Please help me debug my circuit

Question

I am trying to build a guitar pedal but am unfortunately failing. I have about a high school physics understanding of electronic circuits, but the readings I am getting seem impossible to me. I might obviously be measuring wrong, or I might have a flaw in my understanding somewhere, so I turn to you for help.

First, here is the circuit diagram:

My issue is this: If I connect my multimeter to the 9V input and the ground, and measure the resistance, I measure about 500 ohms. This seems about what I would expect, as most guitar pedals consume somewhere in the range of 10 to 200 mA, and so this circuit would to my understanding require 18 mA given a 9V power source.

However, when I connect my power source and measure the voltage from those same two points, I read about 0.8V. The adapter I'm using can supply only 200mA of power, and so by Ohm's law there should only be a 4 ohm resistance. I own another power supply, which is supposed to supply 1700 mA. Plugging this in leads to a voltage measurement that fluctuates between 0.5 and 0.7 volts.

Now, if I were to have a short circuit somewhere, I would still expect to read 9V over the whole circuit. If I did not have a full connection, I would expect to read infinite ohm resistance, and no voltage either. My only explanation is that there is a complete circuit, but the real resistance is much higher than I am measuring. How could this be?

EDIT: Here are two pictures of the top and bottom of the PCB:

Answer 1

You have diode D2 installed backwards:

The blue/green arrow points to the banded end of the diode as marked on the PCB. The red arrow points to the banded end of the diode as you have installed it.

The 1N4001 can handle a good bit of current, so it probably wasn't damaged.

D2 is there as a reverse polarity protection. The idea is to short circuit the power source if it is connected backwards.

With D2 installed backwards, it shorts the power supply when you connect it correctly.

Remove D2, and install it as marked on the PCB.

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The 0.8V you measured was the give away. That's about the forward voltage of a diode conducting a large current. I know that guitar pedals often have a reverse polarity protection made using a diode across the power supply inputs, so I looked for one in your schematic and found D2. I then checked your photos to find D2, and found it there next to R13. The photos are fuzzy, but clear enough to see the cathode marking on the PCB for D2 and the cathode marking on the diode itself.

Answer 2

Further to JRE's answer, something else looked odd.

Having found a picture of the bare PCB:

Image Source

It is clear that all of your variable resistors are inserted the wrong way also. The two metal pins visible from the top of the trimmers are each end of the resistor (pins 1 and 3). The middle pin (not visible from the top) is the wiper (pin 2):

Image Source

You have connected your trimmers 120 degrees out from the correct rotation - notice on the PCB there are numbers marking 1, 2 and 3. You've connected pin 1 to pin 2, pin 2 to pin 3, and pin 3 to pin 1.