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Marantz SC-80 Preamplifier Repair

A customer recently asked me to look at his immaculate Marantz SC-80 preamplifier / SM-80 power amplifier combo. Both pieces were playing up and required work. In this article I will look at the preamplifier, the next article will examine work required to remedy the power amplifier.

 

Customer kept the original boxes which is excellent for transportation purposes.

 

The Marantz SC-80, and its partner, matching Marantz SM-80, are quality mid-range separates hailing from around 1990. They are unremarkable in terms of circuit layout and construction, but were well made and thoughtfully laid out, making service relatively straightforward. Unfortunately, both pieces suffered from poor PCB materials quality, having very thin, probably 1oz traces at most, that are very keen to come away from the board. If you are planning on working on either of these components, set your iron to 280 – 290 degrees centigrade.

 

The preamplifier before dis-assembly.

 

The SC-80 features a fully discrete phono preamplifier, with JFET input stage and good parts quality throughout. This unit was pristine physically, but internally she had been repaired before. A previous repairer had obviously battled with the fragile traces because there were several board level repairs to traces that had previously lifted and I reworked all of those to improve them. There were signs of an over-current fault in the phono preamp that looked to have been repaired. The board was also heavily contaminated top and bottom with fluxey residue, necessitating complete removal and what I now refer to as my ‘über’ cleaning process, involving foaming cleaner, water and flux removal top and bottom. Finally there was a dead capacitor that needed a replacement.

 

A plan view of the SC-80 chassis and board layout.

 

First thing I noticed was the ashed resistor, center-frame. Notice the associated semiconductor next to it and overheated board in the vicinity of these two components. As much as possible of this carbon needs to be removed.

 

This cap has overheated and died. All of the goop that came out must also be removed, before replacing the cap.

 

The best way to commence any work like this is to remove the circuit board requiring work and thoroughly clean it.

 

Removal allows one to better see what lies underneath of course too.

 

This is pretty ugly work …

 

As is this, though it is not the previous repairers fault that a transistor basically exploded on the board.

 

This capacitor has been removed, ready for the replacement, after cleaning.

 

Part of my ‘über’ cleaning process involves washing boards with a special foaming cleaner, allowing that to work its way into grease and grime and then washing the boards in clean water. Yes I know it is a silly name, but I’m sticking with it…

 

Foaming cleaner is applied to both sides of the boards.

 

After rinsing the board is left to drip dry, outside where there is a good flow of air.

 

Vertical orientation allows the board to drain effectively. For those who might be horrified to see electronics cleaned in this way, rest assured that boards are often washed at the factories where they are made.

 

Once the cleaning process was complete, I set about reworking the board in various ways. There were lots of dry joints that required rework and the trace repairs all needed attention. There was a charred area of board in the phono preamp underneath a transistor and resistor that had overheated at some point and charred the board. The transistor looked to have been replaced and was working well, but the resistor had gone high and I replaced it. I removed as much of the conductive carbon from around this area as possible as this is a high impedance part of the circuit and burnt board containing conductive carbon acts as a voltage divider along with other high value resistors in the area and must be removed where possible.

I located a capacitor that had spewed its guts out onto the board, so this was replaced and I then set about checking out other capacitors, especially those that had been replaced previously with the correct values listed in the service manual. Unfortunately, two large decoupling caps in the phono preamp had been replaced with incorrect values and caps that were half of the required 1,000uF. This may have accounted for the low-level hum my customer had complained of and I replaced the 470uF parts with audio grade Panasonic AM series caps of the correct value.

 

Here you can see I have replaced the dead resistor and cleaned the board very thoroughly beforehand. Note the dramatic improvement in appearance of the board. The two tall capacitors at the edge of the frame are those that were replaced by the previous repairer and are of the incorrect capacitance. They must be replaced.

 

A close-up of the new capacitor I soldered in place of the part that died. This is also a 105 degree-rated part.

 

Here, the two incorrect capacitors have been replaced. I used Panasonic AM type audio-grade capacitors here.

 

Another view of these replacement capacitors.

 

A view of the reworked phono preamplifier section of board, replaced resistor and two new capacitors.

 

View of finished phono amp section

 

After all the board rework and testing and replacement of only a few parts, I sealed this board with the excellent RS Circuit Board Lacquer. This gives boards that look a little beaten a lovely satin finish that is aesthetically very pleasing and that seals boards nicely against dirt and moisture. Importantly the lacquer also seals boards in areas where they have been burned or damaged.

 

A nice view of the lovely satin finish of the board after treatment with circuit board lacquer

 

And another view. This stuff seals the board, prevents degradation and ingress of contaminates.

 

A quick test showed me that the hum my customer complained about was gone, and the SC-80 worked quietly and smoothly across the inputs and controls.  The phono preamplifier section was nice and quiet and despite not replacing the majority of capacitors in the SC-80, she should now work well for a few more years.

Customers and readers sometimes ask why I don’t just replace all capacitors as a matter of course and there are several good reasons for this. Firstly, many older capacitors in better consumer gear are actually quite good. The Nichicon 105 degree-rated caps used in this Marantz gear are a good example of this. I dislike replacing good parts for no reason, as all it does is increase the risk of damage to boards with fragile traces such as these and serves no other purpose if the caps remaining are in fact all working well.

Sometimes a complete replacement of capacitors is worthwhile and this is always judged on a case by case basis. Much depends on the operating environment of the component, how much it has been turned on in its lifetime, how hot the component gets internally, how old it is and so on. In this particular case the remaining capacitors tested good and I don’t believe they needed to be replaced at this time. There is a cost analysis that also must be made with quite a few to replace, the cost of work can quickly escalate.

Anyway, this repair was very successful and the SC-80 is now operating correctly. Both pieces ready to provide their owner with years more good service. Be sure to check out my Marantz SM-80 restoration and repair also!

 

4 thoughts on “Marantz SC-80 Preamplifier Repair”

  1. Hello,
    ‘über’ cleaning process, involving foaming cleaner, water and flux removal top and bottom
    Seems a nice solution to cleaning.
    What brand are you using?
    Foam?
    Flux remover?

    1. Hi Fredrik, I use mostly laboratory grade chemicals, from Ambersil. These are obtainable from RS, Mouser, Farnell etc. Be very careful if going down the H2O pathway, to thoroughly dry afterwards! Regards, Mike.

  2. Thank you for your article. I also have this matching pair. I am looking for a Marantz sc-80 switch board and possibly the long plastic switch mechanism that has broken. Mine also looked like a cat had pissed on it with similar problems. Thank you, Matt.

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