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Old 22nd Mar 2021, 11:33 am   #79
GeraldSommariva
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Join Date: Mar 2021
Location: Liphook, Hampshire, UK
Posts: 125
Default Re: The Transam Triton Personal Computer

Quote:
Originally Posted by ortek_service View Post
Yes, basically the 1.008MHz oscillator is divided down internally in the VDU IC to produce an external-oscillator gate output on pin10, that lets through 64 cycles (characters) of the (Phase)1 (Approx 1.6MHz) clock that increments its internal address counters.

So pin10 (nINI) should always be going high for a bit less than 64us horizontal line time.
- Otherwise, there won't be any character-clock pulses into pin9, from external gated source.

It could be possible that extra breadboard etc. capacitance / long wires to crystal could upset the 1.008MHz oscillator / stop it from working, but looks like you've checked that's still running.
You should actually also see a similar waveform at pin1, but it's likely that it may be a lower amplitude / putting 'scope probe on this could stall oscillator - So often need at least a x10 probe, to reduce capacitance of the probe down to around 10pF from typ. 65pF on x1
I found IC62 dead (74LS86) and replaced with new one outputs are now good. It also looks like the /8 counter IC63 (74LS163) is dead too I don't have one to try.

With the VDU chip out, there is a clock on pin 2 of IC63 which can be adjusted by RV1 (Display Width) P15 RCO is at 1.5v and P12 OC is 0V.

So far that three TTL Chip dead ?
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