Shared from another forum…I’m in the same situation with my build.
“I decided to power the 99/22 up after triple checking all connections for solder bridges. Having my fingers crossed and nothing. Just a black screen, no tone, nothing.
So, for my first test, I tested all my voltages to make sure they were all reading good on all the chips for their correct voltages. On the 9900, -5v on pin 1, +5v on pins 2 & 59 and +12v on pin 27, all read good. Verified none of the logic chips are getting hot which would indicate a bad or fake IC. Next, I pulled out a logic probe to start testing some things and I have found that pin 6 (reset) of the CPU is being held low. The reset line of the CPU should stay low for a split second then go high when you turn the unit on. This is not the case, it stays low, this will basically cause the processor to halt from my understanding and not allow it to do anything at that point.
I know the processor is good. I have pulled from a working unit the TMS9900, 9901, 9904 and 9918.
Now I'm at a loss. What could be causing the CPU reset line to stay low at power up? According to the troubleshooting guide I need to check U601, which I believe is the 9904 (maybe someone can verify that for me). the 9904 tests good and as I said came from working unit and it was already socketed so it was an easy pull for testing.
suggestions anyone?”