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This PA head came in with no speaker output. I tried the "main out" on the front and there is a pulsating sound. None of the preamp input "peak" leds are lighting up when putting in signal and cranking the gain knob. Also front panel leds "Phantom" "Standby" pulse with the pulsating noise when they are turned on. I do have schematics but I think this is a switching supply and I'm unfamiliar with this issue and the circuit.
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If I read right, your probably seeing the SMPS cycling on and off. It starts up, detects a fault and shuts down, only to repeat over and over. Does that sound reasonable?
Like any system, if power is not working right, disconnect the power amp powr leads from the supply. You MIGHT have to also disconnect a small connector with low voltage supplies to the power amp. Does the SMPS fire up and stay now? If so, your power lite and the mixer lights should work normal.
Over by the mains connections, ther is a relay, K1. It is your soft start relay - it just bypasses the inrush thermistor after the supply starts. Does it click on and stay? CLick on then off and on with the pulsing? Or never click on?
Does the fan ever come on? Or does it pulse too? Or stays dead?
If the SMPS fires up and runs without the powr amp, then you have a blown PA channel. If the SMPS still won;t run, disconnect the mixer power connections too. Does the SMPS now fire up? Fan run?
If no SMPS action, the most common failure on SMPS, unless the drive is just burnt up, is shorted rectifier diodes on the secondary side. D22-25 and D10. They'd be surface mount little square things.
D1-5 are for the high voltage supplies.Education is what you're left with after you have forgotten what you have learned. -
As always thanks Enzo. Okay so here's where I'm at... spent a good hour trying to get into the front of this thing. No fun.
The relay does not click in any way.
The fan is DEAD.
I disconnected the PA from the supply and still the LEDs are blinking.
I cannot disconnect only the preamp power supplies as they are with the same connector as is power to the front LEDs so how would I know if that helps or not?
Thanks!Comment
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It still cycles with no power amp connected? Then look to the SMPS itself. Start by checking all the rectifier diodes on the secondary side. My thinking is that trouble on the primary side would be blowing fuses or burning stuff up.
A shorted rectifier on the secondary side would load the thing, so it would cycle off and then try again.
And by disconnecting the preamp we would determine if it was loading down the supply. You might lose the power LED, but you could still scope one of the SMPS outputs to see if it comes up and stays. Or for that matter, if the fan came alive. If the SMPS still pulses on and off with nothing else connected, then the problem must be in the SMPS, eh?
Don;t connect it, but just hold your scope probe right next to the small transformer the switcher drives. If it is running, you should see some sort of waveform on the scope. Try it on something that works to see it.Education is what you're left with after you have forgotten what you have learned.Comment
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I've work on hundreds if not thousands of smps equipped units. If it isn't a stand alone module it can be a bear. I would start by checking every power supply rail for a short. There are probably several. After that, like Enzo says look for shorted diodes, zeners, and bad caps on the secondary side. If you have a scope you can watch the chipper transistor oscillate then shut down. Remember that most smps are floating with respect to chassis ground. If you have to trouble shoot the supply it's best to isolate it from the rest of the unit if you can.Comment
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Lowell, can you post the schematic?Comment
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Or post the board number for the SMPS, probably an SPS1000.Education is what you're left with after you have forgotten what you have learned.Comment
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Psu18 sms
pcb827110revb/10Comment
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Then look on the SMPS for D10, 25, 24, 23, 22. If I recall, those are little rectangular surface mount guys. ANy shorted?Education is what you're left with after you have forgotten what you have learned.Comment
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Enzo I cannot find any of those on the SMPS board. Could they be on the PA board that has a cover on it? I see T3 on the SMPS board but none of those diodes. I do see U8 and U10 next to T3 which are little square SMD devices. I cannot however see those on the schematic either. Sorry for the confusion/ignorance.Comment
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This may or may not be of any use.
Enzo was kind enough to guide me through one of these power supplies.
Link: http://music-electronics-forum.com/t22470/Comment
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This is surface mount stuff, so did you look on both sides of the SMPS board? They are just little rectangular things.
No, they won;t put the rectifiers on the amplifier board. That would require sending all that high frequency hash down wires into the amplifier, THEN rectify and filter it. No way.Education is what you're left with after you have forgotten what you have learned.Comment
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Okay right on. You're right they're on the back side... sneaky. D10, D25, D24, D23, D22 all check out fine. I also checked resistance of the latter side of each diode to ground and none of the filter caps seem shorted.Comment
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Hey thought I'd post back haven't heard from anyone. So is this the end point for this thing or can you think of what direction to go in? I guess I need to study up on SMPS. I really have no idea how they work.Comment
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HArd to say. It sounds like the thing is trying to start up but quickly kills itself. The most common reason for that in my experience is shorted rectifiers on the secondary. We checked those little diodes, but don;t overlook the larger ones on the heat sink for the main V+ and V- to the power amp. But other stuff can also do that.
There are more of those diodes than the few I listed. Check every damn one of them on the board.
I suspect your main switchers are OK or I'd expect blowing fuses. I could be wrong.
You have the schematic, so look. There are two switching circuits on this. The mains is rectified and filtered, then the DCV-H is fed down to U2 which switches all the stuff in the lower left quadrant through T3. All that is the low voltage supplies. Note in the center of that the little +16v rail, from D27. Then there is a second switching circuit in the upper right quadrant. Instead of a little thing like U2, this high powered switcher uses U5 to control Q1,Q2, power transistors working through T2. Note that U5 runs off that little 16v rail from the low voltage section. In other words, the high power high voltage supplies won;t come on until the low voltage supplies come up.
SO hold a scope probe next to T3 and see if it is cycling through moments of trying to pulse. And remember that "ground" on the primary side of these suplies is NOT earth ground. And try the same thing next to T2, just to see.
The secondary outputs are referenced to ground, but scope them and see if little pulses show up each time the SMPS tries to fire.Education is what you're left with after you have forgotten what you have learned.Comment
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