Boss DD-3 not working: weird 5v regulator problem

[drummer4gc]

Hey all,

I've got this DD-3 (square chip model) that won't turn on. Initially, it worked but included a rhythmic BEEEEP along with the repeats (and when not playing). After taking it apart to inspect, now it won't turn on.

I am noticing the the 5v regulator is only putting out about .2v. It's getting a solid 9v in, but not much out the business end. I've replaced it with the same result, and both regulators worked fine outside the circuit. Yes, they were connected correctly.

I'm guessing the voltage is being pulled down somehow - my DMM shows continuity from the 5v trace to ground, but there isn't a short I can see. I'm wondering if an old electrolytic might be leaking to ground...does that sound like it might be causing this issue? Any other thoughts about why the voltage coming out of the regulator would be so low?

Thanks!

Schematic: http://www.synfo.nl/servicemanuals/Boss/DD-3A_SERVICE_NOTES.pdf

[bluebunny]

my DMM shows continuity from the 5v trace to ground, but there isn't a short I can see.

If there's continuity from 5V to ground, then there's a short.  Nice try, but you can't deny physics!   

[antonis]

But you can cheat by disconecting the electro cap..

[drummer4gc]

If there's continuity from 5V to ground, then there's a short.  Nice try, but you can't deny physics!   
[/quote]

Totally - what I meant is that there does not appear to be an "external" short due to leads touching or stray solder, but rather an internal short in some component(s). I'm just wondering what might be the most likely component(s) to short so that I don't have to disconnect everything between 5v and ground, although maybe this is my only option.

[R.G.]

Finding a short in a circuit with lots of components is a classically tough problem for techs. There is a way, but most people will not go there.

Electronics is taught to as in a manner that I call "voltage-centric". We measure voltages and deduce things from that. This is probably because we have easily available meters that can measure voltage without tinkering with the circuit itself. Finding a short somewhere in a network of parallel components means that you either have to open possible current paths to isolate the one that's causing the problem, or have some way to follow current without opening the connections.

This last - following currents without opening paths - is possible, but demanding. There are two ways I have seen, and both rely on the magnetic field caused by the current flowing.  One is to feed the shorted connections an AC current and use a tiny magnetic tape head followed by suitable amplification to go where the current leads you. The other is to use a Hall effect probe to sense the magnetic field, AC or DC. There are commercial instruments that do this, but you won't find them at Harbor Freight or Radio Shack, and you need lots of cash to buy them where you do find them. You're left with building your own instrument.

In your case, removing and replacing electros is a good start. Electrolytics have a built-in failure mechanism even with proper use. And if you reverse them, they die even more quickly. In your case, I'd order a set of electros and replace them one at a time. No sense going to all the work to take an electro out and put it back in when for another $0.25 you could put in a new one.

[bloxstompboxes]

We have used these, and the older model, at work for finding shorts detected during an ICT, Inner Connectivity Test. They are an awesome tool to have, especially since they can measure down to something like an ohm. You can do things like removing an inductor to isolate what side of it the short is on. Inductors do have a small amount of resistance. Of course, this might be to expensive and too seldomly useful a tool for our smaller and simpler circuits.

http://eds-inc.com/product/leakseeker-89/

[drummer4gc]

I went ahead and lifted the leg of every component touching the 5v traces and still found continuity from 5v to ground....until I (very, very carefully) lifted pins 33 and 75 (VDD) on the main MN51010 chip. No more continuity from 5v to ground! Replaced every other component leg and left these two on the chip lifted, again no short. Success!...sort of. Is there any other possibility than a dead/internally shorted chip? Remaining hopeful as I don't think they're available to buy (and I don't really want to solder an 84 pin SMD chip).

[PRR]

What is "continuity"? 99 Ohms or 0.0 Ohms?

Most things are connected to most things in a circuit. Different meters have different notions of "continuity". Continuity mode may be useful for rapid go/no-go checks, especially in a system like trailer lights which "should" be all near-infinity and near-zero. But in electronics, Ohms are useful numbers.

[drummer4gc]

Good call. I measure 2.5 ohms between 5v+ and ground.

[PRR]

2.5 Ohms, across a chip, with modern digital meters, seems *very* low.

(If you use an old needle-meter made for testing cars, the high voltage *will* cause a low reading one way, a higher reading the other way.)

[Vitrolin]

i understnd why you dont want to solder a 84 pin chip, it would propably be pointless, i would guess that it needs some firmware to work as a DD-3