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McIntosh - Hints and Kinks

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Maybe this is the place to post this:


I came across 2 C8's. One in excellent shape one needs a little a TLC (cosmetically). Both are electrically fine and restored. Seems a little moisture made its way into one if them and has caused the volume, treble, and bass pots to turn like they are in hardening concrete....the other one in melted butter.

I hit it with some DeOxit D100 (the non solvent variety). Didnt seem to do much. Any ideas on how to re-lube these? Is there a way to open the potentiometers's up and clean the gunk or corrosion out without having to damage them? Should they just be replaced? Or just leave as is. I mean they do work...just not smooth.

Now on the stepped potentiometers, it looks like there is a ball bearing that pops between metal notches for all the selections. Pretty basic, pretty cool. Since there is no electrical connection on that part and it is the source of the friction, can I just hit it with a light coating of bearing grease (I have some high end stuff for bicycles) or should another lube be used.

Any suggestions appreciated....
 
Binding is likely on the shaft bushing at the front of the pot. Tip up the front of the chassis so the shaft lets a very small of amount of oil run down into the bushing. The less you can use, the better.
 
I ordered one of those small tubes of FaderLube so will give that a try. Any thoughts on the ball bearings on the stepped pots?
 
MC 2105 Meter Calibration Control Adjust.

Hello_
During the cleaning of the amp, I accidently touched/turned the 1K pot/s for meter outputs (R213 and/or R214), I cannot find any calibration procedure on this. What should the resistance values be set at? Thanks in advance for an help and guidance on this.

Paul
 

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Meters are calibrated using a signal generator and a dummy load, I use a Sound Technology 1701A, an 8 ohm load, and monitor the output with an oscilloscope. The meter pot is set for +3dB indicated on the meter at an output of 105 W RMS (25V RMS into 8 Ohm load).
 
dewickt said:
Meters are calibrated using a signal generator and a dummy load, I use a Sound Technology 1701A, an 8 ohm load, and monitor the output with an oscilloscope. The meter pot is set for +3dB indicated on the meter at an output of 105 W RMS (25V RMS into 8 Ohm load).
Click to expand...

Thanks for your reply Dewickt. Since I have none of the mentioned special cal tools, can you provide a rough resistance range where I can manually set them to? Will 1-5ohms work OK? I don't listen to my music loud at all.

Paul
 
No known way to preset them as the setting varies from amp to amp. The quick way to at least get the 2 meters to read the same is use a Y cable to feed the identical signal to both channels then adjust so the meters match.
 
dewickt said:
No known way to preset them as the setting varies from amp to amp. The quick way to at least get the 2 meters to read the same is use a Y cable to feed the identical signal to both channels then adjust so the meters match.
Click to expand...

Thanks muchh for your suggestion:thmbsp:. I'll give that a try.

Paul
 
Adding a headphone to Mcintosh MX-113 setup

Hi Folks, I Have an MX-113, Mc-2100 Just service by Terry :music:, Mq-101 on it's way to Terry:thmbsp: And The ML-1c's... The only thing I was missing was a Headphone section. I run everything through my Mcintosh including TV, Ipod, ETC... and late at night when others are resting headphones would really help for TV watching, etc. I happened to have a little Rolls PM-50s Laying around and went Tape out- to 1/4" stereo Monitor in. The rolls has a separate volume control on it and it does the job just fine. I now have a headphone amp and can watch and listen to tv etc. late at night without disturbing the peace!
 
Dilbert said:
I popped it open, disconnected all the cables, and no change. It is still stuck. Would verifying the front panel switch commands are actually get send to the turner be a logical next step in the trouble shooting? Looking at the inside, it is troublesome to get at any board to do some work. Thanks
Click to expand...

Using a scope, I traced the front panel keyboard commands all the way to the input of the 80C51, and there's no output signal from any of the output pins. I then applied a 5V to the Toshiba turner chip's FM and AM frequency turning up and down pins, and other control pins to confirm that the turning chip works fine. I then pop the 80C51 into a EPROM reader, some how the chip is full of all 1s.

Ordered a 80C51 from McIntosh, and all are working well now.
 
Need help with intermittent signal spike.

The spike I refer to is a dramatic increase in volume in the right channel (both amps). There is no deterioration of sound quality - only increased volume. As a test, I pulled the C29 out slightly via the pan locks. I remembered that just behind the front panel on the top of the unit are adjustment (I suppose they could be called pots) for the left and right signal output to the amps.

When the right channel is in the 'spiked' condition, I have to reduce the right signal pot to zero (all the way counter-clockwise) before the sound level matches the left channel. Therefore, the left signal pot is all the way up, and the right signal pot is all the way down.

At some point, the right channel will drop back down to where it was before the spike, and I will have to turn the right signal pot back up all the way (the way it comes from the factory). All of this sounds like something is causing the right channel to be boosted markedly for some reason and it is intermittent. Is there anything that would cause a signal 'boost' to the main outputs?

I have reversed the left/right 'main' output cables to the amps and the problem moves. If I swap the cables the problem does not move. I have noticed that I can 'force' the problem to occur by playing with the treble control. No other controls seem to have any impact. All of my untrained testing has indicated that the problem is with the C29.

Any suggestions would be greatly appreciated.
 
dewickt said:
Meters are calibrated using a signal generator and a dummy load, I use a Sound Technology 1701A, an 8 ohm load, and monitor the output with an oscilloscope. The meter pot is set for +3dB indicated on the meter at an output of 105 W RMS (25V RMS into 8 Ohm load).
Click to expand...

Hi Terry,

I too am trying to calibrate the meters on my MC2300. What I did was feed a 1V RMS (.707VAC) signal into the amp, turn it all the way up and then adjust the meters to +3dB. I don't have a dB meter but I do have a signal generator and an ocilliscope. Can you please provide more specifics on the proper way to calibrate the meters?

Thanks,
 
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Input level is not important because you have the gain controls on the front panel, I put in about 2V RMS and adjust the gain control to get an output of 300W then adjust the meter calibration pot for +3 on the corresponding meter.
300W = 49 VRMS across 8 ohms or about 139 V peak to peak
Power = RMS squared / R
(VPP / 2) / 1.41 = RMS

It is better to measure the RMS with an AC voltmeter that yields a more precise reading than a scope can give on a peak to peak reading that can not be easily read to the nearest volt.

Attached is a quick table of peak to peak voltage across an 8 ohm load yielding Watts RMS.
 

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MAC 1500 MPX Stereo Drop Out

Way back in July I started a thread asking how to correct MPX stereo fade out on a MAC 1500.

MAC 1500 drops out of FM stereo

That thread should have properly been posted here so I'll continue that thread here. Some of the answers in that thread were about possible bad tubes or diodes in the MPX circuit.

Turns out to be just a simple adjustment. In fact, I had sent email about this to Terry DeWick and he had responded back in Dec of 2007: ". . . try adjusting the MUX lamp adjust pot that is next to the mux tubes." I had forgotten this advice (my memory is good for about a week or so) or actually remembered it as advice for just the lamp and not the actual stereo sound.

I found the email. I also found a MAC1500 block diagram in a pdf of the owner's manual that shows that the Lamp control circuit also controls the oscillator on-off control. So the Mux lamp adjust does more than adjust the lamp. The demux part of the block diagram is shown here with the Lamp Control at the far right.

attachment.php


So setting this adjustment a little higher fixed the problem.

I guess if you set the adjustment too high, the light might stay on even for a mono broadcast. Since I can't find a station that is broadcasting in mono, I don't know just how high to go with this adjustment, so I pushed up until it started working and not much more.
 

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I go to the aux input then adjust the threshold till the lamp just goes out, if all is in alignment this should work.
 
dewickt said:
Input level is not important because you have the gain controls on the front panel, I put in about 2V RMS and adjust the gain control to get an output of 300W then adjust the meter calibration pot for +3 on the corresponding meter.
300W = 49 VRMS across 8 ohms or about 139 V peak to peak
Power = RMS squared / R
(VPP / 2) / 1.41 = RMS

It is better to measure the RMS with an AC voltmeter that yields a more precise reading than a scope can give on a peak to peak reading that can not be easily read to the nearest volt.

Attached is a quick table of peak to peak voltage across an 8 ohm load yielding Watts RMS.
Click to expand...

Hope this is not out of line. You can use an Excel spreadsheet to enter peak or RMS voltage, calculate and plot measured power.. in this case for output power from 20 Hz to 20 kHz. It will do all the math and is very customizable.

Power_Output_Sheet.xls

The errors were from line sag and the DVM not being true RMS. You can do this for any and all frequencies, just edit the spreadsheet. This can be an excellent tool with professional test gear!!


-Gregory
 
I made the listing on a Commodore 64, written in basic long before Excel existed, if I was to do it today I would use a spread sheet, but then why do it again if I have the old data...just showing my age.
 
As yes, that warms the cockles of my heart ;) The Commodore 64 was quite the beast. Keyboard computer, TV monitor and cassette tape drive for storage. Those were the days. Drive by the old Commodore plant just over from the Commodore Berry Bridge quite frequently (wonder where the computer name came from :rolleyes:).
 
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