Leaderboard

Wow, I can't believe I haven't updated this thread for nearly 4 years! Anyway, back in the workshop this board came in as dead and it was, totally. First check was the 5v rail and it wasn't there! No problem as quite often the LM723 fails except in this case it was a later mod with a 5v reg TO3 transistor. Checking it had 15v (ish) on the input which it did but nothing on the output. Unusual as these things are supposed to be short circuit protected. To cut a long story short there wasn't a short on the board but the acual reg tranny had gone short on it's output. OK no problem there, slip in a new 78H05 and off we go, happy days etc!! So on power on we get about 4.5v slowly rising towards 5v (that's not right) followed by a 'blimey that's getting hot' smell. Quickly turning it off I found two of the 6820's had turned themselves into hot plates. Whip them out and replace with 6821's (slightly less rare than 6820's). Switch on again, nice 5v and nothing! I'm now off the opinion before the reg went short it passed 15v out on the 5v rail! The other two 6820's were both faulty too but cold. Once new chips were fitted there was activity so the CPU looked like it was OK? Due to the PIO chips being damaged I decided to check out all the TTL chips too. Two of them were bad but I decided it would be prudent to swap them all out as a precaution. So now I have all the chips socketed and Bingo she runs (after a few silly bits of diagnosis that we won't go into here ) 6820 CPU's are become scarce now so I was pleased it was holding out and has done after several hours per day of soak tests. There's no thyristor over voltage protection on this mod but it was brought back for MPU3 no doubt for good reason.