Anthem MRX 300 av receiver stuck in protect. Need help.

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Hello all,

My Anthem mrx 300 av receiver entered protection mode one day without apparent reason. When powering on, the relay clicks, then it says "AMP protection powering off" on the display after about four or five seconds.

I'm a speaker building hobbyist and don't know much about amps, but would like to have a crack at fixing it anyway. Help is much appreciated, but please: layman's terms :)

So far:

I've disconnected and reconnected several boards: no luck

Checked the output transistors with a multimeter on diode setting. They all seem fine.

Checked all the fuses I could see: all fine.

Then I turned my attention to the standby power board, because somebody had marked a few components there with a marker. An earlier repair maybe? There is a power output socket for the cable which goes to the dsp/analog input board. According to the info on the board, one pin should measure 5,6 vdc in standby (it does). Another pin should output 12v. However, when I measure it when powering on, I get only 7v.

Could this be the problem, or have I just stumbled on something minor and unrelated? And if it's the culprit, how do I fix it? If not, how do I continue the troubleshooting?

By the way: I tried finding the service manual online, but it's not available.

Thanks!
 
Still looking for help. Any advice that can point me in the right direction is appreciated.

Update on the situation: unsurprisingly there's unwanted voltage all over the output stage. I measure about 44v on the emitter pins of all transistors (the transistors themselves do check out though when tested on diode setting and I measure +55 v and -55 v consistently on the center pins). The same 44v is measured in the little white sockets that are apparently used in the factory to read out bias transistors.

So the problem is obvious, but still got no clue where the fault lies. Any suggestions?
 
So are these 44 V present on multiple or even all power amp channels? If so, you may have to hunt down a faulty ground connection.

Only having 7 V on +12 V definitely isn't a good thing either. I would look out for the corresponding regulator (maybe a 78M12 of some description) and inspect / reflow its solder joints. Some manufacturers' parts are also somewhat notorious for going bad, notably JRC/NJR.
 
Usually centre pin of output transistors are the collectors- where power supply voltage is 3rd pin is Emitter

See pic - most outputs are either of these formats
 

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Yes, 44 V on all channels. I'll go hunting for a faulty ground connection later today. Not quite sure what i'm looking for, but i'll give it a go.
For starters, check whether you get continuity between ground at the big filter caps and (a) input jack ground and (b) speaker terminal (-) (assuming it's not a BTL output). The center tap of the transformer secondary feeding the +/-55 V supply should also connect to this point. (If there is a set of +/-12..15 V split voltages, same applies there.) Finding power amp input ground would be very interesting, too.
 
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