MX114 problem

calman46

calman46

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I'm taking another try at this MX114. Two issues . The left side plays at lower volume and has a less bass than the right. The right side plays louder with noticeable more bass, but. The other issue is with the volume full CCW you still hear the sound in the right along with a hum (sounds ground related )The left is silent .So it appears both sides have there own own problem .The MX114 has always had these problems. I did ,clean controls , recap (film & electrolytic), replaced volume control (from McIntosh) ,replace main output jacks . Could it be those modules for the bass & treble circuit ? If I touch them I hear it in the speaker.I'm also looking at the push buttons again. Any suggestion will be greatly appreciated . You can tell it can sound great. Thank you
 
It could be the modules but I would check out a few things first.

Check the value of the resistors from the loudness tap to ground and that the ground is bonded to true ground.

The value of these resistors establishes the taper of the control.

Did you pre or post load the volume control......the generic replacement has it's loudness tap at 50%, the originals were at 40/60 or 60/40. Again this affects the taper and the loudness compensation amount.

To really check out the modules you will need to establish very specific output levels at various frequencies......before and after the module.

What kind of test equipment do you have available?
 
I didn't lift one side of the 62k loudness tap resistors ( I probably did at some point in this quest) I get 65k on both in circuit . I have ground to chassis . Is it normal for those modules to be touch sensitive ?
Test equipment I have ,Sencore SC3100 (100Mhz), a 20Mhz o-scope , dual AC millivolt meter, Function Generator/Counter.
I'm not sure I understand "pre or post" load volume pot? before or after what? What did was take out the original and install the replacement the same way. McIntosh service department was very good but I got the impression didn't know this vintage unit.
I did input a 1Kz signal and checked the output on the preamp board #22/#18 the imbalance is there . I was looking into measuring the signal going into the preamp board . The problems are on all inputs and FM so started with line level inputs.
 
I happen to have a MX115 schematic at hand and it shows that the loudness tap is not centered but offset so there is greater resistance towards the input (pre) than toward the ground (post). I have had to preload (add resistance) to these generic volume controls in the past.

You really need to do a spectrum frequency sweep to confirm at what frequencies there are actual discrepancies between channels.

I have gathered the parts to build a discrete module replacement but have not had the time to build them. I did have to build one 25 years ago. The original was crackling in a MA230. They were unobtanium even then.

I would expect your finger's aurora to affect the non shielded module.
 
I did some solder reflow on some of the wires( I/O) on the preamp board. Now the tape and aux levels (1Khz test signal) are even R&L but I still have that hum on the right ,with volume full CCW. The phono is way off the right is a lot higher than the left. I'm thinking now solder joints as the culprit and the right side of the volume pot .I get that hum and sound at Full CCW where the left is silent .
 
The power supply is high . The 75VDC power for the preamp boards is 90VDC .I knock down the mains to 117VAC but power to preamp board is still 85VDC. I passed a1Khz test signal(at the rated voltage) through and look at the output on the Sencore . The tape/Aux waveform look good the right was slightly higher in voltage . The phono waveform looked good also but the right was 0.25volts than the right. So I'm off to check the power supply voltages.
 
I went thru 3 volume controls from Mcintosh on my 114 before we got one that worked close to correct tracking wise. Mcintosh gets these controls from Canada and knows they aren't up to snuff. MX114's are nice once you get them working right. Good luck with yours Cat!
 
MC115 003.JPG MC115 003.JPG MC115 003.JPG MC115 003.JPG Rebuild that PS board, most if not all the problems I see in that model are in the power supply.
 

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The first volume control McIntosh sent me was shorting to the AC switch. The next one the shaft was too sort ,but I fix it used it . The PS is recapped but I need to take another look at it , I think 90VDC is too high . The hum I get is ground loop I think . If I disconnect the left MX114 output the hum in right side stops .
 
You can't just "recap" that supply board. Your just setting yourself up for future failures. Replace the diodes and get rid of the carbon comp resistors. Use a good film caps for bypass and coupling. The TO-66 is still available from mouser, they have a limited supply and there not inexpensive. You can add a heat sink to the TO-220, the MJE150XX is a perfect replacement for the original.
 
Engineering actually came up with a TO220 mod for this supply configuration back in the 80s.... I do not remember which unit though. Mc113 or 115 most likely.

You are disconnecting the LH main out to remove the RH noise?

We waited over a year for the Alps replacement volume control bck in the 80s...alps and Mac must not have been able to reach a agreement for a continued supply.

The current replacement is generic and must be modified for each of the 6 units that can use it......it is not plug and play.
 
McIntosh told me that they had to build the pot . If that included the modification they didn't say . I now I have three pots , the original ,the first one they sent, which I think I'll take another look at. Its all sealed you can't take it apart unlike the third pot which seems to be of cheaper build.
But first the power supply . I may have done the diodes I'll have to check .
When I disconnected the L main out the noise in the R stopped. I see that the PN# 132516(Q202) is wrong and 132028 can be used .
 
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Are there any errors on the MX114 power supply schematic ? Can someone verify if the values on C210c & C210d are correct on the schematic .
 
You can replace that multi cap on the bottom some Mundorf's, the pay off is high compared to general purpose caps. All listed values in the manual are correct, mx115 003.JPG mx115 003.JPG
 
So I have it wrong , I have the orange wire on 47uf/ 200v cap and the violet on the 200uf/ 160v cap .I think must have mixed up the cap voltages with the microfarads. My MX114 wiring color matches the schematic so the schematic is correct ,thanks . Maybe I didn't have enough filtering where its need .
 
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I have IC MPW caps in the two preamp boards. There in there but its crowded, I was thinking about replacing them with Wima caps. I checked a few leads spacing and they would drop in. Wima MKP 10 is the series I have checked so far. Does one Wima series sound better MKP 2, MKP 4 ,MKP 10
 
I found no real change using any of the WIMA MKP/MKS in that instance. The bottom line is the LS, dose it fit. Other than that if you want to tweak a bit more use a full wound PP like the SoniCap. Then you have to be creative on how to fit them into the build without making it look like Frankenstein.
 
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