Winter Project - 3 MC-40's to restore / Update?

Discussion in 'McIntosh Audio' started by chinacave, Dec 6, 2011.

  1. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    A good friend of mine has entrusted me to refurbish some of his audio gear over the winter. He was a distributor / importer of pro audio / studio gear from the 60's -'00's & has a wonderful set-up. Has not been really used for many years, hence my labor input.

    He has already been most generous in letting me help him clear out some of his spare units and speakers.

    I need all of your help and knowledge here!

    First up are three Mac MC-40's

    They all work, and only one shows any signs of even a hint of flaky behavior, even with the tubes they were put away with. (Unit #01397 has -156v dc on Pin 5 of V6 - all the other voltages are withing 10% or so of specified...)

    I will be doing a complete re-build, and would like to solicit opinions. experience, rants etc on how best to approach this, and what your experiences has been with these or similar amps.

    I understand from a post here that the large black Sprauge, I think,.22uf coupling caps should stay, but that all the stripped ones should be replaced.

    A few random questions for the Gurus, Sensei and wizards here, in no particular order.

    -Should I wire in a grounded 3-wire power cord or leave the 2-wire configuration (I plan to use a new, non-crusty one)

    -Should I replace the power supply caps with the same value, or increase in size, and if so, all positions or only after C16A?

    - What rectifiers does everyone suggest using?

    - On two of the three amps - there are yellow apparent capacitors that seem wired in parallel with C8 & C9, yet they are not on the schematic, and have no markings - will detach one and measure - weird...

    -I'd like to replace the press-fit RC input Jack, any advice on how to remove the sucker?

    - Inrush limiter on the input line?

    -Also the sockets - do you suggest keeping existing, cleaning and re-tensioning, or drill the rivets out and replace with new?

    -The owner would like me to get a base-line set of tubes fro one amp so as to have a consistent set with which to compare the re-worked units - short of grabbing something quick from Antique Electronics, is there a preferred set / brand for each position?

    - I have the owner's and service manual, but cannot find a Sams for this - it seems that there is none (checked on Sams website..) what would really be helpful if anyone has annotated it already is a labeled photo of both sides of the tagboard - and as one of the units seems to have been cooked with heat, should I get a replacement tagboard of the same size (assuming I can source one?

    -ANnnnnnd the obligatory question about what are the 'right capacitors and resistors to use. I assume you all have done some / much experimenting in this area - I / we do not want to break the bank with silver-foil in extra-virgin olive oil caps rolled by Druids under a full moon, but would like to use better than 716P orange drops......And should I *really* keep the existing caps at C8 & C9...... Metal film resistors or carbon comp?


    Anything else I am missing?

    Photos & progress reports to follow......

    Some of the other gear after the Macs are two Marantz 7C and an 8B... Threads on those to follow when I get to them....
    Thanks!

    CC

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    Last edited: Dec 13, 2011
  2. dshoaf

    dshoaf That high voltage buzz Subscriber

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    Sounds like you have a nice backlog of gear to work through.

    I've copied your post and will insert my responses below......I don't consider myself a wizard, although I've rebuilt a hand full of Mac tube amps.

    Here goes:

    A few random questions for the Gurus, Sensei and wizards here, in no particular order.

    -Should I wire in a grounded 3-wire power cord or leave the 2-wire configuration (I plan to use a new, non-crusty one)

    >> Dshoaf: No. Don't recommend it. Keep the grounding scheme both inside and out as found. Inspect the AC cord, however, for breaks and any dangerous situations. Replace with a similar cord. You may need to file down one of the blades as they are all polarized these days. I've not had to replace one of these.

    -Should I replace the power supply caps with the same value, or increase in size, and if so, all positions or only after C16A?

    >> Dshoaf: Yes, these caps are approaching 50 years of age. Consider it a safety issue as you do not want them to explode. See my posts over in the Mac forum for discussions on this.

    - What rectifiers does everyone suggest using?

    >> Dshoaf: As many opinions on tubes to use as there will be brands of capacitors as there are posters. Much like wine, there's a lot of views on the subject. I _do_ suggest you maintain a set of known-good tubes, however, for troubleshooting and verification. Take extra pain to keep these separated to ensure they're always _known-good_.

    - On two of the three amps - there are yellow apparent capacitors that seem wired in parallel with C8 & C9, yet they are not on the schematic, and have no markings - will detach one and measure - weird...

    >>Dshoaf: Post pictures as this sounds like some sort of repair or mod.

    -I'd like to replace the press-fit RC input Jack, any advice on how to remove the sucker?

    >>Dshoaf: I've swapped them when one of them broke after I tripped over an amp sitting on the floor with an RCA cable plugged in. The ceramic insulator will crack under severe stress. Otherwise, clean them and leave them alone. Consider this a minor update rather than something to do during restoration.

    - Inrush limiter on the input line?

    >>Dshoaf: These will likely be quite brittle due to their age and the heat they generate while powered up. They will typically break as you brush past it doing something else. These are thermistors and they're available from most places like Digikey and Mouser. I use CL-90s, typically. Google 'thermistor GE CL-90" for more details.

    -Also the sockets - do you suggest keeping existing, cleaning and re-tensioning, or drill the rivets out and replace with new?

    >>Dshoaf: I clean them well, retension and then run them until other evidence indicates they need to be replaced. One AK guy, Sloober at the Mac forum, successfully replaced the pins from those he found in an old HP scope. Keeps you from having to deal with the rivets. Again, you should consider this like the RCA connectors; replace when there's clear evidence of failure.

    -The owner would like me to get a base-line set of tubes fro one amp so as to have a consistent set with which to compare the re-worked units - short of grabbing something quick from Antique Electronics, is there a preferred set brand for each position?

    >>Dshoaf: As per the discussion about capacitor brands, plenty of opinions. Suggest you focus on the _known-good_ stash first.

    - I have the owner's and service manual, but cannot find a Sams for this - it seems that there is none (checked on Sams website..) what would really be helpful if anyone has annotated it already is a labeled photo of both sides of the tagboard - and as one of the units seems to have been cooked with heat, should I get a replacement tagboard of the same size (assuming I can source one?

    >>Dshoaf: Take lots of pictures and notes first - all angles, wide and close-up. Don't forget to note wire colors, too. Trace all out on a copy of the schematic. Confirm with ohmmeter testing. Reason is that there were minor changes across many of the Mac products. No schematic should be considered the 'source of truth' for this reason.

    -ANnnnnnd the obligatory question about what are the 'right capacitors and resistors to use. I assume you all have done some / much experimenting in this area - I / we do not want to break the bank with silver-foil in extra-virgin olive oil caps rolled by Druids under a full moon, but would like to use better than 716P orange drops......And should I *really* keep the existing caps at C8 & C9...... Metal film resistors or carbon comp?

    >>Dshoaf: Suggestions - 1) keep to the original values. See below for exception. 2) Do not derate the operating voltages.

    In order of priority, replace caps:

    - Power supply: All electrolytics for safety.
    - Bias voltage supply: Bias voltage cap where you can bump it by ~50% and be ok.
    - Interstage coupling caps: Post a pic as some of them _may_ not have to be replaced - the 'dog bone' caps, specifically.

    I've used Solens, Illinois Cap and Orange Drops with excellent results. Refer to the above analogy re:wine tasting....

    Things you didn't ask about:

    - Resistors: Expect some drift in the old carbon comp resistors. They can crack as you manipulate the board swapping the caps. Heat them up a bit and see if they change value as I've seen that before.

    - Operating voltages are critical: Take a full set of voltage measurements prior to beginning rework so you have something to refer to. Make sure that all operating voltages are within spec once you've completed the rebuild. Next, run the amps for a few weeks after you're done and then recheck the operating voltages. There may well be resistors that have begun to drift due to the thermals of power up/down. If all of the operating voltages are in spec, the amp's going to sound great and be quite stable.

    - Selenium rectifier replacement: Do this and then refer to the Mac forum Hints and Kinks for adjusting the resistor network values. It helps to have a resistance decade box to make this easy. Don't overlook it as it _will_ affect how the amp sounds. See the Hints and Kinks stickey for details on how to do this. You'll need a variac.

    Hope that helps.

    Cheers,

    David
     
    Last edited: Dec 6, 2011
  3. wgrr

    wgrr Super Member

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    I have some pictures around here somewhere of the work Terry did on my MC40's, here it is.

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    As you can see some of the old bumble bees are still in the unit. Bear in mind that this is not a full restore. He just bought the units up to spec when I bought them from Tom.

    Tom (mcintoshaudio.com) sells original McIntosh neoprene power cords. I would avoid a grounded cord because of potential loop feedback better known as hum.
     
  4. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    Mc-40

    David -

    Wow. Thanks for taking the time for such a detailed response!! - This is what makes AK such a Kick-butt outfit.My comments / questions surrounded by <<<< Text >>>>>



    << very Helpful!>>>>



    Annotated Schematics as-is as-found

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    Last edited: Dec 13, 2011
  5. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    Wgrr -

    Thanks for the photos!
     
  6. dshoaf

    dshoaf That high voltage buzz Subscriber

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    Checked in on your operating voltages. Observations:

    - Power supply voltages are all low. Indicates a 'tired' power supply. All electrolytics should go. Sonically, the amp should produce sound but the bass will sound quite flabby when pushed harder. Higher levels of noise may well be evident.

    - V4 voltages are out of spec. I'd suspect leaky coupling caps if the sockets are clean and tight. At lower volumes with efficient speakers you may not hear any distortion but it will be measurable under full load.

    My recommendations posted earlier stand, though. I'd recommend you rebuild the power supply first, take another set of voltage measurements as you have done and see the difference. Listen to the amp, too. Next, rebuild the bias supply and do the same. Lastly, do the coupling capacitors with the same following steps. It is an enlightening learning experience.

    Sometimes half the fun is in the journey.......

    Cheers,

    David
     
  7. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    Hi David -
    Thanks for your amplified comments!

    >>>Chinacave - I could have asked this question more clearly - I plan on installing all new PS caps - I want to ask if I should increase their value to larger than per the schematic - IE the 1st one after the choke is 80 mfd, should i use a 100 mfd here, and so forth..<<<<


    I am about to order needed parts for the re-build. Is there an advantage or any potential harm in adding additional Power supply capacitance? This seems to have been added to chassis 2499 (there are three annotated schematics in the attachment above)

    And does anyone have any advice on the preferred type of resistors to use? carbon or metal film? Old school carbon comp?

    May get to investigate the yellow mystery caps tonight.

    Thanks again!
     
  8. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    The three amps in question

    Here are the three MC-40's I am re-building

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    Last edited: Dec 13, 2011
  9. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    Individual guts shots;

    no. 02499 with recently unsolder-ed .22uf yellow unmarked caps that were wired in parallel with V3 12BH7 to V4 12AX7 coupling caps #C8 & C9 (also .22) There is also an added 50uf @450v cap at C16B (blue sucker), and triple can cap C16 might have been replaced
    [​IMG]

    no. 01397 did NOT have the two paralleled extra caps, and, I just realized, does not have the separate choke the other two amps have - the schematic I have shows the PS choke as enclosed within the power transformer - PT = T2A (M225) and the choke is shown as T2B..this unit seems to me the only one of the three where this is actually the case.
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    no. 02500 -Again, with the bonus extra yellow caps and the separate choke.
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    Last edited: Dec 13, 2011
  10. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    C.S.I. - Solder terminals

    I am thinking the yellow .22 caps were not a factory thing, based on they way the were soldered;the leads were just placed on the terminal and tack=soldered on, everything else had the leads wrapped 1 1/2 times around the post to make a good mechanical bond before solder.

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    The cap leads just pulled away with their own weight when the joint was heated.
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    Typical wrap detail

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    Yellow cap lead
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    Last edited: Dec 13, 2011
  11. drtechno

    drtechno Active Member

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    I remember rebuilding one of those, but I used orange drops and solens and panasonic electrolytics, replaced all resistors with mills resistors and went with a 6A10 full wave bridge rectifier . added 220 uf cap on the output of the bridge rectifier and lower R10 to 8k

    if you plan to stick with the stock voltage doubler, increase the power supply caps by 2 and use 6A10 rectifiers and add a 500 ohm resistor to the output of the voltage doubler (the dotted line resistor after the rectifiers)

    went with 4 times the value on signal coupling and bypass caps.

    decreased c10and c11 to 5 pf to give the amp more "Air".

    when compared to the stock design, the stock built amp sounds cheap in comparison.

    I wouldn't change the power cord unless its cracked/deteriorated. It is non polarized for the reason if you have some ground loop issue you can turn around the plug.
     
    Last edited: Dec 13, 2011
  12. dshoaf

    dshoaf That high voltage buzz Subscriber

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    Your #2500 abd #2499 both appear to have had something more than just remedial work done to it. I see both the capacitors but also some paralleled resistors that do not appear original.

    First order of biz is to get it back to stock. I'd be heading for full power supply rebuild with standard value parts. Then rebuild the bias circuit. The bumblebee caps also gotta go. Lots of evidence that the resistors have been warm so be careful with flexing the boards as they'll be more brittle - especially around the bases of the terminals.

    Cheers,

    David
     
  13. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    Thanks

    Thanks for the input, guys!

    Really appreciate it.

    I have ordered all parts, decided to use Solen caps, will re-build as per the original schematic first, and then perhaps start adding suggested mods (increased PS capacitance, full-wave bridge etc.)

    More updates & pix soon.
     
  14. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    Some Progress

    Back at this project after holiday hiatus and dealing with the steam heat system on a 100 year old building @ work... in 13 degree weather.

    Slight diversion also while I got up to speed on basic amp tests beyond measuring voltages.

    Initial amp under the knife, w/ sine wave, eyes on scope, 10.5 ohm hill-billy rigged dummy load*, and DVM VOM (NOT yet a VTVM,)** [all of the ones I have and the ones I picked up from my friends' workshop are flaky, to say the least..] gave the following basic output @ just before observed clipping on the scope.***

    #2499
    -With tubes as found - 11.63W @60 hz
    13.84W @600hz.
    (Scope got funky after switching to 6k hz....new probes on the way, will clean controls)

    With new set of tubes (re-issue Tungsol 6L6's re-issue Mullard 12AX7, NOS 12AU7 & 12BH7 from AES)
    - 27.78W @60hz.
    29.01W @600 hz.
    26.46W @6K hz.
    14.47W @20K hz. (! - Voltmeter issue?)


    OK, so I see changing the tubes produces an apparent change. I will call that progress.

    I have NO idea if the instruments I used will produce meaningful results, but this is a first attempt. Any, uh, input or comments would be most welcome. I am trying to be more exacting and have some better and more meaningful metrics to judge my efforts than simply "replace everything replaceable" and measure voltages. Though that has been working pretty well for me.


    Now I am making a diagram of the tagboard, noting components, connections and where the wires lead to. Fun. But a good exercise. I am sure many of you are old hands at this. Guess I do not trust even Macro digital pictures - no 3-D info. Sure would be nice if there was a Sams Photofact of this amp...

    * Waiting for real 8 ohm and 16 ohm dummy loads... used 2 OLD variable resistors in parallel / unknown watts but <<Look>> like about 20W each, so as they are a little marginal, I only did this test briefly. They got Hot.

    **I mention this as I understand that less sophisticated DVM's are only really accurate on AC at 60Hz, so I started there. Do I really NEED to get one of the VTVM's working, or is there a modern Solid-state and under say $100 meter that will serve the same purpose as a VTVM?

    *** Test gear= Craigs' list Tek 432 dual trace scope, GenRad 1210-C tube Signal Generator (Seems spot on on frequency...) Extech Minitec-26 DVM for Freq. and AC volts


    -Thanks for reading

    Chinacave / Ian
     
  15. wgrr

    wgrr Super Member

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    Dealing with a 100 year old steam heat system @ work. Are you an HVAC/R mechanic. If you are, you are not the only one here. We may have to start a new forum to b!tch about customers that will not update equipment.:D

    The MC40's are looking good. They do look like some of them have some weird repairs/upgrades?
     
  16. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    Wgrr -

    I guess I am one of those customers!

    I bought this 1910 building in 2000 for my business, and long story short, it had a new boiler then but all old pipes. Well, they have not aged well. Just put a new / new boiler, and then the condensate pipe started popping holes. While everyone was on vacation, and it was 13 deg F outside.

    This is after the new flat roof, septic system and parking lot, and now shoring up some structural slumpf that I KNOW is ancient, but one of my staff is freaking out about......

    Thanks for the comments on the MC-40s'

    One is almost totally dis-assembled after painstakingly drawing it out so I can put it back together....

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    Last edited: Jan 24, 2012
  17. dshoaf

    dshoaf That high voltage buzz Subscriber

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    Greetings Chinacave, sorry to hear about your heating adventures and can understand why those would take your time of late.

    On the amps, I'm not sure I'd completely trust the results of the measurements just yet. What voltages are you getting on the plates of the tubes at idle and then again when your run a sine wave through it? I suspect you're going to find that those voltages will start to drop as the amp is put under more and more of a load. Did the sine wave clip symetrically? At least you know it will pass a sine wave, though.

    It is sort of like an old car with low compression on the cylinders - it will idle ok but doesn't produce the horsepower when you try to drive it.

    When you've got one rebuilt, though, I think you'll come back with different results!

    Cheers,

    David
     
  18. chinacave

    chinacave Its Like a Jungle Sometimes; It makes me Wonder Subscriber

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    Dear David -

    Thanks again for the comments -

    I am not putting too much stock in the readings either - as I mentioned , I was measuring the AC voltage with a mid-low cost DMM, so no idea how accurate any of it is. Also not too sure of the accuracy / output of my sig. generator and my scope-fu!


    I will check the delta of the plate voltage as you suggest - thanks!

    I am also re-capping a Heathkit AV-3 Audio db meter and will calibrate it if THAT effort is successful.


    I am still curious about a few things -

    1) IS there a modern equivalent of a VTVM for measuring wide-band audio range AC? I really don't want to get involved (right now) in restoring test gear, but at least the Heathkit is the simplest of the several I have.

    2) Do any of the PC (or Mac, AudioDon..) -based test programs or utilities have a VTVM mode, or are capable of measuring audio AC accurately?

    3) The terminal board of the one amp I have taken apart seems pretty close to fried. Has anyone used the DIY ones from AES ( Item # P-HTUR-A )for use in Macs? Any comments or experiences?

    Thanks to all for reading & commenting - Much Appreciated, as always!
     
  19. dshoaf

    dshoaf That high voltage buzz Subscriber

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    On your questions:

    1) Wide-band audio (for an RF engineer, this is an oxymoron) signals can be read with most any of the better digital volt meters (DVM) these days. For instance, my Fluke 117 specs a 5Hz-50KHz bandwidth for AC volt measurements. Most all but the cheapest of the DVMs I've seen sport input impedance specs that are like what a VTVM used to carry. So, just getting a decent DVM is about all you're going to need unless you have very specific testing requirements. BTW, the Fluke 117 also has a frequency counter, too. Really reduces the number of instruments you need on the bench.

    2) PC-based measuring devices usually have some sort of 'front end' box that does the A/D work needed so the software package can do its job. Since you can get a really functional DVM for less, that's the way I'd recommend mainly because you can take the DVM where you'll not likely get a laptop and the dongle/outboard box setup. One thing I do usually have, however, is a small USB or Firewire device so I can use the PC as an audio signal generator. Room EQ Wizard has a host of nice utilities for sweep frequency and other things that get interesting for a work bench.

    3) How fried is the board? If it was due to the larger resistors being mounted close to the board, just mount new ones standing away from it and you're set.

    Cheers,

    David
     
  20. wgrr

    wgrr Super Member

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