[SOLVED] Mixing console lights up and fades away.

Hi everyone, first thread here

I have this mixer console a friend asked me if i could have a look at it, and i'm a bit dummed out with it, hoped someone could give me a hint where to aim.

I believe the console is a yamaha mg102c and what happens with it is I plug it in, hit the switch, and the leds flash bright and then fade away. It happens pretty fast, no sound, no more light, nothing else.
After searching the net if anyone had the same issue with no results, I decided to open it up and see, nothing burnt at eyesight, tested diodes, and a few voltage regulators, some had wonky readings in circuit but once i desoldered and checked them out of circuit they seemed okay.

I then tested the power transformer. It is a yamaha "pa-10" ac transformer(2x18.5v 35w) the output conector has 3 pins (ac, ac, gnd).
First i checked for continuity, beeping only between fases (ac) and NO continuity between gnd and either fase. Then I tested it live for voltage and only got a 41v output between fases, and again, nothing through gnd.

So i guess my question is, is it normal to get a zero volt, no continuity at output gnd pin from such a transformer?
....It can't be an earth lead if the mains don't have ground wire... can't it?

Thanks in advance!

A mixing console processes audio signals. This suggests AC signals, op amps, a bipolar power supply. Also positive and negative voltage regulators, etc. The convenient way to make a bipolar supply, is to start with a center-tap transformer. Can you locate a wire which connects the center tap to the circuit board? If so then the center tap ought to show continuity inside the transformer. However your tests show it does not, is that correct?

My thoughts are not a sure thing, of course.

5inc said:

I plug it in, hit the switch, and the leds flash bright and then fade away. It happens pretty fast,

Click to expand...

Hi,

More than one experience of this kind suggests a possible suspect is a shorted capacitor somewhere, first and easiest is just looking for a bulging electrolytic cap (top). If it's a polyester or ceramic capacitor causing the fault, not sure how you'd check which one(s) are faulty, bad method is to go replacing each capacitor (if none look suspect, and nothing else is causing the fault) until device functions correctly again.

It may not be a dodgy capacitor, but it sounds a very possible cause: Device starts up, then immediately goes off, and/or keeps trying to start up.

whoa! what a quick response! :shock::shock::shock:

Well Brad, that is correct, there is no continuity on the gnd pin from the transformer, I guess that would be the center tap. I never checked power input pins on the mixer though, or with the actual mixer connected to the transformer either, I could do that.


Then d123, thanks, I allready looked at the electrolytics, they all look in great shape, I have no capacitance meter yet, I do have one of those cheap esr meters that tells you about some components, so when i get back home i'll check those.
Ceramic and polyester caps are still one big question mark inside my head. I know they are there, and that they do things, but i never got a shock from one of them, or seen one split open and unrolled unlike i have with electrolytics, so I'll check those aswell when I get there, get the ref and order a bunch.

But before I order those I think I'll wreck the plastic cover from the transformer to see what's going on with that no conductance through gnd, because as brad said, there should be continuity there.


thanks guys!

It certainly sounds like a power supply problem. The normal transformer connection method is one wire is the center tap of the other two so from that connection you should get testmeter continuity to the other two and an equal AC voltage from them. Before delving too deepling into the capacitors which I suspect are not the problem in this case, check the transformer plug itself, it isn't unknown for the joint or crimp in the connector to come apart.

If a capacitor was at fault, the symptoms are more likely to be intermittent operation or buzzing but not a quick spurt of power follwed by nothing.

Brian.

ok, I'm back home allready, cut the case of the transformer open, effectively the green wire does go to the middle. I can't read continuity even in the solder joint on the actual transformer, so as you said, Brian, I guess it's cut off a few mms inside, under other windings before the two wires split appart, that's why there's continuity only from end to end... It looks fine though :bang:

Anyway gonna order a new transformer and when it gets here i'll see if that's all. Will write when I have news.
Thanks for the tips!

Before you order a new one, see if there is a fuse attached to the green wire joint. It won't be a conventional glass fuse, usually they are a small 3mm (1/8 inch) diameter metal tube with a glass plug in one end like this one: https://uk.farnell.com/thermodisc/g4a01084c/fuse-thermal-84-c/dp/476572
Sometimes they are buried inside the windings but still removable. Why I'm suggesting this is if the green wire went to the center tap, normal construction methods would be to wind two wires simultaneously then link opposite ends together to make the center, that implies there must be two breaks, not one, which is highly unlikely. If the joint you can see is from the green wire to the fuse, not the windings, it would indicate the fuse had burned out but the windings are probably OK.

Brian.

Okay, looking for this fuse i found that there is none, but i have found what looks like a open circuit before the terminal, will try and solder it, test conductance and plug it in series with a light bulb, see what it does.

SUCCESS!
Once the transformer was case-free I couldn't see much untill I cut open the two first layers of insulation tape and carboard, and found what appeared to be an open circuit. I was expecting to find a fuse there, and hopefully some reference. I made a little extension using electric wire to hook up the ends of the open circuit and testing it gave me 20.4v through it on both ends. Hooked it up to my lamp-in-series circuit and the bulb merely glowed(good sign), so hooked it off and straight into ac mains, and there she went!

So it all ended up in a simple solder joint in the transformer.

now I have one question left, shall I also add a thermal fuse there? what do you think?
Thanks guys

Good going! A victory! Perhaps what went bad was a fusible link wire? In any case it would pay to think about what caused this failure. Was it overmuch current due to some condition elsewhere on the board?

Even though you now achieved success, you should continue to look for any sign of the previous mishap. Failure of the power supply frequently indicates it was taxed beyond endurance.

What about an easily replaceable fuse? If there is one then it should have blown. Is there some reason it did not?

shall I also add a thermal fuse there?

Click to expand...

This depends on whether the failure was caused by excessive heat, or excessive current. If you can track it down then you know better what is the proper type of circuit protection you should install.

If it didn't have a thermal fuse before I wouldn't think adding one now would make it safer. Usually, if a thermal fuse blows there is plenty of evidence of overheating such as brown 'burn' marks or discoloration of the wires. They do not work by overcurrent but by overheating and even a low temperature one takes long enough to pop that there is normally other evidence of cooking nearby.

If you want to add a fuse, I would add one in the green wire but outside the transformer. Snip the wire and add an in-line fuse holder with a normal replaceable glass fuse. Without knowledge of the circuitry I can't be sure of what rating to use but if this is only a mixer unit, the current consumption would be quite low, maybe 0.2A or so. If you added a 1A anti-surge (time lag) fuse it should be adequate.

Brian.

BradtheRad said:

What about an easily replaceable fuse? If there is one then it should have blown. Is there some reason it did not?



This depends on whether the failure was caused by excessive heat, or excessive current. If you can track it down then you know better what is the proper type of circuit protection you should install.

Click to expand...

the reason the fuse didn't blow is there WASN'T a fuse there, I guess it was wired together with solder wire, and by excessive heat it just melted. It doesn't look like there was any sparks or any explosion, there's no blackness anyhere, just some wires that did'nt even look like they had to be connected.

Could I assume it was because of abusive using? It is a fact it has been heavily used.

betwixt said:

If it didn't have a thermal fuse before I wouldn't think adding one now would make it safer. Usually, if a thermal fuse blows there is plenty of evidence of overheating such as brown 'burn' marks or discoloration of the wires. They do not work by overcurrent but by overheating and even a low temperature one takes long enough to pop that there is normally other evidence of cooking nearby.

If you want to add a fuse, I would add one in the green wire but outside the transformer. Snip the wire and add an in-line fuse holder with a normal replaceable glass fuse. Without knowledge of the circuitry I can't be sure of what rating to use but if this is only a mixer unit, the current consumption would be quite low, maybe 0.2A or so. If you added a 1A anti-surge (time lag) fuse it should be adequate.

Brian.

Click to expand...

Okay, the transformer says it delivers 35w on 2x18.5v.
So to get intensity we divide the power(35w) by the voltage(18.5v) and it gives us 1.89A. (We don't divide it by 2 because it holds both ac lines don't we?)
My big question is, if the glass fuses are marked for 250v, can you use in this case a 2A(I don't know if there are 1.9Afuses) glass fuse and expect it to trigger correctly? because for example car fuses usually carry a much thicker wire for 10A at 12v for example, than a glass one marked for 250v. Can I assume there is some relation to voltage and thickness, or is it because DC is different?

2A glass fuse then? or am I going too fast?

2A would be fine. The rating is the maximum the transformer can handle but in real life a good designer would run it well below maximum to allow a safety margin.
I would be suprised if it even consumed 1A if it is only a mixing console, they are normally quite low power devices, in fact most of the power on ones I have worked on goes into things like indicator lamps and meter backlights, the remaining electronics needs very little power.

Conventional solder melts at around 280 degrees Celsius so if the transformer had reached that temperature to break the joint, there would be lots of burn marks and the plastic insulation on the green wire would be charred or melted back as well. I think this is most likely to be a case of poor manufacturing quality and after time the expansion and contraction due to normal temperature changes just fractured the connection.

Brian.