Marantz PM-50 Amplifier Repair (Replacing the STK and Transistors)

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I’m attempting to repair my very own 1990s vintage Marantz PM-50 amplifier that had some problems with the hybrid preamplifier overheating and subsequently damaging some of the power amp transistors. In retrospect, it may all have started with the loose connection on the STK (I first discovered it a bit into this video) that caused the components to fail due to intermittent current flow and voltage peaks.

I’m exchanging the faulty STK 3062 MK IV for an STK 3082 MK III that has a slightly higher voltage rating. After adjusting the idling current there’s no problem whatsoever with that. It sounds great and runs way cooler than the old STK.

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31 thoughts on “Marantz PM-50 Amplifier Repair (Replacing the STK and Transistors)

  1. Some notes for anyone who might be watching this video and repairing their own PM-50:

    The power transistors used in this amp (2SC3280, 2SA1380) are obsolete, and commonly faked. Unfortunately most of the Toshiba power transistors sold are fakes unless you get them from a reliable source like Digikey. Onsemi NJW0281G/NJW0302G work well in this amplifier.

    When you replace the output transistors in a power amplifier, you should re-adjust the idle bias current through them. Ideally you should start by turning the bias pots (R719 and R720) to maximum resistance, so that there is no bias current. Then connect a voltmeter in millivolts mode across the two outer terminals of the emitter resistors R743/R744. Adjust the trimpot until 14mV appears across the emitter resistors – you will need to adjust it further as the amplifier warms up. (edit: OK – Jan does do that.. but ideally you should reduce the idle current to zero BEFORE you power on for the first time 🙂

    A nice trick to prevent things going bang, is to get an 60W incandescent (old filament type, no LED/CFL/energy saving) bulb, and wire it in SERIES with the Live connection of the mains input. If the amplifier is operating correctly you should get a brief flash followed by a dull glow. If the bulb lights brightly, there is a problem.

    The service manual is available on Hifi Engine: https://www.hifiengine.com/manual_library/marantz/pm-50.shtml

  2. Hello
    I have the same amp in which the speaker 1 does not work but on the other hand the speaker 2 works very well. could you tell me what this failure comes from thanks in advance

  3. I am in Vietnam very much looking forward to receiving your reply soon I am having a hard time repairing the marantzpm50 amplifier sincerely thank you

  4. hi my friend currently has a marantz pm50 amplifier it is broken main power can you help me the circuit diagram because the ball burned out all the stats then thank you

  5. Hi Jan,
    I'm new to hifi-technics, especially repairing them.
    I have a marantz pm-50 but the sound output on one of my speakers is down by 90 %. Do you have idea what might be causing this and how I can fix it? Kind regards

  6. never use other component then advertize by manufacture. Its all calculated and those components can take high temperature..

  7. Those stk modules are a pile of absolute peice of crap they have no proper thermal or overload procection, and just randomly fail.

  8. Lipstick on a pig…cheap fake Marantz junk. If you compared the sound to a REAL Marantz…you would probably chunk it.

  9. Fallen into a rabbit hole; watched a recent audio fix video from your Patreon and now found myself addicted to your older audio repair videos too. Really enjoy them, they're a welcome change of pace and direction from my day job!

  10. Hi Can you please tell me the equivalent transistors of B1254 (2SA1263), D1894 (2SC3180). its really hard to find to me..

  11. Hi. In my amp stopped working and the resistor (RGB33/0.18 Ohm KX2/NOBLE U54) is heating up and starts smoking.

  12. Normally, when we see modules, we drop the unit quick for scrap, and advice against any repair, as it in most cases will not be reliable over time. However, when the customer insist we can do a much better (sound) and reliable job with LM3886 or eq. amplifier boards.
    Therefore, taken this time to repair with compatible original parts, the whole board should
    have been replaced with LM3886 boards, a much, much better alternative in the same time!

  13. Those STK chips were made by Matsushita for Technics. They sound pretty clean , but no bass and they blow like crazy.

  14. I guess much safer way is to remove the power tranny first b4 adjusting the idling current, once adjusted, input an audio source without power tranny , just put the volume to minimum if u hear a good soft sound means its ok to operate finished..

  15. what kind of desoldering toll your using ?and i ave a cheap 200$ pyle 2000watts amp…what can i do to beef up the capacitors transistor ? and what is the outcome of that ?

  16. Must be replaced in matched pairs, I don't understand why multi-turn variable resistors weren’t used to adjust the idle voltage.

  17. Make sure we all give E-bay sellers Really bad reviews and complaints when selling fakes. PayPal will ensure refund money. Sock it to the crooks. There are a lot of fake STK chips and discreet transistors

  18. Jan. Great video! Following your instructions I managed to fix my PM-55. Replaced the STK with exactly the same STK as you used. (all markings numbers the same on EBay imported from Germany to UK). None of the power amp transistors needed replacement mind you. The old broken STK delivered an incorrect high voltage to the left channel power amp only – right channel tested to spec. I guess the incorrect high voltage on the left channel was detected by the protection chip and prevented the speaker relay from operating.
    I did have one disaster ..managed to short-out the small power supply board during testing of the old STK (probes slipped!) which blew the 1 ohm fused resistors on the power board. Replacement 1 ohm 1/4 watt resistors continued to slowly blow (I got through 12 of them!). So I completely recapped the small power board but the 1 ohm resistors continued to blow. Apparently the power rating for 240v supply UK 1 ohm resistors SHOULD be 1/2 watt not 1/4 watt as per service manual. The lower 1/4 watt spec is maybe for 110v USA only. I got around the problem by adding 3x 1 ohm 1/4 watt fused resistors in parallel replacing the blowing single 1/4 watt resistors. Now all is stable and well.
    The PM-55 is a lovely sounding amp well worth saving!
    BTW I also bought the same ZD-915 desoldering station as yours – works great! Do you know if it is possible to buy a soldering iron to plug into the ZD 915? Would be useful to use it as a multipurpose solder/desolder station rather than just for desoldering.
    Many thanks for this video. Regards Peter.

  19. STK modules are disasters waiting to happen. I hate them. So called "new" ones are even more unreliable, usually counterfeit junk.

  20. I've got a kit of 2 cheap stubby short screwdrivers, a slot head and a Philips #2, i think i got them elsewhere but saw them at EuroShop for 1€ recently, same shitty quality, but good enough for something you almost never need. Also i got a bit screwdriver with ratcheting and tilting mechanism at… ehr what was it, Hagebaumarkt? not too long ago, i wouldn't normally use it, but it can be helpful when space is at a premium or you need a lot of force.

  21. One of the main issues with the driver chip is heat. Often the chip does not fail but the solder connections become cracked and open circuit which upsets the bias to the output transistors turning them hard on which blows them up. As you noticed the replacement driver does not get as hot as the old one did so extra heat sinking is probably not required. Thank goodness the use of these hybrid drivers were short lived.

  22. I recommend that you upgrade the heat sink on the stk if it runs hot, no reason why you cannot strap on some more to keep it cooler.

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