XR1212 Power Issues

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welldigger99
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XR1212 Power Issues

Post by welldigger99 » Tue Apr 23, 2019 11:01 am

Hi,
I have been asked to take a look at a XR1212 mixer unit by a musician friend who knows I have some limited electronics experience. He reports that switching on the unit a couple of weeks ago resulted in a "blue flash" from the rocker switch, and the unit went dead.
I have confirmed that when switching the unit on, I can hear a very brief "click" from the relays, after which the switch refuses to latch in the ON position.
Any ideas on this please? Do these switches fail, and if so, is it worth obtaining a replacement or is it more likely that it's warning me of a more serious failure elsewhere on the power amp board. Close visual inspection of the circuitry and components reveals no evidence of burning or overheating.
Any views greatly appreciated,
Thanks guys.

jim h
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Re: XR1212 Power Issues

Post by jim h » Tue Apr 23, 2019 1:24 pm

The switch is a magnetic breaker, so if it won't stay latched it is either defective or the supply is demanding high current (something likely shorted).

I would check the amplifier output FET's, they sometimes fail shorted to the negative rail. That's kind of a wild guess...but it would definitely demand a lot of current from the supply.

welldigger99
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Re: XR1212 Power Issues

Post by welldigger99 » Tue Apr 23, 2019 5:54 pm

Thanks for that. I will do as you suggest. Need to somehow get hold of a schematic now for the XR1212. Hi

welldigger99
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Re: XR1212 Power Issues

Post by welldigger99 » Sat Jul 27, 2019 1:59 pm

Thanks for the earlier feedback. I have replaced the output FETs which were indeed short circuit. I thought that this would provide a solution but -alas - no! On power up after a few seconds there was a flash and a loud crack - similar to what happened when the fault first developed.
On inspection I can see that Q3 and Q4 have both failed in spectacular fashion - the cases of both blown apart! Does this give any of you guys any ideas on where I might look next?
Any views greatly appreciated!

macouc
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Re: XR1212 Power Issues

Post by macouc » Fri May 26, 2023 4:36 am

hello

I have similar situation if not identical : unit doesnt power on , relay " clicks" then shuts down. I changed the relay , no improvement. Then tried to measure resistance with (cheap) ohmmeter that gives near-zero at BOTH circuits of the relay (the 24v as well as 250v pins). It looks like a short circuit somewhere.

I read you guys comments and you mention FET transistors but I cant figure out which components. I forgot to mention: I am a musician , not engineer :-(

I have dowloaded the schematics but understand little.

this thread is the *only* source of info Ive found. Do you guys have found other helpful ressources on this subject? For instance this usit is a XR1212 , but exact same problem could also occur in other peveay powered mixers / amps?

Any idea how to "debug" my power supply?

thanks in advance
PS : welldigger99 did you sort it out finally?

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dak
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Re: XR1212 Power Issues

Post by dak » Fri May 26, 2023 6:10 pm

macouc wrote:
Fri May 26, 2023 4:36 am
I forgot to mention: I am a musician , not engineer :-(
I am afraid that this is an engineer's job by now: even if the original fault may be a single part, there is a bunch of subordinate parts that have gone up in flames and you need to track down the possibly affected parts from the failure (as well as possibly damaged parts), measure them and exchange what is dead or likely near-dead.

Either some part went dead, or something touched that shouldn't. One possible candidate are the power transistors which are mounted isolated using mica and isolating grommets. Depending on the kind of contact, there will be visible traces.

But as I said, that's the starting point and you get to chase down through the schematics what might have gone downhill from there.

macouc
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Re: XR1212 Power Issues

Post by macouc » Mon May 29, 2023 5:12 am

@dak
thanks a lot for your quick reply !

I've just managed to identify the power transistors , no visible trace of anything special.

But in order to test them, how to remove them ?(and even put them back..?) : the middle pin is absent and the back seems to be glued (the mica you mentioned ?) . I have ordered new transistors. I suppose I have to also buy some mica as well. I'll google that mica stuff to get clued up.
Next thing I'm gonna try to remove them for testing.

I also ordered the big electrolyte capacitors. No harm in replacing them.

I forgot to mention something that may be of relevance :
During about 2 years before the current fault, everytime the weather was humid , one of the power amp channel was producing a loud hum for a few minutes before working normally. During these few minutes I just disconnected the speaker cable for the duration of "drying process" . Because it was linked to humid air I assumed it was "drying" taking place.

jim h
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Re: XR1212 Power Issues

Post by jim h » Tue May 30, 2023 5:06 pm

There is no mica or thermal grease on the output devices. These are D2Pak FET's that are soldered directly to the pcb.

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dak
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Re: XR1212 Power Issues

Post by dak » Tue May 30, 2023 5:34 pm

jim h wrote:
Tue May 30, 2023 5:06 pm
There is no mica or thermal grease on the output devices. These are D2Pak FET's that are soldered directly to the pcb.
Ok, I now found the schematics. Class D amplifier, power MOSFET IRFS4227 with 26mOhm on-resistance. That indeed does not sound like it would need a whole lot of cooling, so forget about the mica and heat sink. Sorry, I didn't check the schematics before and had just been assuming class AB.

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dak
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Re: XR1212 Power Issues

Post by dak » Tue May 30, 2023 6:15 pm

macouc wrote:
Mon May 29, 2023 5:12 am
I forgot to mention something that may be of relevance :
During about 2 years before the current fault, everytime the weather was humid , one of the power amp channel was producing a loud hum for a few minutes before working normally. During these few minutes I just disconnected the speaker cable for the duration of "drying process" . Because it was linked to humid air I assumed it was "drying" taking place.
That could be some almost-but-not-quite short by dew. Maybe do the warmup in the dark and look for some tiny sparking (of course, don't risk touching anything in the dark by accident that may be under high voltage). Could be a thermal thing as well, like a cold solder joint.

macouc
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Re: XR1212 Power Issues

Post by macouc » Wed May 31, 2023 6:20 am

thanks jim and dak,

"... thermal thing as well, like a cold solder joint." : I never heard about cold solder joints b4. Now I'm reading about it, fascinating!

About the transistors, since they are soldered underneath, it seems that the repair is totally beyond my skills AND equipment! I'm sending to repair shop.

jim h
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Re: XR1212 Power Issues

Post by jim h » Thu Jun 01, 2023 3:07 pm

Good plan...

dmurji
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Re: XR1212 Power Issues

Post by dmurji » Tue Aug 08, 2023 1:07 pm

You state that Q3 and Q4 have blown.
This is possibly due to a short on the +/-80v supplies.
I would check the output FETS Q103,Q104,Q203,Q204.
Also if the driver chip (Ic102 or IC202) has blown and left the drive to the Gate of the above FETS floating, then that can damage them.
This would cause Q3 and Q4 to blow up in a spectacular way. Together with Q3 and Q4, I would replace IC11, D7 and D26 as they would be damaged too.

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