QSC PWM IC

[Enzo]

By startup resistors they mean the high value resistors from the main DC somewhere. The chip cannot power itself by switching until after it started, so they allow a trickle of current from the main supply into it as a kick start.

[Jazz P Bass]

As a rule, I do not use a lamp limiter on a SMPS.

It will either work or it won't.

As I stated before, the TOP 210 is a 'housekeeping' ic.
Low voltage supply for the rest of the circuit.
If that does not startup, neither will the rest of the supply.
If the TOP210 did fail, there are associated capacitors that should be checked for shorts (on the output buss)

If nothing on the supply PCB is burnt, I usually have pretty good success getting these up & running.

If the PCB is that bad, I just order a new amp less chassis from QSC.

[nosaj]

Pulse width modulators often go bad in smps's.

Check all the associated oscillator circuitry and the mosfets etc.

What kind of symptom is associated with a PWM going bad? sometimes it will start up and sometimes not?\

Thnaks,
nosaj

[drewl]

They usually just fail, either not working or causing the output voltage to be too low or too high.
At least that's what would happen to certain types I repair for work.

Alot of those would fail due to small lizards and bugs and critters crawling inside to get warm and they would short out something.

I'd find a burn mark and a little lizard skeleton inside.
Probably not the cause of your failure.

[lowell]

Thanks guys. Here's a pic of what I've repaired. The red dots signify bad components that I've replaced. The TOP210 was replaced as well as the top IGBT Q96, and the K1 relay too. I'll check for that 18v on D62 cathode. All the K1 relay does is short out the R324 thermistor (mov?) correct?

[lowell]

I am getting ~ -140vDC on D62 cathode, BUT it isn't steady it is continuously falling. I measured PriHI and PriLO, no reason really other than seeing what they are. They are +/- 170vDC. Not sure if that matters or not.

[Enzo]

Thermistor, hence the R designation. it is a series element, MOVs are parallel elements. It is the inrush limiter. It starts out with some resistance, and as it heats up, the resistance drops, so it allows the circuit to start up a little softer that without. But it still stays warm while operating, so it adds heat and still has some small resistance, so the relay shorts across it after the supply starts up, now it neither drops any voltage or gets hot, at least until the relay drops back out.

If the two gate resistors on the IGFETs burnt up, how is the health of U18, the IR2110? After all the current to burn up those resistor had to come through that IC. Oh, I see, just one of them, still applies.

[lowell]

Other than checking voltages on that IC I'm not sure how to test it. The datasheet http://www.irf.com/product-info/data...ata/ir2110.pdf recommends:

VHO (pin7):
between -25v to +525v for "floating output voltage"

VLO (pin1):
between 0v to -170v(PriLo)

My readings:

1: -170v
2: -170v
3: -170v
4: 0v
5: -135v
6: 0v
7: -170v

I believe this IC may be compromised... but not sure. I really still don't understand how this circuit works, and that's my problem.

[Jazz P Bass]

Both gate pins (pin 1 & 7) are at -170V????

[nickb]

Other than checking voltages on that IC I'm not sure how to test it. The datasheet http://www.irf.com/product-info/data...ata/ir2110.pdf recommends:

VHO (pin7):
between -25v to +525v for "floating output voltage"

VLO (pin1):
between 0v to -170v(PriLo)

My readings:

1: -170v
2: -170v
3: -170v
4: 0v
5: -135v
6: 0v
7: -170v

I believe this IC may be compromised... but not sure. I really still don't understand how this circuit works, and that's my problem.

What do you have your -ve meter lead connected to? It should be PriLO. You won't get any meaningful readings at U18 until you have the auxiliary switcher U16 working. Reconnect your meter correctly and check for +18V at D62. If that is wrong, check for +PriHi Voltage at U16 pin 5.

[lowell]

Ok, yah see I don't understand this circuit. I've read quite a bit about smps, and tried to understand them, but there seem to be so many different types, and then within those types they even seem different.

Stby.

[lowell]

Voltage in reference to PriLO at D62 is +5v and falling.

Pin 5 U16 is +341v.

[lowell]

LO and HO on U18 are both +20mv referenced to PriLO.

[Jazz P Bass]

Voltage in reference to PriLO at D62 is +5v and falling.

Pin 5 U16 is +341v.

Not possible!

Pin 5 is the output pin of the TOP210.
It should read +16Vdc.

I see no connection to the +rail that could possibly make pin 5 read +341 volts.

I think the PCB may damaged beyond repair.
Place a call to QSC & see what they want for the replacement board.

Edit: $514.00 http://www.qscparts.com/plmoas.html

[Enzo]

I suspect he might be taking voltage readings to chassis. The switchers use V- and the common, so if an IC has -170v on one pin and -165 on another, it really has 5v across it.

And that is why switchers are so dangerous to work on, the common sits at -170vDC with respect to earth.

Without studying the schematic, I wonder if some variation on this idea is what he is facing?