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Man O War DX modulation fix

Started by madbean, November 06, 2019, 01:31:22 AM

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Problem: When the modulation switch is set to the center position to turn modulation off, the rate LED continues to blink for a while before fading out. This also means the LED/LDR combo is active until the LED goes dark.

Cause: The IC pin attached to the center lug of the On/Off/On switch was left floating in the off position. This means any current sitting on that switch continues to drive the LEDs attached to it. Because there is a path to ground through the resistors that current will drain off but can takes several seconds.

Solution: Put a 2M2 resistor from IC7 pin3 to ground. When the switch is in the off position, this has the effect of nearly instantaneously draining that pin and making the LEDs go dark equally as fast.

Why the mistake happened: Most likely because when I originally prototyped the MoW I used an AQB MOD PCB for the modulation and no switch (rather I used a resistor in line with the LDR so the modulation essentially turned off when the depth was turned all the way down). Even though I went further and built both the MoW and MoWDX production boards in addition to my prototype I did not catch the design mistake. Most likely because those pesky LEDs stay lit in sync with the peaks of modulation. IOW, if the LED is fully lit and you put the switch to center, you have the staying lit problem. If the switch is put to off when the LED is in the fully dark part of its swing, it stays dark.

So, my apologies for the mistake. It was pretty noobish on my part and once someone reported the problem I pretty much knew right away why it was happening. Finding the easiest solution actually came from forum member Cybercow. Thank you so much for figuring this out, Mark!

Two images below:
Put a 2M2 between the pin in the orange box and one of the two ground points in the purple boxes. The lower one is a ground via and even though it is very small it is possible to solder to it if you are careful.
Second image is the fix on my build.


Also, and more accurately: with the 2M2 installed and the switch in the center position the resistor pulls down the voltage on the open pin to 300mV or so: sufficiently low enough to eliminate our problem.


Another fix worth noting came from my notes from my Aqua build: if you get artifact-like distortion from the delay...drop the value of C11. I used a 150n and was a nice balance of clarity was just a touch of grit when I hammer the strings. Lower than that was very clean but I found the delay started to sound lifeless.
Gordy Power
How loud is too loud?  What?