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McIntosh - Hints and Kinks

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Hi Terry.

I am working on Mcintosh MC75 and need part number for main circuit board or board itself.
Any suggetions.

Thanks.
 
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Agreed Murray. will answer anyway, have never seen the terminal board available from the factory, possibly a custom one has been made, just have to hunt...
 
Weird MC30 idle durrent

I am working on a very old mc30.

The 2nd 12ax7 driving the 6L6 tubes just has 220/1-watt resistors going to ground, no caps from the cathode coil back to the 12ax7 cathodes.

I measure 16.5 ohms DC on one side, 6L6 cath to ground, and 20.5 on the other side to ground.

If I have .75V from 6L6 kath to ground, divide by 16.5 = 45.45 ma times 420 volts plate = 19 watts.

The original 1614 tube is 21 watts, so 19 should be cool?

I can't find any, what the volts is for the out tubes on this monster anywhere

I was half worried that the Mac circuit has the out tubes off, and turns them on like a B-class circuit??

Any help most gratis

GridLeak.blogspot.com

gEorge
 
ElectricMan said:
I am working on a very old mc30.

The 2nd 12ax7 driving the 6L6 tubes just has 220/1-watt resistors going to ground, no caps from the cathode coil back to the 12ax7 cathodes.

I measure 16.5 ohms DC on one side, 6L6 cath to ground, and 20.5 on the other side to ground.

If I have .75V from 6L6 kath to ground, divide by 16.5 = 45.45 ma times 420 volts plate = 19 watts.

The original 1614 tube is 21 watts, so 19 should be cool?

I can't find any, what the volts is for the out tubes on this monster anywhere

I was half worried that the Mac circuit has the out tubes off, and turns them on like a B-class circuit??

Any help most gratis

GridLeak.blogspot.com

gEorge
Click to expand...
Please post these questions in the general section of the forum so this doesn't get clogged up with repair questions.

Murray
 
I believe my question has enough of a general character to post it here in the Hints and Kinks section, so...

Is it safe to power up a tube amp without the output tubes installed?

I want to determine the current draw of the output stage of one of my MC75 monoblocks, and I have been given the advice to power the amp up without the output tubes installed, just with the pre/driver tubes. Is that something one can really do or is one going to run into trouble here?
 
If you are using a variac set to about 100V I would say yes, you should measure the unloaded B+ and make sure that it does not go to high and stress the filter capacitors.
 
Need help on MC2500

Hi Terry,
The right channel meter needle on my MC2500 doesn't appear to be in synch with the left channel when the Meter Range knob is set to Watts mode, its doesn't move at all. When set to any of the Decibel modes it functions normal.
Is there any issues and troubleshoots ?
Thanks.
 
Lets start with the easy -- hard.
1- Meter board not seated well or dirty connector.
2 - Out of adjustment or dirty pot on the board.
3 - A bad IC on the meter board (LM201/301)

The meter board is accessed by removing the top cover, it is the left board.
 
McIntosh 2205 kink

Hi folks, im new here and hoping to find some answers about my MC 2205. I got this unit about 2 years ago from a reputible dealer/tech. Lately when I turn it on the right channel doesnt play( no sound no meter ) however if I swich the speaker control off and increase power the meter jumps and i get a normal output from both channels. This continues for about 20 minutes untill the unit warms up. then every thing is fine. Any Ideas? The tech. I bought it from says its nothing serious and its not worth moving the heavy unit to have it checked. He suggested that I remove the bottom pannel and check contacts of the low voltage wires on the right channel side. Does that sound like good advice or should it be checked by a Mac. tech.? Any advice is apriciated and thank you in advance for your help!!!


My system: McIntosh 2205 Power amp, McIntosh C29 Pre amp, McIntosh Mr 78 receiver, 2x McIntosh MI 3 Performance indicator
 
Put it in the back of your pick-up truck and run it through the car wash. If you don't feel like doing that, then you might want to get a can of FaderLube and spritz all the pots and switches. There is a good tutorial on using FaderLube (and Deoxit) as a sticky in the Vintage Solid-State forum. At least that is where I would start. You might also want to read member "Gregory's" MC2205 threads. Very extensive and informative on problem solving and refurb'g an MC2205. They are in the McIntosh forum.

Oops........... just looked and that Faderlube/Deoxit tutorial has disappeared. Follow the directions on the can. Many pictures of the various stages of undress of an MC2205 in Gregory's threads. Check them out and then ask questions like, "where do I squirt the Faderlube" once you have.
 
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The MC2205 has a speaker relay, if it has dirty contacts you would loose the speaker but the meter will still work. The stereo mono switch ties the left to the right so if the right is dropping out it is not the stereo/mono switch. This gets us to the driver board on the bottom of the amplifier, the odds are high that you have an anuler ring on one of the connector pin solder joints. The fix is pull the board and resolder all the connector pins on the bottom of the board.
Note: If you do one driver board you may as well do the other because it will soon follow and fail.
 
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aball said:
Assuming someone else might make the same mistake I did, I thought I would post it here: If you replace the voltage doubler caps in the MC240 (or similar) with non-twistlock caps, don't forget to ground the positive cap to the chassis...

...!

Arthur
Click to expand...

I'm hoping to get an explanation of what you mean. I will be working on a pair of MC240's soon and anticipate finding that the voltage doubler caps are original. Believing that electrolytics of this vintage are probably near the end of their operational life I anticipate replacing the doubler caps. My experience with voltage doublers is that the negitive terminal of the lower cap is grounded, but no other terminals, so I do not underwstand the nature of your remarks about grounding the upper cap.

Please clarify, which cap you mean and which lead?

Thanks

Shelly_D
 
The twist lock caps had the can (outer foil) connected to the chassis (=ground) by design, so, unless special measures were taken to isolate it, the cap's negative pole was connected to ground once the cap was installed. Any parts or wires connected to the cap's mounting taps were thus grounded.

If you mount a newer isolated cap with one of those clamps you have to make sure that you make a connection from one of the cap's taps to the chassis in order to have a ground connection. There are pictures in my Mc75 refurb thread of that exact scenario.

I believe that's what Arthur meant.

Edit: I attached a picture, the cap on the right has it's negative lead connected to the chassis (=ground).
 

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Rodeodave said:
The twist lock caps had the can (outer foil) connected to the chassis (=ground) by design, so, unless special measures were taken to isolate it, the cap's negative pole was connected to ground once the cap was installed. Any parts or wires connected to the cap's mounting taps were thus grounded.

If you mount a newer isolated cap with one of those clamps you have to make sure that you make a connection from one of the cap's taps to the chassis in order to have a ground connection. There are pictures in my Mc75 refurb thread of that exact scenario.

I believe that's what Arthur meant.

Edit: I attached a picture, the cap on the right has it's negative lead connected to the chassis (=ground).
Click to expand...

Thanks. That's very clear and exactly what I needed.

Shelly_D
 
Dead meter on MC300

After about a year of normal operation (I bought it used), The right meter on my MC300 amp suddenly died and the needle sits at zero. The amps on both channels seem to work normally. What might I look for beyond just the continuity of the meter coil? I haven't taken the cover off yet but I am really only familiar with the internal design of an MC2205. Replacing components, reading schematics and fine soldering is not a problem for me.

Thanks,
John
 
Need Help on my C15 Preamp.

Does anyone please help me on how to order the light bulbs for my C15?
part # / sources? 2 of them have burned out last week.
Thanks ! Don
 
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