McINTOSH MR71 vintage tuner upgrade

The McIntosh MR71 vacuum tube FM stereo tuner was in production from 1963 to 1969 and retailed for US$399.  (Today a good MR71 can still command a price 5 times that!)

The MR71 was the last tube tuner made by McIntosh and, together with the earlier MR67 model, is regarded as the best tube tuner that McIntosh made.

This example is Serial No. 82B84 and is housed in an original McIntosh Walnut wood-veneered case (utilising McIntosh’s ‘Panloc’ sliding rail system).

Apart from re-tubing and tuning alignment, there are some obvious circuit areas that can benefit from simple component modernising after 50 years of existence!

Parts to be used are of good technical and audio quality but reasonable cost – this is not a cost-no-object exercise (no Duelands here, sorry).

Here is the Block Diagram (with proposed upgrade areas highlighted in yellow):

Close-up schematic (Part 1):

Close-up schematic (part 2):

Close-up schematic of the Audio Amplifier stereo output stage, showing proposed component upgrades, the coupling caps:

Underside of the MR71:

The Power Supply  first filter cap is a 40uF 200v Sprague electrolytic – 40+years old, most likely it will be a little dried up inside.

This has been refreshed with a modern axial electrolytic replacement- 47uF 200v Kemet/Rifa.  (Modern technology is less than half the size.)

Notes:

• The original Selenium Rectifiers will need attention – more later.
• The original 4-section 50uF filter cap has already been renewed.

The 10uF non-polar Grounding cap attached to the AFC filter box…..

….. is replaced with a Panasonic 10uF 100v PET (metallised polyester film) cap:

You will notice that the film cap (above) has been checked & marked (black stripe) indicating ‘outer foil’ end and orientated for lowest induced noise pick-up, best shielding.  The Outer Foil is connected to the lower impedance side of the circuit (coupling caps) or to Ground.

From experience, there is often also a significant audible improvement (in dynamics).

A simple method for checking with an oscilloscope, the Outer Foil of wound film-type caps is explained by Aiken Amplification here – http://www.aikenamps.com/index.php/where-to-connect-the-outside-foil-on-capacitors

In the Audio Output stage, the existing caps are:

• Mallory 10uF electrolytic output coupling caps (2) – connected directly to the output socket.
• Sprague 1uF electroytic output coupling caps (2) – as above & also part of the feedback loop.
• Philips 0.01uF 400v MKT (metallised polyester film) caps (2) – coupling between tube sections.

Note how the output coupling caps are arranged at 90 deg. to each other – to minimise ‘crosstalk’.

Replaced with:

• Panasonic metallised polyester film (size is a tricky fit but OK – from RS Components).
• JFX metallised axial polypropylene (also Outer Foil tested – from Hifi Collective).
• ‘Orange drop’ style metal foil & polypropylene film (on the 6BL8 socket – from Hifi Collective).

The preceding Multiplex section is on a separate small PCB with some components on the 6U8 tube socket.  These are ceramic disc caps which can be sonically improved by swapping in film caps.

The replacements (yellow) are Mallory metallised polyester film.  (Again, Outer Foil marked.)

The sound:

Noticeably improved.  Still some McIntosh tonal warmth but now more HF information & treble presence.  More ‘sharpness’ but not sibilant, a more ‘ambient’, ‘spacious’ sound overall.  More neutral tone.  Overall excellent clarity .

Things still to do:

• The HT voltage is around 10% low.  This is most likely due to aging of the original Selenium Rectifiers, which need to be replaced – with Silicon Diodes or with an equivalent circuit.
• The Mode Selector switch is quite ‘tight’, excess friction.
• The 2-wire Power Cord needs to be replaced with a 2 core + Earth cable to be compliant in this country (New Zealand).

Footnote:

Thanks to my pal Lloyd Anderson for continuing valuable advice and encouragement!