Date   : Sat, 04 Dec 2004 00:06:19 +0000
From   : "Nick B." <nick@...>
Subject: Re: Eprom programming problem

On Fri, 3 Dec 2004 20:43:07 -0000, "John & Helen Simpson"
<john@...> said:
> I think I have worked out what the problem is - I have used a CMOS
> 27C128 whereas the instructions for the programmer seem to imply that
> you have to use a plain 27128. I reckon that the higher programming
> voltage might have fried my poor CMOS eprom. Do you think I am on the
> right track with this diagnosis, and if so is it still possible to buy
> 27128s? I think I probably bought the CMOS one because it was the only
> type of 128kBit eprom in the catalogue at the time.

I suspect you have nuked your EPROM. Looking at the design the Vpp
voltage supplied to pin 1 of the EPROM socket is 21V. The older NMOS
27128 EPROMs were 21V but the newer CMOS ones are 12.5V.
I had similar experiences many years ago when i built a programmber for
a C=64. the quick fix would be to fit a 12.5V zener (9v1 and 3v3 in
series) as a shunt regulator at pin 1 fed via a 43 ohm resisitor. Make
the shunt regulator switchable and that will allow you to programme 21v
devices as well.

Some circuits in the zip file on this page:
http://ca.geocities.com/nickbeee/epromer/index.htm 

HTH,

-- 
Nick B.