The power circuit of the Atari Lynx II is a ground switched circuit. Unlike most consoles where the positive voltage rail of the system is normally switched on when turned on, the Lynx II is different and it disconnects the battery/DC ground from the rest of the system until turned on.

The entire circuit is quite complicated and a full breakdown of how it worked is best explained in this YouTube video.

It is very important to correctly measure the voltages on the Atari Lynx.

To measure input voltages for the battery and to U6, because they use the battery ground as potential, connect your ground probe to the battery ground spring, then measure your points of interest such as battery positive wire or U6 pins with the red probe.

To measure output voltage, and generally the rest of the system, connect your ground probe to the metal shield (not battery spring ground), then measure your points of interest.

Short pins 31 and 33 on cartridge slot.

Apply 5V (not 9V!) to battery springs using bench power supply or batteries.

Measure U6 pin 14 with multimeter, black probe on battery spring ground, and red probe on U6 pin 14. If you do not get 5V check all these components are good (D9, L16, L14, D10, power jack pin 2 to 3 shorted).

U6 pins during initial power applied but no power button On triggered yet:

If you do not get the values above, short pin 13 to pin 14 to force the system to turn off and measure again to confirm.

If all IN/OUT pins are high or low, U6 is likely bad. If any IN/OUT pair (such as IN1 and OUT2) are not opposites (such as IN1 = 5V, OUT1 = 0V), then again U6 is likely bad, or traces between them

If tests above are ok, U6 is possibly good.

However, it can fail at the oscillation stage later. So to fully test it, solder a wire from U6 IN6 (pin 13) to VCC- (pin 7) to keep the system on. Use an oscilloscope to see the IN1/2/3, and OUT1/2/3 pins.

If they are not oscillating and instead are solid 5V or 0V, and specifically they are not inverting (meaning IN2 and OUT2 are both 5V or 0V, or IN3 and OUT3 are both 5V or 0V, or IN1 and OUT1 are both 5V or 0V), then U6 is likely bad.

Short pin 13 to pin 7 on U6 (sending pin 13 low), and keep it low (solder a wire between them while power is off). Now measure U6 pins again.



Except IN/OUT 5 and 6, all other inputs should be self oscillating due to the feedback on the base of Q8 controlling IN1 which cascades to IN2, 3 and 4.

Using a multimeter you will read a DC voltage less than 5V and more than 0V (4.8V and 0.2V are just what my particular meter reads, and is based on active load on the console also).

If you see a solid 0V DC instead of pulsing on IN1, then Q8, R72, R74, C43, L15, D13 or Q12 are bad, or traces between them.

Check if the gate of Q8 is high (turning it on). If it is not, check if it has continuity to U6 pin 8, and if U6 pin 8 is 5V (which it should be when turning it on).

A common fault I have seen is the gate of Q12 not connected to the via so not connected to U6 pin 8.

Typically this is a dead Q12 or missing traces between the initial power on state.

Check Q12 is good by measuring resistance to ground spring and the gate pin. This should be open circuit or 100k resistance. A bad Q12 typically reads low ohms resistance between the battery negative spring and gate (less than 1k), due to a breakdown between the Source (ground) and Gate pins.

If that is ok, check the R69 resistor, and all connections (check out Default On State section below).

Finally, confirm it isn't simply that the U6 pin 8 is actually going high for some other reason sending the gate of Q8 high turning it on.

If you see a non-spiked oscillation using oscilloscope on IN1 with a multimeter, and close to input voltage (5V) when probing with ground on metal shield (not battery spring) and positive on battery positive then R72 or C43 are bad, or traces between them.

A bad feedback network won't stop operation, it will just make the buck converter less efficient.

If you see oscillations on IN1 (pin 1), but a lower (or higher) than expected voltage on output, then your D13 diode could be on backwards or faulty. Typically a faulty diode that tests correct in a multimeter (but as it is a zener, so cannot be accurately tested that way) results in higher voltage. An working zener but installed backwards install results in lower voltage.

Also Q12 could be faulty. Test with multimeter in diode mode, red probe to battery spring ground. With black probe to Source should get solid beep. With black probe on Gate you should get 0.70V. With black probe on Drain you should get 0.52V.

Anything more than 0.1V difference is an indication of a problem.

Short pin 13 to pin 7 (sending pin 13 low), and keep it low (solder a wire between them while power is off). Now measure U6 pins again.

If you see a solid 5V DC instead of pulsing on pin 1 (IN1) then the Q8 transistor has to be permanently conducting.

This means either its base is always significantly lower than the emitter caused by a faulty R74, D13 or short circuit, or broken traces between them.

Or, Q8 is bad, with a short between emitter and collector.

Some models of the Lynx II have a switch between pin 33 (5V) and pin 31 (ROM detect) of the cartridge slot.

When a game is inserted the cartridge shorts pin 31 to 33 and provides power to the U6 hex inverter chip on the power on circuit, which in turn controls Q12 enabling power on.

To bypass this part of the circuit (just the U6 hex inverter and requirement for a game inserted), short pins 31 and 33 of the cartridge slot.

Your console should now power on without a game inserted and show INSERT GAME on the screen if everything else was working fine.

The key player in the on/off circuit is U6, which is a CD4069UB Hex Inverter.

Power to U6 comes from the battery positive, through the cartridge connector pin 31 (which is shorted to pin 33, battery positive when a game is inserted).

Ground is directly battery spring ground (not "powered on system ground after Q12).

Make sure when measuring U6 to take measurements with your multimeter from the battery spring ground, not system ground.

The U6 chips pin 6 & 8 ultimately controls Q12 N-Channel MOSFET, sinking its gate to turn off the MOSFET and so the system, or pulling it high to turn on.

When the system is on the pin 6/8 are not solid high, they oscillate based on R74 resistor feeding back the output voltage into the base of Q8.

So the higher the output voltage, the more voltage drop on R74, the lower the base on Q8, the more current flows through Q8. Once Q8 turns on, U6 pin 1 goes high, keeping pin 2 pulled low.

When Q12 is on (pin 1 low), U6 pin 10 is pulling pin 1 low, while Q8 (thanks to R74) is pulling pin 1 high. This causes the self-oscillation based on current through Q8 which is based on R74 voltage drop.

With Q12 off, the energy in L15 powers the output circuit until the voltage drop over R74 is low enough (due to current starvation when L15 runs out of energy) that Q8 turns off.

With Q8 off, then U6 pin 10 sinks pin 1 to ground, turning Q12 back on and repeating the cycle, forming a basic buck converter.

Never just short Q12 Drain to Source. This would turn on Q12 all the time, putting the entire input voltage (usually 9V) into the Lynx which expects 5V. This is also why if Q12 or Q8 fails it can totally kill the console!)

The feedback between U6 pin 2 and Q8 base via R72 and C43 is to turn on/off Q12 quicker by pulling Q8 base low or high through the output of U6 pin 2 going low or high at the moment Q8 starts conducting. So it is a fast on/off feedback circuit, which causes this spike.

If the feedback network is bad (R72 or C43) there will be no spike and worse regulated output voltage.

A bad feedback network won't stop operation, it will just make the buck converter less efficient.

To turn on the system starts with getting power and ground to the U6 chip (explained above), and then setting pin 13 to high or low.

To turn off the system U6 on pin 13 needs to be pulled high.

This cascades through an RC circuit to create a Schmitt Trigger.

The default state of the system at power on is pins 6/8 being low keeping the system off.

This is because until the U6 chip is at operating voltage, the inputs all rise with the system voltage. Once VCC gets above 1.2V to 1.7V the U6 starts operating and releases the input pins so they can be controlled.

However during that moment of cross-over, pin 13 input goes high, triggering pin 10 to also go high, which feedback through R69 with a delay to high, allowing a tiny pulse high to turn into a stable high until it powers up and its locked in high.

This high from pin 10 feeds into pin 1, which in turn outputs low on pin 2.

With pin 2 low, that makes pin 4 go high, sending pin 6 and 8 low.

Pin 6/8 ultimately turn the Q12 on when high, and off when low.

Once Q12 is on, Q8 is responsible for using U6 pin 1 to turn on/off Q12 to buck regulate the output voltage. This is done using the feedback from the output voltage at U6 pin 2, into the base of Q8.

For On, the button ribbon shorts pin 6 (POWER_ON) to pin 7 (POWER COM, U6 pin 13). Pin 6 is connected to ground through R68 resistor, so effectively discharges the voltage from pin 7 to ground, sending pin 13 low ultimately outputting high on pin 6/8, turning on the output for Q12 and so the system is on.

Once released, the Q8 controls U6 pin 1 via the oscillation feedback network takes over and controls the Q12 gate keeping the output voltage buck regulated.

Adjusting the value of R74 adjusts the output voltage higher or lower.

For Off, the button ribbon shorts pin 7 (POWER COM, U6 pin 13), to VCC.

This pulls pin 13 high ultimately outputting low on pin 6/8, turning off the output for Q12 and so the system is off.

Once released, the feedback from pin 10 back to pin 13 via R69 is what keeps pin 13 in its current state (high) without it sinking back to low.

If you removed R69 or it was faulty, the console would only turn off while holding the off button, and come immediately back on once released.

The Hayato / Mikey ASIC has a !Power On (pin 20) that is pulled internally to VCC via a 470k resistor inside the ASIC.

Once power is applied and the ASIC sees ground potential and can operate, this pin is sunk low to keep the U6 pin 13 low in a "powered on" stage.

The ASIC can release this pin and send it high, triggering a power off, if it sees fit.

If Q7 is faulty and shorting it could keep the power up circuit from ever turning on. If it is open faulted the ASIC has no control over the power up circuit when needed.

For debugging you can remove the Q7 transistor and the power up circuit will function without it, with the exception of ASIC control is lost.

Before you get into diagnosing a faulty power circuit, faulty ribbon or many other parts that constitute getting the power circuit to work, you can do a very quick test to power up the console without needing any ribbons, screen, or even a working power circuit.

This test helps quickly identify if the console is working with the exception of the power up circuit or components.

Get a bench power supply and set it to 5V. Connect the ground lead to the shield or one of the solder pads where the shield used to be.

This is important as the shield ground is after the Q12 switching transistor which bypasses the power up circuit.

The shield is on the green trace side, while the battery springs and DC jack are on the blue side which requires a fully working power on circuit and ribbons.

Now attach your red lead (making sure your bench is set to 5V, not 9V!), to the battery positive spring.

This spring goes through the 9V rail which gets lowered to 5V by the power on circuit. By setting it to 5V in the first place and bypassing the ground, we do not need anything to work in the power circuit to power on the console, we are simply providing a regulated 5V directly to the system.

Once power is applied the console should boot up fully, it should show INSERT GAME on the screen, or load the game if one is inserted, and audio should play.

If you have just a speaker connected, no LCD, and bypass the power circuit as shown above, a generally working console should draw around 200mA.

If you connect a screen it will jump up to 350-400mA, and anything above that in general is likely a component failing, shorted or faulty.

If you have excess current, use a thermal camera or IPA to detect where the short/fault is.