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McIntosh MC240 Help, Please

pantoramasan

pantoramasan

AK Subscriber
Subscriber
I recently purchased a McIntosh MC240. Checked all tubes etc. Began to variac the amp. Connected speakers, pre-amp and tuner. Set the variac to 30 and after about 5 minutes I hear a popping sound and I see a spark through the hole near the second power tube. I turned off the variac. Unplugged, removed the bottom plate and set on it's side and resumed the process so I could see what was going on. The photo shows the resistor (correct me if it is not) which I saw arcing. Just a couple of times. Below is the photo of the culprit. Any idea what may be going on? any help would be appreciated.
 

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Remove all tubes, plug in to 120 volt and check all voltages as per the schematic. Report back.

thanks,
Ron-C
 
Those are not resistors,they are chokes. They are connected between the output transformer and the output tube plates. The ones in your photo do not look like any of the originals I have ever seen,and both have different values (2.7 & 1.5),neither of which is correct according to the McIntosh service information,which gives 2.2.

These could quite possibly be replacements,and might not be insulated well enough to handle the 400+ volts in this area. Also,check around the wiring they pass over for burn marks.
 
arts said:
Those are not resistors,they are chokes. They are connected between the output transformer and the output tube plates. The ones in your photo do not look like any of the originals I have ever seen,and both have different values (2.7 & 1.5),neither of which is correct according to the McIntosh service information,which gives 2.2.

These could quite possibly be replacements,and might not be insulated well enough to handle the 400+ volts in this area. Also,check around the wiring they pass over for burn marks.
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It's quite possible they are replacements as the unit was serviced way back in 2004 and unused since that time according to the previous owner. I did examine the rest of the unit and appears that some of the capacitors also had been changed.
There does appear to be a black spot on the solder joint to that choke connected to the tube plate. I don't know if that qualifies as a burn though?
 

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costerdock said:
I wouldn't risk it. Have a McIntosh tech fix it and freshen it up.
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That is more likely what I will do. Just thought if it was something simple ,replacing that choke or re soldering the joint, I can do that without fear of screwing it up. But if it gets more involved, then it's off to the tech. There are many here on AK that are quite helpful and I gave it a shot.
It is evident that some changes had been made from what I see, compared to stock units.
 

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It obviously been worked on. Just a few questions.
Did you buy it as working?
Did you buy from a reputable shop?
Without getting too specific where are you ?
EDIT (I see AZ) a lot of people don't show.
Some people (shops) don't like taking stuff that other people have had their hands in. That is, Its tougher to figure out what the last guy did wrong than fix it right from scratch.
That being said, it doesn't appear the last guy was a butcher and these are pretty simple and straight-forward. And access is excellent. Assuming they didn’t kill the transformers (highly unlikely!) This should be a quick easy fix. I'd bet any decent tech would at least give a good once over the original work done while he's under the hood.
Good luck!
I'm sure this will turn out well!
 
I know local (relatively) techs. The west not so much. I'm sure one of our western brothers will chime in.
IIRC, Damacman lives in AZ. I'm sure he can point you in the right direction.
 
nj pheonix said:
It obviously been worked on. Just a few questions.
Did you buy it as working?
Did you buy from a reputable shop?
Without getting too specific where are you ?
EDIT (I see AZ) a lot of people don't show.
Some people (shops) don't like taking stuff that other people have had their hands in. That is, Its tougher to figure out what the last guy did wrong than fix it right from scratch.
That being said, it doesn't appear the last guy was a butcher and these are pretty simple and straight-forward. And access is excellent. Assuming they didn’t kill the transformers (highly unlikely!) This should be a quick easy fix. I'd bet any decent tech would at least give a good once over the original work done while he's under the hood.
Good luck!
I'm sure this will turn out well!
Click to expand...
I bought it knowing it may or may not be working. It was sold by "Audio Classics
LTD" out of Vestal NY to this fellow in Colorado in 2004. The box had not been opened since. Still secured to the plywood platform, double boxed and plastic wrapped. So I was hoping that it would be a good working unit. The person who acquired it and sold it to me knows even less about electronics than I do, so I can't blame him. It was my risk.
I am in Tucson, AZ. As far as i am aware, there are NO good or trustworthy tech's in this town. I could drive it up to Phoenix or close to it.
Tanks for the positive thought.
 
nj pheonix said:
Sort of hard to believe AC would sell a dead unit. If you know buyers name, I'd bet AC could give you some background on what you have.
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Yes I do know the buyers name. I have the purchase receipt from them. That was my next step. Lot's could have happened between now and 12 years,even in storage. Could have been dropped or banged in shipping etc. who knows.
 
Because the power supply was rebuilt I'd be inclined to power it up. Do you have a DBT?
Sometimes output tubes arc and then are more predisposed to Arc again
 
nj pheonix said:
You might be fine maybe needing a set of output tubes
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I did test all the tubes and they were good. The only questionable tube though was the last output tube which shows a couple of shorts on my tester. I believe #8 & #11. I have a B+K 6600 SS tester. So were you saying to fire it up anyway, disregard the intermitten pop and spark?
BTW, I just got off the phone with Audio Classic, They found the name of the buyer but had no other info as to what they may have done to the unit. Said it went back too far.
DBT? sorry, my ignorance...
 
If the tube showed shorts on the tester, do not use it. You may wind up cooking something a lot more expensive than a tube like one of the transformers. Wait till you get some new tubes.
If that MC240 sounds anything like my MC40 monoblocks when your done, your going to have a big grin on your face. When I found mine they were a mess. NJ Pheonix ^^^ helped me out and overhauled them with parts kits from Jim McShane. I couldn't be happier.

BillWojo
 
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