Thursday, 18 January 2018

Fixing PSU issues on old BOSS guitar pedal

I recently bought an old BOSS GE-7B graphic equalizer pedal off eBay. It all looked in great condition and worked fine from a battery but not from a 9V supply. Since I don't want to keep changing batteries (the thing is drawing 12mA even when switched off) I couldn't be doing with that, but decided to investigate a bit more before assuming it was faulty

Luckily these pedals come from a time when manufacturers still thought about making things serviceable, and BOSS openly provided the schematics (I think they used to be printed in the instruction sheet - imagine that these days..). These schematics are all over the internet - for example I found one here

Expecting that some component might be damaged, I looked for anything that might be part of the power supply circuit from the DC socket but not from the battery, and immediately saw a resistor and diode (D1 and R1 indicated below)

Wierdly both components looked and tested out fine. I also did some tests on the power socket in case there was an issue there - all checked out fine...

I was at a bit of a loss so did a bit more searching online and found this interesting article

So actually, BOSS's ACA240 supply, labelled 9V DC actually puts out an unregulated voltage that is more like 12V DC and the diode and resistor are a little kludge to reduce the voltage seen by the pedal electronics! Who knew it? I guess it did not hurt BOSS's sales of their own pricey supplies either when generic 9V supplies wouldn't work with the pedal (and don't get me started on the plug polarity...). 

Back to task in hand... to run the pedal on a proper regulated 9V supply I simply jumpered over D1 and R1 to remove them from the power circuit. This is easily done by bridging pads labelled 2 and 3 on the PCB using a short piece of wire

The pedal now works a treat and I'm glad I held off leaving eBay feedback on this till now, since its actually in perfect working condition! I think a lot of older BOSS pedals (those labelled for use with ACA supply rather than PSA supply) can benefit from this little hack (but make sure you check the schematic of other pedals first before assuming the pads are labelled the same!)

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