A couple of years ago I came to my hands for free! although damaged, a great HiFi amplifier from the early 90s, the Marantz PM-50, 70W per channel, with tone control and input selector, including those of Phono with a small preamp.

A few days ago I finally decided to bring it back to life (or at least try …) because when the Power button is pressed it does not react and some light should shine on the front panel. This has served to get to know it in depth and observe that it has some critical points where all the amplifiers of this model probably end up failing.

Damaged PCB by resistor heat disipation

The equipment has two independent linear power supplies, a + -46V main for the power stage and a smaller + -18V secondary for the front panel and the basic phono preamp. This one, mounted on the front panel incorporates a resistance whose dissipated heat is outside acceptable limits (perhaps with good ventilation in cold environments such as Iceland will not cause problems …). In my case this failure was the first thing I observed with the naked eye. The welding of this component had deteriorated until it lost contact with the printed circuit track and I proceeded to repair it with a generous amount of tin to ensure good heat conduction. After that, excessively optimistic, I thought that “maybe” the bug had already been fixed and my amplifier was returning to the world of the living. Nothing is further from reality…

Output of the power stage where the loudspeaker protection system relays are appreciated

Now, yes, a LED comes on when the button is pressed, then something has improved the thing. However, you cannot hear the control relays of the two speaker systems when you select them, which does not look good. Indeed, there is no output at the speaker terminals, but an infinite impedance characteristic of the open relays.

The cause of the relays not acting can be of a different nature, for example that they are also fried, which I know after consulting forums such as audiokarma.org or diyaudio.com. However, it is typical that this type of equipment incorporates a speaker protection system that disconnects them if a risk situation is detected as an overcurrent or something fatal as a component of direct current in the output for which the speaker impedance is minimal and This could destroy them in a few moments. It is very possible that there is a situation like the latter due to a more serious failure.

Fortunately I found the service manual of the equipment which has facilitated my understanding of it and made the task of finding a correct diagnosis relatively simple.

Once the schematic is studied, it is clear that the power stage consists of two parts: a two-channel differential voltage amplifier, the STK3062 IV (which can be considered a power operational) in 15-pin linear encapsulation and a final stage based on bipolar transistors

Power stage detailed schematics

The first thing in these cases is to verify that the output transistors are not short-circuited by measuring the resistance between emitter and collector, if it is zero it is a bad thing and should be replaced. However, this has not been the case, so I have to check the STK and with a simple multimeter. I check the voltage of its pins referenced to ground, which is logically connected to the housing. In the service manual the voltages that should be present appear and I observe that in pin 2, which is an inverting input and should be at 0V, I have -16.5V and analyzing the electrical scheme I am sure that it can only be due to the failure of some transistor of the STK.

For just 10 euros I order a STK3082 III that is designed for a slightly higher voltage and is perfectly compatible with 3062. It should be noted that the Roman number that appears III or IV corresponds to the version of the IC and I guess it will have better performance The higher it is. What I have read out there is that at least in the case of 3062 version I and II have a certain internal RC network that in the III and IV must be added externally.

Amplifier module replaced

After carefully desoldering the damaged circuit and replacing it and adding thermal grease to its heatsink and welding its pins correctly to the PCB, it is only necessary to adjust idle current according to the service manual. I checked the value before the replacement and it was correct but now it was completely out of place and I adjusted it correctly. This is done by measuring the voltage at a “cemented” resistor and adjusting its value with a trimmer on the PCB. It must be adjusted to 14mV.


 

The moment of truth has arrived and, as always, something nervous about uncertainty, I press the power button and … after a few seconds, the click of a relay is heard. I have not yet connected speakers but the output presented by the amp is no longer dangerous for these and the protection circuit has given it free way.

Once the speakers are connected, they will certainly be part of another entry in this blog, and using an audio signal from a BT module the Marantz PM-50 finally, as Lazarus got up and walked. No, seriously, working perfectly with a very clean output and perfect operation of the tone, volume and balance controls. Ready for another 30 years!