Nov 272016
 

Got this original Juno First (by Konami) for a repair :

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Board played fine but some sounds were missing, the most noticeable was the laser shot:

Here is a MAME record for a comparison:

All the sounds are generated by the AY-3-8910 chip.Probing it revealed that three I/O (pin 19-20-21) were stuck high, these should exchange data with same number of I/0 lines of the Intel 8039 MCU like shown in schematics:

80393r

The 8039  was dead,  clock was missing on pin 2 and 3.I replaced the 8MHz quartz and the two 10pF capacitors with no change.This MCU has no internal ROM so it needs an external one :

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Using a dummy ROM file in MAME reproduced exactly the issue, some sounds were missing like in my board.This lead me to think the MCU was bad so I removed it:

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The 8039 is not widely used on arcade system but I was luck and found one on a Gyruss bootleg.Installed a socket and chip:

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and all mssing sounds were back.End of job.

 Posted by at 10:47 pm

  2 Responses to “Juno First repair log #4”

  1. Great stuff, just got one question: is that a 6Mhz CPU running on a 8MHz quartz ?

  2. Yes, the M5L8039P-6 can accept clock up to 6MHz but this was only for a test.