QSC PWM IC

[nickb]

Should mention there IS 5v on pin 4 of TOP210.

If there is 5V then the 16v rail must be up. Check your measurements. There is a zener that gets bypassed to reduce the 16v to 6V so it might be low but the 5V comes from the 16v so the TOP210 has to be working.

Edit: I overlooked that the TOP210 does generate it's own 5V during startup. So, I guess something is still pulling it down. Do you see any ACV on pin 5 of U16? It should be switching as it retries.

Musta had my meter on a low range. 16v supply now measures 12k to PriLo. I just lifted U13 from 16v supply and the 12k is now 27k. Gotta order that IC I guess.

I don't think that measurement tells you anything of value. You can't draw any conclusions from that. If it measures a few ohms that would be one thing.

Can I turn it on with U13 disconnected without causing harm so I can check the 16v supply?

Lets not cross that bridge for now. I expect U13 is probably OK.

[lowell]

Pin 5 has 340v and there is a pulsating DC across D60.

[lowell]

Actually pin 5 has 400v ??

If D66 zener was open or shorted could it cause this? I dont have a replacement and am not confident it is working.

[lowell]

I subbed a discrete leaded 10v zener for D66. Didn't help so I put D66 back in.

[nickb]

Actually pin 5 has 400v ??

Well that is good. It means the chip is trying to startup. Being a flyback converter means that the the primary voltage will spike above the supply. The TOP210 never gets it's 5V supply fed so it shuts down and after a while it retries. Did you read Jazzs's post 71?

You have 1.6V at the 16V supply - this is good - a not better than nothing. Since you've already had one blown up part on that rail there could easily be another. If you are going to try removing the chips one at a time and as you have replaced the IR2110, I suggest you remove the IGBTs for now to avoid any chance of them being damaged.

Also there is the 6V supply. If something were loading that it will stop it from working. It's fed from the TOP210 too.

[lowell]

I did see JPBs post. The only thing that doesn't add up in regards to the bias supply seems to be that T1 primary only measure 1ohm. Not 15ohms. It could be shorted? The troubleshooting guide doesn't mention this as a possible cuplrit, but I guess it's possible. I have U16 installed still, but don't see how that could affect this measurement.

[lowell]

They are available here: XF-000064-00

This seems like it may be the problem. U16 seems to be working, all other components seem ok.

[nickb]

I did see JPBs post. The only thing that doesn't add up in regards to the bias supply seems to be that T1 primary only measure 1ohm. Not 15ohms. It could be shorted? The troubleshooting guide doesn't mention this as a possible cuplrit, but I guess it's possible. I have U16 installed still, but don't see how that could affect this measurement.

I'd cross check your meter. Grab a resistor 10 to 22 ohms and measure with your meter to see how accurate it is. If meter is OK, then retest the transformer and, if still wrong, again with it off the board, just in case.

[lowell]

I thought about that but I was indeed able to get 10ohms on some resistors in the bias supply. I just confirmed with a 15ohm resistor. Meter is good.

[nickb]

I thought about that but I was indeed able to get 10ohms on some resistors in the bias supply. I just confirmed with a 15ohm resistor. Meter is good.

How very sucky. Still check with with it out the board. I'd also 'ring' it if you can. Put your 'scope on one of the secondaries. Quickly touch and release a 9V battery on the primary. You should see the output ring several times on a good transformer and hardly at all on a bad one. Now compare the number of rings if you short out the other secondary (short wire soldered to do this please). If it doesn't change much, it was bad.

If you haven't a storage 'scope this will be very hard to see. $30 is a lot to put down unless you are sure.

[lowell]

I'll try this out Nickb. Curious why this wouldnt damage a transformer. Couldnt it cause arcing or cause the winding to open?

[nickb]

I'll try this out Nickb. Curious why this wouldnt damage a transformer. Couldnt it cause arcing or cause the winding to open?

The internal resistance of the battery to pretty high so that will limit the current - you'll only be doing it for a fraction of a second anyway so it won't open it. You can add a neon across the primary to protect from back emf.

These sorts of transformer usually have very low DC resistances and I was very surprised by the 15 ohms number. It may well be correct as it's fairly low power but I just don't know for sure hence an abundance of caution.

[Jazz P Bass]

Could have had a new board by now.

[lowell]

Owner wouldn't have paid for it.

Anyone want a couple of these bias transformers? $30 + shipping for 5 of em. I'd keep 2 or 3.

[Jazz P Bass]

Owner wouldn't have paid for it..

Well then **uk em.

But you of coarse are expected to beat yours brains out over this thing for free.

So what happens when you do finally get it working and it blows again?

I think QSC blew the pootch on this amp by not separating the SMPS from the power amp.

And the PCB tracks are thin.

I have been there & done that with these amps.
When the SMPS blows, I bail.