Posts tagged: DAC-312

Tempest repair log – AVG (main) board issue

Status: FIXED

Repair cost: $2 (questionable if part was really needed)


Symptom:  Game board exhibiting strange display behavior – random vectors scrolling back and forth across screen.  Game plays blind.

Diagnosis:  Problem located on AVG board.


  • 08/21/2010 – AUX board and cable verified good.  Tested individual boards on a working setup.  Problem is located on AVG board.
  • 08/22/2010 – Cleaned and reseated 5 socketed RAMs, the 2 vector ROMs, and the CPU.  Board now has no XY output voltage.
  • 08/24/2010 – Tested several chips in the vector generator data shifter section, which confirmed good.  Logic probe pointed to DAC-312 @ D10.  Replaced with no change.
  • 10/30/2010 – Began with game in test mode, no display.  Swapped CPU with known working 6502 and got a single blue vector on screen and activity on the slam switch (switches test screens) and long steady beep.  Confirmed activity on CPU pin 37 & 39.  Activity on E2 pin 1 & 2.  Confirmed ROM activity on the CS1 & CS2 lines.  Theory of operation doc said long steady beep in test mode points to bad vector ROMs.  Flip-flopped the ROMs (socketed) and test pattern appeared.  Adjusting X/Y position, linearity and BIP controls brought test pattern into view.  Went out of test mode and game worked.

Notes: DAC replacement could have had an effect, but hard to tell since game was dead – DAC outputs never changed, but inputs upstream were still bad at that point.   However, after the DAC replacement, I never saw the pattern shown in the video again. Original CPU was later confirmed good.  Bad/dirty sockets ended up being the culprit despite cleaning of chip legs and using contact cleaner before they were inserted into their respective sockets.