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Problem with LUXMAN L-309V Intergrated Amp

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Trying to fix the issue with this Luxman amp.

Takes a while to get out of protection on start-up. When outputting more than about 1W, speaker makes popping sound, if left playing, makes one really loud pop and turns itself back to protection mode.

I took it open and got some pictures.

Seems like the amp had been repaired before. One of the amp board looks like it's been blown up before.

the main power transistors looks different on the power amps, is that a concern?  The 2SA679 is swapped for 2SB655A and 2SC1079 is swapped to 2SD675A.

Any ideas what would be causing the issue?

Any help is appreciated.

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If the L-309V has pre-out/main in RCA's then remove the joining links and plug your source/phone/pod... at minimum volume into the main-in RCA's, slowly increase volume, listen and check for pops which would indicate a power amp problem. No pops suggest a preamp. If pops are on both channels then likely to be a power supply issue. Suggest get a can of freeze spray and give each transistor (in the suspected area pre/power/psu) a 1-2second spray, wait 15-30 seconds for the transistor to recover before moving onto the next one, note any effect on the pops...

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Keep in mind that the freeze spray "shuts down" that and any downstream transistors, so best to work from drivers then back for power amp issue.

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Thanks for the tips.

I bypassed the preamp, buzz and pop is still present in the left channel. The amp still shuts itself back to protection at relatively low volumn. going to get the freeze spray and try it.

What exactly am I looking for when shutting down transistors?

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