I don’t really work on HiFi, why you ask? It’s a fair question and the reason is that there are just so many different makes, manufacturers that it is difficult to gather any experience on them all. So there are a few makes that I like or have success with and I will accept them for repair projects.
I really love the sound of the old Marantz HiFi amps from the 1970s. Walnut veneer chip board surrounds, glowing lights what a joy. I don’t usually work on HiFi but here I make an exception.
These amps are made to the highest standards and feature great transformers and parts top to bottom.
A couple of issues I have come across and are worth sharing to help others.
Crackling no matter volume level
Seen on Marantz 4240 but applies across the range. In this scenario the amp works as normal but produces a fairly constant rustling / sizzling bacon style crackle. The fault may increase gradually folowing switch on or be permanent. Even with no input connected the fault is present and not effected by the volume control.
The cause is leaky tone recovery transistors on the PCB driving the amplifier blocks.
Everywhere on the internet people talk about replacing capacitors. It’s a bit over stated. Often their are visual signs like rupturing. leaks and yes these should be replaced in pairs but shotgunning the whole amp is wasteful of time and money and unlikely to be worth it.
So concentrate on where it is needed.
visibly worn decoupling caps
interstage electrolytic coupling caps
If you have one of these amps that you would like to sell either for restoration or parts donor I would be interested to hear from you.