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.

[Showey]

What should I expect to pay to get this thing fixed if I drop it
off at a repair shop?
It will need at least a couple of xstrs and probably a resistor or two
(Ball park)
Thanks guys.. I just haven't had the time to get back into it
and my daughter needs it.

[Enzo]

In my shop in Lansing Michigan, I'd expect in the $60-100 range. You would need to check rates with your local shops.

[Showey]

can someone give me a price quote on the following?
1 007-0144-000 GP PCB ASSY POWER AMPLIFIER ( STUFFED )

[Enzo]

Are they even available? I'd call Fender and find out if they are, and also find out if that number is actually what you need to order. (You don't want to order that number only to find it has been superceded or that the stuffed board doesn;t include the power semis) They can also tell you list price. Then any Fender dealer can order it. Up to them if they charge less than list.

[Tilman]

Hi,

I have a Passport 250 were the power supply did break the second time. When it broke the first time, I ahve ordered a new one from Fender (about 270$).
Now, after 1,5 years, I got the same trouble again.
I found a schematic for the power supply on the net, but I have no idea how a switching suppy works.

Could anyone give me a hint for which part to look for in the power supply (Mosfet,Transistor)?

Thanks for your help!
Tilman

[Enzo]

Not sure what the same problem might be. No sign of life in the amp? Power light but no sound? Protect light stays on, no other function? What? Can't help you look if I don;t know what you are looking for.

Switching power supplies usually have a ton of protection. Before diving into the switcher, first make sure it is what has the problem. The main power supply leads run from the SMPS (Switchmode Power Supply - easier to write) up to the power amp. Pull that connector off the center of the PA board. Does the SMPS now wake up and run the mixer/preamp? In my experience, amp failures are a lot more common than SMPS failures in these Passports.

SMPS are DANGEROUS. They DIRECTLY rectifiy and filter the mains. Grab this and it can KILL YOU.

The SMPS rectifies the mains and makes about 340VDC. That is switched off and on at high frequency through the small transformer. The resulting ugly waveform is rectified and filtered on the secondary side to provide the working voltages for the system. The switching transistors could be blowm, or the starting trickle might be missing so the system can't wake up. SO many things could be happening, and that is assuming the SMPS itself is at fault. I do not assume that up front.

[Tilman]

Enzo, thanks for your help.

Sorry for being not very precise!

Here is the trouble:

- there is NO LED light (not the protection, not the power)
- the SMPS fan does not run
- the main fuse is OK
- I have disconnected the power amp and mixer ; there is NO DC voltage at any connector.

I had the same symptoms 1,5 years ago. I bought and installed a new SMPS from Fender. The system was working fine after that. Inside the SMPS is no fuse. No obvious blown parts nor black colored parts.

I was hoping someone could say: “Look for T103 and T104!” (example). Checking every single active part inside the SMPS would be a lot of work and even if I would do that – it could be a transformer or something inside the SMPS.

Regards!
Tilman

[Enzo]

OK, well at least the SMPS will run by itself on the bench, it will run without loads. I will attach the P-250 SMPS schematic. I am assuming youi have a P-250, not a PD-250.

Nope, no fuse in there. Your mains fuse serves for that.

Only the secondary references to ground. That is the stuff on the right side. Everything left of center is primary. It is NOT referenced to ground. There are a couple ground symbols - the white empty triangle - but it refers to a commopn, not chassis. Just assume anything on the left has 170vDC on it. You can't connect a scope to that side unless you use isolation techniques. I don't want you hurting yourself or damaging your gear.

The large majority of things that go wrong with an SMPS result in the "dead" symptom. It doesn;t suggest one or two things.

First things first, is it getting mains power? Mains comes in lower left. The fuse and switch are in the cabinet. You got mains at teh thre pin connector? At the top end of the two series inductors? Does that NTC inrush limiter have continuity? That is a early suspect on any dead system. And above that is ther mains on the top of that transformer-like thing with the large "N" in the center? And finally is ther mains across the AC side of the main bridge? And make sure the 110v switch is set.

OK? Then are the DC filters charging up? Got about 170VDCV across each or 340VDC across the pair?

Let me take a tangent. I know I haven't described how it works. But the SMPS won;t start all by itself, so look to the right of the mains filters to the Gas Arrestor - I just love that - and follow the wires down to little transformer T2. It makes a small power supply on the secondary that gets the control IC running. Note it feeds right into the top of the IC. Filter caps a little to the right. Note ther is a winding on the main transformer that feeds the same supply. It really takes over once the system starts. Is ther DC voltage on that little supply? I have no idea what voltage, most likely it will be present or absent, not probably wrong.

Now, the way the thing works is it rectifies and filters the mains to make 340vDC, then the four MOSFETs switch that supply off and on at high freq through the primary of the main transformer. Makes a very ugly waveform. The secondary side is really quite conventional, rectifiers, filters, regulators. Looks odd, but no mystery.

If the mains supply is OK, my first suspects are shorted rectifiers on the secondary. I see 7 of them, just check them all. ANy one of them shorts, and the SMPS will shut right down.

Then ther is that little starter supply we checked.

I have to think your MOSFETS are OK, or the fuse would be blowing.

[Tilman]

Hi Enzo,

wow - you are the man! Even if I would fail to fix this thing - I now have an idea how a SMPS works...great!
Thanks for taking your time helping me with this problem.

I will do as you say and check the supply step by step. I have a main power isolation transformer at my bench I am going to use. I will be very carefull still.
Since I live in Germany, I will set the voltage selector to 220V.

Regards!
Tilman

[Tilman]

Hi,

I did some first tests on the Passport power supply.Up to the DC out of the bridge rectifier, that rectifies the main voltage, everything seem to be OK.
I measured 262VDC over R2.
The strange thing is, at the VdC pin 7 ,at the control IC, there is 0,75VDC only.
At the input of the small transformer which provides the start up voltage is only 35VAC. At the secondary only 2,2VAC.

Could it be that the gas arrester (for what is this for ?) has a too small resistance. The primary voltage for the small IC transformer is divided by the arrester and the resisters behind the main rectifier since the voltage selection switch is open (for 220V).
Should I take out the arrester for a test ?

Regards!
Tilman

[Tilman]

I took out the gas arrester - no change!

[Tilman]

Checked the secondary diodes - they are OK ....

[Enzo]

The gas arrestor is a spark gap. Darn hard for those to go bad.

OK< 2,2VAC still ought to get you 3VDC at the IC.

Power off, measure continuity through the two windings of that transformer T2. Even an open winding can result in a measurement on an AC meter. Just touching the probes to your finger will produce a reading.

it is also possible the secondary there is loaded by a failed IC or leaky cap.

[Tilman]

Enzo,

thanks for your help. I measured 3,8 ohms resistance to ground at the ICs Vcc pin 7 . I took out the IC and there is 3,8 ohms in both directions between pin7 Vcc and pin 5 GND. The data sheet says that there is a 35V protection zener between Vcc and GND. I guess this one is short.
Without the IC, the secondary T2 is 15VAC and at the cathode of D18 17VDC. This is a reasonable value for the IC (12 - 25V).
I have ordered a bunch of new ICs KA3842B and will see what happens after installing a new one.

It is very strange that the zener inside the IC breaks during normal operation though.....

Regards!
Tilman

[Enzo]

Who knows, mains transients feed directly into that little transformer - hard to say what abuse that IC suffered at some point. Most things fail during "normal operations," after all.

I assume you no longer have the old dead SMPS from before? If you do, a quick check might reveal that IC still good on it.

[Tilman]

I installed a new control IC today.

Before I did that, I checked the resistance pin7-5.
The instruments shows a diode between these two pins.
The old IC was dead - this is clear now.

When I started the SMPS with no load the fan did run for 0,5 seconds an than my 16A bench main fuse did blow.
I took out the IC and now "only" the 10A fuse blows.
The IC measures OK.

I guess now the switching MOSFETs are dead. I will give up at this point since I do not know the reason for this. Even if I would replace them now - they might die again...and again...

Enzo, thanks for your help!

Tilman

[David Hawn]

I'm trying to figure out the voltage output of the Passport DC/DC converter. I don't own one and was trying to figure out how difficult it would be to make one. There are 4 wires, one red, one blue, and two black. The 2 black wires are probably both ground. From looking at the power supply diagram posted on this thread, it looks like the 120VAC is turned into a +/-15VDC. The "External Input Voltage" on the right side of the diagram shows two connectors, one with +/-36VDC and one with +/-51VDC. It makes sense that the DC/DC converter voltage would need to be higher than +/-15V so the 7915/7815 can regulate down to 15 volts, but 36V and 51V both seem quite high and inefficient.

Does anyone have a DC/DC converter and could measure the output voltage?

Thanks!
David

[Enzo]

High and inefficient? Well that would be if all they had to do was power the preamp. But those power amps will be pretty well starved if you try to run them on +/-18v.

The power supply makes not only +/-15VDC for the preamp, it also makes +/-51VDC OUTPUT for the power amp.

If you are running on external power from batteries, the INPUT voltage is +/-36VDC. The power amps run on that as well as the preamp regulators. The thing doesn;t pretend to produce full power on batteries. The two connectors are shown in parallel, though they are not near each other. Only the 36v inputs are labeled external input voltage.


My service documentation for the Passport series does not include the DC pack, but if you contact Fender and ask, they might have it to send you. Then again they may purchase the battery units from outside suppliers.

[David Hawn]

Enzo, thank you for your response! I didn't realize the +/-51V was also an output, but it makes sense now.

Thanks again!

[bazb_uk]

Hi, All,

I am currently working on a Fender SMPS... SO Thanks so much for the Curcuit Diagram..

Our unit kept going into protection after 5 secs of been on.

I found 2 pre amps blown on the mixer so guessed high voltage had been plugged into an input.

Although the PSU generated both +/- 50V and +/- 15V When a load was connected it was dropping to about 3v +/-

When inspecting the board I could see both the + & - 15 V regs were charged, and the output resistors. one mesured 150 Ohm, the other 210 (both labled 150) So i have replaced both Regs, and resistors and smoothing caps...

Now I get my correct voltages, But as soon as i load the - channel the voltage dropps to 3 volts. The voltage is there, but there is no power present on this channel only.

Would this point at the SM - Transistors on the Primary side of the transformer? Or could it be a problem with the output voltage sense / drive???

Any food for thought would be awesome!