MC240 - Back to Life

chrisinsc

Active Member
Hi folks. Been doing a lot of research before posting. Picked up an MC240 and an MX110 for 300 USD from a guy cleaning out a house. They were in rough shape physically since its close to the ocean, the MX110 is visually good now and operating. MC240 is now a rat visually but I'd like to run it.

The MC240 plays after bringing it up slowly. After hours of just audible running the far right transformer was pretty hot. All the smaller caps are except a few are Sprague 160p caps. It looks like a couple of the big cans have been replaced and perhaps some other components.

I will resize my images and take measurements and share.
 
Last edited:
Wow, $300!
You've got a deal. No meter the condition, you will be able to sort these somewhat.
Pictures would be great. You can find a lot of info on mx110 and mc240 in archives.
Good luck.
 
I had a fisher 400 which I foolishly traded to used audio dealer who did not reveal its value 15 years ago. I believe the mcintosh find was a karmic repayment with interest!
 
Well worth restoring! The MX110 is, to my mind, the epitome of preamp / tuner combination. The MC 240 is no slouch, get these up to spec and I think you're done with the right speakers!
 
Resizing images now. Sorry.

I will use JBL 4410a studio monitors and an Empire 598 turntable with a Shure V15 IV cartridge as a source along with the FM.

I'm going to make a switch box from a dual gang electrical switch to bypass the 5.1 rig which uses the JBL's as mains when I want to listen to analog.

Next post will have images.
 
Images. This is after two hours of cleaning...
 

Attachments

  • 5d.jpg
    5d.jpg
    57.9 KB · Views: 268
  • 4d.jpg
    4d.jpg
    56.1 KB · Views: 194
  • 3d.jpg
    3d.jpg
    58.5 KB · Views: 205
  • 2d.jpg
    2d.jpg
    70.2 KB · Views: 191
  • 1d.jpg
    1d.jpg
    72.3 KB · Views: 190
Voltages with very low volume music playing:

V1-A 12ax7
1 108 spec 103
3 .9 spec .7

V1-B 12ax7
6 108 spec 103
8 .87 spec .7

V2 12au7
1 250 spec 205
2 108 spec 103
3,8 115 spec 108
6 240 spec 205
7 88 spec 85

V5 12au7
1 237
2 108
3,8 115
6 254 spec 205
7 86

V3 12bh7
1 338 spec 350
2 23 spec 18
3,8 39 spec 35
6 345 spec 350
7 22 spec 18

V6 12bh7
1 350
2 21
3,8 38
6 338
7 22

V4 12ax7
1 433
2,3 -49
6 433
7,8 -49

V7 12ax7
1 433
2,3 -49
6 433
7,8 -49

Power Tubes 6L6GC's - ok used tubes - none matched

All pins 3 and 4 433
All pins 5 -49

Pins 8 as follows:
V8 .87
V9 .68
V10 .62
V11 1.2 double checked

Well that's the current situation.
 
unit has a metal badge on one side of a transformer that reads:

McIntosh Clinic
Performance Certified
12/3/71
 
Bunch of old leaky caps in that amp. You will need to follow the service manual for resistances with the amp off and voltages with no signal. Critical capacitors should be replaced with matched new ones and the voltages should be very close to the service manual when finished.

Thanks,
Ron-C
 
Your MC240 should look like the inside of this one when finished..
 

Attachments

  • IMG_1516 (640x480).jpg
    IMG_1516 (640x480).jpg
    114.4 KB · Views: 227
Parts for the power section on the way today. Maybe they will come in Saturday's mail and I can swap them out.
 
$300 ;_;

I was happy to score the same 2 components for $2k. Always happy to see 240 threads though as I prep to do my own in a few weeks.
 
Does anyone have a layout map for the boards, rail, and chassis for this amp? Im willing to make one but im older now. Lets pretend I have the chops and gumption and you send it to me anyway :)
 
These guys were craftsmen. Plenty of extra wire looped rewiring the new large capacitors.

I will post some pics. Need some advice on the c24 c23 situation.
 
So, some images and some questions:

1) Is it no big deal to remove the rivets and take out the diamond shaped pieces of metal the twist locks cans mounted to and replace with clamps.

2) I have JJ 100/100 caps in place of the 100/30's taken out. They look like they need to be grounded to the chassis, can I just ground to the new clamp hardware along with the black wires designated for the specific cap?

3) Looking at the schematic it does not look like there were enough resistors in the old c25/26 arrangement or is the third one somewhere else?

4) the other two large caps I'm replacing (23 and 24?) are causing me greif because one of the old ones says its a -250/+250 double cap and I only have a single cap for that slot and this old one does not look grounded at all.

Thats it for now. Starting to get a feeling for the circuit but I would like some feedback whether the "old" components and execution was original or whether trying to recreate it is not what I need to do.
 

Attachments

  • c23_24_old.jpg
    c23_24_old.jpg
    44.8 KB · Views: 93
  • c23_or_24.jpg
    c23_or_24.jpg
    42.2 KB · Views: 83
  • c25_26_old.jpg
    c25_26_old.jpg
    49.3 KB · Views: 94
  • diodes_rect_old.jpg
    diodes_rect_old.jpg
    41.8 KB · Views: 93
One approach to replacing all of the old electrolytic caps mounted to the center bar is to remove the bar itself.

Once it is out of the chassis, you can now use a suitable size nail punch and a ratchet-wrench nut socket to drift out the rivets. Then, use mounting clamps for the JJ caps oriented so that the holes line up with ones from the rivets. If the replacement caps have a plastic covering on them, be sure to wrap the area where the clamp will contact it with a couple of extra wraps of electrician's tape. This will ensure that any small burr on the clamp metal doesn't penetrate the cap's plastic and short it out over time.

Re-installing the completed bar will involve reconnecting the wires. Be sure that the ground sections for all of the insulated caps are now connected to the same ground point on the chassis. You can use one of the clamp's mounting screws to add a solder lug for ease of soldering.

Replace any of the resistors that get dislodged from the rework, too, on general principle. Remember that a couple of the components on those solder lug strips are actually chokes but have colored bands that make them look like resistors.

Make lots of pictures and notes to where the connection wires go, of course.

Lastly, if you haven't planned for it, the thermistor will also need to be replaced. That's the disk-shaped item in line with the AC input wiring. It may fall apart on you during all the other doings in there if it is brittle enough from heat - many are.

Hope that helps.

Cheers,

David
 
Thanks to time on the phone with david i am studying the circuit from the beginning and replicating the schematic. There were issues with the existing wiring that made direct replacement of components not a good path. Better for the project that I learn the circuit anyway. Dismounting the choke and the center chassis rail allows for tracing and labeling the cabling easier. Punching out the rivets holding the large power section cap plates is even easier if you use a dremel grinding wheel and grind down the female side of the rivet flush with the mounting plates first. I did not totally dismount the center chassis rail, I left the gray and gray/white wires connected and just swung the rail out and used a piece of wood and a 13 year old boy to stabilize the whole mess while grinding and puching out rivets. I did label all wires after tracing with blue masking tape and a sharpie before remounting the rail and adding the clamps. I used 6/32 hex head cap machine screws and aircraft lock nuts to mount the clamps. Once I determine the best layout I will add a ground lug to one of the clamp mount fasteners. It was incredibly helpful to talk to someone about this endeavor. Thanks david.
 
Back
Top Bottom