NEC uPD7802 does not reset successfully

Thread Starter

Jojo_Smith

Joined Dec 9, 2022
2
I've got vintage Casio keyboard which upon power on gave scrambled sounds, command states, etc., stuck with it. I've managed to reset old uPD7802 in it with grounding short it's reset pin and everything started to work. After not using it for some months it got scrambled again but this time the reset doesn't work any more. It just changes state in to different scrambled. Found out that it's a common issue with this keyboard.
Data sheet says - "29 RESET (Input, active low) RESET initializes the uPD7801. "
1. am I doing reset the wrong way?
2. did microcode went bad?
3. can some failing discrete element on board prevent normal reset?
 

JohnInTX

Joined Jun 26, 2012
4,712
Welcome to AAC!

Just a guess: your description is typical of an incomplete reset due to the RESET/ signal being too short. Processors of that vintage typically take many clock cycles to do a complete reset and they will come up stupid if they don't get it.
The RESET/ signal is likely generated by an RC network connected to the RESET/ pin. The capacitor may have dried out and failed. See if you can trace the circuit from the pin to the reset network and test or change the capacitor. Mind the polarity.
Sometimes a diode is present across the resistor to dump the cap when the power is removed. A shorted diode would bypass the resistor and make a short reset pulse.
Since you can manually reset the uC, I'd suspect that the resistor and diode are OK.
If you can, post a schematic or the model number of the keyboard.

Good luck!
 

Thread Starter

Jojo_Smith

Joined Dec 9, 2022
2
Thanks for the welcoming John!

I've managed to get the exact circuitry for the reset even with the explanation. Pulled out the capacitor and it tested good. How can I test this circuit further?
2022-12-09_192218.png
 
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