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Finding out what year a pedal was manufactured is a relatively simple process. Usually, you can take a look at the circuit board by opening up the pedal and looking for markings on various electronic components and any other telltale labeling inside.
Many ICs have markings indicating which factory the chip was made in as well as the year and the week of the year the chip was made. On older pedals, potentiometers usually have date markings on them as well. Sometimes you will find a capacitor that has a date code on it as well. Here are some examples:
DOD Metal Maniac FX58
In this pedal we find that the "89" is printed right on the circuit board so we might assume that this pedal was made in 1989. However, upon closer examination of the components inside, we see some obvious date stamps that make it seem that some of the components were made in 1990. Logic would dictate that the pedal was made after the latest part. So we might guess that this came out of the factory around 1990. Odd thing about this pedal as well is that the original markings on the circuit board itself read 'FX56', which is an American Metal pedal, not a Metal Maniac!
DigiTech PDS 20/20
In here we find a lot of different ICs with different date codes stamped on them. We can tell that some were made in 1988 and others in 1987 so we can assume that this pedal was made in 1988 or immediately there after.
This is an older pedal with only the back of the potentiometers marked with any telltale signs of its age. One would think that the pots in this were manufactured in the 25th week of 1980.
Your best bet in finding when your pedal was made is to take a guess at when the pedal was made then look inside the pedal to see if the date codes and other markings make sense and are possibly close to when you think the pedal was made. 99 times out of 100 you'll be right on!