Mar 312020
 

I picked up a Juno First bootleg PCB at the end of December but have only had chance to take a look at it in the last two weeks.

On initial power up the game would not boot. It was displaying a solid colour with lines covering half the screen. On checking the 68A09E CPU on the bottom board I could see the CPU was watchdogging (constantly resetting).


The first basic check that I performed was to verify all the roms on the upper and lower boards.  All of them checked out ok. Next I noticed a capacitor was missing on the lower board in the top left hand corner. Based on the schematics of the original PCB and the other capacitors that appeared on the bootleg PCB, I guessed that this was probably a 220uf 16v capacitor. I also noticed a missing tantalum capacitor, but found pictures online which confirmed it was missing on other boards. I also noted a missing transistor on the top board but this was only to do with a coin counter so was not important.

Next up I started checking the clocks on the processors.  I always get confused with the 6809’s and it’s variants so I wasn’t sure what pins the clock should be on.  It could be either 34/35 or 38/39.  On my board the clock was only showing on pin 34 which I thought was a bit odd.

My full results were:

39 – L
38 – H
35 – H
34 – P

I asked a fellow collector to check his bootleg PCB which partially worked, to see what he had on his pins.  He confirmed the following:

39 – L
38 – P
35 – P
34 – P

So I knew my first issue was one with the clock on the 68A09!

I checked the actual clock was working by looking at one of the pins on the crystal before I started looking any further.

The next problem is that there are no schematics for a Juno First bootleg, only an original board.  Luckily Porchy had previously had an issue with the clock on a Juno First bootleg and mapped out a schematic here:

Jammarcade.net

This made it VERY easy for me to track down where the problem was.  I started at pin 35 at the CPU and then went backwards.  I could not see the clock at pin 8 (E6) but I could see the  clock at pin 9 (E6).  As E6 was an LS14 hex inverter I would expect the same results at pin 8 that I could see at pin 9, and clearly this was not the case, so it seemed as if this hex inverter was not working correctly. I swapped it out and was pleased to see the game boot!

Just when you think the board is 100% and you try and play a game I noticed only partial sound when I started the game. Before digging too deep into this problem, the first thing I did was swap the AY-3-8910 in case this is where the problem was.  I’m glad to say swapping that restored all the sound and the bootleg board was fixed 🙂