Hello,
In my practice, I once encountered a similar problem while repairing printing equipment
VEB Polygraph Buchbindereimaschinenwerke Leipzig -
VEM NUMERIK "KARL MARX" Karl-Marx-Stadt, a country that no longer exists - GDR.
In practice, I had to reverse engineer all the boards, including the six-layer CPU board (U880 - Z80).
EPROM was made on UV U2716 24 pieces (MC2716)
During the repair, dumps were taken from all EPROMs.
And, the programmer that I recommended to you (Advantech LabTool 48) allows you to do final verification of the EPROM contents, using a check when increasing and decreasing the power supply from the nominal value.
As it turned out, - a very effective verification method that identifies vulnerable UV ROM cells.
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And so.
I would like to offer you a method that involves supplying additional power to the PCB in the +5V circuit with an acceptable excess of +5.5V, in parallel with the already present - from the standard PSU.
If, of course, you have a reliable laboratory power supply.
The goal is to provoke a change in the situation or reaction, including TTL and everything else.
In this case, you can manipulate the "virtual RESET button" that
@Harry_22 talked about.
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In fact, you do not risk anything if you do everything consciously and carefully.
If there is a result, then a further strategy can be developed.
@222Lab_Test222 I do not insist on anything.
It's up to you.
Good luck.