Workflow suggestions for refurbishing MC2125

Discussion in 'McIntosh Audio' started by ManhattanUp, Apr 18, 2015.

  1. ManhattanUp

    ManhattanUp Active Member

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    Greetings!

    I'm looking to put a little love, time and $ into my MC2125. It appears to be working well enough though I note two anomalies: one (minor I think) is that the Watts position on the left meter doesn't seem to be working at all and second that it appears to be putting out much less power (as measured in DB using a pink noise test signal) than my recently refurbed MC2105. It's running with the faders all the way open while the MC2105 is set about 2-3 o'clock for equivalent SPL. OK, possibly that's a red herring but I figured it's worth putting out there as another variable. Mostly I am guessing that at it's age, some of it's parts could use replacement and I enjoy these projects.

    I have already read through a number of relevant threads on work performed by other MC2125 owners and have begun putting together a parts list for the refurb, though I need some help with the caps. However, I see two possible workflows in front of me and wanted to get some opinions from folks wiser and smarter than me before doing anything. I want to do this right and efficiently. So…

    Workflow #1 would simply entail shotgunning parts. To me that means replacing caps, pre-driver, driver, and output transistors, as well as the emitter resistors. It's a simple and straightforward approach which possibly replaces components that might otherwise be working just fine. On the positive side, it minimizes the number of times I'm inside mucking about inside with tools and soldering iron. On the downside, I might be missing some other less obvious details that need addressing.

    Workflow #2 would entail spending time tracing and measuring signal through the left and right circuit pathways, looking for weak and/or failed components and only replacing these. I would likely need some assistance interpreting my results as well. The risk of this approach, if I had to hazard a guess, is that as weak/failed components are replaced, they may in turn put new stress on marginal down-circuit components that precipitates their failure and necessitates replacement. So I'm possibly having to perform multiple "surgeries" where in Workflow #1, I'm in and, hopefully, out once or twice at most. On the plus side, I'm catching more details and possibly replacing some less obviously important parts.

    That's it. Looking forward to your input on this…

    Michael
     
  2. jeffs79

    jeffs79 AK Subscriber Subscriber

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    Remember - the 2125 has power guard while the 2105 does not. Have you compared the two with the power guard "out" on the 2125?
    I read somewhere (possibly the McIntosh "For the Love of Music" book) that the reason that this 2120/2125/2200/2205 had a power guard switch to disable it was that the 2105 generally sounded "louder" when compared to the 2125 with power guard engaged. I have a 2100 and a 2120 powering very similar speakers (AR-3a's on the 2120 and AR 10pi's on the 2100). The 2100 always sounds "louder" although I have not used a db meter to measure. On the bench, the 2100 puts out more power than my 2120 when power guard is active. If I turn the power guard off, they measure pretty close in terms of power.

    Eitherway, I'd recap and go through the driver boards. The last time I checked mouser had the CEN-U07/57 pre drivers in stock so I would replace them if I was doing the caps. I tend to leave outputs alone unless I know there is an issue. Just my 2 cent. Best of luck!
     
    Last edited: Apr 18, 2015
  3. ManhattanUp

    ManhattanUp Active Member

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    Thanks for the reply. I will check out the effect of switching PG out. i would say I would be surprised to know that PG is reducing rated output by that much if at all. I'm certainly not pushing the amp in any obvious way nor do I ever see the red PG lamps come on (and I know they are good, I replaced them and the caps/diodes behind them recently).

    But, assuming for the moment that PG is limiting output w/o illuminating the lamps, maybe very subtly, might that also be symptomatic of problems elsewhere?

    Michael
     
  4. 1moreamp

    1moreamp Active Member

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    Power guard tends to trip into play at about 90% of the amps rated output power level by its electronic design. < quoting Mac sources here > And yes its main action is to reduce the amps input stage gain so the amp is backed off from being over driven into clipping. " UN Adjustable by its design Auto gain control is one possible best description " It like any other gain reduction circuitry can malfunction in several different ways. None of which are easy to determine off a test bench setup.

    I tend to check that if the amp is driven by a mono input to both channels of the same gain control levels, and then into a common matched output like a dummy test load both channels should deliver a equal RMS AC voltage across that known test load. < you can't go by front panel meters as they also have to be aligned properly inside of the amp and they too can be out of alignment as seen by other posts here on the AK >
    I then test drive each channel till the point power guard kicks in and levels get reduced to see if indeed they are operating in concert with each other and exactly alike.
    But this takes a test bench setup for best overall results not typical home speakers which they might also be at fault internally for one reason or another thus skewing the test results..

    I have seen others here report uneven gains or channel outputs as one failure mode and also silent channel output is another failure mode. The opto-coupler is 99% of the time the culprit and they are no longer made and only later replacement upgrades are typically available nowadays. < I would replace both since there will be differences between new and old parts that needs to be addressed by replacing both at the same time >
    I would disable the power guard and see if there is any difference with crossed fingers that taking the circuit offline will alleviate the symptoms of its possible failure mode.

    Other Mac amps sounding louder is because they are not being automatically gain reduced like power guard amps are, so there is no musical compression and power reduction taking place in the NON power guard amps and they are being driven into clipping while power guard amps are not and are in power limiting to say it bluntly as to how power guard works.. And while Power guard has it fan base, and I am not trying to stir their gyyr here its does exactly like I described above...


    And if you want to add power guard to any other brand of amp, just go looking for a DBX compression limiter from yesteryear or its modern equivalent today, and place it inline in front of any amp brand made by any company and turn it on and set it up to limit drive capabilities to the main amp section of any amp built and you have added power guard to say Sansui or Pioneer or Kenwood/Trio etc....Only difference being the outboard compressor limiter is ADJUSTABLE...a very needed ability IMHO Duh!

    Mac innovated this idea long ago to keep their products from sounding terrible like most other solid state amps being driven into clipping. And while its truly a Mac unique item its results can be duplicated by adding a simple compression drive limiter like those used in studio recording setups since way back in the 60's . DBX made them back then, and I am sure if you search you can find other brand names of them out there all recording studio grade mine you, and all doing about the same exact thing to your musical content and that being Dynamic range compression of those super expensive recordings by top studios I am sure everyone here owns plenty of. cause your music is only as good as its source and its playback gear together combined. So folks buy high end super dynamic range content CD and digital content and the play it back on amps using dynamic audio compression LOL LOL LOL....Makes no sense to me but to each his own i say... If it floats your boat then do it....

    I might be won over by power guard if it was adjustable , but its setup by Mac design to be exactly what it is and no more and I find its inflexibility to be it weak point to me and my thinking,,,but I am a the kind of person that i want everything especially for the price Mac gear sells for.... Oh and I am old and grumpy too lol lol lol...
     
    Last edited: Apr 20, 2015
  5. ManhattanUp

    ManhattanUp Active Member

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    @1moreamp: good tip on the mono signal/dummy load testing. Needless to say I would never trust the meters to do more than impress friends w/ retro coolness. I'll do the PG test tomorrow and report back.

    Also: want to confirm that the resistive opto-isolator (vactrol) used in PG circuit, is Mc part # 144-070, as shown on the schematic. Are there specs for it? I recall reading another thread elsewhere on AK regarding it and no one ever gave a replacement part number. There are several variants of VTL5C available.

    Michael
     
  6. 1moreamp

    1moreamp Active Member

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    VTL5C is the only replacement I have seen mentioned here on the AK and its available in the US and in England. I found two source for it, one on that auction site and the other using Octopart search engine.. And if one is bad then replace both because the newer parts will not operate like the old one did so in order to keep both channels in sync so to speak both will need to changed out.


    Hope some of this helps, The other master tech's here on the AK, are a great source of info. c-dk, Audio clinic, Dewickt, and a couple others, just about anything they have posted will be of help to you. everything they have posted has always helped me in one way or another....
     
  7. c_dk

    c_dk Super Member

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    Have you read the dozens of posts detailking updating the MC2205/MC2125 family of amps?????

    1) Replace ROE caps on driver boards
    2) reflow ALL Molex pins!!!!!!!
    3) Replace axial caps on power supply board!!

    These are must dos whenever any of these amps come in for anything.....read the posts there are more.
     
  8. ManhattanUp

    ManhattanUp Active Member

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    @c_dk: Thanks for the reply...though no need for all the '????' and '!!!!' I think. :)

    As per my OP, replacing the caps was already part of my plan. What I was trying to get a better read on was how to approach other critical components: blindly replace or assess quantitatively? My inclination was toward the latter workflow but maybe that would be a waste of time, i.e. in a 30+ year old piece of electronics, I should just replace pre-driver, driver, and output transistors along with emitter resistors and be done with it. There seems to be some difference of opinion on this matter in those threads you mentioned. And now there are some suggestions that PG may need some attention too. What's your take?
     
  9. ManhattanUp

    ManhattanUp Active Member

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    So below is a link to a Perkin Elmer data sheet for their line of Analog Optical Isolators. On page 46 is a table listing 10 variations of the VTL5C. Among other differences, they can have widely different response times. There was another thread that this came up in already but I don't believe the question regarding which variant should be used for the PG circuit ever gets answered.

    http://pdf.datasheetcatalog.com/datasheet/perkinelmer/VT500.pdf#page=46&zoom=auto,-14,778

    On the other hand: this might be a perfect case of 'Just buy from Mc and they'll send you the right part' full stop. ;->

    Michael
     
  10. 1moreamp

    1moreamp Active Member

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    Well contacting Mac might be helpful to see if they have a replacement and or a suggestion. Since only they have the characteristics of the original device to begin with. My gut tells me that there is no perfect replacement and there may be needed mods to get any replacement to operate correctly. I have been lucky and have not had to deal with failure replacement on these.

    Given my druthers about Power guard in the first place if it were my amp I would simply remove the circuitry's ability to interact with the amp and leave it so some future "must have it original type person" can deal with the defective fault and its rebuild issues. But this is just my "if it was mine" point of view, and that seems to rarely be the way Mac owners feel about their gear... So take my point of view with a grain of salt please, as its just that my point of view. I wouldn't run power guard if I owned any Mac amp that had it anyway, but that's just a me thing....
     
  11. c_dk

    c_dk Super Member

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    The LDRs are not a critical part in the MC2205/2125 family. The first issue with the Clarix LDRs was in the MC2255/2155/2250 family by yours truly, in 1983 or so. Mac used LDRs in muting circuits in many units, but not the MC2125. I have yet to see PG not working by a LDR not conducting to ground but there could always be a first time. Just need to observe on a scope and distortion analyzer, and I do not test continually in PG.

    Molex pin issues have ate many techs alive with intermittent issues in all units of this vintage.
     
  12. ManhattanUp

    ManhattanUp Active Member

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    @c_dk: Good stuff to know. Thank you.

    Oddly, I am right at this moment reading an old thread from 2010 ("McIntosh restoration/repair: Caps and grounding") where it seems you also found yourself asking the similar kinds of questions regarding the when and why of just replacing parts:
    Good to know I'm heading down the right path.
     
    Last edited: Apr 21, 2015
  13. ManhattanUp

    ManhattanUp Active Member

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    Poly wanna capacitor?

    A quick question: I'm putting together a shopping list of all the caps I'll need but am stumped by what Mc means by "poly" vs. "film" esp. since there are at least five film caps dielectrics that begin with the word "poly." I've read through a dozen or so threads which mention recapping and "poly" but can't get the matter clarified. I'm sure this is a "duh" question for more experienced hands but would appreciate the help.

    Thanks!
     
  14. 1moreamp

    1moreamp Active Member

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    new caps and new materials have clouded and complicated your buying choices since cap technology has evolved a lot and dozens of newer materials of construction now exist.
    Some so new they have no track record of lifespan and usage yet. I feel your pain in this matter. Each different construction material is promised to deliver all sorts of better performance, but remember they are selling something to you also so take some of what they say with grain of salt.

    I remember when Tantalum's were touted as being so great, and now they are the scourge of audio repair tech's because of how they fail and the damage they do. I can't tell you how many Orion car amps I repaired just because of Tantalum cap failures used as Bypass caps on the lower rails on op-amps they blew out the entire amp when they shorted internally. Just a terrible cap usage IMHO

    The great poly materials debate your worried about is not easy to explain or decide as opinions vary just like gas mileage lol lol lol.. I'm a Mylar film guy myself, Just love them and the SQ they impart IMHO. Most poly caps have match's in the Mylar film group so I fall back to my old trusted favorite when in doubt. Mylar film caps got history all the way back to the tube era. But even with all that history I check each one individually before installing them, and cross my fingers they had no flaws internally when they were built...

    I wish I could help un-cloud your query about which poly is which but I think that would take a huge amount of AK forum page space just to debate which materials are best, and then that would only be a best guess opinion from just about anyone. Most of the AK master tech's here actually listed a part number and or brand name which should guide you directly to what they found useful and reliable in their work. Their work like them have history and track record so their opinions are for the most part very good IMHO...

    maybe someone else would like to sit and list all the different poly cap materials and their pro's and con's which can be very use specific. What and where and how they get used effects them so much it would take a ten year study to be sure of anything and therefore doubt free.

    Wish I could be more specific but each circuitry placement deserves its on engineering considerations to take in to account for, and so a shot gun way of thinking might not be the best thinking on the subject.
     
  15. ManhattanUp

    ManhattanUp Active Member

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    @1moreamp I'd be happy for a recommendation or two from you. While there are lots of AK discussions about specific replacements for Mc small electrolytics, I can find almost nothing on the film and poly parts (and these terms are frequently used interchangeably). The Mc parts in question are as follows:

    064-136 (.068uF, identified as "Mylar" in the service manual)
    064-142 (.01uF/2% identified as "Poly" in the service manual)

    I'll be using Nichicon audio grades for all the 'lytics.

    Michael
     
  16. jayvis

    jayvis Well-Known Member

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    I have a workflow suggestion. Ship the amp to DeWick or AC, pour a shot of Dickel, go to the fridge and grab a beer, sit in the recliner and turn on the tube. :D
     
  17. ManhattanUp

    ManhattanUp Active Member

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    The 3rd Worflow Option!

    @jayvis: Well, I can buy the whiskey for both of us (a little better than Dickel, I promise) if you'll pay for Mr. DeWick's services. Deal? ;->
     
    Last edited: Apr 22, 2015
  18. 1moreamp

    1moreamp Active Member

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    Well , now this is just my personal opinion here again, but it would appear that your attempting to replace certain film caps that Mac already placed in critical spots.
    Now I may be just spit balling here but If those caps are already high grade film types which the part description says they are, then unless they read bad and or have shorted out or failed simple testing methods I would leave them as-is and worry about all the wet caps, the ROE's and any tantalum devices only.

    I see that one of those caps is already very high tolerance at 2% and IMHO Mac would not have placed such an expensive cap in that place if it was not absolutely necessary. And so if its still on value its probably 100% AOK and will probably live another 40 years in this amp in its usage.

    IF you must replace perfectly good film type caps just to satisfy your grail chase then I would follow as closely as possible what MAC did, and that in my memory is a polyester film type but very tight tolerance. < very expensive for its time back then mind you >
    As I recall from my recent 2205 expedition/restoration those poly caps are located probably inside the amps bias feedback network area. I just checked them for ESR and value and left them as was per MAC. There is a old saying that may apply here well, and that is "If its not broke, then don't fix it"... Not being rude mind you but to my memory the only poly caps with 2% were in a critical area and they were in great shape when checked so I left them alone.

    A quick check of the 2125 docs on site and your amp appears to be a cut down MC2205 schematic are clones of the MC2205 with only a few minor voltage dependent variations as I see and read it.

    MAC amps have been designed to play into those auto transformers and they get compensated internally to deal with those output load considerations so they are stable and do not break down into oscillation playing thru the auto transformers to God only knows what speaker loads the speaker makers have sold you.
    SO I tend to be very careful when dealing with what MAC did in the amps internals on a recap. Now pulling those brown or red ROE caps and replacing them with Mylar film bipolars or electrolytic bipolars is a no brainer. I just verify each cap before I replace so they are on value and tolerance matching for both channels, so going by Mac's values it should work nicely, as their sole purpose was DC decoupling. I also check to see for even the slightest ringing or noise on a sine wave to make sure my replacement caps have not caused a nightmare on elm street.

    Again now just my personal opinion here, but I would pay attention only to the replacement of the wet caps and the ROE bipolars, and any Tantalum caps are a must be gone thing, and just test and verify the higher grade caps Mac already used. Unless the ceramic disk, Mylar, or Poly film caps exhibit bad test results, I say leave them as the amp is stable with them in place as is.
    Just my 2 cents worth of advice and others here my not agree and I welcome their input also. But those caps are the least likely to ever cause you any trouble and they are the longest lifespan devices your money can buy, or has already bought in this case. Worry about the ROE and wet filter caps and any tantalum types you find. The rest should be golden as MAC intended IMHO...

    Oh and I am a 30 year anniversary McCallen guy myself, but a nice 25 or 28 would be ok also...lol lol lol...neat please and only in a doubles portions lol lol lol..


    If you search that auction site use this description to find WIMA 10 ufd Mylar film caps for all those 10 ufd wet caps the description is: " 1pcs - WIMA MKS2 10uF (10µF) 50V 5% pitch:5mm Polyester Film Capacitor "
    These will last pretty much forever and they are bipolar and they are not cheap either but if it were MY amp these would be going in not Nichicon Muse.

    There is also a 0.47 at 50 cap in there in a couple places and they would be WIMA film also for my money. I offer Nichicon or Elna SILMAC II to clients but I am a WIMA fan for all these simple decoupling caps . but this is my opinion and 2 cents worth and personal choice..others may disagree but I am a WIMA guy....

    Oh others here say replace those 0.47 wet caps with 1 or 2 ufd caps for stronger bottom end or bass enhancements. Upping the value will drop their roll off effects and could alter lower bass sonics which has been suggested in another thread here on the AK. I never tried it myself, but others here have and say its a good alteration sonically.. maybe something to play with if your time window allows..

    Oh least we we not forget right behind the RCAs on the back of the amp on a little board the RCAs connect to, Yes two bi-polar wet caps , please dump those for WIMA also IMHO... WIMA's will last forever and never dry out and WIMA has got them that fit there nicely all pretty and red like they are. or blue or green depending on other brand names available..if color is an issue lol...
     
    Last edited: Apr 22, 2015
  19. ManhattanUp

    ManhattanUp Active Member

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    Well, that was my inclination as well but I'm also looking to collect as much information as possible during this 'pass' as well as get it recorded in a thread for future generations. ;->

    P.S.
    Good stuff. Try a Glendronach 15 or 18 if you haven't already. Another great sherried highland at a better price point. (And where is the cocktail glass smiley?)
     
  20. 1moreamp

    1moreamp Active Member

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    Had Glendronach very strong Sherry flavor, lots of oak also, I like Glen Roth for the same price point and it tells you on the front of the bottle what it tastes like before you buy so your not disappointed. A little smoother also.
    Their Nose's pre-taste and place description on bottle label...Never been disappointed with that and very reasonable price point at around $60+ a bottle Bevmo has it for sure, if they exist near you... Very drinkable and affordable for everyday sipping, over a nice conversation and maybe a nice Cohiba in hand also..lol:thmbsp:
     

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