The power circuit of the Atari Lynx 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 is different and it disconnects the battery/DC ground from the rest of the system until turned on.

The entire circuit is quite complicated, but matches the Lynx II circuit identically (just a different layout) 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 wire, 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 drain of Q11 (not battery wire ground), then measure your points of interest.

Short pins 31 and 33 on cartridge slot.

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

Measure U6 pin 14 with multimeter, black probe on battery wire 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 IN6 (pin 13) to VCC+ (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 IN6 (pin 13) to VCC- (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.

Ideally use an oscilloscope to see the oscillations, which are usually pulses going from 3V to 5V. If not, below are the readings you likely see if using a multimeter.

Except IN/OUT 5 and 6, all other inputs should be self oscillating due to the feedback on the base of Q13 controlling IN1 which cascades to IN2, 3 and 4.

NOTE: The oscillation patterns below can vary depending on the circuit, power draw and feedback. The important thing is you should see oscillations where shown.

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 Q13, R54, R56, C37, L6, ZD1 or Q11 are bad, or traces between them.

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 source of Q11 (not battery wire) and positive on battery positive then R54 or C37 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 than expected voltage on output, then your ZD1 diode could be on backwards or faulty.

Also Q11 could be faulty. Test with multimeter in diode mode, red probe to battery wire ground. With black probe to Source should get solid beep. With black probe on Gate you should get an open line OL. With black probe on Drain you should get 0.52V.

Anything more than 0.2V difference is an indication of a problem with Q11 or traces to it.

Short IN6 (pin 13) to VCC- (pin 7) of U6 (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 Q13 transistor has to be permanently conducting.

This means either its base is always significantly lower than the emitter caused by a faulty R54, ZD1 or short circuit, or broken traces between them.

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

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

Showing the path here on Lynx II schematic as the Lynx 1 schematic is not clear to show this.

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 Q11 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 wire ground (not powered on system ground after Q11).

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

The U6 chips pin 6 & 8 ultimately controls Q11 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 R56 resistor feeding back the output voltage into the base of Q13.

So the higher the output voltage, the more voltage drop on R56, the lower the base on Q13, the more current flows through Q13. Once Q13 turns on, U6 pin 1 goes high, keeping pin 2 pulled low.

When Q11 is on (pin 1 low), U6 pin 10 is pulling pin 1 low, while Q13 (thanks to R56) is pulling pin 1 high. This causes the self-oscillation based on current through Q13 which is based on R56 voltage drop.

With Q11 off, the energy in L6 powers the output circuit until the voltage drop over R56 is low enough (due to current starvation when L6 runs out of energy) that Q13 turns off.

With Q13 off, then U6 pin 10 sinks pin 1 to ground, turning Q11 back on and repeating the cycle, forming a basic buck converter.

Never just short Q11 Drain to Source. This would turn on Q11 all the time, putting the entire input voltage (usually 9V) into the Lynx which expects 5V. This is also why if Q11 or Q13 fails it can totally kill the console!)

The feedback between U6 pin 2 and Q13 base via R54 and C37 is to turn on/off Q11 quicker by pulling Q13 base low or high through the output of U6 pin 2 going low or high at the moment Q13 starts conducting. So it is a fast on/off feedback circuit, which causes this spike.

If the feedback network is bad (R54 or C37) 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 IN6 (pin 13) to high or low.

To turn off the system U6 on IN6 (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 U6 OUT3 (pin 6) / OUT4 (pin 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, IN6 (pin 13) input goes high, triggering OUT5 (pin 10) to also go high, which feedback through R70 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 OUT5 (pin 10) feeds into IN1 (pin 1), which in turn outputs low on OUT1 (pin 2).

With OUT1 (pin 2) low, that makes OUT2 (pin 4) go high, sending OUT3 (pin 6) and OUT4 (pin 8) low.

OUT3/4 (pin 6/8) ultimately turn the Q11 on when high, and off when low.

Once Q11 is on, Q13 is responsible for using U6 IN1 (pin 1) to turn on/off Q11 to buck regulate the output voltage. This is done using the feedback from the output voltage at U6 OUT1 (pin 2), into the base of Q13.

The On and Off buttons have a shared pad which joins to U6 IN6 (pin 13), which joins to TP18 near U6.

When the On button is pressed, it shorts the U6 IN6 (pin 13) to battery ground through R2 resistor, so effectively discharges the voltage from U6 IN6 (pin 13) to ground, sending it low ultimately outputting high on OUT3/4 (pins 6/8), turning on the output for Q11 and so the system is on.

Once released, the Q13 controls U6 IN1 (pin 1) via the oscillation feedback network takes over and controls the Q11 gate keeping the output voltage buck regulated.

Adjusting the value of R56 adjusts the output voltage higher or lower.

The Off button shorts U6 IN6 (pin 13) to VCC.

This pulls U6 IN6 (pin 13) high ultimately outputting low on OUT3/4 (pins 6/8), turning off the output for Q11 and so the system is off.

Once released, the feedback from U6 OUT5 (pin 10) back to IN6 (pin 13) via R70 is what keeps IN6 (pin 13) in its current state (high) without it sinking back to low.

If you removed R70 or it was faulty, the console would only turn off while holding the off button, and come immediately back on once released.

The 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 IN6 (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 Q4 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 Q4 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 source of Q11 (big tab).

This is important as the Q11 source bypasses the power up circuit and buck regulation.

The main system ground is on the green trace side, while the battery 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 wire.

The battery positive wire 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.