I'm using a dual powered Willem Programmer to read old EPROMs (M27128A, 27256). Unfortunalety I get a lot of reading errors. Some days ago with a 27256 where I thought of a faulty IC, today a 27128 with the same problems. Reading CMOS-EPROMs like 27C256 works.
As I read the power supply can be a problem. I use an external AC/DC adaptor instead of USB supply. On the powered IC on the willem board I can measure 4.88V between VCC and GND.
I also tried to change the timings without success.
Anyone with suggestions what could have gone wrong?
4.88V seems a bit low. The older chips draw more current and may be pulling your Vcc down. I'm not familiar with the "dual powered" but my PCB60e is USB/External powered and I use a 12V 2A wall wart. Does your board have a 7805 regulator? If so, does it get warm?