Jun 072016
 

I got this faulty Pyros (US version of Wardner) PCB from Ebay :

DSCN3385

It was not really cheap but it was worth the buy since, on the basis of the seller description, I was confident it could be an easy fix.Board showed an ‘I/O ERROR’ upon boot:

DSCN3384

I knew from my experience that this board produces the mentioned error when, indeed, there is some troubles with the inputs during the startup sequence (also a simple buttom pressure would cause it).So for first I went to check with my logic probe all the inputs probing pins on the JAMMA edge connector.All of them were high except the ‘COIN 2’ (pin 16 solder side) which was stuck low.I could trace it back to pin 17 of a 74SL240 @20N :

74LS240@20N

I could measure only few Ohms of resistance between this pin and ground, this meant internal junction was nearly shorted to GND:

DSCN3387

Once desoldered the chip, it obviously failed the test:

74LS240@20N_testing

With a good chip, the board entered in game with no further issue.

DSCN3392

 Posted by at 11:08 pm

  2 Responses to “Pyros repair log”

  1. Interesting way to see if internal junction is correct or not by measuring its resistance. 🙂 Nice fix !

    • TTLs are based on transistors and so diode.So, you should measure a voltage drop of 600-700mV between GND and I/O.Few ohms means the internal junction has gone bad by shorting to GND.The best would be to analyze the junction with an Octopus curve tracer.