Fix for oscillating power amplifier (Output static fix)
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Hello everyone.
I've noticed a really annoying intermittent hardware bug related to the power amplifier. If you've connected power to your Mega MIDI module and have noticed no "FM sound" and just a bunch of static (or other popping noises), feel your power amplifier IC (U8 on the far right side). If it is getting warm, you are likely experiencing an oscillating amplifier.
The reason for this is as follows:
- The LC filter values are off. C31/32 should be 0.68uF, not 68nF. L3/4 need to be 22uH, not 2.2mH. This is an artifact of my Mega Blaster design that boosted treble, but in this case, it causes instability.
- My routing in that area is less than ideal. I think the inductors are a little too far away.
Solutions:
- Simply unplug the power supply, wait a moment for the power capacitors to drain, then reapply power. Occasionally, this will work.
- Replace the components. Unfortunately, this is a little involved. I will give a step-by-step down below on how to replace your components.
- Tap a 100nF capacitor (rated for at least 16V) against U8's pin 19 and 20. This should give the amp a little capacitive kick to drop out of it's oscillation.
- Permanently apply 100nF ceramic disc capacitors across U8's pin 11/12 and pin 19/20. This is not recommended as you are essentially shorting the power amplifier. This fix is a "band-aid" and will cause the amp and caps to warm up to around 70°C. You can usually stuff the caps within the IC socket then add the amp chip back over-top. Again, this method is not recommended, but is listed since it's one of the easier fixes.
Replacing the components
In order to access the necessary components, you will need to remove the display and plastic back plate.
- Begin by unscrewing the four corner screws on the LCD module.
- Using a heat-gun or soldering iron, quickly heat as many pins as you can (applying lots fresh solder helps!) and apply a firm upwards pressure to lift the display off of it's pins.
- You will need to use a solder-pump to remove the excess solder blocking off the LCD pin-holes. You can also use a can of air-duster/compressed air to blast the solder out, but show caution as hot solder may fly everywhere.
- Remove the five back plate screws and the 4 standoffs, then remove the back plate.
- Locate L3 and L4. Apply heat to both of their pins and pull from the other side to remove.
- Remove C32 and C31.
- Replace C32 and C31 with a 0.68uF 0805 capacitor rated for 16V+
- Replace L3 and L4 with 22uH radial inductors (8.7x11mm)
- Here are some! https://www.digikey.com/product-detail/en/samsung-electro-mechanics/CL21B684JOFNNNE/1276-2976-1-ND/3891062
Ensure that your LCD header pins sticking out of the main board are clear of any excess solder.
- Screw the back plate back on using the 5 screws, then screw-in the standoffs. Show caution not to snap-off any of the screws or standoffs by over-torquing them or bending them!
- Align your LCD onto the standoffs and make sure all of the pins are in their correct location.
- Re-solder the LCD.
All done!
What now?
Any Mega MIDIs ordered from me after June 13 2019 will already have this fix applied to them.
A future revision to the Mega MIDI is in the works that will optimize the board layout and hopefully provide better A/D separation. The amplifier circuit will also be replaced with a design based on the Mega Amp Mod. This will simplify my BOM and will make it easier to support YM3438s.
Stay tuned, sorry for the inconvenience! Thank you so much for your patience and support.