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.
All the way back from 1941 this is got to be one of the oldest amps that I have worked on. Pair of 6L6 output power valves red chassis, lots of rust – sounds good as gold.
Bit of history with this amp as it was stationed in Pearl Harbour back in the day and presumably saw some action. Later on it found its way over to the UK at some point where it has been waiting for restoration for some time. Virtually all of the old resistors were drifted so much in value that the amp would barely output anything. That restored and a few worn out valves replaced and the amp is back to full power. Not surprisingly it sounds great and distincively to a different era.
Wow what a treat. This 1966 AC100 needs a service.
Well after all these years yes I agree it’s about time.
One owner from new, original cover and used twice weekly in the band. The sort of thing vintage amp enthusiasts dream about.
This rarity is one from the erase when pop music was really developing. The Beatles had stopped touring and concentrating on recording Revolver, Jimmy Page was coming out of the studio. Pop was progressing and diversifying.
100 watts doesnt sound like much by todays standards. But don’t think this is a slouch though. This amp drives speakers very deeply indeed and even at low volumes is absolutely room filling.
Few gut shots below and keep your eyes open for one of these in your attic.
Fantastic active monitor speakers that sound great and make you feel right at home.
A couple of issues and tips I have noticed on these are:
watch out for overheating resistors in the power supply (R2, R3 and R17). They cause damage due to the heat travelling up the board, look at the pictures for the smoke damage on this (working board). They need to be removed from the PCM cleaned and re-soldered to improve conductivity and ventillation.
When replacing the output devices it is critically important to use matched devices, otherwise they will fail usually within the first hours use. Matching does not take too long using the Nelson Pass method but remember to allow two minutes for each device to warm up a little before taking a reading. You need a stock of about 20 devices to get a good set of matches.