Here's a Boss HM-2 with a switch that selects between a Boss PSA or ACA power supply.
When you use a PSA power supply and put the switch in the PSA position then the pedal works great. When you use a PSA power supply and put the switch in the ACA position, like the stock circuit, then the pedal is starved for voltage. The circuit isn't quite getting enough voltage to work like it was designed to. It sounds very lo-fi and ratty. In a mostly bad sort of way.
One of my earlier posts talked about the ACA to PSA mod. This pedal has a switch to select the option of engaging the mod (PSA mode) or going back to stock (ACA mode).