Fender Passort PD-250 stuck in protect mode

[rrflorida]

Ι have a pd-250 that is stuck in protect mode. I bought it this way, just hoping to get it working. I read through this thread http://music-electronics-forum.com/t13287/ - which is very informative.

I measured the power amp transistors, none appear to be shorted. I then disconnected the power from the PA board (CN17), and it is still in protect mode. Not sure what to do next. Any advice appreciated (I'm not a technician, so type slowly...)


I got this useful input from "52 Bill":

"My first guess would be that you have a short in the power supply. Be very careful as this has a switching power supply that can do you some real damage.

The first test that I do is to check all of the diodes in the power supply. Unplug the supply and check to see that the caps have discharged. Then use the diode test function on your meter to test the diodes."


So I'll make this check and report back.


I had originally posted this as a reply to the thread mentioned above, and on the counsel of a senior member of this forum, started this new thread.

Thanks
randy

[rrflorida]

All the diodes on the PS PCB measure good. That is, very low resistance one direction, very high resistance the other direction - incuding the monster diodes d10-d13. Looking over all the PCBs, I don't see any dark areas that might indicate excessive heat. I'd like to pursue further if it's possible without sophisticated bench abilities.

Any input appreciated.
Thanks,
randy

[rrflorida]

One more data point. Just connecting the PS to the power switch and the small protection PCB, the protect light comes on. So it sure seems like a PS issue to me. I'm not qualified to work on the PS "live", so unless there are additional passive measurements to make, I may be at the end of this investigation....

Thanks,
randy

[52 Bill]

There aren't many things that you can test passively with a multimeter in this power supply beyond the diodes that you have already tested. You could look for open circuit traces or resistors.

Don't expect to "see" a bad part, most failed parts look quite normal.

[rrflorida]

Thank you for the input Mr. Bill. I'll look around for those things. Some of the solder joints look a little "cool" to me, so I suppose I will hit them all with an iron - at least it will feel like I'm doing *something*...

[rrflorida]

Passively measured everything I could get to, one false positive on a diode that measured fine out of circuit. Touched most of the joints with an iron. Then decided to make a few live measurements. When I turn on the power, the big inductor (L5 on the schematic, part KS-1554) makes a small spark every time - I can hear it, and see a very small spark. Surely that's bad, right? I had my meter on the 51v output, and it sort of looks like it starts ramping up, and then stops when the spark happens. To be fair, it happens so fast that I might be imagining this part..

So I suppose I'll try to locate one of these things, unless I hear that this is normal behavior.

Thanks again,
randy

[g1]

For starters, I won't warn you like others have about how dangerous these switchmode psu's are when live .
If the spark is from L5 it may be cracked or could be arcing over to a trace under it (if there are any). Is the spark near either end of the inductor? If so I assume you have tried resoldering it. I would suggest removing it and inspecting it and the board for signs of arcing. If the part itself is cracked right near the end it may be possible to stretch & remount.
Otherwise it sounds like the supply is treated as a module by Fender so the part may not be available from them. Perhaps you could find someone with a dead supply for parts.

[rrflorida]

Thanks for the response g-one. The spark is on the front of the coil face about half-way between bottom and top. I turned it on a few more times (in the dark so I could really see it, after dialing 9-1- on the phone , and a spark shot straight sideways from the coil about an inch long - did not appear to be actually arcing to anything external to the coil.

I spoke to Fender just now, and this part retails for $3.99. Oh, but they stopped carrying it in 2007. Sales guy checked with tech, he said I either have to buy a new PS ($400 retail), or find someone to build the coil.

So unless someone sees this post and has a broken PS laying around as you mention, I guess I'm in the market for a coil manufacturer to build one of these things.... seems unlikely..

Thanks

[Enzo]

Can you see the break in the wires or the arc point? The coil might be repairable. If ther is a break, you might be able to splice it. If ther is an arc, you might be able to slip some insulation in between the arcing points.

And this isn't rocket science. Pull the coil. ANy cnahce whatever the problem might be is near the end of the wire? SO me might unwind one or two turns to eliminate the problem. SO the inductance is a little low, sue me. It ought to work. What is there to lose?

Even if ther is a break in the middle of the wires, if you can access the break ends, and unwind each enough to splice...

[rrflorida]

Thank you for the response Mr. Enzo. At the site of the spark, there was a small discolored area deep in the winding, under the top layer. So, in keeping with the spirit of "what is there to lose", I disassembled the coil. The expoxy was the hardest part, but after that it was just unwinding. It now appears to me that the arcing was between coil wire and the core. The core is very sharp at its edges. So....

I'm thinking file the edges of the core to eliminate the sharpness, then either spray the core with urethane or wrap it with tape (unless you guys tell me not to), then buy same-gauge magnet wire (magnet wire is the right stuff, yes?), and re-wrap. 52 two-wire wraps total, 20 wraps on the first layer, 32 wraps on the second layer. I can't make it not work any worse, eh....

Thanks for all the input.
randy

[Enzo]

Sounds reasonable to me. If it works, you are a hero. If it doesn't work, oh well, no worse off than before.

[J M Fahey]

I'm thinking file the edges of the core to eliminate the sharpness,

Skip that part, itīs ceramic.
Unless you are talking about the plastic bobbin.
In that case, itīs unnecessary, itīs made not to hurt wire insulation, of course.

[Steve Conner]

If it's made of ferrite, it could be harder than your file! I've used plumbers Teflon tape to wrap cores in similar situations.

What does the inductor look like, is it an E core set (like a little transformer) or a toroid (doughnut shaped)? The E core sets are sometimes assembled with plastic spacers between the mating faces. If you don't save those and put them back in your new coil, you'll blow something. Also, the wire should be on a plastic bobbin (to prevent it touching the core) that should be saved for rewinding.

Assuming you address these points, I think the chances of success are pretty good!

[rrflorida]

That's a hot tip about the teflon tape - I'll do that.

It looks like a doughnut. I just did a quick test, and fortunately it looks like the file will conquer the core.
The gory details are below.

Thanks for the input and encouragement - I will order the wire (I measured it as .04", so 18 AWG) today, and should have some results around the end of the week.


The coil before:


The bare core (sharp edges with burrs - but not for long....):

[Steve Conner]

That looks like one cheesy inductor! It wouldn't surprise me if the wire insulation got damaged during the winding process, causing it to short out later.

The yellow/grey colour code means the core material is iron powder. You can get the cores from Micrometals.

The inductor is probably part of the PFC circuit, the thing that cleans up the line current to comply with whatever regulations they thought up recently.

Yes, it's magnet wire.