McIntosh MC2505-2 Help: need schematic, FSM, SBs please

Discussion in 'McIntosh Audio' started by SaturationPt, Mar 13, 2017.

  1. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    I have found a couple of copies of the 2505 FSM on this and other forums (same copy, 36K00 and later) but nothing on the MC2505-2. They don't match.

    If you have anything: documents, links, personal notes, I will very much appreciate any help. I listen to this system regularly, and am starting to think I should be checking some of the voltages, currents, bias resistors, etc. to be sure that I can listen to it for a long time.

    TIA.
     
  2. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    1,744
    Location:
    France
    Hello Jeff

    That's the problem with the web everybody copy everybody without knowing what they do ...

    Where are you ?

    You probably don't know that there is 11 different models of MC2505 , MC2505-2 included .

    Usually I don't need schematic of the unit except if evevything is burned and then difficult to guess .....

    I don't think that you will need it to repair it . If this is not your job give it to someone who knows what electronic is . I will not recommend you to do it yourself if it is not your job .

    You have to think about that before to do anything inside .

    There is no bias ajustable or offset ajustable in MC2505 , and MC2505-2 included

    Don't you have qualified tech close to your area ?
     
  3. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    I do appreciate your concern, am an Engineer who spent part of the early '80s as a Tech repairing stereo equipment part-time during school, then a career designing digital and analog electronic equipment. Most of my home shop equipment is kind of vintage (CRT scope, remember those?) but is adequate for what I want to do, I stop at RF since I do not have the equipment nor the experience for RF work.

    I am in North East Indiana, USA. No service centers nearby, and my McIntosh dealer in Michigan went out of business in the early '80s.

    As far as the multiple versions of the 2505, I am aware that there are multiple versions, and mine being the -2 makes it more difficult. The one SB that I did find is for the MC2505-2 10K01 through 11K54, and none of the components match my (SN: AT1024) unit.

    I have sent an email to McIntosh Labs to see if they have any docs that I can buy, but I haven't received a response. If I can get documents for any updates or Service Bulletins, and a Service Manual would be wonderful so that I can check any voltages etc. in this unit.
     
  4. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    1,744
    Location:
    France
    Jeff

    Give me the number of your driver pcb or post a nice picture of it (both sides )
     
  5. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    I will take a couple of pictures tonight and post them, thanks for the help!
     
  6. c_dk

    c_dk Addicted Member

    Messages:
    5,489
    Location:
    West Michigan
    Is the amp clipping symmetrically? Under stress do the supply rails collapse?

    Any sign of notch distortion? Is there excessive 60 and 120 hz buzz or hum?

    I do not believe any of the semiconductors used in these amps have been in production for 30 years so it is not uncommon to find any number of strange combinations used over the years.
     
  7. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    No hum, and I only run the volume up to about the 12:00 mark occasionally with no audible distortion.

    I have lost a gain pot however (only works at full gain) on this amp, and would like to replace / rebuild the pot, and while it's apart I'd like to check it over as thoroughly as I can.

    Where are you in West Michigan? I moved down here from Holland / GR, and do get up to GR occasionally.
     
  8. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    A couple of photos of the output boards 048916, and an overall shot into the bottom of the chassis showing the power supply resistor configuration different than the -2 schematic I've found (and different than the upgrade / SB schematic pre-11K55).

    I look forward (eagerly) to finding technical / service information on this revision!
     

    Attached Files:

  9. c_dk

    c_dk Addicted Member

    Messages:
    5,489
    Location:
    West Michigan
    McIntosh never differentiated a I or II, they moved the class A transistors to the driver boards late in the 7 year production run.

    You are making this overly complicated without doing the most telling performance checks. These amps should put out close to 70 watts; if it does not or if it clips unevenly, you have issues. If the power output drops, and the supply rails collapse at low frequencies you have power supply cap issues.

    Yes, all those axial caps are way past their expected life and should be replaced. There are many in this forum that extol the sound benefits of replacing the old Mylar caps with new PP caps. I know from personal experience that a 3 ohm drift in value of the bias cap will throw off the distortion level of the amp.

    How do I know.......I have been bench testing these amps for almost 40 years. It is not more complicated than that....if you have the test gear put them to use.

    PS your pictures show nothing different than a typical later production old MC2505 vintage '72-'74. Looks just like the one I bought in '74.
     
  10. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    You are correct, it is a late '74, bought new in '75.

    The Model Number on the chassis is MC2505-2, do you mean that this was a change to the P/N only and wasn't documented as a model update/change? I really don't remember that part of the history, and my McIntosh salesman is long LONG gone IIRC (but his picture is still in my mind, worked at Electronic Sound in Holland, Michigan).

    I can calculate power output and look at waveforms at frequencies, but would very much like to start with an accurate schematic, ... or better still an accurate Service Manual. If there are SBs also, it seems like that would help to understand any deficiencies or improvements identified after my unit was produced (although being near the end of production, most were likely already rolled into production).

    If there's information out there, I can identify any special parts (such as inductive, fuse-type, wirewound, flame-protected etc.) parts and replace per the original design.

    Otherwise, I will do my best, just order resistors, capacitors, diodes, and hope that I have replicated the original design (I'm too OCD to feel good about that).

    Also, the MC-2505-2 SB for early P/Ns says "Although the MC2505 is rated at 50 watts (sine wave) per channel, it will normally put out at least 75 watts (sine wave) per channel. However, it is possible to drive the amplifier even harder than this. The following modification will improve the power output ...)" and "Amplifiers having a higher serial number than 11K55 have been modified".

    Doesn't this indicate that I should be able to drive it with clean sine-wave input higher than 75watts without distortion? The modifications shown (and pre-modified) in this SB don't match my power supply configuration either.

    I'd really like to find documentation.
     
    Last edited: Mar 14, 2017
  11. c_dk

    c_dk Addicted Member

    Messages:
    5,489
    Location:
    West Michigan
    The quote you are referencing is from a 1967, '68 service bulletin #2 and has nothing to do with amps produced later in the production run.

    Looking a your PS input board your unit is a very early later version, note the 470 mfd axial caps. The last version had 680 mfd.

    Unfortunately back in the 70s Mac did not write service manuals for OCD owners....only for frazzled service techs dealing with said owners. I doubt you will find the expected documentation you expect to find.

    Like Patrice said, Mac sent us a lot of service manuals for Mc2505s, I would hazard a guess that there are more MC2505 manuals than any other Mac unit. Thank goodness they stopped this practice and compressed their manuals with later models of amps.
     
  12. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    I do appreciate your input, but it seems like you're telling me that I don't know what I'm looking at, without actually telling me what I'm looking at.

    If this is an early production due to the 470uF caps, and the later had 680, is this a recommended change? Are there other components that would change to balance this higher capacitance?

    If the Class A transistors are on the driver board does this indicate a late production?

    Are there service manuals? If they are available how will I be able to get a copy? If not, is this then all tribal knowledge not to be shared?
     
  13. c_dk

    c_dk Addicted Member

    Messages:
    5,489
    Location:
    West Michigan
    I would guess there are over 50 threads highlighting restorations of MC250s, MC2505s and their bigger brothers that share many circuit ideas, the MC2100 and MC2105. The search function is your friend you will find part numbers, ideas, and actual restorations.

    Personally I am looking for new ideas that others have been trying. I have used my personal MC2505 to try some things, some good, some real bad.

    Bring new ideas, bounce them off us if you wish. Realize I have 6 turntables to refurbish, a SX1250 to finish up and have to plan out restorations for a MA5100, a HK730, and a Marantz 1250. I am sure I am forgetting something!
     
  14. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    Thanks again. You have many projects, keeps you busy!

    I've spent a couple of evenings searching MC2505 threads & postings, nothing yet. I guess I haven't gone back far enough. It really isn't that complicated a circuit, I'll make some choices and see how it sounds.

    Is there any reason to look at replacing the gain pots with newer high-end audio pots? Seems to make a difference in some old tube gear. The signal path is pretty simple in this amp, I really don't see much optimizing there, and there are already some nice caps, ... I'll be replacing the 470uF with 680uF, doesn't seem to be anything dependent that will need to be changed. The multi-cap would be the tough one, I've found multi-caps for old tube gear new, but those are pretty specific. I believe that I can always go up a little in capacitance in these filters provided it isn't enough to cause an inrush-current problem for the transformer.
     
  15. kevzep

    kevzep Its all about the Music Subscriber

    I had the same problem trying to find service information, I gave up and went ahead completely rebuilding a MC250 with NO information....
    Personally, I think they are the simplest amplifier on the planet to work on, everything is pretty straight forward....you can make your own schematic they are that basic.....
    The one I did was so badly damaged, I couldn't really tell what was going on with the bias circuit (didn't correlate the schematic I did have so that was useless), so I just made it adjustable, the original bias resistor had completely fried up, I thought there must have been a trimmer somewhere....
    But no, anyway, the point is, it was 5 years ago when I rebuilt this amp, and its still in used daily with its "wrong" output transistors, "wrong" TO-220 drivers, adjustable bias, and a bunch of other stuff I did in the power-supply...
    Puts out 78watts per channel, stable, no oscillations, basically turned me onto McIntosh....Didn't want to give it back to my friend!!
     
  16. SaturationPt

    SaturationPt AK Subscriber Subscriber

    Messages:
    851
    Location:
    N.E. Indiana
    Perhaps I am looking at too much like an Engineer. I have many options (for example) in capacitors, I can match the labeled capacitance and voltage, but what about ripple current ratings? More might be better, or will it adversely affect the sound, ... same with ESR, can change the sound.

    I'm not as concerned about the amp as I would be re-capping the tone section of the preamp, but still it would be nice to know what the diodes and caps are supposed to be.

    So much easier with vintage Pioneer/Marantz/Yamaha (sorry to swear here), as the information is readily available. I've chosen to "shoot from the hip" on many of the components in the amp as it is a pretty simple amp, will need to reverse engineer the original circuit design a bit more with the tuner and preamp.
     

Share This Page