Monday, 29 May 2017

Atari Fire Truck PCB Repair

So after having a good few years of reliability of my Fire Truck game, it decided to develop a small fault after a 100 mile journey to the Revival Retro gaming show last weekend... The lower part of the screen where the score text is displayed, showed corruption for the top half of the letters. There was nothing I could do over the weekends event and it didn't seem to disrupt the game plays over that weekend. Any way back to having the cab home I broke out my small test monitor and worked on the game from the back of the cab.

Image of the fault at the show, as you can just about make out, the bottom section of the text is correct with the top half showing incorrectly:

The part of circuitry that deals with the text generation on screen is shown below, eight data lines are fed into the 74LS174 at N3 from shared RAM N2 & P2 (Not shown), they are clocked through the the 74LS174 by the *HRDWR signal. When the signal is low, the CPU is stopped by the STPCLK signal, and data will pass through into the Prom at R3 where its parallel output is converted to serial and is then summed with the other video signals.

I was hoping that the Prom faulty as that would have been more time consuming to fix! As it turned out, it was OK, I knew from documenting these schematics previously that when in test mode, no text above or below the play field should be visible, so lets take a look at what my test mode screen showed.

Clearly some sort of text was still visible, The outputs of the 74LS174 should all be held high in test mode essentially making all the bits of the Prom output no data. Checking with my logic probe output pins 5, 7, 10 & 12 were held high but pin 2 was pulsing away with data and triggering the Prom to display garbage data on the screen all the time. I temporarily linked pin 2, to the 5v supply (forcing the signal high) and sure enough the garbled text went away

I removed the device, fitted a socket and replaced with a new 74LS174. Upon re-powering the game everything was working as it should be. Replaced IC shown below.

Thanks for reading, Mart :)