Bringing new life to an old classic marantz stereo receiver with a class D amp board.

Picture of Bringing new life to an old classic marantz stereo receiver with a class D amp board.
*this instructable takes for granted that you can solder, understand basic electronics, and have fixed electronic gear before.*

so you acquired an old stereo receiver but it has issues. there are many things that can go wrong with an old stereo receiver but one of the common issues can be blown outputs. sometimes the cost of parts is prohibitive, making it not worth the effort of trying to fix a piece back to its original design.

what to do? junk it? NO..

this instructable is intended to be an option for those that have a piece not worth fixing. vintage stereo gear can be valuable, even  broken so don't just dive in till you know what you have. you may regret your actions later if by chance you had something quite valuable, even if broken. finally, this instructable is focused on transistorized stereo gear from the 70's and 80's. old tube gear is a totally different animal and not covered here. throughout this instructable an effort will be made at trying to solve some of the common ailments of old stereo gear. there's always a chance you might get lucky and what ails your unit is covered here and you are able to fix it instead of mod it.

with that out of the way, let's see what's up with this old marantz.

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Add images link not working for me, let me know if I can email you
Pertaining to my last post, closeups of burned resistors and empty capacitor slot.
Thank you for this. I have a Marantz 2420 that my Dad bought in 1970. In my first floor dorm room, it could be heard on the third floor. Beautiful piece of equipment. Not knowing what I was doing, in 1999, I hooked up my speakers (8 ohm) and my friend's truck box (4 ohm). There was a strange chirp and it gave up the ghost. It's been boxed up since then, but I recently repaired a TV and thought I might have a crack at the receiver. I'm currently on step 3. The transistors failed the continuity test. Two had two way continuity and two others had none at all. With the transistors removed and a new fuse in place, it successfully powered on. There are no good local stores for electronic components, but I ordered some that should arrive by the end of the week. This beast shall live again! Pictures to follow when it is up and running.
ke4mcl (author)  joseph.johnston.39561 month ago
Cool! Good to hear you may save it. Dont forget to replace the thermal grease and put any insulating spacers back.
So.... New transistors installed, the unit comes to life. I start with all EQs centered, high and low filters and loudness off, volume all the way down, speakers connected to main, main on, remote off. Bringing the volume up slowly everything sounds fine. Tested balance and filters, all good. After a couple minutes there was some crackling noise from the right side speaker. I turned it off, and switched the speakers to the remote channel to test it, and also to identify if the crackling was a receiver or speaker problem. No sound, and right away a small puff of smoke from the right side (volume knob side). I shut it off, and reconnected to main, hoping the problem was limited to the remote channel. No such luck. Smoke, and one resistor almost immediately red hot (R727). Further inspection shows R716 and R728 are scorched. I think they were the first two puffs of smoke and R727 was about to do the same. The fuse did not blow. I think (wish/hope/pray) that the resistors started to cook off but hadn't failed so badly to burn the fuse or fry anything else.
Also came across this anomaly: above R728, the board has holes and symbols for a capacitor (C713) but there's no sign of it having been there. It is present on the schematics as a 220pf 300V capacitor. Any ideas on what is cooking my resistors and what's up with this mystery capacitor?
I have a ceramic non-conductive thermal compound for CPUs from Radio Shack that I planned on using. Should that work?
ke4mcl (author) 28 days ago
Yep. That will work fine.
Hatredman1 year ago
The only problem with this instructable is that the sound quality of a D-class amp is no rival to an original AB-class Marantz power amp. Your receiver will sound louder, indeed, but poorer.
ke4mcl (author)  Hatredman1 year ago
id have to respectfully disagree with you. i not only tinker with old stereo gear but also collect vintage tube audio gear. audio is a very subjective hobby and ultimately one should buy what sounds good to them.

i have found some lousy class d boards but have also found a great many excellent sounding ones. in particular all the boards i have gotten from Sure Electronics in the 15wpc class and above have sounded great.

were class d falls short is when it clips. it goes from beautiful sound to cats fighting on a chalk board really fast. its very important one does not drive class d amps into clipping.

the mod that was done on this marantz is fully reversible. if you keep your old parts, there's no reason somebody cant go and undo what you did, repair or replace the original amp board, and wire it all back up the way it was.
Sure, there's no questioning on the subjective part. But Class D amps, even the better ones, have its issues - as all types os amps have, even single-ended class A tube amps.

D's are not good with very high audio frequencies (God damn you, Nyquist!) and they tend to turn things on the bass side a little muddy - just like tube amps, even the better ones, 'cause the damping factor is too low.

This is no opinion, this is something tried and tested on the road - ask a PA owner or tecnician and he/she will share his/her disgust about Class D amps with you. They still love AB or H for a reason.

Anyway, of course there are marvellous D amps out there, and if the instructable was about some cheap 80's stereo I'd totally support the "transformation". But it was a Marantz. It deserved some respect.
Again, it's not a critique on your instructable, because many will find it inspiring and useful - and it is!

It was just a comment about the receiver being a Marantz, not the average low cost receiver.
from that angle it looks like the cals are buldging
pfred21 year ago
I owned a similar model although mine was Quadraphonic. My unit was rated at 20 watts per channel output. I burnt one of the channels out in it and it was impossible to dig into it to change the transistor due to how it was made. The thing was a total rats nest! It was OK back when I used it but I've had a lot of stereos since so it wasn't worth repairing to me. Not many consumer electronics really are. I did scrap it for salvageable parts though.
ke4mcl (author)  pfred21 year ago
wow.. that would have been an excellent unit to drop a Sure Electronics 4x100wpc board in! i'm familiar with the quad versions of the marantz line in this instructable and they are really well suited for this mod.
Really Inspiring. I am thinking of restoring my 11 years old DAPIC Amplifier! :)