McIntosh output transistors

Discussion in 'McIntosh Audio' started by c_dk, Nov 11, 2013.

  1. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
    West Michigan
    I am sorry......I do not think anyone is trying to create paranoia about completed repairs.

    This is to my knowledge the only forum, albeit an open one, where those of us who have been trying to service our local Mac owners for decades can share information.

    I would hope that more locals will join and share....without their added input that knowledge will certainly be lost.
     
    Searing75 likes this.

     

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  2. Classic_CJ

    Classic_CJ Well-Known Member

    Messages:
    585
    Yes, it does. I'm all but certain that the 'L' designation on the resistor means that it in *non*-inductive. As of right now, I have four different brands of resistor in my supply labelled 139-125. The oldest of these is a single remaining Rockwood labelled '5W L15 5%'.

    After that, came an IRC brand labelled '9601 PW5 .15 (ohm symbol) 5%'

    After that came an unknown brand labelled '0.15 (ohm symbol) J5W NS (or maybe N5) 0205'

    Finally, the ones most recently ordered are labelled 'RCD PW5 R15J V8-211'

    The kicker on those last ones was, as I mentioned previously, that the resistors in the bag labelled 139-105 had *exactly* the same labelling.

    All of the above were ordered from McIntosh parts, specifically because of the inductive/non-inductive issue. The inductives are easy to find from any number of vendors, but not so the non-inductive, at least in small quantities.
     
  3. SoundOfSound

    SoundOfSound Super Member

    Messages:
    4,051
    Mills makes .15 Ohm non inductive 5W resistors. There are a few places that carry them.

    http://www.partsconnexion.com/resistors_mills_mra5.html

    No affiliation.
     
  4. jplch

    jplch New Member

    Messages:
    6
    Hi everybody,

    I am trying to repair the blown output stages of my MC-2500 amp (bought it this way). In what concers output transistors, the blown ones are rca's mac part number 132188/9 that are according to service manual MJ15003/4 so no ambiguity here, but should I go for a specific brand, or whatever one (of course not a fake one) is fine (variations in hfe value) ?

    As for drivers, these are mj3237 and mj3247, obsolete by now... Even if this has been already adressed (various answers), may i ask again which substitues should I consider ? Manual says drivers are "selected" types, what does it mean in practice ?

    But now comes a harder issue. The emitter leg resistors are the rockwoods 5WL.15 and the manual says part number 139125 which is non-inductive. However, according to discussion above, the WL might mean inductive... It just happens that i do not have the pages with output stage schematic corresponding to my serial number CS3771, however the schematics for older serial number seem to correspond to the circuit of my mac. So maybe the discussion on the circuit modification above does not concern the mc-2500 amp ? In a nutshell, should I use exclusively inductive resistors, or this is not mandatory (I do not have the laboratory equipment to check possible oscillations...) ?

    When I shall (hopefully) complete the rebuild, I shall need to set the values right, and I do not have either the manual pages concerning this. Could anyone supply me with the complete manual, again, this is an MC-2500, serial number CS3771 ? Thanks in advance for all your answers, I am an amateur willing to learn...
     
  5. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    1,938
    Location:
    France
    1) welcome on AK !
    2) what numbers are "stamped" on your ( both ) driver boards ?
    3) by the fact that you found " rockwoods 5WL.15" inside your unit, did you see that they look like replaced resistors ? somebody worked on this unit using a bad reference resistors ?
     
  6. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
    West Michigan
    A MC2500 with blown outputs.....that is a rarity. Did the whole channel fail in a cascade?

    I am sure that factory parts can supply you with the correct schematic.

    Hopefully your driver transistors are OK.

    I now always replace the darlington bias transitors when ever I have a MC2255 family of amps near the bench with a hot iron nearby.

    The issue with inductive/ non inductive resistors were with the early series of MC2205 family. They need to have the inductive resistors to insure they do not oscillate.

    As always your local tech should have the facilities to properly test these amps under stress to assure you get what you are paying for and are repaired correctly.

    McIntosh engineering did not discover that the resistors manufacturer had changed their build design until the amps were tested under stress and found to need that little bit of inductance to maintain stability.

    It was after that mutual discovery that the amp circuit was modified to allow the use of non inductive resistors.

    I believe that subsequent amp designs (after MC2205 family) could use either. The MC2505 amp family (includes MC2300) need .33 and .56 inductive wirewounds......
     

     

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  7. jplch

    jplch New Member

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    6
    Hi, the heatsinks correspond to what is pictured on the schematic as boards (1 to 4) 045630, 045950, 045951, 045634. As for input boards, these are marked 045928. The amp looks inside as totally stock, not a single intervention as far as I can say (unless everything was changed at once a long time ago. I would then think that the rockwoods are then appropriate to this model, or at least they have been mounted at the factory. Now, I still do not know if WL means inductive or, on the contrary non-inductive. In the discussion above, opionions diverged on this topic, or at least there seemes not to be a clear consensus...
    One driver has e-b shorted, it is labelled 198 Malaysia 8526, I think this is MJ3237 since the other channel has the corresponding driver (not blown) marked as Motorola MJ3237.
    I do not know the history of the amp so no idea what happened. Some output transistors seem ok, but I shall change them all, no chances takes here... As I wrote to clinic-audio, I still don't know what about the WL meaning... The Rockwoods sure seem stock...
    As I wrote, one driver is blown, looking for substitute. Thanks for the hint on the Darlington, I shall change them, they seem easily available. Concernig the WL issue, do I have to understand reading your post that in my case inductive or non-inductive are both ok ?
    Working on the mac is part of my hobby, so I shall try to cope with the situation by myself, or at least going as far as possible without however making things worse. Hence all my questions.
     
  8. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
    West Michigan
    I have experimented with driver transistor substitutions with some of my own amps to see what would happen but would never mess around with someone's else's.

    Just buy the driver transistors from Mac parts knowing that what they send you will have been approved by engineering and gone thru their Q/C.

    If the WWs are original and have not gone open reuse them. If you have some open ones, again buy them from Mac parts and move on. I am sure that at one time batches that Mac received were marked with a L for the inductive ones.....but those shipments probably got used up decades ago.

    Without test gear your project could be a expensive learning experience.......
     
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  9. jplch

    jplch New Member

    Messages:
    6
    In my country unless you are a professionnal, you cannot buy spare parts from manufacturers so the only way to have my mac repaired is to leave it at the shop. Apart from the fact that it is very costly over here, I want to try myself as part of my hobby. Should I try directly with Mcintosh USA to get parts ? Also, I am willing to take chances with your advices concerning substitues, of course I will not blame you if something wrong happens ;-).Now, I have bought in between some wirewound resistors at the right values, and I am assuming they are inductive since there is no explicit mention to the contrary. I have a scope and a signal generator and know how to use them, so if there is oscillation I should be able to see. What I was looking for were specific advices concerning my problem from people more experienced than me and knowing better macs than me.
     
  10. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    1,938
    Location:
    France
    hello
    where are you ?
    You did not answer to my questions if you want some help , see #25
     
  11. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
    West Michigan
    Over the last 1/2 century, due to the restrictions and complications many countries have placed on the importation of American products into those countries, McIntosh has made exclusive import agreements with local importers who have then navigated those local import rules. McIntosh still honors those import agreements. That is why in many cases parts are only available through local distributors and the factory can not ship to end users directly. Don't blame McIntosh, blame those making your import rules.

    I personally would worry about the driver transistors much more than the wirewounds, I would not replace them if they are not open. There can not be much inductance there anyway in a .15 ohm resistor, and besides the problem with this part stemmed from them not having enough inductance in a amp that was built from 1977 to 1980.
     

     

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  12. jplch

    jplch New Member

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    6
    Hello,
    I leave in Switzerland. As I mentioned above, I cannot see any intervention on my unit, everything seems completely stock. Hence I think the WL resistors are genuine and not the result of some previoius repair. As for the driver boards, they are stamped 045928 if you mean the boards which drive the darlington, the driver tansistor MJ3237, and MJ3247 etc. Thanks anyway for your willingness to help.
    Hi, I was not blaming specifically McIntosh for this policy, this is a general situation in Switzerland save for some rare exceptions among brands represented here. When I was in the States I used to marvel at the liberal policy where one could buy almost everything one needed to repair one's gear at home...Anyway, let's stick to the topic, I have already ordered some standard wirewounds making much the same reasoning you did. Indeed, a good number of the WL are open and need to be replaced. As for the replacement for the drivers MJ3237 (and possibly the MJ3247) I am still looking. I have located some internet vendors claiming they still have them available but I am suspicious about the authenticity andf, most of all, the quality of these parts. Regards.
     
  13. dshoaf

    dshoaf That high voltage buzz

    Messages:
    10,033
    Location:
    Santa Cruz, CA
    Agreed! There are fewer of the guys who were there when this gear came out who know and cal recall the details of each model's quirks and uniqueness.

    This thread is an _excellent_ example of that. You guys are to be congratulated for taking the time to share.

    Cheers,

    David
     
  14. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
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    I would trust a NTE replacement before I would a eBay seller. Like I said I have experimented with a substitute pair in my own amps....I believe Jeffs79 has also.
     
  15. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
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    There is no NTE cross for MJ3237 or 3247......my data sheets from the late 80s show them available in limited quantity from Mac parts even then and they were changed to a different part with circuit changes it appears for the 2nd gen MC7270.

    I have had no reason to order from Mac parts to see what the now ship as a replacement.
     
  16. jeffs79

    jeffs79 AK Subscriber Subscriber

    Messages:
    300
    Location:
    Upstate New York
    The devices that c_dk & have used on the 212X/220X/215X/225X series have worked very well for me.
    I recently replaced the drivers in my MC-2500 but I was fortunate to have MJ3237/47 on hand.
    I believe the MC2500 uses drivers that are also the Monitor Amp output devices in the C33 & C34V. I think Mac replaced them with TO220 devices TIP42B & TIP41B.
     

     

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  17. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

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    The C33 uses TO-66 outputs. The C34V uses the TO-220 plastic parts. The aluminum L shaped heatsinks have different holes as do the circuit boards for the pins. I own one of the first C34V's off the line and it has some "leftover" C33 parts on it including longer knobs and the older monitor circuit board.
     
  18. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
    West Michigan
    This thread has gotten way off the track that I started trying to figure out what devices Mac was using in a particular MC7270......turns out they were Harris devices caught up in the GE take over of RCA after they consolidated Harris.

    That said the MC2500 did use TO66 devices as drivers for the MC2500, which were also used as outputs for the C33 monitor amp.

    The MC2002 also used TO66 devices as drivers commercial #s 2n4240 and 2n6423. I will guess that if you ordered any of these TO66 devices from Mac parts you would receive NTE devices that they had curve traced. Only NTE has enough buying power to get TO66 case devices made to spec at this time, in my opinion.

    My guess is that you would receive a NTE38 and NTE175 as replacement parts. Hopefully someone has ordered C33 or MC2500 parts to confirm.....
     
  19. jplch

    jplch New Member

    Messages:
    6
    I received the replacement drivers for my mc-2500 and they seem ok under tough multimeter testing. Let's hope for the best once I put them in the circuit. Now, I have another question concerning the actual circuit I am working on. There is a small component connecting the darlington emitter to the base of the negative driver but it does not show in the schematic where the connection is pure wire. It appears to be open so that I have to replace it but the color code is a 6 band one with ambiguous reading. On the other channel corresponding heatsink this component is not open and shows roughly 50kohm but this does not match the possible reading of the open one (besides it does not make sense either). To start with, is this a resistor or an inductance or else ? Could it be part of the modification of the output circuit once non-inductive resistors on the emitter legs of the output transistors were used ? Thanks in advance for any hint!
     
  20. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
    West Michigan
    What parts did you order?

    It is evening here so no access to schematics. When you are referencing a darlington are you referring to the bias transistor?
     

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