When the CPU initially powers up the LED is lit... even before the CPU starts 
running. Now, once the CPU gets a valid <reset> signal, it goes out the the game 
ROM (U6), and starts executing code. One of the very first things that the CPU 
does to acknowledge that it is running, it to turn off the LED. That is what the 
flicker is. After this the CPU then tests each section of the MPU board and if 
that section passes, the CPU flashes the LED. Hence, if a section is bad, the 
LED fails to flash for that section and you can start troubleshooting more 
intelligently. When your LED is always ON (like mine on my Centaur currently) 
That would mean that the CPU is not even getting any data from the GAME ROM 
(U6).

These are the steps that the MPU board goes through to start up the CPU.

* Power Applied (+5V rises to within tolerance)
* Valid power detector triggers after voltages have stablilized. This
  send a logic LOW to the CPU's reset pin for about 10milliseconds or
so.
* The Valid power detector's output to the reset pin rises to logical
HIGH
  (+5V) and the CPU now goes out to the GAME ROM (U6) via the address
bus
  and starts executing instructions. (the CPU actually does something a 
  little more complicated than this but for now just understanding that
  the CPU *reads* data from the ROM is good enough).
* The CPU reads program data back from the ROM (U6) and proceeds to
init.
  the PIA chips and turn off the LED.

So somewhere in these steps your CPU is getting lost and not completeing
it's first task of turning OFF the LED.

** Things to Check **

Make sure your +5V is between 4.90 and 5.25V, put your voltmeter on AC
volts
and measure across +5V and ground as well, it should be *under* 0.05
volts.
If not, replace your 11,900 uf 20V capacitor on the Regulator board. If
the 
+5V supply has too much 60 Hertz noise on it, the CPU may not boot
because 
this noise transfers onto the Address and Data busses and makes the
signals
garbage.

Make sure you have approx 11.9VDC at Test Point 2 (TP2) on the MPU
board.
If this is not present, the valid power detector will not produce a
valid
reset signal to the CPU. A common problem here is that the 82 Ohm
resistor
R11 opens up.

If you have a logic probe, check the reset pin (pin 40?) while you turn
on the 
game. It should pulse low and remain high forever. If not check for bad
transistors
in the valid power detector circuit.

Make sure the CPU halt line is HIGH (pin 4?) always!

Make sure you do not have a pin bent under on any of the socketed chips.

Other things start to get tricky, you pretty much need a logic probe
here. Let me know if you
want me to ramble on with more info. I have been through this board
*heavy* today so this is
all nice and fresh in my mind. But I still don't have mine fixed. :-(
  jess