Fender Passport pulsating problem

[KCman]

When plugged into a light bulb limiter, the light flashes on and off fast.

When the power supply is disconnected, the light doesn't light.

I hooked it back up each connector separately, CN203 on the amp board makes the light flash (also a sound pulses with the light and no source)

I tested the outputs and found one I believed was bad. (cricket test with an old sencore). Not sure how to test these and/or if the sencore works for these.

I replaced both, the one I thought was bad didn't do anything, the other made it flash dim instead of bright.

The guy said it got rained on while on.

I've never had the light pulse, not sure what to do next.??

[Jazz P Bass]

Not to sure that trying to test the unit on a lamp limiter is such a good idea.

Most SMPS supplies do not react well on a limiter.

[KCman]

Thanks, I will check it without the limiter. I don't have experience with these power supplies.

Thanks again for the quick reply.

[Enzo]

Your SMPS is reacting to the bulb. It has a soft start, and when it kicks in the momentary current surge lights your bulb, which drops the voltage, which causes the SMPS to try to cycle up again. And on it goes.

What is the actual problem we are trying to solve?

There should be a three or four wire cable from the SMPS to the powr amp board - that is the main V+ and V-. Disconnect that. Does the SMPS now power itself?

Unless you are blowing fuses, ditch the bulb.

[KCman]

Sorry, I didn't explain more off the bat.
It is making a pulsing sound with no source.
I disconnected the v+ v- and the voltages were correct.
That's when I decided to hook up the bulb.
With the v+ v- connecter not connected, no light

With no bulb and everything connected, pulsing sound through the speaker, and I can hear a mechanical clicking from the supply that matches the speaker.

Any ideas if it's the supply that has a problem when a load is applied, or if it's something else?

I haven't opened the supply yet. I will take a look for something obvious, but I have no experience with SMPS.

Thanks

[eschertron]

Sorry, I didn't explain more off the bat.
It is making a pulsing sound with no source.
I disconnected the v+ v- and the voltages were correct.
That's when I decided to hook up the bulb.
With the v+ v- connecter not connected, no light

With no bulb and everything connected, pulsing sound through the speaker, and I can hear a mechanical clicking from the supply that matches the speaker.

Any ideas if it's the supply that has a problem when a load is applied, or if it's something else?

I haven't opened the supply yet. I will take a look for something obvious, but I have no experience with SMPS.

Thanks

with no load, the SMPS voltages are correct?
with the amp (load), the SMPS cycles? can you put test loads onto the SMPS to see at what point the voltages collapse?

[KCman]

I'm not sure exactly what you mean by test loads?
How should I do this?

[eschertron]

I'm not sure exactly what you mean by test loads?
How should I do this?

I don't know the specifics of your amp, but supposing the SMPS delivers +80vdc to the power amp module. hooking a 10k/10W resistor across the v+/v- will draw 8mA. Peanuts. The SMPS still working, no voltage droop? hooking up a 1k/10W resistor will draw 80mA. Starting to get into significant current draw territory now. How does the SMPS behave? Ensure any load is not going to draw too much current, ensure any load is not going to burst into flames.

My null hypothesis with this series of trials is that at some point - below the current draw of a normally functioning amp - the SMPS voltage will collapse and demonstrate the "blinking" behavior you noticed. Bad SMPS. If the SMPS is OK at any load up to that which you expect from the normally-operating amp, then the amp becomes suspect.

[Enzo]

I don't disagree with esch here, but in my experience, that SMPS either works or it doesn't. I think you have a bad channel on the power amp, The SMPS tries to start, finds a excess load, shuts itself down at which time it tries to start, finds excess load, shuts itself down, etc. The pulsing and clicking you hear is the result of the SMPS cycling. it COULD be a SMPS problem, but I will bet my lunch money you have blown outputs on the power amp.

I see this is really a mono amp? I usually see the larger models. In this the left and right are the same amp output.

[eschertron]

this ^^^
My idea was to test the SMPS to prove one way or the other. If Enzo says that voltage present is the test for good SMPS, then go with that.

[KCman]

Before I read any posts (I don't have internet or service at my studio/shop), I opened the SMPS and checked test point 1. It says it should be 300vdc. I got 81vdc.
This is right after BD1.
I get 120vac at the input of BD1
But, at test points 2 and 3 I get 35 V+ V- (supposed to be 34v+-). So that's good
This was with no load on the supply.

Doesnt this point to the rectifier?

By the way, I replaced the outputs 2sk1058 and 2sj162. They are hard to find. These don't seem to test on my old tester. Any insight to these?

[Enzo]

I suspect you are measuring 81v to ground. But the input from that bridge is not grounded. I bet you have about 170 volts across the filter cap. The "ground" of that primary side is actually the negative side of the rectified mains.

Where does it say 300v? I don't spot that offhand?

As far as I can tell, this SMPS should run on any mains voltage. At 120v it rectifies to about 170vDC. At 240vAC, more like 330vDC.

Were the old transistors shorted?

[KCman]

I tried to attach the service manual in the first post.
On the SMPS schematic page it shows tp1. On the following page it shows the test points.

I had one transistor not test on my sencore tf46 so I found a set of both and replaced them. When I got them they didn't test right either. Maybe I got ripped off. I haven't had problems testing any other transistors out of circuit with my tester??

[Enzo]

Yes, I downloaded your manual, thanks, I see the big red TP1 on the SMPS schematic. Next page appears to be the board layout, but I don't spot 300v on it.

The transistors are MOSFETs, not bipolar transistors. SO they won't test like a 2N3055 or MJ15003 or similar. But just using an ohm meter, were any of the old ones shorted between any two legs?

[The Dude]

I'm with Enzo. I think your SMPS is fine and it's pulsing because of a short or imbalance in the amp section. Those outputs are FET's and may not test with your transistor tester- not sure what tester you have and if it's capable of testing FET's. If you had shorted outputs, it wouldn't be unlikely there are associated components also shorted or burnt open because of the shorted outputs. I'd start checking other components in the amp, i.e. D205, D206, Z204, Z205, etc.