Fender PA problem. Passpoprt 250

[Showey]

I have a Fender Passport 250 that let the smoke out of a couple of the
transistors on the power amp board and went into protect mode.
I replaced the failed transistors (very carefully) and it worked!!
BUT, once I really put the screws to it for a minute or two, (singing etc)
it smoked 'em again. (seems fine if left idle or spoken short bursts through the microphone.)
Where do I start? What have I missed?
Any help is GREATLY appreciated. This P.A. is used in a church and for my daughters newly formed band.

[Enzo]

Isn't that a serious pain in the butt to take apart and service? Really not the type of thing to make you happy to have to do again.

First, what transistors did you put in it and where did you get them? COunterfeit power transistors are a serious problem these days, and if you buy from these little discount places that send out flyers all the time, be very wary.

SO far I have not gotten any counterfeits from MCM nor from B&D Enterprises
www.mcmelectronics.com
B&D Enterprises - electronics, projection lamps for hitachi, sony, mitsubishi, panasonic, samsung

ANd if you used NTE parts, shame on you.

Otherwise, when you change burnt power transistors, also check the ballast resistors for opens. Ballasts are those power resistors in all the amps of .33 ohms or something similar. "Emitter resistors." Always check the driver transistors, and for that matter the rest of the resistors associated with the output stage.

[Showey]

Man they sure are a pain!
The first time, I swapped used ones from another board
(trasistors checked OK) The 2nd time, I bought ALL new ones from Mouser.
So now... I need to ask, how do I bias this board? Do I NEED to bias after I replace the 242's and 1962's?
I am not stupid but I AM somewhat new to this type of repair.

Thanks in advance for all the great info..
(I Love this forum, y'all ROCK!)

[Enzo]

This is a P-250, right?

Did you verify all the ballast resistors were OK? A output transistor burns out, and it often will take the resistor with it. If you replace the transistor and the resistor is open, it is as if the transistor was not there, and the other one will have to carry the whole load, and will fail.

When one xstr burns out, one from the opposite polarity usually fails too. But on each side there are two per type, so when one shorts usually it hogs the current and protects its mate. But they can both die. Make sure ALL xstrs in the channel are OK.

Yes, the amp must be biased. TR202,204 are the two bias controls for the two channels. TR or TP, I forget. Then TR201,203 are the DFC offset.

Bias is set like on any other solid state power amp. You can either bring up a test signal and adjust the bias until the crossover notch just disappears and no farther. In fact I would back off a hair from that point.

Or you can monitor the mains current draw, and turn the bias controls the direction for least mains current to start. Then advance one control watching the current as you turn up the control. At some point the current will start to rise with the control. Stop and back off. The point where current starts rising is the adjustment - it is in fact the same point where crossover distortion disappears. Set both channels the same way. And again, I;d back it off a hair. NO one will hear a tiny bit of crossover anyway.

[Showey]

Fantastic answers Dr. "E"
I'll go back through this beast a bit more carefully.
I am still in the learning stages of repairing this stuff. I have had great luck so
far. I have a little electronics education / knowledge but there is a lot more
to learn. Thanks you for your assistance along the way!

[Showey]

Well, I cant find it and I know its mostly die to my lack of experience.
I have two separate power amp boards. One will take out one set of xstrs (Q212 and Q215) while the other board takes out Q228 and Q231.
Obviously both board have the same issue, just in different places.
Checking the Ballast resistors seems to be a problem. R220, R223, R221, R224, R249, R252, R250 and R253 all have the same resistance and that resistance
seems inconsistent with the way they are marked. I am doing SOMETHING wrong. I would just drop it all of with the local amp guy but
1) I am in the car business and I cant afford it right now and...
2) I love learning this stuff.
I have the complete schematics in PDF format.
More help please?
Thanks in advance.!!

[Enzo]

Well, help us out here.

R220, R223, R221, R224, R249, R252, R250 and R253 all have the same resistance and that resistance
seems inconsistent with the way they are marked.

WHAT resistance do you measure? HOW do you think they are marked? And what is the inconsistency. All I want to know is are they open or not. They are only supposed to masure 2 tenths of an ohm.

In the case of the two transistors, are you sure BOTH are burning out? They are in parallel, so when one shorts, the other is usually OK. If 212,215 fail, make sure 214,216 are OK. Q211 drives them, is it OK? How about 100 ohm R2129? And zener Z202?

[Showey]

Ok. Here goes. Q211 is fine, R219 is fine @ 100 ohms R220, R221, R223, R224 at .2 ohms. Q214 and Q216 are fine Z202 is fine.
You were right. It only smoked one xstr it was either Q212 or Q215
Sorry I thought they were both bad so I don't know which one it was.
(I know, I know... Duh)
I have removed each component so far either completely or at least partially for testing.
Whats next?

[Enzo]

How is all the bias circuit? And about parts shorted to the heatsink?

[Showey]

I will work thru the bias circut tonight.
Hopefully I will find it there.
All the Xstrs are insulated to the heat sink via what looks like strips of duct tape and thermal grease.
When I reinstalled them all, I completely cleaned all
of the old grease off and applied new stuff and reassembled it just as I found it.

[Showey]

Okay,
I ended up on the bias adjustment pot and found that it was set basically to 0 ohms or very very close on the left chanell and closer to 9 ohms on the right.
the left channel is the one that keeps taking out one of the xstrs under a constant load. Is it safe to assume, that since I found no other issues that this may be all it was in the 1st place? (back to my 1st post and questions)
I checked the other board that I had and the same was true but in reverse. The right channel (the one that smokes on this board) was set near 0 and the left was at 4ohms.
I set the better condition of the two boards to match each other (bias pots on right and left are set to 9 ohms. I plan to fire it up and set the bias with my scope with the crossover notch method. Please tell me i'm on the right track?
Finally,exactly where should i connect my probes to perform my bias measurements?
Thanks again!!!

[Enzo]

I would think zero ohms to be the coldest setting.

You are working with the speaker load removed are you not?

[jrfrond]

Guys, we repair a lot of Passport units, and I am here to tell you that they are a nightmare on all levels, whether it is disassembly/reassembly, troubleshooting, service access and testing. They suck all the way around. Usually, we approach it on a modular level.

[Showey]

Yeah, I am starting to really dislike this unit.
I will order a power amp board complete and go from there.

[Enzo]

I see more of them than I would like. At this point I have the disassembly thing down. Kinda like work on enough PV Classic 30s and you know how to do it.

I haven;t run into one I couldn;t fix reliably. And I am comfortable working on the switcher. But they are a pain. Fender calls the SMPS a swap out, but they expect us to fix the amp board.