Fender PA problem. Passpoprt 250

[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 (You don't want to order that number only to find it has been superceded or that the stuffed board doesn;t include the power semis) They can also tell you list price.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.