PT-80 Won't function on 9V

[boyersdad]

Hey gang,

I built a PT-80 a while back, and on my bench it worked great. Then I'd take it out and slap it on a friends pedal board, and all I can get is the buffered clean! Check for loose wired etc, and it always checked out. Bring it back home, plug in my testing wall wart, and it'd be fine.

So I start suspecting my wallwart. Turns out the (unregulated 9V) wallwart is delivering 12.3V under the ~35-45mA load of the PT-80. The regulated Boss PSA's my friends use apparently won't cut it.

I hooked up my bench supply, and discovered the cut off point for the delay to kick out is below 10.5V.

Anyone else had similar problems? Could it be my regulator? I'm using a 100mA To-92 regulator.

[head_spaz]

It's the regulator.
The 78L12 is a 12 volt regulator... 9 volts doesn't cut it.
The PT80 is designed to run on 12v and 5v internally, and since it is internally regulated, it requires at least 14 volts on the input for proper operation.

Linear Voltage Regulators work by dropping the input voltage level to a lower "steady state" output voltage. Linear regulators cannot raise ouput voltage level above the input level. (for that job you'd need a buck-boost regulator)
Typically, the voltage drop between the input and ouput of a linear regulator is a minimum of two volts. This is called "Drop Out".
So you're gonna need a minimum input of 14 volts in order to achieve proper operation, by design.

You could alter the design to operate on 9v, but you would probably notice the detrimental effects in the pedal's operation.
I would just use a dedicated wallwart for the PT80 and be done with it.

[WLS]

If you want to use a 9v wall wart. Then use the charge pump design.

Bill

[Andre]

My PT-80 (without charge pump) works fine with 9V.
Just leave out the 7812 and place a jumper wire between the outer pads of where the 7812 is supposed to be on the PCB.