be a battle in itself sometimes. Things now go from bad to worse and then worse still. Trying to work out why it wasn't booting led to both TMS9902 chips being removed and socketed (not a bad thing as it's handy to be able to whip these out for testing). Replacements were tried for IC10 but that made no difference. One of the other 74LS541's that drives the 7 seg looked a bit iffy and once removed and put on my chip tester showed it to be bad. While I was at it I swapped out the last one for good measure although it did test OK. So, power up again and ..... still no boot! Normally you see a quick flash of the peripheral comms on the 7 seg but it wasn't? While checking the connections for that I found it read about 40 Ohms to ground, that's not good! I'd now been on this for a couple of days on and off and thinking I've found it. Get to the bottom of this and it's happy days, how wrong can you be but I digress... After removing all connections from the peripheral comms line the remaining culprit turned out to be IC 11 (CF700045). There are two of these ULA chips and they do all the magic smoke and mirrors that drive all the different sections on the board. The only quick way of checking these is by substitution as there's so many i/p's and o/p's and they all do something depending on something else and I've not got too much of an idea what's actually going on especially at bus speeds! In with the new chip and expecting it all to start happening and ..........still no boot. Hmm, well there's always the Thames I suppose. Another weekend passed and then another day and now I've socketed all the PIO chips, RAM, tapp'd out all the address and data lines to everywhere and checked (and double checked) for any shorts which of course there weren't any. On switch on I could see a quick flash on the 7 seg but it would just drop back to the bottom left segment. It was almost like it was booting seeing something it didn't like and stopping. Next step was to remove the CE lines from the PIO chips to see what would happen. Confusion now reigned as, which ever chip I did, it allowed the 7 seg to switch to the top left segment but nothing else. Now we get to this morning and someone was shouting at me from way up above saying 'have you checked the Power fail line' ? Hmm I did check this a few days ago and it was high, so no power fail. (pay attention here as I might ask questions later) I've now checked all the SIL resistors and all others that lift a pin to 5v just in case any were broken. Attention was now directed to where the battery used to be as most of the vias here get corroded. All the tracks looked good but I've been caught out there too many times in the past so a meter check is always the best policy. There are quite a few links here but most of them are dead ends to the 64 way connector, so never used. I then found that the ground connection which joins up with the battery connection was actually broken from top to bottom and another link that joined IC5 (TMS9902) to somewhere else (one of the ULA's I think it was) was also broken. A few patch wires later and it was time to switch on again. ARGHHH, still no boot. Thames he we come. It's funny I could still hear this voice saying check the power fail and then a faint distant memory hit me that there should be a waveform here! Getting the scope on to IC37 (74LS74) showed no output so with guns ablazing a new one was fitted. Before switching on I thought I'd test the chip first as I couldn't take another disapointment. 'Chip failed' it said, by this time the excitement was unbearable. Switch on and BINGO it's working. What a relief, I think I need a less stressful activity, how about a driving instructor! PS: Reading the manual it does mention that lack of the AC voltage sense supply will cause an NMI and it is the most important one so although the supply was good, because the waveform wasn't there I assume it just shut down. Won't be caught by that one again (I hope)

if you take the plugs of the back board on the reels there should be a long red plug and a little green one one is photo and the other is power then you slide the reels out of the machine so you get to undo the big screws bolts so you can slide the reels apart to get to lamps but be careful as the reels get brittle through age hope this helps