QSC PWM IC

[lowell]

This PA is a PLX3102. Doesn't turn on. Found that U16 PWM IC is physically cracked. Not sure if that's the only problem. This IC is an obsolete part - TOP210. I'm being lazy, anyone know a good substitute? If not I'll buy some of ebay if need be. Being this is a SMPS, any advice welcome on tackling this repair. I HAVE been reading up on SMPS and am kinda sorta starting to grasp the concepts...very cool stuff. I am aware that I do not want to ground a scope if troubleshooting a SMPS.

thanks!

Schematic is here: QSC PLX3102 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

[Mark Black]

I would contact QSC directly - they may not only have replacement parts but might also have some service bulletins applicable to that problem.

As an example see this (although not for your model): http://qscservice.com/index.php/download_file/715/147/

Edit - in fact you may not even need to call them (although I still think it's a good idea): IC-000135-00

[drewl]

Pulse width modulators often go bad in smps's.

Check all the associated oscillator circuitry and the mosfets etc.

[Jazz P Bass]

Not too sure if this SB is relevant.

http://qscservice.com/files/7313/811.../PLC0013_B.pdf

[tedmich]

2 for $7.98!

IC-000135-00

[lowell]

Thanks guys. Ordered 2 of the IC and 2 of the zeners. $30 including shipping. WHAT A COMPLETE AND TOTAL RIP OFF!!!!!! Ah well.

[mhuss]

$30 including shipping. WHAT A COMPLETE AND TOTAL RIP OFF!!!!!! Ah well.

Is it better to have (expensive) original replacement parts available directly from the manufacturer, or roll the dice on some random guy on eBay? I vote for the former.

[lowell]

Agreed, still, very expensive.

[tedmich]

QSC does have some deals, you can buy all the components (amp dish + 2 speakers, minus cabinet) for their well loved KSub for fully $400 less than the cost of a built one. They do ask for a serial number to dissuade "builders" but those are easy to find...

[lowell]

Ok so the line filter relay, which is connected to the thermistor, is also toast. Ive replaced it, as well as the pwm ic and added the zener per the tech notes. Still need to check the IGBTs and diodes.

I would appreciate some advice on testing and powering up without blowing it in the process.

[lowell]

Ok so here's a schematic of the PLX3002. Loud emailed me back saying it has the same exact PSU. http://qscservice.com/files/4813/6329/4053/plx3002.pdf

I replaced the PWM U16 and added the zener. This IC is in the "Bias" section of the schematic, center of page 5.

I also replaced the K1 relay at top left of page between the thermistor and bridge rectifier. It had arced and taken a pcb trace with it. So those are fixed.

I checked the IGBTs Q96/97. Q97 is shorted gate to emitter. I'm thinking I need to replace both of them in case Q96 was compromised. ?

D70/71 across the IGBTs are good. Shall I replace them anyways while I'm at it?

Do I need to worry about U18/19, top right in the "gate drive" section?

What does PRI mean? There is PRI-HI and PRI-LO rails post bridge rectifier. I know this is the main dual supply that is being charged by the IGBTs and that T2:1 (transformer primary) is being yanked around by the IGBTs on/off switching. Never seen PRI before.

Here is a picture. Components circled in pencil are confirmed ok. Red dots are bad and need replacing. Thanks in advance!

[Jazz P Bass]

On the HOT side of this SMPS, I have always refered "PRI HI' As Primary High.
And "PRI LO' as Primary Low.

You will want to consult the schematic but Pri Lo can be used for your negative reference on your volt readings when monitoring the HOT SIDE.

U16 is a Top 210 ic.
It can be considered the 'house keeping' ic.
It has to pulse T1:1 or nothing else will work.

[lowell]

JPB,

Which side is the HOT side? Secondary or primary?

My main concern right now is whether or not, out of preventative care, I should replace more than the components I've highlighted with red dots. I don't think there's much testing of the ICs without powering on. But I want to power on the RIGHT way, with the correct procedure. Bulb limiter? Variac? My understanding is that this will operate throughout a wide range of mains input supply voltages. So I don't THINK a variac will have any effect.

In short, any suggestions before turning this on after I've replaced all aforementioned components? Thanks again.

[Enzo]

Lowell, this is an SMPS, it is extremely dangerous. The primary side is wired DIRECTLY to the mains and rectified. THAT is the "hot" side. neither side of that DC is ground. Those main caps are being charged directly by the rectified mains, not the IGBTs. The IGBTs alternate turning on the +170 and -170 ssupplies through the T2 primary.

You are correct, powering up a switcher with a variac doesn't work like a linear.


You REALLY should use a bench isolation transformer to power this supply until it is working.

[lowell]

Ok so something like this?

Tenma Isolation Transformer | 72-6670A (726670A) | Tenma

Any suggestions on powering on a SMPS post repair?