MX114 and MC250 Question

Discussion in 'McIntosh Audio' started by AlTinkster92, Oct 9, 2017.

  1. sberger

    sberger Hard Core Geezer Subscriber

    He liked them with the Apt Holman preamp. It's the 114 that's the issue(although the Cornwall does produce better bass, imo, everything else being equal).

    While it does seem to me that Audio Classics dropped the ball here, it could be that the 114/250 combo is just not as appealing to you as is the Apt Holman/250. I enjoy my 250 that I got from Audio Classics with my MX110, but I find that the 110 with my Harman Kardon Citation 22 sounds better to me with both my Cornwall II's, and my KLH Model Five's. So in the end, who knows what will work with our ears, and what won't?
     
  2. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Just a update to mention that my 114 is now in the hands of our fine tech Dave Gillespie....cant wait to see what he finds :) AL
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    As a point of followup, I received Al's MX114 today and did a few quick checks to provide a basic assessment of the preamp section's operation. The first observation was apparent right off the bat: It was dead as a door nail.

    Oops, wait a minute. Now it's on. But now it's off. The internal connection of the original molded AC plug had clearly given up the ghost. A quick snip and a temporary plug attached that's way more vintage than even many vacuum tube pieces, and the unit now had proper motivation. On to some quick scope shots which tell the tale quickly enough:

    Below: A temporary replacement power plug resolved power issue.
    SAM_2279.JPG

    Below: With the Tone and Balance controls centered, Loudness and Filter switches turned off, and the volume control at max, a 250 mV 1 kHz sine wave signal into the Aux inputs produced a very clean, well balanced 2.50 Vac output from both channels. This pic actually shows the output of both channels superimposed on top of each other with the scope in dual trace mode. Hard to argue with this! All the following shots are in superimposed dual trace mode as well.
    SAM_2269.JPG

    Below: Since the proper display of a square wave requires a flat circuit response from 1/10th the fundamental frequency to 10X the fundamental, a 2 kHz square wave sent through the unit as set above will quickly check for response issues from 200 Hz to 20 kHz. With 1 kHz referencing 0 db, actual response at 20 kHz is R= -0.35 db, and L= -0.50 db, with both channels within specification. The slight rounding at the top of the leading edge indicates amplitude loss between the waveform presented, and that of an ideal square wave. The balance between the channels at 20 kHz is actually quite good.
    SAM_2274.JPG

    Below: Carrying on from above, a 200Hz square wave will quickly check for any response from 20 Hz to 2 kHz. Here, some tilting of the wave is to be expected, since the signal is experiencing phase delay from the RC circuits of the tone controls. The important point is that the wave top is still reasonably flat, indicating a uniform amplitude within the band. Again, with 1 kHz referencing 0 db, actual response at 20 Hz is R = -0.40 db, and L= -1.10 db. While the left channel is out of specification, even the most trained Golden Ear would have trouble detecting this discrepancy. Again, as high quality equipment of this vintage goes, this performance is still very good.
    SAM_2275.JPG

    Below: But now, we come to the crux of the matter. Here, with a 1 kHz sine wave presented to the Aux inputs, the scope sensitivity is increased to 50 mV/div, and the MX114's volume control is set at 11:00. The Left Channel output of the MX114 in this shot (0.15 vac) would be driving an 8Ω speaker connected to the output of a 50 watt amplifier with an input sensitivity of 1.0 vac to a power level of just over 1 watt, or to about 200 mW for a 50 watt amplifier with an input sensitivity of 2.50 vac. The former could be too loud in an apartment setting with sensitive vintage Klipsch speakers, while the latter still might produce banging on the walls. This level of imbalance would be very apparent, and destroy the stereo image and sound stage!
    SAM_2270.JPG

    Below: Apparently, somebody has been here at the volume control before. Further proof is the pink sticky note seen in the first pic found inside the unit that clearly notes how to reconnect the wires to the volume control after it was removed at some point. The control currently installed is beyond bad. It is specified as being an 800K unit, with the Left Channel section measuring ~ 950K, and the Right Channel section measuring 1.6 megohm. The loudness tap readings were equally bad.
    SAM_2277.JPG

    A call to Brian at McIntosh parts found that a new volume control complete with AC switch is still available as new part number 134-369 for the MX114 for $100. Hopefully, it will be here the first of next week. Once everything is first repaired on the unit (or determined to be corrected by way of a typical restoration), then a clear plan for actual restoration can be developed with Al for the unit going forward.

    Dave
     
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  4. c_dk

    c_dk Addicted Member

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    Dave,

    What does the noise spectra of the MX114 look like via aux and phono?

    Is the volume control imbalance taper similar throughout the tracking rotation?

    Other than the control mistracking any preliminary thoughts, any suspects so to speak, relating to the owners dissatisfaction of the sound?
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    C -- I was only conducting tests through the high level section with my scope during these quick checks, as that is the type of sources that the OP uses in his system. Once I get into the unit in earnest, I'll be happy to put it on my Spectrum Analyzer (HP 3561A) to get that info.

    The volume control's imbalance is quite poor throughout the portion of rotation that represents where the loudness circuit is operational. Above that point (by comparison) it actually looks half way respectable up to a maximum setting -- as long as the loudness circuit is in fact connected to the loudness tap. The 62K resistors used there basically fix the impedance to ground at an identical level at that point, so that with the well balanced signals applied to the top of the volume control, it allows the upper portion of the volume control to track well enough even though that section of the two controls are well mismatched in resistance. Below the loudness tap however, and because of the fixed impedance provided by the 62K resistors, the mismatched resistance of the upper section of the controls presents a great differential in signal level that appears at the point of the loudness tap, so that tracking below the loudness tap is poor. Interestingly however, the mis-tracking differential below the loudness tap is largely proportionally constant, meaning that for this particular control, an external fixed resistor can be connected to largely compensate for the poor tracking behavior below the loudness tap, while providing minimal mis-tracking of its own above it. A proper repair of the volume control issue however can only truly be effected by replacing the control.......

    Al's comments to me on the unit were that when he first purchased the unit, he was totally blown away with its performance on many levels -- certainly SQ, but also FM performance, smoothness of the controls when operating them, appearance, etc. Therefore, there was clearly an enamored element at play, but by all appearances, there was also a real element that the unit was all he expected it to be, and more. It was also his first Mac piece. His positive impression of the piece remained to the day it was sent off for service. Everything seemed to go down hill when it came back with the broken glass. No doubt, the length of time in sending it back and having that repaired and the snafu in getting the glass ordered plays a part here in the form of an emotional let down. This was amplified by the fact that when he got it back from the glass repair, the unit did not power up -- but did after he reset the fuse. Thinking that resolved the problem, that issue (for all intents and purposes) when away until it wouldn't power up for me either, leading to discovery of the defective plug. Of course, during the extended wait to get his unit back from the glass repair, he had purchased and extensively used the Apt Holman preamp until the Mac found it's way back to him. Therefore, he had an immediate reference to judge the Mac against, and all of the sudden, the channel imbalance became an issue -- but to Al's credit, it is entirely possible that the channel imbalance in fact did not first arise until he received the unit back with new glass. Al is a very detailed person about all of the pieces he owns, so I can only assume he likely would have in fact noticed such an imbalance if it were present when he first purchased the unit. For my part, I find it interesting regarding the sticky left in the unit detailing the volume control's connections, and the clear appearance that the connections to it as now installed have been disturbed. As to who, or at what point in time that event happened, who knows.......

    Once the new control comes in and is installed, then an honest assessment of SQ can be made, and other elements investigated. For now however, I've got to get rid of the big elephant in the middle of the room with a new volume control. Al operates his equipment at flat settings and generally low volume levels. While the MX-114 should shine in that environment through his refurbished MC-250 and highly efficient Klipsch speakers, the extent of the volume control mis-tracking displayed would certainly accentuate poor SQ displayed under those conditions as well.

    Dave
     
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  6. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Thanks Dave for the initial findings, for the life of me I can't believe they sent this back to me operating as it did, they assured me everything was up to "spec" . Will watch and see how things progress. Al
     
  7. c_dk

    c_dk Addicted Member

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    The C26 preamp design of Roger Russell provided the basis for this preamp tuner as well as 4 additional units if my memory is correct. The made in Canada replacement potetiometer is a generic replacement that covers I believe 7 units,interestingly not the C26, and the loudness tap is at 50% not the ratio of the MX114 which is 60/40.

    Everyone of these original pots that I have taken apart after replacement have shown to be a linear taper, except for the C26, when the carbon was not missing. Some cleaner's solvent in days gone by wrecked havoc on these controls..it must have been a popular one with Mac techs or a faulty adhesive for the carbon track used by CTS.
     
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  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    C -- Thanks for the C26 reference. I am almost always on the vacuum tube side of things, but the occasional deviation into SS territory is always a nice refresher. It's interesting that the MX114 looks so much like a SS version of a vacuum tube design -- as in SS designed by vacuum tube people. Once I get the old pot out, I'll see what I can glean from it.

    Dave
     
  9. c_dk

    c_dk Addicted Member

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    The McIntosh engineers back in 1965-67 gave up their precious vacuum tubes kicking and screaming. After the last amp clinic I hosted with DOB, while we where having dinner I was being ribbed about my using 6L6CG groove tubes in one of my three MC240 amps.

    Dave was relating how the culling process was getting too expensive to continue as the quality coming out the RCA plant was getting so poor. When 50% were not meeting Mac's expectations the writing was on the wall.

    I got his last 8 RCA/GE outputs from his personal stash of tubes....they were culled out of over 80 tubes.

    He was impressed by the performance of the rebranded Russian via Canada tubes but did imply I should hide them before Gordon Gow came to visit in a few months.......that's what led to his setting me up with his tubes.
     
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  10. Pio1980

    Pio1980 AK Member Subscriber

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    RCA still making them in Harrison factory then?
     
  11. 62caddy

    62caddy Trust but verify Subscriber

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    The plant closed in 1976.
     
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  12. Pio1980

    Pio1980 AK Member Subscriber

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  13. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Just a quick update on my 114. Dave G has the unit and ordered a replacement volume control for it since the on in it now is shot. He mentioned to me that he thought the new switch was not up to par so to speak, he hooked it to a scope and the signal in it did not track evenly at all if I remember correctly.. I called Gail at Mac parts and they are aware of problems with the new replacement and are sending another at no charge. They didn't even want the other one they sent back. Anyone here had the same problem with these newer pots? Thanks Al
     
    Last edited: Nov 12, 2017
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    To punctuate Al's comments, the control that McIntosh sent:

    1. Is certainly a quality looking piece manufactured by PEC Canada, and includes a power switch manufactured by CTS.

    2. Interestingly has part number "134-364 750K" stamped into it, with the last digit different than part number 134-369 that Mac specifies as the replacement part. The original pot is specified as an 800K unit, while this control measures 758K in one section, and 807K in the other. The loudness tap is placed at 380K and 397K respectively.

    3. While the resistive tracks are apparently graduated quite closely, the mechanical coupling between each section is quite poor, so that small movement of the shaft first moves one section, and then the other. Turning the shaft in the opposite direction, then the opposite effect happens. With a 10 vac 1 kHz signal applied to both sections of the pot, at typical low level positions, the wiper of one section was delivering an output of under 100 mV, while the other was topping a 200 mV output. At elevated operating positions, the difference is lost in the amount of signal supplied, but at low (more typical) listening levels, the difference is very significant. Loose coupling indeed.........

    Dave
     
  15. c_dk

    c_dk Addicted Member

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    So you seem to be describing mechanical slop between the main shaft and the secondary shaft that interconnects the two sections?

    It has been a few years since I installed one of these PEC sourced controls......but I seem to remember that when I disassembled one from a 1900, 6100 and MX115 I measured the raw wafer's carbon they actually measured almost 1 meg when probing the good carbon track and the resistance was linear not logrithimic.

    I have had good luck with preloading the control to move the center tap resistance to more closely to the ratio as shown in the schematic and what I measured via the original track.
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    C -- That's exactly what I am describing. The term "slop" is the term I use as well, but seems.... I don't know.... rather unpolished relative to the quality that Mac equipment represents, so I was just trying to give a more professional spin on the description. :) But slop it is!

    A linear track will never do for a volume control -- I'm not telling you anything you don't already know. But with the discrepancy in the part number that Mac specified versus that on the part I received, it surely makes me wonder if the wrong control was installed in the unit whose control you examined.

    I've included a pic to show the control I initially received from Mac. Note the epoxy used to seal the two sections together to lock in the final tracking relationship established between the two sections after assembly. Were it not for the slop, the control would be excellent. The tracking mis-match between the sections is constant across the entirety of rotation in each direction of travel (except of course at the immediate point of the loudness tap), indicating that the problem is not the resistive elements, nor the mounting relationship between the two resistive elements, but due to the mechanical coupling between the wiper elements of the two sections since the slop produces complementary mis-tracking between the two directions of rotation. The fact that the two sections measure different resistively is not particularly an issue in this case, since the relationship of the loudness tap resistance to the total resistance of each section is relatively the same between the two sections, or nearly so.

    Dave
    SAM_2286.JPG
     
  17. Pio1980

    Pio1980 AK Member Subscriber

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    Looks something like stacked AB 'J' pots.
    Imo, Mac should have used 'J' s from the beginning on everything, until switch point steppers.
     
  18. c_dk

    c_dk Addicted Member

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    I believe the loudness tap to ground via a resistor reshapes the voltage divider....

    By adding a cap when the loudness is engaged the bass is affected and opens the door for further eq options when this setup is used in the C28 preamp.
     
  19. mrz80

    mrz80 I guess it's in my blood

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    Do you recall if the 1700's preamp section is derived from the C26 as well, or was it a completely different beast? Board and circuit topology are similar to the C-26 but nearly as similar as, say, a 112 or 113. I'm asking because the one place I'd really noticed audible degradation in my 1700 before the power supply caps all went sideways was in the phono stage.
     
  20. c_dk

    c_dk Addicted Member

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    As for the volume controls I measured, they were originals and I do believe I might even have pictures of the wafers.

    I experimented with Rod Elliott's "better volume control" idea as a experiment using wafers from a Alpha control that did not have a loudness tap.

    Our local parts supplier had some left over parts from back in the day when they sold a " build your own" replacement. I put a replacement together, installed it and have not yet made the time to test it.

    On trasistor preamp sections, I believe the MAC1500 receiver had the first, at least transistor phono preamp. The MA230s all transistor preamp section followed that.

    The subsequent C24 was the basis of the MA5100 and MAC1700.
     
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