Atari 2600 Repair Help Needed

[ericpskl]

I have an Atari 2600 (Darth Vader, CX2600A) that I've been trying to repair.  Symptoms:
 

• With a known good cartridge inserted, I get a good RF signal on the TV, but only have a black screen.
• Logic probe shows pulsing on the clock pins, but no pulsing on any address or data lines. 
• Clock frequency matches that of a good 2600.
• All three major chips check out okay -- they are socketed and work fine when installed in another 2600. 
• Good +5V coming out of the voltage regulator.
• Known good external power supply.
• All contacts cleaned multiple times.

 

Any recommendations on what to check next would be appreciated! 

[+Stephen Moss]

If you perform a search of the internet or the 2600 forum you should be able to find a copy of the 2600 field service manual which may help you to diagnose the problem.

Did you check for the clock on all the relevant pins, TIA pins 4, 11 & 26, 6507 pins 26 & 27 and pin39 of the RIOT?

Also check that pin 32 of the TIA is taken to ground and that the reset pins of the 6507 and the RIOT (1 & 34 respectively) are pulled high to 5V.

  

[ericpskl]

Stephen, I owe you a pint!

 

Followed your instructions and found that the RIOT reset pin (PIN 34) was at 0.4 volts.  Pin 34 connects through a 22K (R227) pullup to +5V with a 0.1uF bypass cap (C238) to ground.  Pulled and tested both components and found the cap was bad.  Replaced the cap and she booted right up!

 

 

[+Stephen Moss]

Another Atari lives again.

You did most of the work, with the information you provided, for me a missing clock or reset issue seemed the mostly likely explanation and thus the next things to check.