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Showing content with the highest reputation on 05/19/21 in all areas

  1. Another head scratcher solved today. This one was a bouncer as it was extremely intermittent. It would run fine then freeze for a bit and maybe carry on or just lock up. Other times it wouldn't boot then a short while later be as good as gold?? Trying to nail down faults like these can take absolutely ages and it's all to easy to give up and 'BER' the board. As it goes I was lucky with this one and whilst monitoring the various lines on the scope I noticed that the E line was low when the fault was apparent. If it hadn't of shown itself at that exact moment I would have missed it. Anyway checking out the E line showed it to be sitting at approx 1.9v when it was good and about 3v when faulty. With the power off you could see with a resistance reading that it was leaking to the 5v rail within a couple of hundred Ohms. Then it would go back to normal and measure over 1M Ohms. Now the E line feeds lots of chips so where does one start? First port of call was the buffer chip IC 21 (74LS04) and the output from this chip was a little ragged on its waveform but not that bad. Anyway a new one made no difference. I didn't want to start on all the PIO's as they all had cropped leads (MOD4) meaning they wouldn't go back into sockets. A real pet hate of mine but that's for another day! So, the E line also goes to the 68B40 and one other chip. I should really have gone here first as the chip is right next to the battery. IC19 (74LS22) was whipped out and a replacement sorted it. I have one other board which is giving me the run around which had an odd fault I've never seen before. It runs absolutely fine but for there's no alarm sounds or switch bleeps when using the diags. These sounds are generated by the 68B40 and not the synth chip so the sound check on the diags works as it should. The odd thing is I do get the initial bleep tone on boot up which also comes from the 68B40. A new chip hasn't solved it nor has the multiplexer chip (74LS138)?? Still plenty of head scratching to come I reckon.
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