SOLVED Help needed with Aionfx Lumitron

[pmerritt]

Hi everyone,

I’m new to pedal building and am having a great time. So far I’ve built a Big Muff Pi, Tube Screamer and Honeybee from Aion. All worked as soon as they were plugged in. My luck has changed with pedal number four a Lumitron, Mu-Tron iii clone. My understanding of how circuits actually do things is almost nonexistent though I do understand the fundamentals of most components and I can read a schematic. I have a lot of troubleshooting experience, but in systems very different from pedal circuits. I have been using a Fluke multimeter and an oscilloscope for signal tracing. I have attached the schematic I’m working from and have made no modifications to the circuit.

So I set the board up for testing without any of the offboard wiring done. Everything on and battery connected… nothing. Disconnect the battery and I get signal through the unpowered board (though I’m not sure I understand how that is possible). I have triple checked all component values and orientations. All good. Voltage measurements are where things go off course. Using a battery, I got about .5V drop when connected to the circuit which draws 12mA. Seemed ok to me. What has me stumped are the voltages at the LT1054CP. Pins 1 and 8 get PS voltage as I expected. The -VA at pin 5 makes no sense to me. Looking at the data sheet (and the -VA designation on the schematic) I believe the 1054 is working as a voltage inverter. Therefore, I would expect to measure the same voltage as input except it should be negative. I admit my understanding could be totally wrong. What I measured was about 8.6V input and 0.8V at pin 5. This doesn’t seem right to me at all. The pin 8 and 5 voltages make it to the opamps as expected. Can anyone give me an idea of what to expect for the pin 5 voltage and, if it’s wrong, suggest possible sources of the problem?

I’ve done some more things but don’t want this to be too long for people to want to read

Thanks for reading I greatly appreciate any and all help you can provide!

Peter

 

[jwin615]

It sound like you may have a short on the -VA rail or a bad charge pump. Possibly a bad opamp but assuming you got them somewhere reputable, doubtful.
First, make sure you have ground integrity. Need photos to help much more there but if you are testing unboxed, there's a few ways you could mess up ground.
Check C15s orientation.
Check for solder bridges. Clean, check again.
If -VA is still MIA,
Pull the opamps out and look again.
No go? Is the charge pump hot after being plugged up for a couple of minutes?
If none of these things work, please come back with clear pics of both sides of the board and associated wiring.
Good luck.

 

[pmerritt]

Thank you for the quick response. The chips came from pedalpcb and Aion. Should the pin 5 be the negative of VA as I expected? I will post pics tomorrow.

 

[jwin615]

Oops. Check C14, not C15
Yes, between pin 3 and pin5 should be -9ish
Make sure you have you battery -negative on the power ground pad and not a signal ground pad. Aion runs separate groundplanes sometimes and that could cause an issue

 

[pmerritt]

Well I'm here to report that my Lumitron works perfectly and the reason it didn't was me.

jwin615 hit the nail on the head in the previous post. While I gave myself credit for being able to read in my first post, this clearly isn't the case. In my rush to get the pedal tested, I failed to connect anything to the power ground. In fact the schematic indicates the charge pump is connected to power ground. In my defense, the three dirt pedals I built from Aion have two empty pads next to the PCB out pad. If I had paid attention, these pads on the Lumitron are indeed important. One being for the filter LED, and the other... power ground! Once I added these wires, all worked fine. All in all a learning experience, but frustrating that the mistake was so obvious and preventable.

Many thanks again jwin615!