MX113 Service Recommendations

Discussion in 'McIntosh Audio' started by jlovda, Mar 15, 2017.

  1. jlovda

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

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    I have a beautiful MX113 that is need of alignment. The FM is somewhat weak and I never get a steady stereo signal regardless of the signal strength. The lamp is always blinking. I live in Canton, OH. The Mac website shows two authorized service centers near me; Protec Electronics in Cuyahoga Fall and Progressive Electronics in Pittsburgh. Has anyone used either of these businesses. Any recommendations or comments? I just bought a new front panel for $150 and don't feel like shipping the unit, even to my favorite spot in TN.

    John
     
  2. c_dk

    c_dk Super Member

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    Jon, I will only remind you once that I did offer you the use of my Sencore Sg165......it is just setting there in it's box since I got a Soundtech 1000.
     
  3. jlovda

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

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    I made a decision a year ago (due to an MC2200) that I will recap and change some components by anything requiring test equipment goes to a knowledgeable service pro. Besides, if I needed it, I would only need it once.

    Just for the heck of it I will experiment and change the LDR on the multiplex board if the part (144-179) is still available. Just tweaking some coils with an alignment tool couldn't be that complicated but changing the LDR and other components per the service bulletin below would really jack up the cost. As beautiful as the unit is, I got it very cheap and it is rather unimportant in my component collection. I got this info from a search. It's about five years old.

    The MR 67, 71, 73, 74 and the MX-112, 113, 114 and the MX-115 all used an OptiCoupler (LDR) - (Original Pt#: 144-013 and 144-042) in the multiplex circuit. This part is no longer available. In December 1982, McIntosh issued a Service Bulletin #039406 which specified a new LED replacement (Pt#: 144179) to replace the older part. This Service Bulletin also covered the circuit change necessary to install the new part. The updated LDR is still available from the McIntosh Parts Department ( 800-538-6576).

    This change involves the insertion of a 1K, 5%, 0.5 watt resistor (Pt#: 141180) from Pin 13 connection on the actual printed circuit board, to the Anode side of the new LDR. The new LDR lead has a "+" Marked on it. Connect one end of the resistor to the un-marked lead. The (+) is the cathode side of the LED, inside of the new LDR. This side is connected to the junction of C406 ( .005 uf ) and R410 ( 470 0hm ) resistor. The resistance portion of the LDR is connected in the same manner at the original wiring.

    There also has been noted that the failure of Q402 (Pt#: 132-090) has also been observed. This failure would exhibit motorboating of the audio signal. Since the original part is no longer available, the replacement is a MPSA-27, which should be available from various, online sources. It is suggested that this device be replaced, even if the issue is not noted.

    Another issue that has been seen is an open condition of D404 (1N-4148). This should be replaced even if operation of the Multiplex circuit is proper.

    The final suggested update involves Q403 (Original Pt#: 132-057). This device should be replaced when the LDR is replaced. The replacement device is a 2SC-2240B.

    It should be also noted that the identifier, on the schematic, may be different with the other affected models, but the circuit is similar.

    Hope this is of some help.

    Mark
    Group Moderator
    McIntosh Factory Restoration Center

    'Changing the 1K resistor sounds a little confusing. I picture of a completed replacement would be worth a thousand words.
     
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  4. Classic_CJ

    Classic_CJ Well-Known Member

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    Changing the 1K resistor sounds a little confusing.

    Not really. The resistor must be added because the original part used an incandescent lamp as the light source in the LDR, and the LED operates at a much lower voltage. The polarity of the LED in the new device is important because the original lamp didn't have one relative to the DC power supply driving the part.

    The actual possible hassle is that there are usually cables and wires in the way underneath that you have to move out of the way and sometimes unsolder from the board connections when they can't be moved to sufficient clearance for your de/soldering work on the LDR.

    The final suggested update involves Q403 (Original Pt#: 132-057). This device should be replaced when the LDR is replaced. The replacement device is a 2SC-2240B.

    Wasn't aware of this one, but it's very likely the 2SC2240 has a different pinout then the original device, so be aware of that when installing it.
     
  5. jlovda

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

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    Just to clarify things.... One end of the 1K resistor is soldered to the pad between C406 and R410. The other end "floats in space" with the (-) or unmarked wire of the LDR LED side soldered to it. The + lead goes back into the pad hole as normal. This means that one PC board hole from the original LDR is left empty?

    I just ordered one from Mac. It was $20 plus $8 shipping.
    Classic_CJ:
    I cannot figure out how to send you a private message. If you have changed an LDR on an old Mac tuner I would appreciate you verifying my comment above.
    Thanks

    Thanks
     
  6. Classic_CJ

    Classic_CJ Well-Known Member

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    You're close, but no, there is no unused hole left on the board. Locate the yellow/white wire coming to the #13 pin on the MPX board. This wire supplies +24vdc that is first routed through the front panel muting switch to the board then to the LDR. Once you locate the wire, you can follow the short trace to the pad where the LDR attaches. (It's the only thing this wire connects to on the board).

    Remove the old LDR. On the new LDR, you can tell the LED end by two things-- the leads are much closer together where they exit the plastic body, and there will be a polarity marking. The photocell end has wider spaced leads and there is no polarity marking. The LED end will be placed closer to the center of the board, just like the lamp was in the old LDR. You will attach the 1K resistor to the hole with the 24v supply, standing upright. Bend the leads on the new LDR so that the + or anode side of the diode will stick upwards to attach to the resistor. The other three leads go downward in the usual fashion through the board. Solder them, then hook the resistor lead to the LED lead and solder them together "in space".

    The other side of the LED (the cathode end) goes to C406 & R410. R410 connects to Q403, which connects to ground. Q403 is what switches the mute drive in and out by grounding the LED/resistor circuit.

    +24v ---> 1k res ---> [ ->I- ] ---> R410 ---> Q403 ---> GND
     
  7. jlovda

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

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    Thanks!!
    I'm waiting for the opto coupler to arrive from Mac.
    John
     
  8. jlovda

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

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    Well, I changed the old opto-coupler in my MX113 with a new 144-179 replacement. Nothing changed however. I can get reasonable signal strength (up to 6 on the meter) but the stereo light still flickers like the signal is very weak. I tried rotating the "FM level" pot on the multiplex board but it didn't seem to have much change. Do I need a full scale alignment? Any other components on the MPX board that are known to be problematic? Thanks

    John
     
  9. c_dk

    c_dk Super Member

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    You have not established a baseline so your signal strength number is relative to .......what? Is the dial on frequency? Do you get a better MPX light off station?

    As for "tweaking" some variable inductors (coils) and the corresponding variable capacitors.......the cost of knowledge, the experience, how to evaluate what the the expensive test equipment measures, that is were the expense is.

    Replacing a LDR is the simple part, in this case a not needed replacement.

    Terry, a retired RF engineer is the resident RF expert on this forum. I would bet even he needs 2 hours at a minimum to do a complete alighnment of a Mac FM tuner.
     
  10. jbailey930

    jbailey930 AK Member Subscriber

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    Good luck, John. I am following your thread as I plan on refurbing my 113 soon. I don't have any issues with the tuner section of mine so that is good not to mess with. Cap and other updates must be performed per spec.
     
  11. jlovda

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

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    c_dk:
    Well, this is the second project where I rolled the dice and lost. I will probably sell the unit as is regardless of how pretty it looks cosmetically. I have about $400 into it now, it will cost me probably $80-$90 to ship it to Terry plus the 2-3 hours work plus the risk of brand new glass being broken (or a new $75 carton if I want to do it right). This will add up to $700 or more and they are not worth that much on ebay. Funny how you can get obsessed by a $225 "deal." I bought it with the sole proposition that it would not get shipped anywhere.
     
  12. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Jlovda, heck I have 500 in my MX114 and still shipped it off to Audio Classics, worth it to me as I like the unit and it will be a keeper. Get a appropriate shipping box and get it fixed....my 2 cents. I love these old Macs...:) Al besides, it was my first one.
     
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  13. 62caddy

    62caddy AK Subscriber Subscriber

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    If it was in rough shape I'd say cut losses and run but if it's as nice as you say, why not just ship it off to Terry and let him do his magic. So what if you're in it for a couple hundred too strong? It's a classic tuner preamplifier with the regarded MR74 tuner section.

    If nothing else, it's good to have around for backup when something else goes on the fritz.
     
    Last edited: Mar 24, 2017
  14. jlovda

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

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    Hey guys. I think you may have talked me into reconsidering. It is a BEAUTIFUL unit. (When I got it the chassis was basically perfect but the glass front panel had half the paint peeled off. It looks like it just came off the assembly line now with the new glass.). Also, I have done quite a bit of "open heart" surgery on it already and I have a psychological attachment to it. I'll start with buying a box. My finances will determine the pace to completion. I'll let my wife worry about how much it's worth after I die. As you can see from my list, I already have more units than most AK addicts here dream about.

    I should give you some history about other "lucky" acquisitions. A couple of years ago at a local flea market someone was selling a pair of AR 2-ax's for $10. The cabinets and grilles were pristine. I got it home and found out the woofers were Radio Shack replacements and the tweeters were shot. Bought woofers off ebay that needed new surrounds. Bought the surrounds and put them on. Found out one basket was slightly bent and the VC rubbed. Bought a third woofer. Bought two Parts Express replacement tweeters and the required inductors. Also bought caps and L-pads to replace the corroded rheostats. The units look perfect and (to my ears) sound perfect. Final cost of $10 speakers?....over $200.
     
    Last edited: Mar 24, 2017
  15. jlovda

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

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    Hooray! The problem is solved. The part I replaced was Q403 on the multiplex board, a 2SC2240B. See the service note above....

    The final suggested update involves Q403 (Original Pt#: 132-057). This device should be replaced when the LDR is replaced. The replacement device is a 2SC-2240B.

    Be aware. The MX113 multiplex board in my manual is slightly different from the board in my MR74 manual and was slightly different from my actual unit. On mine the collector (center pin) is off the board and is soldered to the top of a vertically mounted 470 ohm 1/4 watt resistor. The connections are very tiny. I screwed up one new transistor by breaking a lead but had a second one. I suggest if you need to change this transistor and the orientation is like mine, buy two transistors and two resistors just in case.

    The stereo lamp is now steady when a stereo signal comes in. Going across the dial, all the stations were a constant 1/8" off so a couple of turns of the screw on the side corrected that. The only thing I have left to do is tweak the signal strength meter for full scale and mid scale deflection. There is a very strong station 1.5 miles from my house that is great for FS and another 25 miles away for MS. I avoided a 90 mile drive (twice) to Pittsburgh for service.

    John
     
  16. 62caddy

    62caddy AK Subscriber Subscriber

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    Glad to hear you sorted it out.

    Let us know how it compares with some of the other McIntosh equipment in your signature.
     

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