Frequently Asked Questions - Troubleshooting

You are using MAX4558 multiplexers without clamping Schottky diodes(bat 85), aren't you? Although MAX4558 has ESD protection,it
doesn't work well when there is overvoltage on inputs and you still
need clamping Schottky diodes. So in reality MAX4558 isn't any better
than a regular 4051 multiplexer.

If this a latest version of MegaDrum without an LCD contrast pot than
you need to adjust the LCD contrast from the MegaDrum menu. After MegaDrum is
powered up, press UP/DOWN to adjust it. If this is an older version of
MegaDrum than there is a 10k VR1 in it. You set LCD contrast with it.
The potentiometer, you have, might be a multiturn type so you might
have to make several revolutions to set correct LCD contrast.

Before trying to troubleshoot the problem, swear that all resistors
(measured with a tester), capacitors and an inductor of correct values
and you have no short circuits or breaks. After that tell about the problem on the forum and I'll try to help. I'll be frustrated if it will have turned out to be a short or a break after all.

From experience some samples of PIC18F2550 don't work properly with the USB firmware version from http://www.megadrum.info/content/downloads. Try my alternative USB MIDI firmware for PIC18F2550 from this thread: http://www.megadrum.info/forums/viewtopic.php?f=3&t=268 . Note, that in this case pin26 of PIC18F2550 must be connected to the ground over a 22k-47k resistor.

No they cannot but there is a workaround. See a guide in the Documentation.