Restoration and Upgrade of a Late Production C26

[Leestereo]

AK member @BilboBaggins brought in a really nice McIntosh C26 for a complete restoration and upgrade; its a late production unit from 1977 (based on the capacitor date codes).

Here are couple of pictures of the unit (with the covers off) as received:


The glass epoxy driver boards are numbered 045-085 and 045-086. Note the generous use of polyester film box capacitors ("cutting-edge" for early 70s solid state). The tone control circuit is composed of discrete components (instead of using the poor sounding 130-027 module). Also interesting is that this late production C26 uses a RIAA feedback circuit that resembles the one in the C27.


Note the use of garden variety ceramic capacitors (top left) on the treble tone control.


The power supply can capacitors are in excellent shape and do not need to be replaced. However, the original carbon composition 1.5kohm resistor had drifted to 850ohms (which resulted in the 2nd can capacitor being subjected to ~120V instead of the prescribed 98V).

 

[BilboBaggins]

Oh good! I've been looking forward to seeing this thread! I absolutely love what you did for my C26! I spent three weeks listening to the Tangent RS-08s after I pulled out the C24 and put the fully restored C26 back in. The C26 now lets me play music louder with a much cleaner presentation than I ever could before. The bass is more punchy and more present, but not overly so. Perfect balance in my opinion with the controls flat. Back to driving the Kef Concerto, only now, I'm no longer looking for speakers to replace them.

 

[Leestereo]

Happy to hear that you continue to enjoy your restored C26; its really a wonderful unit.

 

[Leestereo]

Here is a picture of the restored/upgraded Power Supply components on the underside of the chassis: the diodes were upgraded to UF4005 (soft recovery types); the final 100µF/16V axial filter capacitor in the
regulated supply was replaced with a 220µF/35V capacitor; the original carbon composition power resistors, 1.5kohm/2W and 8.2kohm/0.5W, were replaced with 1.5kohm/3W and 8.25kohm/1W metal film types, respectively.

 

[Hipocrates]

 

[kevzep]

Looks like a Sansui AU999 inside!!

 

[Leestereo]

Here are some pictures of the restored/upgraded C26 Driver and Power Supply boards.


On the power supply board (on the right), 2 carbon composition resistors, 27kohm/0.5W and 4.7kohm/0.25W, were replaced with 27.4kohm and 4.75kohm metal film types (respectively); the original 75V zener was replaced with a 2% precision, low noise variant and the 100µF/16V filtering capacitor was replaced with a 220µF/35V low ESR type.


A combined total of 77 components were replaced on the Driver boards. All of the original polyester film capacitors have been upgraded to polyphenylene sulfide (PPS) or polypropylene (PP) types. All of the original signal path ceramic capacitors that weren't already C0G/NPO types were upgraded to C0G types (small blue "blob" capacitors). The original 100µF coupling capacitors in the phono stage were upgraded to Nichicon MUSE ES bipolar types and the 100µF emitter bypass capacitor was replaced with a Nichicon MUSE KZ type. The power supply resistors rated at 0.5W or higher were replaced with metal film types. All of the resistors that McIntosh had specified with a 5% tolerance were replaced with 1% metal film types. Also, in order to lower the background (Johnson) noise level, any signal path/feedback loop resistors which were 47kohm or larger were replaced with 1% metal film types.

 

[MACKIE1975]

Nice work. This is what c_dk has always advocated for a full restoration.

 

[c_dk]

Yes and no......

I do believe that a C26 carefully updated can make for a great less than $1000 entry level McIntosh preamp, maybe closer to $850 in the real world.

Do we really, I mean really KNOW, that the tone modules sound bad? These modules were used in dozens, maybe dozens of dozens, treasured tube and solid state preamp sections throughout the 60s and into the 70s. the one I had to replace in the 80s when Mac parts no longer had them was replaced with a made up discrete circuit. I wish I would have cracked it open to see what was inside but did not.

All the pico farrard caps inside the cement coat could have been silver micas for all I know. I do know that Mac parts went through a number of suppliers over the years and I believe stopped using them because the supplier quit making them. For all we know the lovely " tone " craved by vintage collectors is because of these modules.

I have the discrete parts collected but not the time to try them out in my project C26.

The C26 had some issues that needed sorting from the beginning that Roger Russell highlight in his post at his site.

Mac engineering had a few updates they sent out for RF interference issues.

My use in the late 70s before being replaced by a C32 pointed to a noisier phono stage and preamp section and crosstalk issues between sources. You can hear a tuner or CD player in the phono background.

I have found that replacing the electrolytics with KZ muse caps to be SOP.

Replacing the multicaps with multiple 105C Nichicon PW caps with added bypass has shown a measurable lowering of the noise floor in both the phono and line stages by 10+ dB.

It has been beneficial to replace the phono eq resistors if you test and find the phono RIAA eq curve to be off. I am still using one of DOBs reverse RIAA circuits to test all the preamps that choose to work on just like we used to do at amp clinics.

I am still pondering the crosstalk issues and someday I hope to try the module replacement.

I applaud all the efforts made to update these classic units to perform closer to the standards of today........but 80 plus parts would put this type of rebuild beyond the budget of most everyone not a DIYer.

I just got through reading a statement paper by regenesis audio on supposed updates to the PAS phono stage by a company modding to a marketing ideal.

I have opened many a unit that seemed to be built to a checklist of "approved" parts.

I hope to not fall into that quagmire.

 

[Leestereo]

Here are pictures of the upgraded signal path capacitors installed on the "Mode", "Loudness" and "Treble" controls. The original polyester capacitors on the "Mode" and "Loudness" controls were replaced with polypropylene film types and the garden-variety ceramic capacitors on the "Treble" control were replaced with C0G types.

 

[Hipocrates]

Is always a pleasure to see your work, can't wait to read your conclusions after the updates.

 

[c_dk]

Those preamp boards were highlighted in a MX113 service bulletin MX113-2 from 1983......

I happened to pull out my c22 schematic and a quick count showed they used I believe 9 of the modules in it. Phono, filter,tone circuits all had them in active service.....There was also a note about changing suppliers from Erie to Sprague in 1965 I believe.

I wonder what Mac did for the reissues since Sprague stopped production in 1982 or so......

 

[Leestereo]

....I do believe that a C26 carefully updated can make for a great less than $1000 entry level McIntosh preamp, maybe closer to $850 in the real world. Do we really, I mean really KNOW, that the tone modules sound bad? These modules were used in dozens, maybe dozens of dozens, treasured tube and solid state preamp sections throughout the 60s and into the 70s. the one I had to replace in the 80s when Mac parts no longer had them was replaced with a made up discrete circuit...I applaud all the efforts made to update these classic units to perform closer to the standards of today........but 80 plus parts would put this type of rebuild beyond the budget of most everyone not a DIYer...

Yes, as this example demonstrates (at least IMO), a restored and upgraded C26 is a very capable and excellent sounding unit. The discrete components in the tone circuit of this late production unit made upgrading this circuit much simpler than it would have otherwise been.

Although this particular restoration upgraded all of the original polyester film capacitors with polyphenylene sulfide (PPS) or polypropylene (PP) types, this process can arguably be considered "optional" (however, it is very worthwhile considering the performance gains). This is what should be replaced, IMO, in a C26 restoration/upgrade: all small electrolytic capacitors (the large can capacitors can be kept if they test OK for capacitance, ESR, leakage); all signal path ceramics that are not NPO/C0G types; carbon composition power resistors; feedback (including RIAA) resistors. Also, the McIntosh service bulletin(s) regarding RF interference should be implemented, if applicable.

 

[c_dk]

What actual performance gains are to be appreciated by swapping out all the film caps?

You are talking a major expense of time and while I do know the net is a buzz with talk of capacitor "tone" way to much is based on conjecture not actual documented performance.

 

[Leestereo]

The dielectric properties (e.g., %DA) of PPS and PP film capacitors are well documented via their respective data sheets; also relevant is the work of Cyril Bateman which examined the impact of capacitors on sound quality: https://linearaudio.nl/cyril-batemans-capacitor-sound-articles.

 

[c_dk]

About what I figured.....

 

[BilboBaggins]

What actual performance gains are to be appreciated by swapping out all the film caps?

You are talking a major expense of time and while I do know the net is a buzz with talk of capacitor "tone" way to much is based on conjecture not actual documented performance.

Interesting.

When I first contacted Leestereo about doing this work, it was with eyes wide open. I love my C26, and I believed that optimized to current technology where it made sense could resolve all the issues that people criticize about this preamp. Leestereo was willing to do so, and I trust his work, his skills, and his knowledge. And best of all, he is local to me so I don't have to play Russian roulette with the shipping system! I had a budget, and leestereo stayed within 10% of it.

Recently, I had the pleasure of auditioning an updated Dolan PM1 preamp at request of Richard Dolan. I'm fortunate enough to work with him, and he was eager to hear what I thought, and in particular, to hear comparisons of the updated PM1 and the updated C26. I had the PM1 in my regular McIntosh system for about 4 weeks. My wife and I enjoyed listening to a good bit of music during this time. Auditory memory is funny, because you can listen to something, make a small change and then question whether you like it or not. In our case, we changed the preamps back and forth a few times. Once it was because one channel seemed to be fading in and out. It was, but it was a dirty pot on the MC2205. Problem resolved and although the Dolan PM1 performed well, I prefer the restored C26. What can I say, it is my baby!

At the end of the trial period, I invited Richard over and we had a listening session first on the McIntosh system, then with the Dolan PM1 in place, and we both agreed there was very little differences between them. Mind you, this is with ears that are at least 55 years old, but it says a lot!

Finally, I would like to comment on the crosstalk and phono noise. When I asked Leestereo to upgrade it, I could hear the crosstalk and noise in the phono section. These were two areas I wanted addressed. Well, I can comfortably say that both of these issues were addressed very satisfactorily. I hear no more crosstalk, and the noise on the phono inputs are not within my hearing ability. A complete success as far as I am concerned and Leestereo's workmanship is just top notch! Did you note how beautifully he worked the restored boards? Well worth the money in my opinion!

 

[Leestereo]

About what I figured.....

Not sure what your response implies; the Bateman articles are technically advanced and clearly demonstrate the advantages of the better film capacitors with respect to measurable distortion levels. I am not suggesting that a single capacitor substitution is the be-all and end-all, but having more than a dozen polyester capacitors in series (per channel) upgraded was readily audible to me; of course YMMV.

Edit:
Here is a link to a Cyril Bateman article that documents the poorer measured distortion for the types of capacitors that were stock in the C26 (i.e., conventional ceramic capacitors and polyester types) versus their replacements (i.e., C0G, Polyphenylene Sulphide and Polypropylene types): https://linearaudio.nl/sites/linearaudio.net/files/Bateman EW 11 2002 mar 2003 caps 100 nF to 1uF.pdf

 

[c_dk]

Personally I am looking to lower the noise floor by 3,4 maybe even 6 dB through out the frequency range......and lock in the accuracy of the phono equalization.

And keep the expense of time reasonable.

 

[Leestereo]

Recently, I had the pleasure of auditioning an updated Dolan PM1 preamp at request of Richard Dolan...and although the Dolan PM1 performed well, I prefer the restored C26....At the end of the trial period, I invited Richard over and we had a listening session first on the McIntosh system, then with the Dolan PM1 in place, and we both agreed there was very little differences between them...

Very interesting story about the Dolan PM1 (a highly thought of "high-end" preamplifier, originally from the late 80s); even more gratifying (for you and me) is that the C26 held its own!

I had the opportunity to work on one a couple of years ago (http://audiokarma.org/forums/index.php?threads/dolan-pm1-preamplifier-restoration-upgrade.629192/). Here are some of the pictures that were originally in the thread: