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G6Tanuki · 2nd Jan 2018, 3:33 pm

Have a look at the oscillator xtal and see if there are a couple of small capacitors [22 33 or 47pF being typical] one from each xtal pin to earth.

I recall a case in the past where one of these failed and it was enough to unbalance the circuit and stop the oscillator from doing its thing.

There's also a phenomenon known as "Sleeping Sickness" in crystals - generally believed to be due to contamination during manufacture. Basically if the equipment isn't used for some time the crystal becomes _very_ reluctant to oscillate. Confusingly, if you remove a crystal suffering from this and put it on a test-rig that drives it a bit harder than the equipment it was fitted to, it 'wakes-up' and works just fine, as it then will when refitted to the equipment. But if the equipment's not used for a few months the problem recurs.

[I had a batch of VHF "Stornophone 500" walkie-talkies that were afflicted by this. Annoying, since they were specified for emergency-use so typically spent six months languishing in their holsters unloved and unused, just long enough that the crystals went sleepy and the radios then didn't work when needed]

2nd Jan 2018, 3:33 pm	#19
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: 8085 processor questions Have a look at the oscillator xtal and see if there are a couple of small capacitors [22 33 or 47pF being typical] one from each xtal pin to earth. I recall a case in the past where one of these failed and it was enough to unbalance the circuit and stop the oscillator from doing its thing. There's also a phenomenon known as "Sleeping Sickness" in crystals - generally believed to be due to contamination during manufacture. Basically if the equipment isn't used for some time the crystal becomes _very_ reluctant to oscillate. Confusingly, if you remove a crystal suffering from this and put it on a test-rig that drives it a bit harder than the equipment it was fitted to, it 'wakes-up' and works just fine, as it then will when refitted to the equipment. But if the equipment's not used for a few months the problem recurs. [I had a batch of VHF "Stornophone 500" walkie-talkies that were afflicted by this. Annoying, since they were specified for emergency-use so typically spent six months languishing in their holsters unloved and unused, just long enough that the crystals went sleepy and the radios then didn't work when needed]