Sep 172017

Another board from the box of faulty ones, an Irem M92 motherboard (with Undercover Cops license seal)

For my testing I used the ROM board from a working Undercover Cops (but I could use a whatever one).On boot I was greeted by this screen, self-test failed  reporting errors related to some RAMs 

There are two  6116 SRAMs @IC43 and IC44 whose data bus is tied to the two graphics custom ASIC ‘GA22’ and ‘GA21’  :

When I went to piggyback the RAMs, error changed in this:

Probing them with a scope revealed the outputs were dead on both despite the address lines were properly toggling:

Both chips failed when tested out-of-circuit:

With good RAMs the board successfully passed the self-test and entered in game:

But sound was missing at all, I could hear only a slight buzzing noise:

Using my audio probe I was able to verify that both FM sound and PCM samples were properly generated and then pre-amplified/mixed by OP-AMPs but signal got lost somewhere not reaching the input (pin 1) of the MB3730 amplifier.The whole audio section was previously recapped by someone so I ruled out electrolytic capacitors.But a closer inspection revealed heavy corrosion on many 1/8W resistors (probably due leakage of original ELNA capacitors), here are pictures of them taken with a microscope:

Measuring them gave me infinite resistance sign that they were open:

Here in details the bad resistors I replaced:

  • R201 –  1KOhm
  • R203 –  100KOhm
  • R205 –  4.7KOhm
  • R208 – 4.7KOhm
  • R209 – 2.7KOhm
  • R211 – 10KOhm
  • R213 – 10KOhm

This restored full sound, board 100% fixed.End of job.

 Posted by at 11:58 am

  2 Responses to “Irem M92 motherboard repair log”

  1. Excellent work! keep going the good job!

  2. Thanks for reading!

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