marantz sc80 sm80

Marantz SM-80 Amplifier Repair & Restoration

In this second of two articles on the Marantz SC-80 preamplifier and SM-80 power amplifier, we look at the SM-80 power amplifier. Specifically I detail the work required to restore this amplifier back to full working condition. The earlier article looks at work done to matching Marantz SC-80 preamplifier.

The Marantz SM-80 stereo power amplifier is a reasonably powerful, well-made, mid-range consumer amplifier from around 1990. It typically sold with its companion, the Marantz SC-80 preamplifier and I have just finished repairing one of those so be sure to have a read of that article if interested.

The SM-80 is a typical class A/B design and runs a fairly low bias current. It also uses a FET input for very high input impedance and is a very easy load for any preamplifier to drive. The power supply is reasonably beefy, with 4 x 12,000uF capacitors feeding +/-53V rails.

The SM-80 was designed with serviceability in mind, unlike a lot of modern gear which is throw-away, should anything substantial go wrong. The SM-80 was delivered to me with a fault where the protection relay would not allow correct operation until some 15 or 20 minutes after turn-on. This had progressively gotten worse over time and indicated that an overhaul was likely required. My initial thoughts were a leaky cap, or an open or high ESR cap in the circuit associated with the protection chip.

Additional Details:

Before we go too much further, if you want some more information about the Marantz SM-80, or the Sc-80 for that matter, pay a visit to The Vintage Knob for more details. The HiFi Engine kindly also provided these specifications.

Power output: 100 watts per channel into 8Ω (stereo), 300W into 8Ω (mono)

Frequency response: 20Hz to 20kHz

Total harmonic distortion: 0.02%

Damping factor: 200

Input sensitivity: 1V

Signal to noise ratio: 121dB

Dimensions: 420 x 132 x 334mm

Weight: 13kg

Year: 1990

Here she is, with her satin black clothes still on…

Initial testing confirmed the owner’s description of the symptoms and I immediately opened up the amplifier and looked around for anything obvious. Nothing except a few dry joints jumped out at me, so I reworked those and retested the amplifier – same symptoms. This indicated to me that this particular amplifier would most likely benefit from an overhaul  and replacement of all the capacitors except the main filters.

And here, naked for the fist time since manufacture. Note the large EI type transformer to the left, bank of four 12,000uF @ 63V filter capacitors for smoothing the 53.5V (tested at around 55V) rails, behind the transformer and the amplifier module to the right, with two large heatsinks. This amp was designed with good thermal management in mind.
My attempt to show some dry joints, please excuse the images all shot without access to a macro lens. Note also the heat-damage in these areas, where the board is brown.
Some more dry joints around components that run hot
Mains inlet board. I reworked this one as a matter of course
This factory kludge blew me away. Note how poorly executed the factory board repair is, and the fact that, even when brand new and worked on in the factory, the damned traces on these crappy circuit boards lifted!
And another, probably because I couldn’t believe how bad it was!
The other driver PCB is better, but still needs a little attention to some of the joints

I proceeded to remove the amplifier module from the chassis, a job made easy by the excellent attention to detail and serviceability elements built into the design by Marantz engineers. With the amplifier module removed, I inspected the protection relay. This seemed good. The next step was to remove and replace each electrolytic capacitor on the output module, the peak indicator and the secondary power supply module.

Here we have the chassis, minus the amplifier module
The amplifier module, a nice, integrated unit, very easy to work on now it is out of the chassis

The work was difficult as the main amplifier PCB traces were even more reluctant to stay attached to the cheap substrate and I had to recalibrate my soldering temperatures several times from their already very modest settings! To be frank, I don’t lift traces, so to find them peeling off at 290 degrees was disturbing. In the end though, all the work on the main board was completed without too much drama and some high quality audio-grade capacitors and 105 degree rated parts now grace the restored amplifier board.

Some lovely Vishay / BC capacitors, from my stock. I used plenty of these in this restoration
Various other capacitors that will be used to complete this job – there were around 30 capacitors used, of five or six values
Here is my workstation – my god this needs a tidy up after back to back restorations and repairs on various equipment!
Here we can see some re-working of the board, still with plenty to go
An overall view of the mostly reworked main amplifier board
Two lovely Nichicon Fine Gold audiophile-grade capacitors I have replaced in this critical area of the circuit
Here we have a group of four of the BC / Vishay premium capacitors I have used elsewhere on the main amplifier board. I used around 16 of these in total
A few more of these great parts can be seen here
Finally a few more, near the output protection relay. Note that all the other small capacitors you can see here in this shot are new replacements for the original parts
The finished amplifier module, with all electrolytic capacitors replaced, ready to reinstall in the chassis

The next job was to refurbish the secondary power supply, which actually feeds much of the amplifier with two additional rails – an 82V rail, and a 62V rail. The main power supply feeds the output devices and a few other areas like the protection circuit and generates +/- 55V rails. The secondary supply is located behind the front panel and runs pretty hot. As with the main board, there were distinct signs of heat damage. Replacing these capacitors was straightforward as the board pops out for ease of servicing. I used two gorgeous Panasonic FC ultra low impedance, high temperature capacitors here, to replace the already very nice Elna Cerafine caps used by Marantz. The rest of the capacitors were replaced with various high-temp, low ESR parts I use in all my work.

This is the secondary power supply board, showing original parts and still connected to the amplifier chassis
Here it is, out of the amp and ready to be carefully inspected
The reverse side of the board again shows signs of heat damage. All these joints will need to be reworked and I will replace all the capacitors
Here is the reworked, restored board, with all electrolytic capacitors replaced and soldered joints remade
The smaller replacement capacitors
The heart of the secondary power supply, the two replacement Panasonic FC filter capacitors. These are as good a capacitor as can be bought, for these applications.

After reworking and parts replacement, I carefully flux-cleaned each board and checked for correct orientation of parts, before reassembly. I then hooked up all the board to board interconnects and powered her up, hoping to hear that lovely protection circuit relay click closed, but nothing! I took the opportunity though to test other amplifier functions and found both channels to be working perfectly. I set bias currents in both channels while looking at the schematic to try to understand why she was not coming out of protection.

These are the capacitors I replaced on this SM-80 restoration. Looks like fewer than there actually are in there – around 30 in total.
Probably a better view of the old removed parts, showing many of the smaller capacitors buried at the bottom
The amplifier module back in the amplifier chassis here, with all board to board interconnects in place. The little daughter board you see in the middle of the image drives the peak indicator LEDs.
Final view of the amplifier ready for testing. This testing took some hours as I traced a fault in the protection circuit and allowed me to set amplifier bias over a long period of time which was helpful. I usually set bias according to the service data initially, and then fine tune by taking heat-sink temperature reading with a Fluke infrared thermometer.

After some hours of investigation and testing, I found that a critical 55V rail was not appearing at the top of the resistor feeding the protection circuit controller, a device called a TA7317P. The resistor in question divided the 55V down to 3V required by TA7317P, but without 55V at one end, the protection circuit controller chip would never operate. In the end I traced this fault down to a minuscule crack in a PCB trace near a capacitor I had replaced. This was invisible to the naked eye and I only found it by carefully probing the area with a multimeter and looking for breaks. Once this broken trace was repaired, the amplifier came back to life, needless to say I was quite relieved!

This brings  up a point I made in the Marantz SC-80 overhaul article about the possibility of damaging traces when replacing parts. Despite my being perhaps the most careful and meticulous solderer of boards, this hairline crack held things up and was completely invisible to the naked eye. Even under magnification it was barely visible. This highlights the need for care when replacing parts, especially when one suspects the quality of PCB traces and material might be poor.

This Marantz SM-80 is now working perfectly. She comes out of protection after about 5 seconds which is the correct time interval. Having been completely recapped, this SM-80 should be good for many more trouble-free years of service.

3 thoughts on “Marantz SM-80 Amplifier Repair & Restoration”

    1. Hi Paul, thanks for your question. Every job is different so I can’t really give you an estimate until I inspect the amp and figure out what is required. The Marantz needed a fair bit of restoration work and this pushes up the cost. Much depends on the number of parts that need to be replaced. Kind regards, Mike.

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