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Need Some Help with an Odd EHX Bass Micro Synth Mod...

JanusZarate

JanusZarate

Low End Avenger
Feb 21, 2006
10,198
671
40
Boise, ID, USA
#1
I'm not an electrical expert by any means, so bear with me here. Through some reading, testing and experiments, I've learned that the EHX Bass Micro Synthesizer:
  • Has a 2.5mm DC jack, instead of the usual 2.1mm
  • Is wired to be center-positive at the jack, instead of the usual center-negative
  • Seems to run just fine at 18VDC
I've recently replaced the original DC jack with a new one that's 2.1mm, and I've wired it to be center-negative.

I tested it out, and everything powered up fine, and using just my bass and a computer, the effect works fine.

Now, here comes the weird part.

I plugged it in with the rest of my effects on my pedalboard, plugged it into my Dunlop DC Brick with everything else, and it wouldn't power up. I tried it at 18VDC using an 18VDC output on the Brick - nothing. I tried it on 24VDC using a 9VDC outlet and my Godlyke Power Pump - still nothing. But my other effects powered just fine...

I decided to unplug the patch cables from my BMS, and voila - the light came on! I plugged one cable back in, and the power to the BMS was knocked dead again. I unplugged it and tried the other one in the other jack - same thing.

So, here's what I'm seeing so far:
  • The BMS powers just fine by itself, and works when used by itself (no other effects in the chain)
  • All effects power up with the Brick as long as the BMS is not in the effects chain
  • The power to the BMS is cut off as soon as a patch cable is plugged in to either jack (input or output)
The temporary replacement jack I installed is an all-metal panel mount type - the original was plastic. I have a plastic one coming this week, but the all-metal one was temporary. I covered the chassis that would come in contact with that jack with electrical tape as a precaution, so there's no contact of any kind at that point (at least, I hope not).

The input and output jacks are wired together for grounding purposes, as they're both linked to the gray ground wire you see in the photo. That wire goes to the chassis. I haven't changed any wiring other than what goes on at the DC jack and the LED; all the other wiring is intact. The LED wiring has never been an issue - I changed it to a blue LED months ago.

What might be going wrong? Why does it work by itself perfectly fine, and power with all the other effects when it's not in the chain? Why does it not receive power when it is in the chain?

Is it my DC jack? Is it the grounding? Is it something else altogether?
Is it something I can change?

It's all yours, TB - see if you can solve my little dilemma so I don't have to go back to a wall wart! :help:

[Invalid or Expired Link Removed]
 
#2
I'm going to take an only-marginally-educated stab and say it's the grounding. In a bubble, the reversed polarity of the power jack is "theoretical", it shouldn't really matter which side is + as long as you connect the + pin to the correct place on the circuitboard. But as soon as you connect the ground to other powered devices via the patch cords, all bets are off. Now all of their circuits are connected as one big device, one where the mis-matched polarity from one component to the next is a deal-breaker.
 
#7
Stinkfoot web site said:
When you reverse the power connections for use in a PNP/positive ground pedal, the battery negative is now wired to the DC jack (to be switched out by the adapter plug) while the battery positive is wired to the input jack (to be connected to ground through the plug). Since the adapter jack is the same (the switching part is still on the "sleeve/outside" connections), the jack's connections will also have to be reversed. Simple, eh?
Click to expand...

Wait, does that mean I could do something about the way the input/output jacks are wired to fix the problem? Or am I reading this wrong?
 
#8
MysticBoo said:
Wait, does that mean I could do something about the way the input/output jacks are wired to fix the problem?
Click to expand...

No, you can't rewire the jacks. The instrument signal chain is always negative ground. If you reverse them anywhere you'll short out your signal.

Think of instrument signal and power as two separate paths. Your DC power supply can only be either negative or positive grounded. It can't be both. The BMS probably uses the reverse polarity supply becasue the circuit is positive grounded. Why else would they use that supply?

That quote is talking about switching jacks to turn the battery off and on, but the BMS doesn't use a battery or input jack power switching so it doesn't apply.
 
#9
sonicvi said:
No, you can't rewire the jacks. The instrument signal chain is always negative ground. If you reverse them anywhere you'll short out your signal.
Click to expand...

Okie doke, that's what I thought it was.

Hmm... it's back to the separate adapter, then. I don't have the room on my board for any of the isolated-plug power supplies on the market...

Plan B it is - a mounted surge protector!
 
#10
Vewwy Interwesting... I thought something looked different in your photo... Have you played around with your synth, other than your current mods? I ask, cos mine is different....

I have 3x extra caps and an extra wire...

basssynth.jpg
 
#11
Yours looks older. Yours looks like it's got silver screws on the PCB (mine are black). and yours has a DPDT switch (mine has a 3PDT switch - perfect for true bypass mods).

Mine is stock other than that blue LED.

The pedal is also fried now, but that's a completely unrelated thing :hmm:
 
#14
343 Salty Beans said:
Fried like not working after you re-reversed the power jack? Or fried like since you did that mod its not working?
Click to expand...

I think something shorted out when it came into contact with my kitten and a pair of tweezers. I had it plugged in and open at the time, and she was playing with the tweezers.

It's... uh... complicated. :(

The kitten's fine, though - it didn't even faze her, although she apparently didn't like the flash of the spark. :D

I'm going to contact Electro-Harmonix to see if they could fix it. The board looks fine, but the On/Off switch smells odd. I don't know for sure if that's it, though.
 
#19
MysticBoo said:
I'll give that a shot when I get home. I think something else was fried, too - probably an IC, because I can't see anything visibly burned, but I can smell it on the board.

This sucks either way. :(
Click to expand...

That does suck. I'm sorry for your loss.

But to the problem, I just learned recently that in center negative effects, the negative is connected to the ground, but only when the input cable is in place. They do this by using a stereo input jack and wiring the negative the the middle one (called the ring), therefore when a mono cable is in it is connected to the ground, making the effect able to turn on.

It sounds like the BMS might do the same thing except by connecting the positive to the ground, but then again, I don't know why switching the power jack out would mess it up. Theoretically nothing would change to the rest of the circuit, as long as you wired it correctly...

One thing I had a problem with was that there were three prongs on the power jack i used recently. I still don't know what the third one was, some sort of ground maybe? But I connected it to the wrong one the first time, so things weren't working, changed it and it worked... maybe thats what you did too
 
#20
Sir Edward V said:
One thing I had a problem with was that there were three prongs on the power jack i used recently. I still don't know what the third one was, some sort of ground maybe? But I connected it to the wrong one the first time, so things weren't working, changed it and it worked... maybe thats what you did too
Click to expand...

Nah, the jack I used was a metal panel-mount type - only two prongs on it. On the old one, only two were hooked up (the third was left untouched). I think the third one is just an alternative connection for the barrel, but I didn't bother looking at it further.

The pedal worked fine after the mod, up until the freak accident. It only didn't work when it shared a power supply with other pedals.
 

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