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McIntosh - Hints and Kinks

Discussion in 'McIntosh Audio' started by dewickt, Jul 1, 2004.

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  1. MX117.MC7150

    MX117.MC7150 Listener Subscriber

    Messages:
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    Have you checked out any late-50s, 60s & early 70s U.S. made automobile prices recently? Pretty much the same with them with one notable exception. Most old McIntosh, once serviced, will/can be used daily for the next 40-50 years without additional service. Essentially has an infinite lifetime. Very few similar era automobiles see daily service.
     
  2. Paul K

    Paul K Member

    Messages:
    90
    Just a question about the 240...
    When a power tube goes, what fuse does it take with it or does it even take a fuse out?
     
  3. johndoe1

    johndoe1 Member

    Messages:
    59
    RE: Ground Problem resolved: MA5100 - bass Control

    Terry thanks for the help - I soaked it a lot for weeks and it finally cleared up almost 99%. I did call Mcintosh and they said they have the POT for the bass control at $5 Flat type not knurrled...I think it's fine now.

    Thanks again.

    John
     
  4. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    21,513
    Location:
    Angel Station, Alabama
    C24

    Apologies if this has likely been covered previously.
    I may be tasked with renovating an older gent's shelf of Mac consisting of a C-24, MC-240, and a pair of MC-75. Having heard the sys I think it prudent to start with the pre and expect to clean switches, pots, transistor sockets, and replace all the axial e'lytics with a modern equiv, and probably the PS e'lytics. The power amps will be next as I'd like to keep his pride-and-joy playing for him thru most of the process. He also has a couple tube era Mac tuners presently unused out of service.
    What are the 'got-chas' on doing all this for the C-24 and what replacement signal and other caps are advised/recommended ?
    TIA!
     
  5. MX117.MC7150

    MX117.MC7150 Listener Subscriber

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    Can't change too many caps in the old gear. You are definitely on the right track. These guys http://www.tubesandmore.com/products/capacitors have the multi-cap cans for the amps. You might also want to replace any selenium devices with silicon.
    Did the C24 (no dashes in McIntosh model nomenclature) use transistor sockets? Never seen one in the flesh.
    For the small lytics most folks go with Nichicon's Muse audio series (KZ, etc.). Sold several places, but Mouser and DigiKey never seem to have everything I want so I usually purchase here http://www.hndme.com/productcart/pc/viewcategories.asp?idCategory=73 as he seems to stock most everything.
    When you get to the tubes, the caps will depend on how much money the owner wants to spend. Caps can get expensive very quickly in the higher voltages for tubes. The less expensive Mundorf are pretty nice for the money. Have also read good things about Amp Ohm caps.
     
    Last edited: Jun 11, 2013
  6. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    21,513
    Location:
    Angel Station, Alabama
    Thnx for response, Now have a service manual to study. This is a transistor unit with socketed devices, easily replaceable and probably in need of contact dope and work-in. Pic from another forum;
    [​IMG]
    The critical audio e'lytics appear to be but four all in the line driver stage, all of the rest on the skiz appear to be reliable film types. The PS e'lytiics may or may not be in need of replacement and as always I need to check for any eyelet-to-chassis grounding interface problems. I know the subject unit has a residual hum problem and noisy volume pot that may or may not remediate with Faderlube. The power switch is also predictably defective thanks to the then-standard U.S. practice of not having a snubber cap across them, thus guaranteeing eventual failure.
    Any other suggestions welcome, TIA!


     
    Last edited: Jun 12, 2013
  7. MX117.MC7150

    MX117.MC7150 Listener Subscriber

    Messages:
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    Location:
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    Very simple compared to the C28 and later preamps. One of the issues with later preamps is grounding. The grounding lugs get some corrosion around them which results in a poor ground. Not sure if the C24 using the same grounding scheme, but something to look at if it does.
     
  8. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    21,513
    Location:
    Angel Station, Alabama
    Thnx again!

    I see you have a pair of MC50, I have a single as a doorstop at present from a county ballfield announcers booth. It's my only piece of Mac ever and I don't have a place/use for it, yet, I just like seeing it around I guess as they were soooo droolworthy when I was a lad.
     
  9. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    21,513
    Location:
    Angel Station, Alabama
    I see basically 2 versions of this preamp, the 'later' has additional e'lytics on the phono/tone board. Is it recommended that these be 'updated' when recapping and if so, are additional changes recommended or is it best to leave it as-is with a basic re-cap of the Bakelite tube & other axial e'lytics?
     
  10. MX117.MC7150

    MX117.MC7150 Listener Subscriber

    Messages:
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    Good caps are cheap for the preamp. If were mine, I would change them all and eliminate any worry.
     
  11. oreo382

    oreo382 New Member

    Messages:
    34
    Mac output transistors

    I have noticed that the 132-070 op transistor used on some MAC models is recommended to be substituted by an MJ15003. I was reading a MAC forum thread on Yahoo where a service center rep said the original part is a 2n5303 (which is still available ).Is the MJ15003 an upgrade?
    Specs show the 2n5303 as a 20amp 80volt transistor.Does anyone know about this or have any comments?
     
  12. dewickt

    dewickt Will fix about Anything

    Messages:
    3,376
    Location:
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    If you order a 132-070 from the factory you will get a MJ15003, a common, inexpensive, and reliable part that is still in production by several manufacturers.
     
  13. bogadocarlos

    bogadocarlos New Member

    Messages:
    11
    Hi Terry,

    I have an all original C11 that I bought two months ago. Today started doing a noise (not continuosly, aleatory) on right channel, when function after 20-30 minutes. The noise is short, low frecuency, something similar when carbon potentiometer have problems (sorry my description!)..
    Can you give me some orientation to start analazing the problem?.

    Many thanks.
     
  14. dewickt

    dewickt Will fix about Anything

    Messages:
    3,376
    Location:
    Knoxville, TN
    Could be a dirty tube socket, clean pins with DeOxit D5, to a bad filter capacitor, replace can or temporary bridge the bad section. With these hints and not knowing your technical level I think it may be time to go to the shop.
     
  15. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    21,513
    Location:
    Angel Station, Alabama
    +1 what Terry sed, these things are so full of contaminated contact interfaces that need reconditioning and suspect age-deteriorated components that a meticulous step-by-step renovation procedure is mandatory to restore reliable top performance.
     
  16. bogadocarlos

    bogadocarlos New Member

    Messages:
    11
    Many thanks for your help.
    Was a bad cap of 0.01uF on right output stage, bumblebee that I replaced for a black beauty. Taking oportunity I replace the two in same step.

    Thanks again
     
  17. wanders

    wanders Super Member

    Messages:
    1,227
    Location:
    Northern California
    Sorry to be replying to this several months late. FWIW, last year I sourced all electrolytic caps, including cans, for my c-24 directly from the factory. Seems like the total was around $60.
     
  18. Gary Kaufman

    Gary Kaufman AK Subscriber Subscriber

    Messages:
    733
    Location:
    NH
    Having just read thru 47 pages of hints, I grabbed the information that related to tube related Amplifiers and collected it into one file. I thought this might also be useful for other:



    MC30
    Here is a good starter, MC30 bias upgrade for safety and reliability.
    Replace the Bias filter capacitor with a 10 to 22 uF (I use 22uF) at 450Volt, then replace the bias diode with a

    good silicon one, 1N4007 works well. Now that you have a new bias supply the voltage will be to much, to get it

    back to original replace the 3.3K series resistor (one next to diode) with a 9.1K 1/2W resistor, measured bias is

    exactly what it should be with these 3 changes, and the long life of your MC30 is one step further.

    440V on the plate and -42V on the grid of the 6L6, power transformer type is of no consiquence, replace the

    selenium and change the value of the resistor in series with it to set the bias, about 9.1K should get you close,

    a value either way may be needed to get it right on. I find anywhere from -42 to -43 is good when plate is at

    440V..
    __________________

    MC30 biasing and filament surge
    Turn on surge is common, this is due to cold filiments getting their initial hit of voltage, if this bothers you

    add an NTC resistor like the MC240 uses, thruthfully I would not care since it hasent caused the transformer to

    fail for the past 40+ years.
    My schematic is for the later MC30 drawing # MC-30-4 (#15329 and above, 6-15-56), R30 is the bias droping resistor

    with C13 the - bias filter, the bias is called out at -42V with B+ at 440V, this is the most common version of the

    MC30.
    Watch out on the early version that uses the 2 extra .47uf capacitors and has the square bias rectifier on the

    capacitor side of the board, it does not have a droping resistor and if you replace the selenium you will have to

    add a resistor in the transformer feed side of about 8.2K.
    Remember to much bias makes the amp run cooler but also decreases output power and raises distortion.
    __________________

    MC30 thru MC275 distortion
    MC30 thru MC275 all, if you have increasing distortion as the volume is increased check the second stage, V2,

    12AU7 Phase Inverter, pins 1 and 6 must be at the same voltage (265V), if not replace the .22 uF capacitor on pin

    7. The grid bias is derived from a 2.2 M ohm resistor, if this capacitor as any leakage it will upset the bias

    voltage resulting in the phase inverter outputs being unbalanced.
    __________________

    MC60 Bias update
    When changing out old bias diode to a silicon (1N4007) change the 1.8K series resistor to 5.1K, check bias for -45

    or slightly higher (-46) when B+ is at 435V on the KT88. If you have a fully meterd variac it should draw 1.2A at

    120V.

    Note: some very eary MC60 did not use a series resistor, you may have to experiment to come up with a good value

    for the resistor.
    __________________

    Speaker terminal screws
    If your screws on the speaker barrier strips are chewed up and you want to replace them watch out - if the new

    screws are too long they will short to the chassis.
    __________________

    __________________

    MC240 Bias update/mod
    When replacing the selenium diode in the bias supply with a silicone diode (1N4007) use a 470 ohm resistor in

    series with the transformer lead going to the diode, this should put the bias voltage on the button !! While you

    are at it replace the bias filter, a 22uF 450V cap will stiffen it up and last forever.
    __________________

    Bias Voltage MC240
    The banded end of the new diode should be tied to the transformer, this will give you a - voltage, it may need to

    be tweaked with a resistor in series at this point to get the proper -46V (loaded) when the B+ is 430V at the

    output tubes. When all is working as it should with the AC line at 120V the AC current should run 1.2 to 1.3 amps

    when you have a set of good tubes installed.
    __________________

    Rebuilding Bias Supply on MC-40
    I have 2 MC40's to do this to waiting on the shelf, the resistor will be in series with the diode and the lead

    from the transformer. The value should be about 470 Ohms, you may have to adjust it slightly to get -52V on pin 5

    of the 6L6 with the voltage on pin 3 at 470V. I use a variac to make sure the plate voltage on pin 3 is at 470

    before I measure pin 5.
    __________________

    First thing to check on a MC240 is the mode switch, work it a few times to get the self wiping action to clear

    oxidation off of the contacts.
    __________________

    " I opened up a 240 to see if the selenium rectifier had been changed and confusingly there has been a diode

    soldered in but no resistor..."

    A percentage of the 240s do not need the 470 ohm resistor in series with the diode, variables in the power

    transformer and circuitry can change the voltage. It is best to measure to make sure 430V plate voltage and -46V

    on the grid are the ideal with some variation up to -48V at the grid being acceptable.
    __________________

    MC240 Thermistor
    CL-50 NTC, thermal resistor available from most supply houses, or from the factory parts department 800-538-6576.
    The MC240 uses a voltage doubler type power supply, the NTC is there to limit the turn on surge to the supply,

    makes the power transformers surge less, and doubler capacitors last longer.
    __________________

    Mcintosh Chrome

    McIntosh chrome responds nicely to a good car wax, I use Turtle wax Carnuba paste, just don't rub real hard on the

    lettering, and also gives the black paint a nice shine. If your chrome is getting tired with pits or hazing you

    can use Wenol metal polish (www.wenol.com), it can help revive the chrome to be as good as it can be. Whatever you

    do use a clean soft rag to do it, keep using a clean section as the dirt it picks up can scratch the chrome

    leaving swirls and hazing.

    For chrome I use Wenol (http://www.wenol.com/), it gets the rust and oxidation. For the glass I use Turtle Wax

    Carnuba, can use it on painted surfaces just don't rub it a lot or you will end up with a beautiful gloss finish.

    For general dirt I use clear Ammonia, is very gentile on lettering, both the Wenol and Turtle Wax can eat at

    painted lettering if you rub to hard, the metal ID/information plates are safe.. Do not use Windex, or 409 type

    cleaners as they attack lettering and painted surfaces.
    __________________

    MC225 hot 12BH7 and 22K, 1W resistors.
    If your MC225 has cooked the two 22K, 1W resistors and the 4.7K, 1/2W resistor, replace the pair of .047uF bumble

    bee capacitors feeding the grids of the 12BH7 at the same time you replace the resistors. When the .047uF caps get

    leakage they will raise the grid voltage of the 12BH7, I have seen 96V on the grids, replaced the caps and had it

    go down to 33V, close enough to get the tube running in spec. The second thing this will do is raise your output

    power from a sad 22W to a happy 30W+ as it did for me on a customers 225 that was running all original McIntosh

    tubes.

    Mc225 Thermistor replacement?
    In another thread I'm rebuilding the power supply of my MC 225, and a question or two has arisen. first, what does

    everyone use to replace the thermistor with?

    Secondly, when replacing the bias diode with a 1n4007, which way is the band oriented? The original has one wire

    on one side, and two on the other.

    What is the value of the bumble bee that is on the small power supply board?

    I would use the G.E. CL-80, 47 ohm, 3 amp, NTC, gives a nice soft start.
    The band is the + end, this is the end that goes toward the transformer R46-R47 junction.
    The capacitor is a .01uF use 630V, C24.
    __________________
     
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  19. Gary Kaufman

    Gary Kaufman AK Subscriber Subscriber

    Messages:
    733
    Location:
    NH
    And here's a summary of some of the tuner comments:


    Tuner alignment MR65B
    Here is a useable complete alignment procedure for the MR65B, can also be use for a MR71 and MR67 with some minor

    part number changes.
    Alignment for MR65B

    IF Alignment :
    Remove V11 osc.
    Inject 10.70 mHz into pin 7 V2B mixer
    measure at TP1 and adjust
    T2, T3, T4, T5 for maximum
    T6 bottom for maximum voltage at D2
    (may have to misadjust T6 top to get voltage near .2 V)
    T6 top set for 0 V at D2
    T7 bottom for maximum voltage at D3
    (may have to misadjust T7 to to get voltage near .2 V)
    T7 top set for 0 V at D3

    The top adjustments of T6 and T7 set the center tune points of the discriminators for audio(T7) and AFC(T6) they

    can have more than one 0 point, choose the ome that gives the most change if the 10.7 frequency is shifted (most

    sensitive).

    RF Alignment:
    install V11 osc
    FM Signal to antenna terminals
    keep signal low for good reading at TP1
    at 106 mHz peak C2, C5, and calibration trim C53
    at 92 mHz peak L1, L2, L7, and L11 for calibration
    the calibration at 92 and 106 may have to be repeated several times to make the dial correct.

    Demultiplex Alignment:
    FM stereo signal to antenna center tuned
    T501 for maximum voltage at V14B pin 9 (19 kHz peak)
    The following should be done with the stereo signal set for L or R modulated with the other channel not modulated

    (L only) read output voltage at the non modulated channel output and adjust for minimum output, L501 and T502 (38

    kHz Osc).
    L504 is the SCA filter and is peaked for maximum seperation and SCA rejection.

    If setting of T502 is in question pull V14 and set frequency at V15 pin 6 for 38 kHz by adjusting top of T502.

    All adjustments in the stereo demodulator an be slightly re adjusted to maximise the seperation to a minimum of 30

    dB.

    A 65B in good condition can get as good as 33 to 35 dB seperation and RF sensitivity of 2 uV for 3% THD (30 dB

    SNR).

    For improved distortion you may want to replace C75 & C77 (10 uF) at the volume control, and C74 & C76 (1 uF) at

    V9 and V10 with new quality components.
    __________________

    MR65B, MR67 Increase Stereo Separation
    While giving a MR65B the work over on the bench I found that I could not get better than 28dB of separation, I

    remembered a modification used in the MR71. Since all three models use the same basic demultiplex and output

    stages I figured that the modification would help my separation problem.

    Connect a 27K 5% 1/2W resistor and a .0015mF 10% capacitor in parallel. Connect this network between pin #8 on one

    6BL8 audio tube to the pin #8 on the other audio tube.

    Amazing --- the MR65B now has over 40 dB of separation just like the late production MR71, and a definite increase

    in sound quality…..

    This modification makes up for a slight loss in the L-R portion of the composite signal caused by the SCA filter

    by increasing the difference signal gain in the left and right audio amplifiers while leaving the common signal

    gain unchanged.
    __________________
    MR73, MR74, MR77 Bulb

    OK, here we go, the 73-74-77 are basically the same demux, if it has the 270 ohm resistor use a #1828 (37V), if no

    resistor use a #1835 (55V). Either lamp has a slim long filament that hates vibration, and looks for an excuse to

    break.
    Have gotten several tuners with a #47 in them, is bright for a while till the 270 ohm burns open after the

    transistor switch shorts.
    __________________


    12AT7 in McIntosh Tuners
    Most all the tubed McIntosh tuners MX110, MR65, MR67, and, MR71, use a 12AT7 in the front end of the tuner. This

    tube is used as an RF amplifier and mixer stage and can have a big effect on the overall performance.
    Here is the problem, overall sensitivity is only 5uV and has a tendency to oscillate on the high end of the band,

    is very tweaky when trying to align. You have put in a new tube and expect more than this !! this is a NEW stock

    tube... Well take it out and use an old stock or used tube of the old design without the internal shield, all the

    problems will go away. It seems that the new tubes with the internal shield works fine in preamps at audio

    frequencies, but at RF they will not work!! The internal shield has plate to plate capacitance that will cause the

    tube to oscillate messing up the front end of your tuner.
    This is one case where new is not necessarily better.
    __________________
     
  20. dewickt

    dewickt Will fix about Anything

    Messages:
    3,376
    Location:
    Knoxville, TN
    Thank you for the summary's on amplifiers and tuners, should save a lot of hunting, little did I know when I started this topic that it would grow to 48 pages.
    Opps, just did a check and now it only has 18 pages ...
     
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