Old Person asking a dumb question, please.

[One and a half]

I’ve been watching 12voltvids on you tube quite a lot Lately and the most common problem for vintage amps are 

leaky caps (electrolytic)

open resistors 

shorts on the + - rails caused by leaky caps

leakage on T01 type transistors

 

the protection relay may turn on prematurely due to a combo of the above. The signal from the source would have to be measured and compared with the rated maximum of the preamp especially and for modern devices is 2V , in those days 0.775V or 1V was common.

 

If the amp hasn’t Ben serviced for quite some time, it’s worth to recap and weed out suspect transistors. Marantz receivers now have a long history of what fails over the years, a service wouldn’t cost that much in time, and gives peace of mind for the next 10 years or more.

 

[One and a half]

1 hour ago, sandyk said:

 

 

 Unless C701 and C702 are leaky, they should prevent this from being a problem, at least at normal volumes.

 Perhaps the Marantz protection circuit is unduly sensitive ?

 As you have said, he needs to use a DMM to check for D.C. offset from the source device.

 

 Another question would be, does the output sound distorted before it cuts out, in comparison with the audio directly from the source devices .

Agreed these two are the first to look at since they are involved with a reference.

[One and a half]

Oh, just saw on a computer screen instead of a iPhone screen @philbin, see on the drawing of the amp are numbers in red latters and italics.

These are reference voltages that you can measure at these points. Black lead to a chassis ground and the red lead on the identified component. If the voltage measured is not right, the culprit is close by (sometimes).