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Help me bring my Dad's MC240 back to life

C

chibrew

New Member
New member here and I need help on my McIntosh MC240. I’ve done a bit of research and am still not sure how to proceed, so I am seeking the collective wisdom of this group.

Background:

The MC240 was my Dad’s amp. The last time I can clearly remember it in service was in the mid ‘70’s. I think he might have powered it up for a bit around Y2K, but I’m not sure. I know better than to turn it on (we were not allowed to touch it as kids, though honestly, I can’t remember if we ever did), and want to get it going again. I’ve personally had this on the shelf for about 15 years and want to be able to use it. I’m no audiophile, but appreciate music, quality components, and nostalgia.

I’m not interested in modding it per se, but I want to use quality parts that pay homage to the original design. That said, I’m not a purist.

Goal:

Be able to listen to this amp safely for years to come

Options:

1) Send it in for service – While I can just write a check, this is not my typical style.

2) Through pain staking research and good guidance, test and replace the needed components.

Approach:

This is where I need help. I think the easiest (not cheapest) thing is to:

1) Replace all tubes – (not sure what brands, etc and I see LOTS of opinions on this)

2) Replace all capacitors – (I know there are some pre-packaged kits, but also know the quality of components here matters as much as the tube). Looking for guidance here as well.

3) New power cord

4) Should I replace the rca jacks?

5) What am I missing?

I’m very comfortable replacing parts, using a VOM, but am by no means an engineer. I can build a board off a schematic, but do get lost in some of the theory. In short, I’m a great parts replacer, but don’t always get the why. That said I can follow directions well, and am willing to get tools/equipment (within reason) if needed. I view this as a way to learn more about how tube amps work as well. I have the service manual and owners manual from on-line. Are there and other documents I should look for?

I’m looking for guidance on how to approach this. Ho do you power it up in stages, what is the proper burn in sequence for new parts, etc.

If in the end I need to send it in for service I will.

Thanks in advance for your input.
 
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Welcome.

Honestly I'd hold off on buying tubes. The ones in it may be OK. I would have them tested and go from there.

I'll defer to the Mc experts on the rest of it. You may want to post this in the McIntosh subforum, those guys can probably provide more detailed answers.
 
Since you have not heard this amp in stock form, modding it is not pointless but without some reference not necessary. If you are good at soldering and replacing then you should have an excellent chance of rebuilding the circuits. First thing to check is if the transformers are good. That is best done with a variac at low AC volts. You can trace the AC from the line cord into the power trans and after the PT at the various secondary taps. The secondary volts would all be proportional to the AC input. Example, 12vac = .63vac for the filament voltage. Of course, a schematic with voltages at key points in the circuit is a must.
 
There are some MC40 and MC240 rebuild threads on the Mac forum. That will get you pointed in the right direction. I have a set of MC40's that were rebuilt by a member here that are my end game amps. I purchased rebuild kits from Jim McShane that included all of the parts minus tubes to bring my amps back into spec.
I agree with Gadget, have the tubes tested. Mac's are not hard on tubes and they may still be good.

BillWojo
 
If you brought this to me for re-starting, i would first check all the tubes. Then, I would check each power supply cap for excessive ESR using the Blue ESR meter. This can be done in circuit. If any fail ,they get replaced. Next, I would disconnect each power supply cap. and take a value reading and a leakage reading. If any failed, replace them. (or just replace them all as a matter of course. It depends on how much the customer wants to spend) Finally, i would disconnect one end of each interstage coupling cap and repeat the tests above, replacing any bad caps. Check all the resistor values and replace as necessary. Some you can measure in circuit, some have to have one end disconnected to get a true reading. Finally, I would put the tubes in, hook the amp. to resistor loads, an input signal and start it up with a Variac at about 60vac. This will provide a (very) slow start, while I monitor the amps. outputs using a scope. Even if very distorted, I need to know if its acting like an amplifier ASAP. Once it shows life, I will turn up the variac 10 v. at a time, while monitoring the bias and output trans. voltage.
Once its up and running, I power it for 30 mins. @ 1w, then 30 mins. at 1/3 power. Not until this passes will I connect to speakers.
If you buy nothing else, get a used American Variac. They're indispensable.
 
The tubes are likely fine, and may be much better than new ones.If you are going to keep and use the amp, I would send it to Terry Dewict for a proper rebuild, if you are going to sell it, sell it either lightly tested, or in original condition.
 
Sounds like there's a lot of sentimental value for this amp. Please rest assured that it can be rebuilt to operate as it did when it came out of the box the first time. We've got lots of stories and rebuild documentaries over at the McIntosh forum here at AK. Please take some time to read through each and every one of them.

If you're not up to rebuilding it yourself, you'll find 2 companies pop up over throughout those threads: Terry DeWickt and Audio Classics, who have a special connection to McIntosh itself. As with all good McIntosh restorers, the good ones are busy so stand in line. These guys know their stuff and know what's needed to restore them properly. Given the sentimental value of this amp, be prepared to wait on them.

A word of caution: Please do not get in a hurry to run this amp in its current state. You'll see in the archive threads that a variac is used to do so. This is a safety measure. It isn't a matter of _if_ the amp will fail, it is a matter of _when_.

If you want to restore it yourself, the rebuild threads are the way to go. I, too, recommend Jim McShane as a source of rebuilt parts - he knows the high-performance tube amps of the MC-240 era.

Lastly, what speakers will you be using with it?

Cheers,

David
 
Good on ya, regardless of which way you go.

Totally doable yourself, the Mac section is a treasure of knowledge and help. Take butloads of photos!
 
would agree with Primosounds, the tubes are likely OK, the transformers are most likely good. The only questionable parts are the electrolytic caps.

make up a dim bulb tester and use a variac to bring it up slowly. Even it all goes well, would still consider replacing the power supply caps. have rebuilt a number of MC240s, sometimes the power supply caps still test good, but other times they produce high ESR values (showing they are drying out).

you can buy the replacement caps for the power supply from several on line sources.

If the amp still has the original bias supply , would be sure to replace the cellinium rectifier with a modern silicon diode, other wise you run the risk of trashing a perfectly good set of out put tubes.

the 240 is a nice amp. good luck.
 
Jim McShane has been supplying rebuild kits for the Harmon Kardon Citation series of amplifiers for 15 years and I have been his customer for over a decade. He is well regarding as having the most comprehensive and technically superior rebuild kits for the Citation series of HK products. He will provide a full rebuild kit for the McIntosh amps as well, however his web site does not full describe all the kits or components he carries. It is best to email him and inquire as to availability and what he recommends. jimmcshane@mcshanedesign.net
 
So it's taken me about a year to get started on this. I've ordered a kit from C-tech electronics and it has everything I could need. I plan on only replacing what is needed, but am prepared just in case.
I decided that it was time and started by taking a bunch of pictures, cleaning off some dust with a swiffer sheet, pulling the tubes and putting it in my dehydrator. I'll leave it in there overnight and I think that should ensure that I have any moisture that might be in the transformers out. It's been inside so I don't think I have a problem here, but it doesn't hurt to do it. Tomorrow I'm taking the tubes in to be tested. From what I can tell, it looks like the output tubes have been replaced, but I'm not sure on the others. At least one has the factory McIntosh logo. From the best I can tell, it's completely original, except for the tubes. Looking forward to getting this going.
I'll add pics as I go along and welcome input.

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The electrolytic caps are all a threat to the amp's continued serviceability. They are *OLD* and past their due date. The selenium diode in the negative rail also has to go, they are poisonous when failing, and prone to failure. Get them outta there. The McShane kit is a very fine way to start; it is *MOST* convenient, in addition to being well chosen quality pieces. I got one of his kits for my Citation II. Very well done.
cheers,
Douglas
 
I second the Jim McShane rebuild kits. Almost everything has to go, all the leaky caps and out of spec resistors.
My MC40 monoblocks were rebuild using his kits by a fellow AKer and I couldn't be happier.

BillWojo
 
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It would surprise me if this doesn't need immediate attention. Get the mcshane kit.
jim's Citation kit had everything my tech needed to bring it back up to snuff.
 
I started by having the tubes tested at Absolute Sound Labs. They are McIntosh factory authorized and in my back yard. Evan is a great guy and was glad to help. All good on the tubes. I also have the kit from C-Tech electronics, which is very similar to Jim's kit. No knock on Jim, I've had the other kit on a shelf for a while waiting to get to this. I'm waiting on a few resistors I decided to change and I will be done with the power section. I'll post pics as soon as I'm done.

I tested all the resistors and am debating on changing a few that are out of tolerance. Here is a picture of the resistors that I've numbered and a chart with the values. I think R17, R18, R42, R43 all have a capacitor in line which threw off the readings. There are a few that I can't find (blanks) so if anyone can point me in the right direction I'd appreciate it.

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Your amp is in very nice shape and nice that it's been in the family so long. Looks like you're being very methodical about your rebuild process. :thumbsup:

Having rebuilt 3 x MC240's, 4 x MC30's, and 1 x MC225 there are a few things I would add and comment on. I'm pretty sure not everyone will agree with me 100%, but:

- It's almost certain that all resistors are out of tolerance. Even if they measure close at room temp, the carbon comp resistors heat up their values will shift due to age & moisture absorption over the years. Change them all and be done with it. You will always be chasing bad resistors otherwise. I would use 1% or 5% metal films for this.

- The power supply rebuild kit from C-Tech is okay as long as you have the version with the Authenticap large filter caps and voltage doubler caps. They are very high quality. I'm not a fan of the CE Filter Caps, but they are not terrible and would be just fine.

- Increase the value of the bias supply cap to at least 22uF for a more robust setup.

- There are Bumble Bee caps in there. It's well documented that they were defective from manufacture and are most likely cracked, leaky and unreliable. This has a significant effect on SQ and reliability. Replace them. This is where Jim McShane can certainly help with a carefully chosen selection of film & PIO caps.

- Clean then tighten up the tube sockets using a dental pick or large safety pin to coax the socket tabs a little closer together.

The quality of modern parts is significantly better than the best that were available when this amp was designed and manufactured. Take advantage of that and you will be very satisfied with the end result.
 
"I'm pretty sure not everyone will agree with me 100%"
That is a safe prediction...LOL

Carbon film is available in the required dissipations, and with room to move up a step, which keeps everything slightly cooler. Also, IME it is quite predictably neutral...as in one CF resistor is not going to sound wildly different from another.
cheers,
Douglas
 
There are improvements to be had, but that is for later. The fixed voltage, fixed bias( not adjustable ), really should go. Putting in something adjustable is quite useful. Minimal tear up is required. I am not sure how they got away with the system as designed, and certainly with new tubes being able to set idle current is a very useful thing. DC resistance of the cathode part of the primary is used for the current sense resistance.
cheers,
Douglas
 
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