need help troubleshooting qx-949 with protector problems

jpdylon

non-active member...
I'm toying with a qx-949 I picked up from a friend. Its fine after about 5 minutes from turnon, but then the protect kicks in. it may only stay on for a few seconds, or longer. Its totally random and does not have any pattern. PS voltages are ok, and there is only 3mv on the left and 6mv on the right. Nothing is getting hot, and there are no bad smells. Is there a problem with the protect, or is there a fault in the amp and the protect is doing its job?

Is there a way to see if the protect is working fine and the amp is messed up? I just want to get an idea of what to look for before tearing into this.
 
wow, nobody? That's surprising. I would have thought someone would have chimed in by now... :scratch2:
 
Need to look at the output of the amp section when it goes into protection. Of course, that can't be done at the speaker terminals, you'd have to measure the offset before the relay somewhere to make sure that it isn't an issue with the amp itself. If that looks OK, then the protection circuitry itself is suspect, and in particular the transistor that drives the relay.
 
I checked the transistors on the protector board out of circuit and everything tested fine. Upon checking emitter voltages on the output transistors, the rear right channel output emitter voltages are very erratic and are anywhere from .2 to almost 8 volts at any time. everything else tests at .5v, so it's off to driver board troubleshooting land for me...

**EDIT** I pulled the outputs and tested them. All appear to be ok. no leakage or shorts.
 
Indeed, if the emitters are jumping to 8V, that would certainly piss off the protection.
 
I've pulled and tested all the transistors for that channel, resistors as well. everything seems to check out ok. Is there something I am missing that's just too damn obvious? What causes errtaic or high emitter voltages? Could it be something in the pre-amp like a bad transistor shooting voltage into the amp input?
 
You have what appears to be an intermittent leaky transistor, or dry solder connection. Time to whip out the freeze spray, and/or carefully examine the driver board for bad solder joints.
 
Hit one of the audio amp transistors on the driver board and the emitter voltages went way up to about 8 volts, then became more erratic. I think I at least have one winner, but I'll get back to this thread after a week. between work and school its hard to find time to sit down and play with this stuff anymore.....
 
Got some time to fix the pioneer this evening. Two audio amp transistors had an intermittant short that was revealed with freeze spray. A driver transistor had a junction drop between the collector and base of about 2.7 volts vs the 4.7 spec that the transistor was supposed to have. After some new transistors and a bias and offset adjustment, she's alive and well.

Thanks for the assist EchoWars.
 
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