Jan 082016
 

Some days ago I had on the bench this original Pitfall II PCB:

Pitfall_II_PCB

Board booted but all characters/texts were missing and also sound was totally absent:

issue2

I started to check the circuit around the six tiles/characters ROM and found nothing of abormal until I came across a 74LS273 @IC39  with missing clock on pin11:

74LS273@IC39_

As you can see from the above picture someone had already socketed it but broken the trace going to pin 11, he tried also to patch this but didn’t make a good work ending up there was no perfect continuity (45 Ohm measured) between this pin and above pad :

pin11_resistance

Once established  connection, all missing characters were back again:

fixed

So I moved to troubleshoot the lack of sound.Putting my fingers on solderside pins of the LA4460 amplfifier didn’t produce any noise at all so I was sure that it was bad since I could get sound from its input (pin2) connecting it to an external amplifier but nothing on its outputs (pin 7-9).So, I removed the amplifier and put a good one back:

LA4460_reworking

But I was wrong, still no sound at all.So, I went look at schematics of Choplifter that, though runs on different Sega hardware (System 2 while Pitfall II is on System 1), shares the same audio circuitry:

audio_circuitry

I had not taken into account the pin 6 of the LA4460 which is called ‘MUTE’.This pin when tied low, like its name says, mutes the outputs.So, I went to probe it on the board and, indeed, it was stuck low.I could trace it back to pin 16 of a BA12003 @IC129 (a  Darlington transistor arrays) :

BA12003@IC129

The input (pin 1) was low so signal was not inverted internally (like schematics show) and outputted high :

ULN2003

I desoldered and tested it out of circuit where it failed:

BA12003@IC129_failed

I replaced it with a compatible ULN2003:

ULN2003@IC129

and sound was back again.End of job.

 Posted by at 11:19 pm

  2 Responses to “Pitfall II repair log #3”

  1. Most excellent! Your ability to weed out all the faults never ceases to amaze. Keep it up!

    • Thank you!This has been a little tricky since I never dealt before with a Darlington transistor array.