Fixing Atari's QWAK! (1974)

Atari's QWAK! is the third arcade PCB I bought.

I didn't let my current inability to fix the Olympic Tennis boards prevent me from buying more boards.

I bought this board from the guys at QuarterArcade, with the intention of dumping the ROM, to make it publicly available.

I wanted to dump the ROM so badly, because nobody on the Vintage Arcade Preservation Society's website had marked theirs as working.

Here is the circuit board. This picture is from the Ebay listing for the item I bought.

Source: QuarterArcade

Somehow, the pic from the Ebay listing is still on the internet. Do a quick search:Atari QWAK! pcb.

Oh, wait. I put it on the internet. *FACEPALM*

The schematic on the internet is for the Revision - "F".
From looking at my board, I have the earlier Revision - "C".

(I have already done some repairs to the board.)

After I made a test rig, this is what appeared:

It's not impossible to tell, this board has problems.

  1. No score area
  2. No buzzing sound complaining that someone stole the light gun.
  3. No sound
  4. No duck
  5. The screen is not a solid grey

Later, I discovered:

  1. No ROM Output
  2. -12 volts is not present
  3. The duck is there, but something about the enable isn't right...
  4. The game can't be coined up

Since this was before I became proficient at TTL by creating PERISCOPE, I was very clueless but optomistic about fixing this.

So, I just started piggybacking chips. That was literally my method of finding bad chips. Go me.

Later, I was able to realize that I wasn't going to get the game working without the -12 volts being outputted by the negative voltage circuit.

So, I began focusing my efforts there.

After replacing (almost) all the capacitors in the negative voltage circuit, I decided that I needed to replace the zeener diode, and the ua741. It was after that, I realized that a replacement for the ua741 was the NTE941M. I replaced it and the zeener diode.

While replacing the zeener diode, I realized there was some wierd sort of resistor put where the diode was supposed to be.

See? That is clearly a diode symbol.

Boom. Replaced.

After replacing all that, it still didn't work.

By the way, I feel I should mention that this all is happening about half a year after I bought it, and after I replaced about 20 chips... [Insert sheepish grin :)]

Anyway, I'm out of ideas. I replaced the first capacitor in the circuit, but there is no negative voltage present there.

There is a transistor, Q1, at the top right of the board. All of the pins are reading ~1 volt, so maybe I need to replace it?

If that doesn't do it, I don't know what will...

Help me, Obi-Wan-Ken-2N5193/NTE185-Obi. You are my only hope.

I replaced it, and it still doesn't work. Where is a nuke gif when you need one?

I should probably mention that unconnected pads come off easily when desoldering.

(Hey? It's not like I'm trying to damage my board or anything. I'm looking into alternatives for sucking up solder to see If I can prevent that.)

Okay. I have (unintentionally) shotgunned the whole negative voltage circuit. It still doesn't work.

Maybe there is some other reason why it doesn't work?

At this point, it occurred to me that I have never seen a negative voltage circuit with a straight-up DC power supply input. They always have used an AC supply. I am using a DC benchtop power supply. Maybe that's the problem?

I read the section on negative voltage supplies in Practical Electronics for Inventors by Paul Schertz and Simon Monk, and it basically gave me the message that you can never use DC power supplies to power a negative voltage circuit.

A few months passed. I couldn't find a 25 vac center-tapped transformer.

And yet something about those searches for center-tapped transformers bothered me. I saw "25 VAC" center-tapped transformers, but they were 12.6 - 0 - 12.6 volts. I knew those couldn't be the right thing because Atari QWAK! used those voltages to make 18 volts DC.

Since I already had a transformer that was to those specifications, I decided to try it in case I was wrong. I figured it wouldn't work because I thought I needed a 25 - 0 - 25 volt ac transformer.

I turned it on... and nothing happened. Just the weird white bar on the screen. Just in case my ROM was bad, I used my oscilloscope to test and see if the negative voltage was working.


I quickly coined up the game and I heard these lovely noises:

The QWAK! noise. Not working.

The gun stolen siren. May not be fully working, but I don't know.

The gunshot noise. I think it is correct.

(I was shorting some pins together to get the gunshot noise.)

On some websites, the description of QWAK! said that a little square would appear where your shot was placed. I looked at the schematics awhile back, and there was one circuit I didn't understand. I thought nothing of it. I saw nothing that indicated that feature.

But then I fixed the negative voltage circuit. And that all changed:

I still have no light gun to test the game, so I am just shorting wires together. That is why the square shows up in random places.

Getting the negative voltage circuit to finally work was a bittersweet moment. While I had just heard the sound for the first time, it also verified that my ROM was dead.

My oscilloscope also verified that. Good inputs, no output. I made sure it was enabled, and it was.

This dog will not hunt.

But I really want to be able to play QWAK!

That leaves one thing that I can do: Reverse engineer the ROM section of the game, and design my own ROM.

I considered using an arduino, but that would be too slow.

I considered using 16 of my leftover 82S23s to make a new ROM. But then I designed it in my head and realized that it would take more than 50 chips to accomplish.

I even considered making my own diode array to replace the ROM. That would be too huge.

So, I can either buy a new ROM, which would be nigh-impossible.

Or, I can buy some (larger) Bipolar ROMS and make my own programmer and daughter card.

Stay tuned.

This page was last updated 1/1/2020

I made a spelling correction.

This page was probably published sometime in September 2018.