Ampeg PF-350 Power Amp Shut. Yellow Fault LED on.

[Tubegamblar]

It's going to be Ampeg today... sorry.

I do have to take care of this common thing that seems to happen with this little amps.

The Amp have the Power Amp section shut. I did the research and seems like none of the threads involving this amp is solved so maybe it's interesting to have one solved because as far as I can see is a common problem with this amp. The Yellow Fault LED issue.

I know this LED can be turned on by trillions of things due to the switching power supply.

This amp has the preamp working allright until the Send effects jack and the DI out. The Send and DI XLR out are working, so the amp can be used as a preamp ;D. Input, Gainstage, tonestack, limiter, Audio In, and Phones all work ok. Cooling fan is not working and there's no power amp output. Customer told me maybe the amp got hot playing with an active bass and super loud

Didn't check anything, so Any ideas for tomorrow to check?

So many thanks to all

[Tubegamblar]

Okey! just found that FQPF12N60C switching Mosfet from the Power supply blown. So here are my questions:

1. FQPF12N60C IS Not Recommended For New Design as of 30-Oct-2013... Does this mean that this transi is not recommended to use anymore (as they fail) or that it is not recommended for designing new devices as the supply is getting short? Will it be hard to find?

2. Are This Mosfets have to be paired? Do they have to be a matched pair?

THANKS!!!! ;D

[MarkusBass]

1. FQPF12N60C IS Not Recommended For New Design as of 30-Oct-2013... Does this mean that this transi is not recommended to use anymore (as they fail) or that it is not recommended for designing new devices as the supply is getting short? Will it be hard to find?

Have you tried to find them? It may be difficult because the transistor is withdrawn from most of the shops I know. But Mouser Espania still has them. I think that the problem is not that they fail but currently there are transistors with much better parameters. So what would be the purpose of buying a worse transistor?

2. Are This Mosfets have to be paired? Do they have to be a matched pair?

I don't think that they have to be matched pair but I would use two transistors from the same production series in order to have the same resistance when they are switched on.

Mark

[Tubegamblar]

So many thanks Mark, I'm also a Marc!

If Mouser Spaña have them i'll find out right now.

So nice if they doesn't have to be paired, same batch will be nice and ok.

ANYWAY, Wich Transi can I use If I want to put a better specs one (or pair ;D)? it will be an upgrade heheheh

R9 is also blown, don't know if all blown together due to the massive blob of White Silicone due to heat or I don't know...

[MarkusBass]

Here is the old one:
http://www.mouser.es/ProductDetail/F...hXVANhORA9c%3d
Similar (or better) transistors can be found by clicking the "Show Similar" button on this page (but first select voltage, current and package type check-boxes).
Sometimes with "better" transistors (with lower Rds On parameter) the supply will work differently so the safest way is to use the old transistor type. But you may also try using some other transistors and see what you get.

Mark

[Enzo]

"Not recommended for new designs" means that the part is being retired, and a designer should not expect there to be a reliable supply of them in the long term. It has nothing to do with the quality of the part. They will be available in the market until the stock is gone, so if your supplier has them , then use them. You are repairing a circuit, not doing new design.

[Tubegamblar]

Hi everybody, thanks for the info and the support. Just replaced the blown transistor (Literally blown i mean phisically broken in half!) and the thing is the same... Fault led on and power amp not sounding. The Thing is that I only replaced the blown one. Replaced also the blown 10k resistor underneath the blob of silicone. Didn't have SMD ones but ended soldering nice and clean an 1/4w regular resistor with the legs well bent and contacting with the pads.

Do you recommend me to put the other transistor as well? if they don't have to be paired, well,it does have to work... but this switchin power supplies drive me crazy...

THANKS!

[Tubegamblar]

Ok the thing is that as the amp was like the same... I've changed the other mosfet... and so the 100k resistor blown for another one with the exact 100k ohms and it's doing nothing. I know Christmas is near the corner but i'll appreciate any thoughts about it. Anything I can measure for diagnosing?


SO MANY THANKS AND HAPPY CHRISTMAS!

Marc.

[MarkusBass]

When a circuit like this fails, all other components in the neighborhood of the failed component should be checked. Have you checked Q3 , Q4, D9, D10 and all resistors there? Which 100k resistor you replaced? Wasn't it 10k resistor on the gate of the MOSFET?

Merry Christmas,
Mark

[Tubegamblar]

Hey Mark Thanks! yes, i've changed the 10k on the gate of the Mosfet sorry! it's 10k not 100K sorry about the mistake ;D so I'll check the other components you mention. D9 and D10 interesting... so tomorrow i'm gonna check them all including Q3 and Q4 ;DDDD

Thanks again!
Marc.