Marantz Stereo Amplifier PM-14 MkII: explosive silicon

A heavy beauty, a Marantz Amplifier, PM-14. Certainly worth a detail look – according to the owner, the left channel is not working, and some smoke escaped along with burned smell.

No wonder – the two main transistors of the left channel are blown, say, exploded. And, upon further inspection, also the traces leading to the transistors, at least some of the traces – evaporated.

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This is the full view of the left channel amp (right channel is quite similar, but mirror image – some components placed at slightly different positions.

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Some burnt resistors replaced…

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The board is double sided, but not through-plated, and the quality, well, is not all that good – needs to be soldered with care, otherwise the traces will lift.

After some further checks, fitted a new set of transistors (ISC brand, China – still better than some fake “Sanken” transistors), but to the biggest disappointment, when switching on, these exploded in a loud BANG when adjusting the idle current. Well, turns out that also one of the earlier stages had a short transistor 2SA1145. More testing revealed another defect, a dead Zener (exchanged both Zener, to make sure that there are no issues with temperature drifts etc.).

With all these things fixed, time for a test, using a HP 8903A, and a scope. The HP 8903A is really handy, if your are into any quality audio repairs.

The test setup –

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Distortion measured at several frequencies, and several volts RMS at the output (using a 5.6 Ohm dummy load – just because I didn’t want to take out the 4 Ohm precision load for these power levels). -80 dB total harmonic distortion+noise, not bad.

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The front view – it has the classic Marantz indicator, for best sound, wait until it has warmed up.

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As a reference, just in case you need it, the service manual Marantz PM-14 Service Manual. Or, just send me a message, or the come along with your dead amplifier.

4 thoughts on “Marantz Stereo Amplifier PM-14 MkII: explosive silicon”

  1. Hi Simon ,
    Like your story, the endstage is hardly ever the only thing which is broken , which you prove again looking at the addition parts you had to replace.
    Thanks for the manual , needed this because I came accross a PM14 MKii which blows mains fuses of the in-house electrical installation when switched on with empty elcos. I cannot find any zero crossing or NTC device which should limit the inrush current at “virgin” switch on in the manual. Did you come accross this problem with PM14’s you have seen ?
    Thanks, Chris

    1. Hi Chris, with the 14 MKii, I never had a fuse trigger, but all these amplifiers I test on a common German 16 Amp “B” type fuse, which is not very susceptible to false triggering. Maybe you can check your installation, and use a mains fuse with slower characteristics, if local code allows. With any of the end stage repair, one of the most important learning I can share is that you should run the amplifier full powder for a few hours, before you call the repair done, because some component earlier in the chain or some bias circuit, etc, may have hidden damage. Simon

  2. Simon
    Thanks for the advise.
    Did you find the root cause for the initial blown transistors.
    Thermal design not ok or are the end stage transisitors to be blamed (difficult to detect when exploded)?.

    Rgds. Chris

    1. Unfortunately, no, it just failed when the owner used it, without any overload or abuse or advance notice. The design appears perfect, the circuit boards are a bit low quality for such high end equipment, but the heatsinks are big.

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