intermittent fault - 77 power amp

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18-Jun-2019 12:52 AM

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I've a 77 system with separate pre- & power- amps - the stereo amp (77POW), not monoblocs.


The amp has an intermittent fault, verified by running it on another system which also demonstrated that the fault is present irrespective of using quadlink or rca inputs.


When it's warmed up, i'm getting very loud crackles / "explosion" sounds. These are severe enough I'm surprised the speakers have survived. Sometimes it starts after a little use, sometimes it can be hours. As far as I can tell, it's on both channels - so I suspect input section.


Even if amp is left on, silent, it will eventually exhibit the fault.


I've replaced both the LM317 & LM337 regulators which I believe run the input buffers (these were running very hot so seemed likely culprits for semiconductor breakdown) but fault remains.


I can see there's a 78L05 which I presume runs the cmos logic (4016 analog switches) on the quadlink input. Other than that, there's a smattering of small signal diodes (all look to be 1N4148), single zener, and couple of small transistors amongst the several OP275 (suspect these are final drive to power section), SSM2143 amps (differential - so quadlink/ampbus receivers i believe) around the input section, and LM393 & LM339 comparators(!)


Obviously much of this will be dedicated to quadlink inputs, but it is quite a lot to be speculatively replacing - has anyone here any experience of any of these typically failing or similar faults on these?


Does anyone has a schematic of this input section?


I've found a photo which suggests the 99 stereo amp is basically the same - board layout looks *identical*.


http://www.meridian-audio.info/public/99powint%5b2030%5d.jpg


I'm a bit suspicious of the comparators .. not sure why so many would be needed?.. 10 in total! (2x LM339 quad & a LM393 dual) ... is there a muting circuit here which could be playing up? clutching at straws perhaps - how is the remote power-off implemented in these? (reference in http://www.meridian-audio.info/public/99_series_technical_overview%5b1531%5d.pdf)


Thanks!

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