Leaderboard

Yet another board nearly drove me to insanity. Maybe I'm lossing my touch with age, who knows. Usual battery rot symptoms so all the usual suspects whipped out and renewed. A bit of track patching out needed for the alpha and triac driver links but other than that pretty clean. So on power up (with diags) the triacs all test OK then the tester alpha comes up with MPU4 ALPHATEST OK so all looking good. Thinking I'm in for an early day and just waiting for the lamp tests to start it comes up with an alarm and a bit of garbage on the alpha although I can see it's trying to say 'alarm' but no idea what. The RAM solder joints were looking a bit crusty so I thought that better come out, just in case, which was good as it failed on my tester. I've had similar issues before, like the previous fault above, so it's always a good idea to check out IC16 and put it in a socket which then gives you the option to isolate the CS feeds to all the support chips. Isolating IC5 the alarm was still apparent but the alpha then stopped on a readable 6840 error. Of course changing the chip made no difference. Just a little teaser here, I'd checked the 5v previously and it was a bit low at 4.77v but we all know TTL min rating is 4.5v and it boots OK so I thought I'd come back to that later. Big mistake!! Anyway back at the ranch getting nowhere and contemplating taking out all the support chips and putting in sockets which I did. After each replacement no change to the fault so 2 days later I'm losing the will to live. By now I've made sure all the Address and Data lines are intact plus every other line from the cart to the board. I'm thinking there must be a corroded via somewhere that I've missed. Rapidly running out of ideas and it's now day 3. There's only two options left, the first one is to jump in the Thames but maybe option two first. Thought I'd go back to the low voltage supply and see what's going on there. After stripping down the regulator I thought I'd run a test on the LM323 with my bench PSU and lo and behold it stuck solidly at 5v?? Hmm, what am I missing here? Tried a spare regulator chassis and blow me the board booted and ran fine. After all that wasted work but hey ho it works so put away the swimming trunks. So what was going on with the original regulator? Putting it back with the little board again it's measuring 5v off load but if I put a 12R resistor across the output it drops to 4.77v. It was then I noticed something on the radio. I listen to Caroline on 648Khz and putting the resistor on the output more or less blocked out the radio signal! Suspicion then pointed towards the tant cap that decouples the input to the LM323. When a new one was fitted the voltage remained stable under load at 5v. As the wires to the board power socket weren't attached they probably acted like an aerial otherwise I would have noticed this before. I'm sure someone up there doesn't like me.