Crown XLS 402 trips into protect mode at power up

[cjlectronics]

I searched the forum for repairing a Crown power amp and came up with a few things but nothing like what I am experiencing. A buddy brought in a Crown XLS202 402 602.pdf and as soon as you turn the amp on the fans run and lights turn on for about 3 seconds, then it shuts down. Nothing looks burnt and everything looks normal.

From the service schematic, the tech notes state this amp has a soft start mechanism in it which makes me think the power supply section is good but when the amplifier section is kicked on something in the power amp section is shorted and tripping the amp into protect mode.

Has anybody seen something like this?
Am I on the right track?
Should I start pulling the power transistors to check for shorts?
What would you do at this point as you can't check for live voltages cause it won't stay on to do so.

[Jazz P Bass]

First off.
Slow down.
You are jumping to conclusions.
Start by pulling the power supply header connectors.
This should effectively isolate the power supply.
Now test the power supply voltages according to the schematic.
You should not have to 'pull' the output transistors to locate a gross short.

[cjlectronics]

First off.
Slow down.
You are jumping to conclusions.
Start by pulling the power supply header connectors.
This should effectively isolate the power supply.
Now test the power supply voltages according to the schematic.
You should not have to 'pull' the output transistors to locate a gross short.

Great advise and thanks for the scolding. I was jumping to conclusions. However, I wasn't to far off.

I pulled power cables from the secondary of the transformer and was able to get the unit to stay on when the +85vdc buss was disconnected. I started poking around with my multimeter and found a dead short across the emitter and collector of one of the MJ15025 power transistors. I checked other parts hanging off this transistor and they seem to be OK. However, I don't have a micro-ohm meter to check the 0.47 ohm resistor... It measures like all of the other o.47 ohm resistors so I think it is OK.

I'm thinking its OK to just replace the power transistor and try it??

[Enzo]

Yes, or if that transistor has others in parallel, then power up without it during tests.

Whenever you have a shorted output transistor, always check the drivers too.


I'd bring it up on a variac while I watch manis draw instead of just flipping the switch.

Or at least a "light bulb limiter."



You don;t have to measure 0.47 ohms. You can get closer measurements if you short your meter leads first to measure the lead resistance. Then subtract whatever that is from the resistor reading. If they all read about the same, that is good enough. Most times they are either burnt open or they are OK. They are wirewound after all.

[Jazz P Bass]

Great advise and thanks for the scolding.

It was not my intention to 'scold'.
(more like a light smack to the back of the head)
Enzo, Stan & others have consistently preached have proper troubleshooting technics.(sic)
I was just chiming in.

[Gtr_tech]

Enzo, Stan & others have consistantly preached proper troubleshooting technics.

We're working on turntables now??

[Jazz P Bass]

Oopsy.
I must learn to spell (or use spell check)

[Enzo]

Took me a second there... I'm getting too old.

[cjlectronics]

I replaced the MJ15025G power transistor that had the emitter to collector short with another MJ1502G5. Powered the unit up using a VARIAC and the unit stayed on and ran without any problems. I input a 500Hz signal and then slowly increased the volume through an 8 ohm 1000W load resistor with ~20VAC p-p across the resistor and let it run for 15 minutes. Again, unit ran without any problems. Then I increased the volume some more and popped the 6 amp fuse on my VARIAC. I had the amp configured for a bridge output. Plugged the unit into the wall socket and decreased the volume where I was pushing ~30VAC p-p through an 8 ohm load resistor and let it run for 30 minutes. It ran great. I'm gonna say its fixed.

Thank you to everyone who replied and those that looked at this thread.

CJ