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Marantz 125 AM/FM Stereo Tuner Alignment and Upgrade

I recently treated my Marantz 125 AM/FM Stereo Tuner to a very careful alignment and upgrade. This beautiful tuner, with upgraded power supply and output buffer, is easily the best sounding tuner I have had the pleasure of listening to.

After modification and alignment, I compared Marantz 125 to a Sansui TU-717 and several modern digital tuners including the gorgeous Kenwood L-1000T. The stunning Marantz beat everything, leading me to the conclusion that it is one of the best tuners I’ve ever heard. The 125 tuner has tone. Tone is a hard thing to describe, but you always know it when you hear it. The 125 sounds extremely natural, relaxed, has wonderful warmth, texture and bags of air. It’s a special sounding tuner, from a period of manufacture that featured engineering and workmanship. Sadly, these days are mostly behind us.

Older Marantz tuners are legendary for their warm, smooth sounds and their ability to separate stations packed closely together, This selectivity is something much-needed in North America during the peak in FM radio the 70’s. Marantz spared little expense when they designed this tuner and it shows when you lift the lid. Each circuit block is packed into its own shielded metal sub-block and all adjustments are clearly marked. The tuner features a five-gang FM front end, discrete class-A output stage, special IF filtering and great attention to detail everywhere you look. You don’t even get a discrete class-A output stage in some expensive preamplifiers, but you get one with this beautiful Marantz tuner!

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This is such a good sign in a tuner – separate shielded boxes for each circuit block.
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What a lovely piece of gear – this attention to detail is yet another thing you don’t see today.

The work on my Marantz 125 centered around two areas: a precision alignment and replacement of critical parts in the circuit. Looking at the parts first, I paid extra attention to the power supply and signal path. In the power supply, I removed all the original electrolytic capacitors and replaced them with premium, low-ESR, high-spec parts. In the signal path, specifically in the output buffer, I replaced all the electrolytic coupling capacitors with much better sounding film capacitors.

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This is the upgraded power supply – note the tall Panasonic FC filter capacitors.
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Here you can see the small blue tantalum capacitors I removed from the signal path, with the large orange and white film capacitors in their place. These are sonically better, plus I was able to lower the cut-off frequency of the high-pass filter by using larger capacitance in these DC blocking caps.
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A close-up of the new film output coupling capacitors – they are the big red and grey rectangular packages.

The second and critical part of the process is the precision alignment of the FM front end, IF stages and stereo demodulator. This alignment process is what often brings the biggest performance gains to FM tuners, because they are complex electro-mechanical systems and they drift over time. Often they are not even that carefully aligned from the factory and sometimes huge gains can be realised by a careful alignment, utilizing my precision laboratory equipment, specifically designed to aid in this sort of work.

More pictures below and if you would like to know more, let me know via the contact page.

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Gorgeous Marantz 125 tuner on the bench ready for a full alignment.
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My Panasonic VP-8175A FM/AM Signal Generator was designed for precision, low distortion alignments like this one. The Panasonic generates a very clean, quiet signal, essential when working on high performance tuners.
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This is my Tektronix distortion analyser, measuring the distortion of a signal generated by my Panasonic VP-8175A and output by the Marantz 125. Note the high reading and this was before setting the test up properly.
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And this is after setting things up and doing some tweaking on the alignment – you can see here the tuner decoding a stereo test signal and reproducing it with 0.12% distortion, a very good result indeed for a tuner of this age, or any tuner for that matter!
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The job completed, what a stunning front panel.

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Gorgeous!

2 thoughts on “Marantz 125 AM/FM Stereo Tuner Alignment and Upgrade”

  1. Hi there, thanks for an excellent blog. I have been lucky enough to purchase a Kenwood KD – 600. I have a very good technician cleaning up the machine and again thanks to instructive pages on your blog. I would like to install the same Jelco 750 arm on the deck. Would you be able to supply one and ship to South Africa? If so, please let me know the cost. Thanks & regards, Randall Abrahams

    1. Hi Randall, thanks for your positive feedback, much appreciated. The Jelco arm works very well on the Kenwood, but you will need to make up an arm mounting board to suit. In terms of supplying the tonearm, you would be best to have a look on eBay, set to an international search. You will be able to get one cheaper this way, by dealing directly with a supplier. Kind regards, Mike.

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