McIntosh MC75 overhaul

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I have had two of these since around 1978, and they have traveled the world with me. In the late 1970’s I did a recap of most of the capacitors and replaced all the resistors and the main diodes. Then in the mid nineties I replaced the main power supply capacitors again (the 250uf units), but left in place the lower values (100uf and 30uf) from the earlier refit.

So I had just finished a pair of NAD 2200’s, so I thought now is the time to take these out of my listening rig and give these mac’s some love and attention.

So I have replaced all the capacitors, electrolytic and film, which you will see in the photo’s- The main PSU caps are Cornell Dublier 250uf 350v, the 100uf and 30uf are double 100uf and double 30uf 500v cans.

The 12uf are all Sprague 250v and the films audiocap.

The original diodes are now UF types. Most all the resistors have been changed as well. I also reinstalled the input caps which I had removed previously, though I may remove them again.

Aside from the usual drilling out of rivets, the whole exercise went very well. They received new tubes “golden dragons” and Sovtek 6550’s I had bought in the 90’s as spares.

They do sound great once again , though I am tempted to try some different  tubes, especially that initial 12ax7, and perhaps some other KT88/6550 types.

 

 

 

work in progress

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So I have been very busy with some interesting upgrades and refurbishments. I will post more details latre but as a summary (and aide memoir):

Revox B740

Adcom 545 II

Onkyo A-7

APT Preamp

my own MacIntosh MC75 tube amps

and three NAD 2200’s

descriptions and photo’s to follow

Onkyo A7 Upgrade

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I recently completed an upgrade of an Onkyo A7, which is the most comprehensive one for these amplifiers I have done.

Starting with the power supply section, my customer asked that the 25,000uf 55v main capacitors be replaced, so after an analysis of the available options two Nippon Chemicon 27,000uf 100v units were installed, the original units were also Nippon Chemicon. These units are about 20mm taller, but the same diameter, and sit inside the case with no problems. As these had been done I decided to replace the main rectifier diodes, so Fairchild 1N5402were installed. The power supply capacitors were replaced with Nichicon PW series, except for C706 which is Bipolar, and here a Nichicon BP type was used.

 

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Those new power supply capacitors looking serious

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Completed unit

The main output transistors were fine, however I replaced all the driver and low level transistors on the main amp board. The differential amp was comprised of 2SC1775/2SA872 and these are now KSC1845/KSA992. The bias stabilization transistors were 2SC945 and these are now KSC2383. The driver amp pair was 2SA912/2SC1885 and these are now KSA1220AYS/KSC2690AYS. The complimentary pair were 2SD358/2SB528 and these are now KSC2690AYS/KSA1220AYS respectively.

One thing that appears to regularly blow on these units are the 56 ohm resistors R653/R655 and R654/R656- so I always check them and generally replace them as well. All the electrolytic capacitors have been replaced, CV605/C606 4.7uf 50v with WIMA film 4.7uf 50v and the others (C619/C620 and C633~C636) with Nichicon PW units. The emitter resistors R659~R662 were replaced with Dale 1% 5W resistors.

The tone control board caps (C409/C410) were replaced with Nichicon KL series low leakage units.

On the pre-amp and equalizer board theimg_0649differential amp (Q301~304) 2SC726 were replaced with KSA992 and the tone amp (Q305/Q306) were replaced with KSA1845. All the electrolytic capacitors were replaced, C201/C202/C303/C304 with WIMA 2.2uf 50v film as were C225/C226/C317/C318 with 1uf 50v WIMA film types. C211/C212 were replaced with Nichicon KL types as were C319/C320. C315/C316 were replaced with Nichicon PW types.

All the switches and controls were deoxed , and the unit aligned as per the service manual.

The result is a very fine sounding amplifier!

May-2017- I have just completed another A-7 upgrade, and measure the distortion at around 0.02% THD+N- which considering the spec is 0.1% is quite an improvement. This unit has all new transistors, including the outputs, except for the phono stage, as well as the mods described above.

Onkyo A-10 upgrades

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the Onkyo A-10 is the big brother of the A-7 and A-5. These were certainly over engineered for their time and have both a heavy aluminum front plate (about 1/8″ thick) and very good quality components and construction.

I have only come across one A-10, and it impressed me a great deal, and effortless sound quality, great detail and solid bass..all with great transparency. I mentioned in an Audio Karma post that I proffered the upgraded version to a Krell KAV300i I have and what were the upgrades etc I undertook.

All the electrolytic caps except the large power supply units (2 x 13kuf 63V) were replaced. Mostly by Nichicon PW low impedance types, however the original circuit had a number of low leakage caps fitted, and these were replaced by Nichicon KL types- except the 2.2uf and lower values, which were replaced by WIMA film types. C275, C276 were replaced with ELNA Simlic II. C253,254 were replaced with Nichicon KZ, as were C311,312. C337,339 were replaced with Nichicon FG.

The 2SA726 were replaced with KSA992, as were the 2SC720. The 2SC1681 were replaced with KSA1845FBU

overall the amp sounded very good, and shows I think a very good underlying circuit and quality components

I recap and upgrade Onkyo A-7s whenever I can find them

NAD 2200 mods and upgrades

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I have been recapping and modding NAD 2200s for a while now- and wanted to give some details back to the community:

I have selected a number of mods from AK and DIY Audio and others listings, as well as my own tests, and these seem to make the majority of difference:

C215-218- These support the 18v rails that provide the power to the initial VAS stage of the power amplifier broads, and as such larger values provide a greater local reservoir. Replacing the original 2.2uf 50v with 470uf 50v makes quite a difference, “more bass” and better dynamics, I have settled on Nichicon HE (ultra low impedance)
C111-114- I have replaced these with COG Ceramics and have added 47uf 50v in parallel with each of these (across the 18v supply lines to the IC) and this again seemed to provide better dynamics, these are Nichicon PW
I tried Pilots (posted in AK) swapping of C109, C110 to 2.2uf 50v Wima- but ended up going back to Nichicon FG 10uf 50v- which gave more solid square wave response at frequencies below 100Hz
C107,C108- these were 820pf ceramics (circuit states 1000pf film)- so replaced these with WIMA 1000pf 50v film
C203, C204- I tried a few versions (2.2uf Film, 10uf Film- both WIMA, Nichicon KL, Nichicon FG) and finally settled on ELNA SIMLIC II 10uf 50v- for me this kept the detail and was smoothest- also kept the LF square wave tidy.

I replaced the 2043DD IC’s with LM49860- I had tried the OPA2134 (which is an IC I like), but again for me the LM49860 is better suited to the amp-perhaps as it is BJT as is the 2043DD

All the other electrolytic capacitors have been replaced as follows:

Power supply- Nichicon PW same values
Power amp- all PW except c223, C224, C311, C312, C313,C314- all FG and C203,C204- ELNA Simlic II as mentioned
input amp- C109,C110 FG, C115,C116 Nichicon KA
plus the 47uf mentioned above across the 18v supply lines
10000uf 80v with Nichicon or equivalent
the 10000uf 120v are unobtainable, and the 160V versions are both expensive and dont fit- so I have some 10000uf 125v sourced from China- which I plan to try, though part of me is worried they will explode- so as long as the original caps are within spec(which they are at present) I tend to leave them alone.

for me this is the best NAD mod yet- more bass, better dynamics and measures well, though there is some low frequency phase shift and roll off from about 50hz downwards-the LAB inputs less so as expected.

one thought is to bypass the 10k ufs with a film cap (1uf?)- but again I have not tried this as yet.

Pioneer SX 850

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I have just started working on a defunct Pioneer SX 850. When it arrived there was no audio and no relay “click”. The insides, besides being dusty looked OK.

So I started with the power supply and removed and replaced all the electrolytic capacitors with a mix of Nichicon low impedance and audio types (all 105c). There are two transistors which should also always be replaced as they are often prone to failure. Having done this and with unit connected to a variable power supply with a fats trip and current meter, I turned it on, and still now relay click. So I tested all the voltages across the Power supply, ans everything is now as it should be.

So the next board to work on is the protection circuit, again there are two transistors, that should be upgraded, and all the electrolytics replaced. There are two small electrolytics, Sanyo, that I replace with WIMA film caps. Again turned it on, and hey presto the pleasant sound of the relay kicking in, so I then ran some tones through the aux input and all is good. Next is the phono equalizer, where all the transistors are replaced with low noise types and the small value electrolytics by WIMA film capacitors and the other electrolytics with Nichicon Audio types.

 

so fa so good- now on to the main amp boards- photos to follow

 

NAD 2100 upgrades

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so I have two NAD 2100s in for upgrade, one had a cracked PCB and is now up and working, all with new electrolytic capacitors throughout- The Panasonic 10k uf 80v from Digikey drop right in perfectly.

However the second NAD is being somewhat recalcitrant and I have pulled out nearly all the driver and and output transistors to try and find the problem. The issue is with the High Voltage rails,

and now I have it coming out of protection, which led me to look into transistor equivalents and replacements. Although the main outputs (Sanken) are also avialable from Digikey, some others, such as the 2SB922

appear to be unobtainable, and have very few appropriate substitutes.