Marantz PM-66SE Blown Channel

[Chudley]

Hi,

Recently my amp developed a problem which appears to be fairly common looking at other posts - when switched on the relay wouldnt kick in an therefore nothing could be heard through the amp.

Whilst investigating the problem the main fuse blew & the left channel power transistors were found to be the cause - they were scalding hot and had blown as well.

I've replaced R802 as it was blown, as well as replaced Q761, Q763 & Q751 with equivalent transistors. I've also replaced RN12 as this read significantly far from its intended value.

Switching on now produces an audible click from the protection relay immediately, without the few seconds wait as should be the case. Also, the left channel power transistors are red hot again straight away. I'm guessing I need to replace all other transistors on the channel, but before I do can anyone suggest anything else that may be causing this problem? I've tested as many resistors as I could any all the rest seem fine, and I've resoldered numerous dry joints, which is my guess what blew the channel in the first place.

Thanks

[Stuey]

Hi Chudley,

I've only worked on a Marantz SR50 with a similar problem, but I BELIEVE the basic topology is similar.

I think you'd almost certainly have to replace the bias trimpot - these commonly go shorted. I used a Spectrol 25 turn trimpot with a 0.5W rating for finer adjustment and better reliability.

Cheers

Stuey

[Chudley]

Thanks for that - it was something I had considered and will check