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Marantz SC-80 / SM-80 repair queries
Marantz SC-80 / SM-80 repair queries
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Old 5th November 2008, 12:20 PM   #1
Low Orbit is offline Low Orbit  New Zealand
diyAudio Member
Join Date: Aug 2005
Question Marantz SC-80 / SM-80 repair queries

Gidday folks,

A few days ago I picked up this lovely pre/power combo for a song knowing it was faulty.

I was told the amps were playing up, and now I'm tracking the solutions down.

pre amp

audible pop LH channel on input switching

audible (at 6 ft away from the speakers - I can't get any further away) hum (sounds like mains hum but is also present in the tweeter). This is in the preamp - I've run a Perreaux SA-80 I have as a preamp and doesn't exhibit this symptom.

It gets louder if you swing the balance control to either speaker.

(The headphone output is pretty quiet - slight hiss at high volumes with nothing playing and a very faint hum, but much cleaner than the preouts.)

The volume control chatters through the speakers when moving up and down with nothing playing - sounds like motor interference (with the remote - not by hand). When I had the lid off the other day there is a yellow wire that leads from the control board that looks like it should be earthed to something but I have no clue what. This is sitting in mid air at the terminal end and has been attached to something in the past. I'm guessing the board itself (looks like the indent of a washer on the terminal) and there's a screw very near where the lead sits.


The decoupling caps in the output stage have deteriorated.

The caps and the output transistors have deteriorated.

Now, as a matter of course I'll replace the 4 caps beside the output transistors - would going to a higher voltage in the same package with the same or close values be fine? (I almost doubt you could find a 16V cap these days). If I were to alter the value of the cap up or down what would it do to the sound?

I'll start with garden variety ones first and if that cures the hum then I'll look at upgrading at a later stage.

I'm a little concerned about the pop on input switching but it's not a speaker destroying one. It's the same volume regardless of the music volume, but I would like it gone. This happens through the poweramp and headphones.

power amp

From cold on switch on the RH peak indicator lights and stays on (this was the LH one but seems to have moved across).

After about 10 mins or so (could be sooner) the light goes out and the sound balances itself out - before that the affected channel is a lot quieter and audibly fuzzier than the other.

There is a lesser hum which ceases to be audible about 3 - 4 feet away from the speaker LH channel only. Both midbass and tweeter, although predominantly the tweeter. (Pre amp OFF or MUTED)

RH channel is dead quiet. Can't hear a thing. Not even with my ear pressed into the speaker. (Pre amp OFF or MUTED).

There are a few brown marks on the board, but no obvious scorch marks. These are mainly at the pins of transistors although a few of the resistors look damaged (Can't be sure of that though). The amp has gotten damp at some point as there is very faint surface rust on some jumpers and to a lesser degree, some component legs.

I'll pull the amps out and take some shots and post them up if requested.

Any thoughts / help would be much appreciated as I'd like to bring these stunning little amps back to their former glory. (Yes I'm aware that there are much better preamps to be had and I may investigate that at a later date).

Needless to say it canes the Yamaha (AVR) for presence and slam, even in its current humming guise.

My apologies for the long winded post.

(addenum) I can't source the correct caps locally - they're a 16v 270 uF - we only have 220 uF or 330 uF ... could I use any of these? If so, which would be better? (They're also a higher voltage although I guess that won't really matter).

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Old 26th May 2010, 04:35 PM   #2
techgadget is offline techgadget  United States
diyAudio Member
Join Date: Jun 2005
Location: boston
Hello Low Orbit. I realize this is an old post. I recently aquired a Marantz SM80 that needs some attention. I'm wondering if you ever resolved the problems you were having with your preamp and amp? Thanks in advance.
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Old 26th May 2010, 06:36 PM   #3
Low Orbit is offline Low Orbit  New Zealand
diyAudio Member
Join Date: Aug 2005
Gidday techgadget,

yes, I had the poweramp fully serviced - the main board needed a bit of attention - because of some cap leakage the board was open circuit in some stages causing the peak indicator problem. The technician de-populated most of the board, gave it a good bath and clean and replaced the faulty components.

There was also a random turn off issue caused by the small board directly behind the power switch. This board was completely stripped and rebuilt.

Overall it cost a few hundred $NZD to get done but has been faultless ever since and sounds wonderful.

The preamp is another matter. I replaced some obvious leaking caps in the input and output stages myself, and got it mostly working. The volume control chatter was fixed by reconnecting the earth strap to the board.

This mainly solved most of the issues but I still had a fair amount of hum. I continued to use the amp on and off - mainly as a phono preamp for a few months. Then the main relay started playing up - not switching on / turning off randomly.

One day I foolishly used both preouts at the same time to driver two different amps in a hybrid biamp configuration. That ended badly. It seemed to work fine for the several hours that I had it running but it must have generated some nasty feedback loop as when I went to use it again in the morning (it was left on overnight) there was no audio.

A quick lift of the lid revealed about a dozen or so caps with bulging tops and a fair amount of leakage. Ouch.

Given the state of the boards (it's obvious there will be tracking all over the place) I'm considering gutting the interior and using it for an upcoming project. I believe it would be several hundred dollars to repair, and that's a bit foolish.

Let me know what the problems are with yours - I may be able to point you in the right direction.

Cheers, Shane.
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