"Active" Bass onboard wiring problems

[slashandburn]

So I offered to fix a Bass guitar for a friend of a friend, expecting a loose wire, short, or a blown diode or something else simple. I was greeted with something that looks like its been fixed a few time before.

So the reported problem:  It stopped working. He changed the onboard battery. It still didnt work. He then noticed the new battery had got really hot really quick.   

Looking at whats under the hood (also not being a bassist or even all that experience with active guitar pickups) I think what i have here is simply passive pickups and a basic onboard preamp.




Here's some photo's.  The offboard wiring is as follows: 
Positive lead from the battery clip goes to the grey wire coming from the board.   Negative goes to the jack socket Ring.
Red Wire from the board goes to the jack socket tip.  (somewhat peculiar choice of wire colour)
Black wire from the board goes to the jack socket sleeve.
The 2 yellow wires go to the pickup white wires, while the blue wire connects to both Bare wires from the pickups. 

I'm at a loss. Whith a new battery (which also heats up, quick) my DMM tells me something is definately wrong. I cant see any obvious shorts, which has me suspecting somebody has tried to fix this previously and ^&*$%&£ up the wiring. (the grey wire from the board being used for +9v while the red wire is going to the jack tip sort of reinforces this theory, but again, I'm having to trust that it did previously work wired exactly like this. My battery (outwith the circuit) reads 9.2,   if i pop it in the clip and measure directly across the terminals I'm reading zero. And I can't get any kind of readings from the op-amp.

So theres no power going to the board for a start.  Add to that the  reading of 0v across the battery when its in the clip and today I'm doing a whole lot of headscratching.

Maybe I'm being an absolute simpleton and all i have to do is switch out the protection diode. I probably could've done that in the time I've spent scratching my head and going round in circles, I just have this feeling that something isn't right with the wiring. Anyway, I know this isn't strictly a stompbox but it's in the same ballpark so I thought I'd throw this out there for your help.

Should I just stop dragging my feet and swap out the diode and the IC for new ones already? It's not like I don't have spares.

[bluebunny]

Did you test this protection diode?  If it's a short in both directions, then it's the culprit (assuming it sits between +ve and GND).

[slashandburn]

It does infact sit between + and ground, right at the start of the signal.  My meter doesn't have a continuity tester (I know, I couldn't beleive it either), how else can I check this quickly?

Assuming I'm probably replacing it anyway, I could just snip this offending diode from the board?  If it works, replace only the diode, if it doesnt, suspect more fried components?  Am I on the right track?

[GibsonGM]

If it's not in there, and YOU try to install the battery backwards, you can ruin the whole thing, your call.   

You COULD just put a new one in first, or better yet, quickly check it:   http://www.learningaboutelectronics.com/Articles/How-to-test-a-diode

It won't do the "DC test" as it is not conducting DC in this role...

[slashandburn]

Great stuff! Thanks Mike. And Mark.  It would appear that the diode is infact shorted. 

Maybe I'm being an absolute simpleton and all i have to do is switch out the protection diode. I probably could've done that in the time I've spent scratching my head and going round in circles

Thanks again! I'm off out for a bit, I'll report back after I've sourced some stronger coffee.

[GibsonGM]

Hope it works!

[bluebunny]

My meter doesn't have a continuity tester

I'll bet it tests resistance.   

Good luck with your fix.

[duck_arse]

yer good luck with the diode. but, get the datasheet for that IC, find the ground pin, follow that along until you reach some wires, and you should then know if it's arse-about or tikkety-boo. it's a bad idea to cut a protection diode, and then test with battery.

[slashandburn]

And we're good!    Just the diode.  I "Muntzed" it and the guitar sprang back to life. I've woke up a bit. It all makes sense now. 

I'll bet it tests resistance.   

Good luck with your fix.
[/quote]

Thankfully, yes it still does that! Most of the time. I'm told it only refuses to measure resistance when I've made a mistake but sometimes I swear it just like to mess with me.

..............get the datasheet for that IC, find the ground pin, follow that along until you reach some wires, and you should then know if it's arse-about or tikkety-boo.

Great advice! I'll remember that for next time. I ended up just taking the guy on his word that it DID work and all he did was change the battery.  The wiring did look funky though, the offboard stuff was just bare solder joins, floating around in the cavity, and grey wire for 9v didn't sit well with me.

it's a bad idea to cut a protection diode, and then test with battery.

Thanks, yeah I was considering that somewhat. If there was another short somewhere or a mistake with the wiring somewhere I ran the risk of frying more than just a diode. It was a bit of a gamble, but we're all good. As soon as I realised the diode was my shorting the circuit I just decided to go for it.

Thanks again folks!