MX-113: (rambling) first impressions, and advice sought...

polaris26

New Member
So I am the proud owner of a McIntosh MX-113 tuner-preamp, which I had hoped to pair with my MC-2505 amp. I thought they would look nice and play well together. Before the MX-113, I was using an Adcom GFP-565 preamp for CD and Phono, and feeding in the tuner section from an old Pioneer receiver for FM listening. I was hoping to replace the Adcom and Pioneer setup with the MX-113. Not expecting the same sound, but something synergistic at least.

Although the MX-113 looks really nice for its age and has most of it's functionality, I think there is something lacking to the sound. One thing I noticed immediately was the background noise level, even on line level input (white/pink noise, not so much hum). Not super-loud, but noticeable. I am thinking the old transistors might be contributing to this type of noise.

The other thing I noticed was the cross-talk of the CD that was still playing in the background when I switched over to the Phono 1 input. I imagine the selector switch is not providing a solid ground to the de-selected inputs so maybe some cleaning/Deoxit would help there.

The only function that seems not to work at all is when I switch the selectivity to narrow - the reception disappears altogether. I haven't explored this issue any further yet as I would probably listen in 'wide' even if the narrow setting worked, due to my lack of competing stations here in a rural area, and the typically better sound quality that wide usually offers. Eventually I would want to fix this issue anyway.

I downloaded a copy of the MX-113 service manual and took a look at the preamp section schematic. I have read that the MX-113 sends the FM Multiplex output into the Phono section of the preamp board. This is correct as far as I can see. The trick here is that when you select the FM input, one of the wafer switch sections is used to bypass the frequency-selective parts of the feedback loop around the phono stage with a resistor that lowers the overall gain and flattens out the response. I assume this is done because the output from the Multiplex section is too low to compete with line level sources and needs some flat boost.

I modeled the phono section with LT Spice and the response of the Phono preamp in the "flat mode" is flat within a dB from 20Hz to 20kHz and gain is about 20dB midband. Using a Laplace function to create a reverse-RIAA voltage controlled-voltage source as a simulated input, the response of the Phono section in Phono mode is correct within a few tenths of a dB across most of the audio band.

Interestingly, there is a serious out-of-band resonance peak around 500kHz that is due to the cumulative phase shift reaching 180 degrees inside the loop. It should be noted that I modeled the circuit with the added Service Bulletin components in place (the one that was designed to suppress RFI).

Back to reality - I measured the actual voltages of the power supply on my MX-113, and they were pretty much right on spec, and there is very little ripple on the 75V supply (the 10V and 9.5V are filtered derivatives thereof) - which would explain the lack of hum.

So - my main sonic complaints with this little gem seem to be some excess noise and and overall lackluster sound quality. Maybe part of what I am hearing is age, and part is inherent to the design.

I am guessing that some modern parts could help in the first case. I would be interested to hear what people more experienced in these matters would have to contribute to this pursuit.

Parts I am thinking of replacing:

All of the transistors in the preamp boards -

132-095 = KSC1845
132-096 = KSA992
132-092 = BC549
132-042 = 2N4401

These are subs I have read about on the forum. I would particularly like to hear feedback about these subs; are there commonly-available subs other than the ones listed that you prefer in this case, and why?

There are some electrolytics in the audio path used in emitter bypass and even feedback coupling functions. I would be interested to hear ideas about modern replacements for these (I am thinking something like the Elna SILMIC series?)

I am assuming for now the box film caps should probably be left alone - maybe not?

What about the encapsulated component clusters in the bass/treble section - should they be adapted over to discrete? The pinout should allow you to test the values of the internal components individually to check for out of spec/drifted values.

Perhaps I need to check all the values (R's, C's) for drift - I do have a reverse RIAA circuit somewhere I can use to plot the Phono section flatness - just haven't gotten around to it yet. I confess, all of my observations thus far have been admittedly quite subjective.

What other ideas does anyone have?

My appologies for the rambling post...

Regards,
Dave
 
There's a lot of tired electrolytic caps in the preamplifier stages of the MX 113, MX 114, and the MX 115. Replace those, the background noise will be extremely lower and the unit will sound like it should sound. This was an issue in my own MX 114.
 
In my experiments, swapping the internal cabling with individually shielded cabling made the major S/N improvement.....i measured a 10 dB increase.

Also as the driven shield supply gets noisy it modulates with the signal.
 
Time to get the tantalum's and electrolytic caps replaced, the box caps are normally OK, the preamp boards are a bit awkward to get at but the results are well worth the effort.
 
Thanks everyone for chiming in on this thread! This morning I began to create a substitution parts list for a first round of replacements.

I will begin with most obvious parts in the so-called 'phono' stage - three transistors and three polarized caps per board (leaving the box caps and the later stages alone for the time being).

I realized upon taking a closer look that some of those polarized caps I had seen on the schematic are in fact tantalum types (as Terry just mentioned). I am under the impression that current thinking says that tantalums generally don't belong in audio coupling/bypass roles. If I were going to sub for them, would using SILMIC or similar "audio" electrolytics in their place be a good idea? I don't think there is room in there for large film types, as the capacitance values are quite high.

As for the transistors, I was going to use recent-manufacture A992 and C1845 types, unless someone had a better suggestion (Mouser numbers are: 512-KSA992FBU and 512-KSC1845FTA, respectively). I realize the pin configurations may require a little twisting.

From what I can see on my two boards, some caps and transistors may have already been replaced over the years, as the two sets of parts do not match in appearance, so I don't feel so bad 'subbing the subs', as it were. I will save the old parts and log where they were pulled from, just in case.

c_dk - your idea about cable swapping seems interesting - I will put that on my list of ideas to try.

KentTeffeteller - you mentioned experience in changing caps - what did you use in place of the tantalums?

Regards,
Dave
 
The non shielded cable bunch is where your crosstalk is coming from.

All these vintage units are decades overdue for a recap. I assumed a recap was your first priority.
 
HI all - reviving old thread. I acquired an MX-113 some time ago. It's mainly used in my workshop with a 2100. It's been run hard and put away wet before I got it. Since I have access to @Gregory 's expertise and that its very scratchy volume pot (even after my cleaning) and the FM signal meter is stuck, its now on his bench.

We have access to the schematics and service manual. Besides what he's pointed out in these pix ( i don't know if we have the where with all to replace cables...maybe) what other updates should be made?
Thanks as always - NP_needs_replacing.jpgshadow_old_cap_2.jpg
 
Remember you have to match levels with the MX series like Mac pre-amps they are designed to be used with amps with 2.5 to 4.0 volt sensitivity, having an amp with much higher sensitivity will introduce almost 12 db more noise in some cases. But others are probably right that the unit needs to be restored after this many years. Personally, I didn't like the tuner in a MX 113. I thought a MR-77 was a much better sounding unit. Folks in the know say the 74 was the best tuner at that time period, I'll stick with the 77 and the much more advanced 78. I live in a very congested area with many FM stations stacked on top of each other and the 78 and 80 can root them out much better than the MX series or the older MR tuners. Now I liked the MX 117, but that was more because of the quality of the pre-amp and the graphic tone controls.
 
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Remember you have to match levels with the MX series like Mac pre-amps they are designed to be used with amps with 2.5 to 4.0 volt sensitivity, having an amp with much higher sensitivity will introduce almost 12 db more noise in some cases. But others are probably right that the unit needs to be restored after this many years. Personally, I didn't like the tuner in a MX 113. I thought a MR-77 was a much better sounding unit. Folks in the know say the 74 was the best tuner at that time period, I'll stick with the 77 and the much more advanced 78. I live in a very congested area with many FM stations stacked on top of each other and the 78 and 80 can root them out much better than the MX series or the older MR tuners. Now I liked the MX 117, but that was more because of the quality of the pre-amp and the graphic tone controls.
When I read about all these tuners in one paragraph, I think of a BINGO game with the numbers tumbling around and one ball randomly popping out. Other people have mentioned in the past that even though the MX113 is supposedly the same as the 74, the signal has to pass through the preamp section with some detrimental effect. It's interesting looking at an old white Mac sales booklet with the 77 being introduced on two pages in a blaze of glory while the 74 plays second fiddle on the next page with a muted description.
 
Remember you have to match levels with the MX series like Mac pre-amps they are designed to be used with amps with 2.5 to 4.0 volt sensitivity, having an amp with much higher sensitivity will introduce almost 12 db more noise in some cases. But others are probably right that the unit needs to be restored after this many years. Personally, I didn't like the tuner in a MX 113. I thought a MR-77 was a much better sounding unit. Folks in the know say the 74 was the best tuner at that time period, I'll stick with the 77 and the much more advanced 78. I live in a very congested area with many FM stations stacked on top of each other and the 78 and 80 can root them out much better than the MX series or the older MR tuners. Now I liked the MX 117, but that was more because of the quality of the pre-amp and the graphic tone controls.
hi Twiii - I agree about the MR77. I have one as well and its in my primary system along with MC2255!

I plan on listening to the MX113 through my "refreshed/refurbed" MC2100. I like using the tuner during my office hours but also use the Aux input for streaming.

Greg replaced for me the cap above and a similar one the AM board. We measured voltages and most were close i.e. 23.7v vs. 24v. Next steps are to work through updates on tantalum caps and certain electrolytics.

There is a problem that the Stereo indicator stays lit while moving through the dial. Also, the signal meter seems to be stuck. So, any suggestions on further refresh and addressing the indicator would be appreciated.
Thanks
 
Also, the signal meter seems to be stuck.
The signal strength meter on my MR77 had a strange color when I bought it in early 2014. For some reason the back of the meter where the printed numbers are was discolored. Heat? I called Mac Parts and found out the only meters still available THEN were for the MR78. They were very cheap so I bought one of each type. The meter consists of three sections sandwiched together by two tiny screws; the front window that goes through the rectangular hole in the steel, the meter movement and the back plastic with the printing. The movements are the same but the window size is different and there is more writing on the 78 version. I put the old window on the new meter. I works perfectly and looks fine but the words " signal strength" and "multipath" are also there in addition to the numbers. It took a lot of work replacing the meter. The interior glass had to be removed and the steel plate behind the glass had to be hinged forward. The moving dial pointer had to be carefully rotated upward and flipped over the plate. There is a resistor going across the to meter terminals. Assuming the MR78 meter is still available, you can use the first and last sections of your old meter and put the new movement in between.
 
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