Mikepin emailed me asking if he needed an adapter board to replace a bad PROM on a Xenon Vocalizer board.

He also wanted to know if he has to replace all the PROMs or if there was a way to find just the bad one.

This is an old repair from my Geocities site, so not every pic is a masterpiece.....

vocalizer_b.jpg

The problem with this Xenon, was that when you made the saucer shot, the female voice would say:

"Welcome to *&&%^%#@#@#" with a startling burst of static noise.

All the other voices and sounds appeared correct.

So the sound system was working, but one of the Masked ROMs had suffered from Bitrot - the physical data on the chip had become damaged from age.

The sound for Xenon is a set of two boards.

If you press the red button, it plays all the sounds and speech in order.

I did not want to replace all the Masked ROMs since the "Welcome to Xenon" sample would likely only be on one of them, but I had no way of knowing what samples were burned onto what ROM.

I removed one ROM at a time, then pressed the test button.

The sounds on the now missing ROM play back as silence when you run the test.

The second ROM I removed (U5) proved to be the "Welcome to Xenon" one.

So I burned a U5 image onto a standard 2732 EPROM.

xenon-sound-boards.jpg

Those old Masked ROMs were not erasable like an EPROM is.

They also require a a different pin configuration, so that's where people get the idea that you need an adapter board to replace a Masked ROM with an EPROM.

But Bally designed the board to run with either chip.

1.jpg

In front of each ROM there are 4 jumpers: A,B,C,D

If we jumper A and C, we can use a standard 2732 EPROM on the board.

Because there are jumpers in front of each ROM, we DON'T have to change all the ROMs with EPROMS when we need to only replace one of them.

2.jpg

In this pic we see that jumpers B and D are being used for the ROMs.

Black corrosion is often found on these old ROMs.

If you want to take a desperate stab, you can clean the ROM legs and reinsert it into the socket. About 5% of the time, this actually gets the sounds working again.

masked-roms.jpg

Since you have the de-soldering tools out to move the jumpers anyway, you might as well replace that crappy old socket.

If it was the socket that was actually flaky, and you thought it was the ROM, it could have you chasing your tail for a few minutes.

OLD-SOCKET.jpg

Here you see the socket has been replaced and the B & D jumpers have been moved to positions A & C.

This position makes the board compatible with a 2732 EPROM.

SOCKET-INSTALLED.jpg

I installed the new U5 EPROM and Xenon had her full vocabulary back.

EPROMS have a little window in the center of the chip that lets you erase and reuse them. Exposing them to strong UV light will set all their bits back to zero.

We have to remember to cover the window after erasing, so that light does not damage the new programing.

I will note that I have left an uncovered EPROM out in full sunshine for an entire day and it did not alter the programming (testing the checksum still said the chip was perfect), but you can never be too careful.

EPROM-WINDOW.jpg

Eprom2732.jpg

So this repair demonstrated a few interesting facts:

1. You can replace an old Masked ROM with an EPROM in Xenon's Vocalizer board.

2. You can mix the old ROMs with new EPROMs.

3. You don't need an adapter board to use EPROMs on the board.

4. With a little deduction, you can find just the faulty chip without replacing them all.