Bizarre 4400 melted relays

[grimaldi]

Dear colleagues, nice to meet all of you.

Prior to writing this post, I searched all the internet - including Audiokarma - but with no success.

I bought an untouched Marantz 4400 made in 1976 with wooden cabinet and with a few common issues.

I managed to fix all of them and all channels are working through headphones.

I never tested it with loudspeakers, until yesterday.

Prior to my test - with no loudspeakers connected - the protection relays were "clicking" properly.

First I tested the main's front right channel, with success.

But when I tested the main's front left channel, a huge pop at minimum volume was heard and a continuous loud noise was coming from the loudspeaker.

When I looked to the open receiver, I saw a green cloud of sparks coming from one of the relays.

Later, only the main's and remote's front right channels were working and the relays were "clicking" only at the 125x2 mode.

All channels are still working through the headphones.

And the loudspeaker is perfect too.

The same issue occurs at the remote's front left channel.

I disassembled the relays and you can see the damages in the attached pictures.


My guess is:
The previous owner shorted the speakers' connection sometime and one of the relay's copper strips melted with its adjacent strip - see pictures - creating a permanent short circuit.


My question is:
Where the issue is, at the relays or deeper in the power stage?


Thank you.

 

[wlhd1610]

First off Welcome to AK!

Holy crap that's a serious meltdown!
Must have shorted the speaker wires or sent full rail voltage out to melt the relay fingers like that.
Hook it up to a dbt and check for dc at each input terminal on the relay to check the amp outputs for starters.
If your dc offset looks ok then install new relays and recheck.

And don't ever turn the 2/4 channel selector on the rear panel with the receiver turned on!

Bob

 

[rBuckner]

Welcome to AK!

I have seen this damage and the unit had shorted output transistors in the amp. I would think it wouldn't work with just headphones if it were like this though. Perhaps someone figured out the transistors were bad and replaced them but didn't know enough to check the relay when it didn't work? I'm just guessing but I'd check all output transistors if I were you.

You will also want to search for information on what we call a "dim bulb tester" also known as a DBT. Basically you are inserting a household light bulb in series with one wire of the mains AC power input. It can save you from blowing up parts sometimes. The bulb will be bright at power on and should dim down usually. If it stays bright, the unit is drawing too much current. This is just for when doing repairs and needs to be removed for doing adjustments.

 

[bktheking]

Nothing bizarre, your unit went full DC and cooked the relay contacts, sounds like a full rebuild is in order. 4400's are a full resto piece, repairing them isn't enough. It's a long process to redo them but it's rewarding once they are done.