Post by Owen Y on May 27, 2017 12:42:42 GMT 12
Quite a few years back I paid a bit much for this vintage FM stereo tuner. However, the MR71 is a 'classic collectable', the last vacuum tube FM tuner made by McIntosh during 1963-69. The MR71 is regarded amongst the most desirable tube FM tuners, alongside McIntosh's earlier MR67 & the Marantz 10B. Additionally, I knew the importer was a professional electronic technician, a radio enthusiast. It had been converted to NZ voltage & set up for NZ FM de-emphasis (HF EQ), aligned, re-tubed & some key components renewed. Otherwise, condition was excellent, but not mint.  
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Post by Owen Y on May 27, 2017 12:43:26 GMT 12
 The sound of this MR71 seemed to conform to what most people describe as 'McIntosh sound' - attractive overall tone, on the warm side of neutral, perhaps a bit rolled off in the treble, lacking a degree of sparkle. So after 15+yrs, on the advice of my pal, it was time to upgrade some parts mainly in the PSU & amplifier sections.
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Post by deano1974 on May 27, 2017 21:33:54 GMT 12
I would not be concerned if i paid too much, because she is a real beauty and there are not many around in such good condition, timeless classic!
Manager & Product specialist at Rapallo AV & HI-FI
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Post by colinf on May 28, 2017 9:55:51 GMT 12
Lovely tuners, I used to work on them way back when. Can't remember much about the circuit now but think it used a 6BL7 in the output, is that right?. I remember they sounded warm with a loss of depth compared to the mighty Marantz 10B, and even the trusty old Dynaco FM-3 could show a bit more depth. What are you going to do to it?
AMR-iFi R&D
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Post by michaelw on May 28, 2017 11:59:34 GMT 12
owen; are those pearl tube coolers over the tubes ? i must admit to being attracted by the cosmetics but not the sonics of tube tuners. 
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Post by Owen Y on May 28, 2017 17:41:52 GMT 12
owen; are those pearl tube coolers over the tubes ? Indeed, but not installed by me. The previous owner. Together with an audiophile (Siltech?) power cord - which I will remove.
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Post by Owen Y on May 28, 2017 17:59:19 GMT 12
Lovely tuners, I used to work on them way back when. Can't remember much about the circuit now but think it used a 6BL7 in the output, is that right?. I remember they sounded warm with a loss of depth compared to the mighty Marantz 10B, and even the trusty old Dynaco FM-3 could show a bit more depth. What are you going to do to it? - actually, I've done the bulk of the planned work & it sounds much improved. Previously I had never felt excited about this MR71. Now has some sparkle, better neutrality, spatial ambience, decay. Also found some additional things that need doing. I will outline the process.
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Post by Owen Y on May 28, 2017 18:54:36 GMT 12
This is the 'block diagram' schematic of this device, with the areas highlighted that will be worked on: 
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Post by Owen Y on May 28, 2017 19:20:52 GMT 12
This is what the MR71 looks like underneath & inside:  
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Post by Owen Y on May 28, 2017 19:27:31 GMT 12
As said, the radio sections have been already checked & 'refreshed' by a radio technician after it was imported. However, there are a number of capacitors in particular, which are now either old and/or degraded, or simply not up to todays 'audio' standards. 1) Power supply: The first filter cap is a 40uF 200v Sprague electrolytic - 40+yrs old, it will likely be fairly dried up inside.  This has been refreshed with a modern axial electrolytic replacement- 47uF 200v Kemet/Rifa: (Modern technology here is less than half the size, as can be seen.) 
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Post by colinf on May 31, 2017 7:57:31 GMT 12
Wow forgot it had selenium rectifier diodes! Just found the circuit diagram for it. They run on very low voltage, only 95v on the output tubes. Could you take the output from the stereo decoder directly and see what it sounds like? The output stage is set for a gain of about 11.
AMR-iFi R&D
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Post by Owen Y on May 31, 2017 16:00:02 GMT 12
Hi colinf - yes the Seleniun diodes need replacing, HT is low - more later I will 'refresh' but keep it as authentic as poss.
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Post by Owen Y on May 31, 2017 21:17:08 GMT 12
2) Audio Output stage: This part of the Tuner is the final audio output amplifier, built around a 6BL8/ECF80 triode/pentode double-tube per channel. As in most amplifier circuits, renewing & upgrading capacitors is a good strategy, especially 50yr old electrolytics, which will now be seriously degraded. The 'coupling' (and feedback) caps are also directly in series with the 'signal path' - there are 4 per channel.  These existing caps are: - Mallory 10uF electrolytic output caps (2) - these are directly connected to the output socket. - Sprague 1uF electroytic output caps (2) - (as above & also in the feedback loop). - Philips 0.01uF 400v MKT (metallised polyester film) caps (2) - coupling between tube sections. 
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Post by colinf on Jun 1, 2017 20:08:43 GMT 12
So just upgrading and updating the capacitors made a significant improvement. Which do you think was more important, power caps or coupling ones? Or you changed them all at the same time?
AMR-iFi R&D
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Post by Owen Y on Jun 1, 2017 20:34:57 GMT 12
Hi colinf - yes, I did what I advise others not to do...changed more than one thing at a time! Most of the caps are aged replacemnts, not audiophile upgrades For this exercise, the new parts were the best technically that I could find without going into audiophile pricing & also parts that would physically fit. I don't use it in my main music system, but I should - I need to rig up a proper Yagi aerial first.
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Post by Owen Y on Jun 9, 2017 9:35:32 GMT 12
The above caps now replaced: As said, parts choice for this project is good audio quality but reasonable cost: - Panasonic metallised polyester film - JFX metallised axial polypropylene - 'Orange drop' style metal foil & polypropylene film  Note how McIntosh originally installed the output socket caps (the white ones) at right angles to each other - to minimise 'crosstalk'. Not something that you see often in hifi amps, but McIntosh observed this good practice, being radio engineers.
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Post by Owen Y on Jun 9, 2017 17:34:45 GMT 12
You'll notice that the new film caps have been marked (by me) with a black stripe - this denotes the 'outer foil' lead end. It is good practice for best shielding & to minimise induced noise pick-up, for the outer foil end of wound film caps to be connected to the lowest impedance side of the circuit - either the Ground end or the 'source' side in coupling positions. (Some film caps, due to their symmetrical internal construction do not exhibit this characteristic.) There is also a significant difference in sound between the different orientations - a difference in dynamics & liveliness. Some caps are supplied with Outer Foil end marked. To identify outer foil yourself, you need an audio oscillator or by using an oscilloscope. A significant difference in noise pick-up amplitude, between 2 orientations, when holding the cap body...   Further reading - www.aikenamps.com/index.php/where-to-connect-the-outside-foil-on-capacitors
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Post by colinf on Jun 22, 2017 19:15:04 GMT 12
When I was in Aussie last week what should I need to repair but a lovely condition MR71! The output stage needed work and the tuning knob cord had come off, necessitating the removal of the front panel. The caps had already been replaced and after it was done it sounded lovely. Very warm and classy, and a pleasure to use. But afterwards my mate got me to check over a Leak Troughline with SS stereo decoder. There was nothing romantic about the sound at all and there was so much more depth than the McIntosh! But I would take the Mac home in a heartbeat. It looks fabulous.
AMR-iFi R&D
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Post by Owen Y on Jun 23, 2017 15:45:48 GMT 12
Interesting coincidence, colinf !..... Th earlier MR67 is regarded v highly too, perhaps moreso.
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Post by Owen Y on Jul 2, 2017 20:09:49 GMT 12
There are a few caps in the preceding Multiplex section, on a separate small PCB & on the 6U8 tube socket. They are ceramic disc types which can always be sonically improved upon by replacing with film caps.  The replacements (yellow) are Mallory metalised polyester film (only GBP1.00 ea). (Again, they have been Outer Foil marked.) 
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Post by colinf on Jul 5, 2017 20:31:02 GMT 12
How did the mods on the multiplex board sound?
AMR-iFi R&D
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Post by Owen Y on Jul 6, 2017 7:09:23 GMT 12
Hi colinf, again sorry, this was done together with the output stage cap upgrades . This MR71 is part of an 'office' system & a bit of a hassle to pull in/out!
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