Hi,
I was working yesterday on a 143 Limiter from a friend.
I find quite a few problems with the Built-In Power supply.
First there's Electro Magnetic interference (EMI) between the transformer power transformer and the circuit. MXR tried to minimize that by putting the IN and OUT jacks on opposite side to the transformer. But there will always be some EMI hum induction affecting the audio signal with he transformer inside the case.
Second for 220/240V countries, MXR decided to cut costs on a new transformer, and use the 110V one, so they are dropping 110V to 220V bu using a resistor. It's R40 in the schematic, in my pedal this was a 8K2 resistor, not sure about the wattage I thing from 3W to 5W possibly.
Well this resistor gets really really hot, and also nowadays most European countries are not even 220V , the supply is more close to 240V.
I have a 236V AC supply, the resistor was so hot, the case was really hot, and I was afraid it would melt the solder at the resistor legs.
Also the main Electrolytic Filter capacitor C3 is next to this resistor, and these type of capacitors can fail prematurely when exposed to higher temperatures.
I replace the 8K2 resistor for a 10Watts one, it was a really tight and difficult fit. Still gets pretty Hot
Third and Last, Main Earth connect to pedal case, Audio Ground connects to pedal case, audio ground will connect to Main Earth also trhough the amplifier in the end.
So there's 2 paths to Earth, and it's a potential ground loop situation.
Maybe at home you don't have that problem, but it can happen in a gig, in practice.etc
The solution here could be to Isolate Audio Ground from the case, by insulating the Pots and the Jacks (or using plastic jacks), and then connecting audio ground to the case via a 10R resistor in pararel with a 100nf cap.
like this:
To be honest the Best solution it' to ditch the internal PSU completely and like Ironnman28 advised feed 18V from an external wallwart, that will solve all the problems with the internal PSU.
You feed the 18V right after the Diodes CR1 and CR2, into the regulator and filtering.
You have to remove R40, the transformer T1, and the 2 rectifier diodes CR1 and CR2, and install a DC jack or another connector type for the 18V supply.
Problems solved.
Even if you ditch the intrernal PSU or not I would advise improving the Filtering on the pedal by increasing the value of the axial C3 capacitor from 400uf to 1000uf. and adding 2x 100nf COG Ceramic caps one before the 7815 and one after.
I installed them like this:
I also removed all the rotten and disintegrating foam that was glued to the case back cover, and glued just a plastic sheet insulator :