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Racal 1992

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Yoanndlc:
It is what I was thinking but the power supply are correct.
They use a RC filter to filter the vcc, there is some variation on the vcc pin but I decrease the serie resistor and there is no improvement.
Thanks to you

Yoanndlc:
Hi Andy,
Could i ask you a last measurement ? Could you make a scope screenshots of the pin 3 of ic20?please.

Thank you very much in advance

Yoann

Andy Watson:
What are you expecting? I think pin 3 is part of the data bus - therefore it is an apparently random signal which will include periods of no-drive - i.e. floating. The image below is a typical capture of IC20 pin 3. This is exactly the sort of signal that I would expect.



Note: My scope is slightly over-reporting the voltage. The peaks should be about 4.9V

Yoanndlc:
Thank you very much Andy. Your signal is very close to mine.
I was expected a clean signal with 0 or 5v but sometimes I have triangular signal.
But You seems to have the same.
I think it is coming from IC39 Q3 output, I will try to add a comparator on this pin
I let you know.
Best regards
Yoann

Yoanndlc:
Hi,
Just to wrap up this post:
I tested the pin with a comparator, and it was working. However, I found the IC39 on eBay.

I replaced it, and now everything is working perfectly.

The issue was indeed with IC39.

I’m very happy! Thanks to everyone for your help.
Yoann

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