Yikes...MC250 left channel is suddenly distorting, going in and out...

joekapahulu

Active Member
Well, after 6+ years without problem, my MC250 is suddenly misbehaving. I am getting distortion out of the left channel, the sound is dropping in and out, the volume going loud then soft. I thought it was the preamp volume control but I have swapped around the Preamp inputs, tried a different preamp, switched sources and it always stays out of the left channel. Seems to go away after the unit heats up for awhile. Similar to issues I had years ago on my MA5100 before Terry repaired and updated it. The MA unit was also updated by him when I bought it off the 'bay back in 2010. It went straight to him before being shipped out to Hawaii.

So, based off that description, any likely culprits that can be identified? Also, how big a deal might this be?Is this a must fix right away or it will get worse quickly? What is the risk? It costs a great deal to ship to Terry from the islands. I have found one guy out here that claims to do service work "for any brand" but I have never used him, not sure of his merits. Trying to figure out which way to go. I have a substitute amp so I wont be without music but I do love the sound of the 250.
 
Keeping in mind that I don't know what I'm talking about, have you tried working the GAIN controls back and forth several times to "wipe" them off? (Power off of course.) Maybe Deoxit?

That's all I got... good luck!
 
My suggestion is to start with the easy to check culprits--Deoxit and lube the gain pots and the stereo/mono switch. Mine were really crusty and a good cleaning worked wonders. If those don't take care of it, then you're into diagnosing the internals.

My suggestion is to try to get it rehabilitated now, while it is still somewhat working, than wait until whatever failing part finally fails and blows out a bunch of other components (I recently picked one up that had a transistor that failed, and when it failed, it cooked a bunch of other transistors and resistors--better to replace the suspect part now than to replace the suspect part and a bunch of other items).

If yours hasn't been updated before, remember it still has 50 year old capacitors and components in it, and is due for a rehab. There are 8 electrolytic capacitors on the circuit boards inside that are probably overdue to fail which should be replaced, and resistors have probably drifted out of spec too. Also, the transistors from that era were not as robust as modern ones, and can start to fail after this many years of repeated on/off heat cycles.

The MC250 was hand built, and because of this is not difficult to work on for someone with basic knowledge and skill. If it were me, I'd probably take it to the local tech to fix the problem before it blows up, and then see what it would run to have the capacitors and out of spec resistors replaced (if those aren't the cause of this fault).
 
Do you have the ability to do the cleaning of the switches and pots? That is always the first step even as you put the amp on the test bench to observe what the distortion looks like on the scope. I always expect to do a contact cleaning for any unit when it comes in for a bench test.

Sometimes just manipulating the switch or pot will temporarily restore the function to let you know what might need cleaning.

There are some semiconductors that fail in a fashion that would duplicate your observations but so would a dirty contact.
 
Where is the gain set? Definitely exercise the pots and the stereo / mono switch. Also clean the input jacks while you are it. I need to keep the gains 3/4 to full on my 250 as the right channel is scratchy after thorough cleaning. These are getting close to 50 years old after all! living in the islands does have some drawbacks :eek:Good luck!
 
Keeping in mind that I don't know what I'm talking about, have you tried working the GAIN controls back and forth several times to "wipe" them off? (Power off of course.) Maybe Deoxit?

That's all I got... good luck!
Thx for the input. I did work the gain controls before posting and that had no input. Next step is to move it and DEOXIT. Will have to wait until I can get a neighbors help as my back wont handle that weight anymore. So we'll see. Mahalo for the input.
 
My suggestion is to start with the easy to check culprits--Deoxit and lube the gain pots and the stereo/mono switch. Mine were really crusty and a good cleaning worked wonders. If those don't take care of it, then you're into diagnosing the internals.

My suggestion is to try to get it rehabilitated now, while it is still somewhat working, than wait until whatever failing part finally fails and blows out a bunch of other components (I recently picked one up that had a transistor that failed, and when it failed, it cooked a bunch of other transistors and resistors--better to replace the suspect part now than to replace the suspect part and a bunch of other items).

If yours hasn't been updated before, remember it still has 50 year old capacitors and components in it, and is due for a rehab. There are 8 electrolytic capacitors on the circuit boards inside that are probably overdue to fail which should be replaced, and resistors have probably drifted out of spec too. Also, the transistors from that era were not as robust as modern ones, and can start to fail after this many years of repeated on/off heat cycles.

The MC250 was hand built, and because of this is not difficult to work on for someone with basic knowledge and skill. If it were me, I'd probably take it to the local tech to fix the problem before it blows up, and then see what it would run to have the capacitors and out of spec resistors replaced (if those aren't the cause of this fault).

Thx for the input. Yeah, I know Terry replaced some of those caps as part of his "low level update". Not sure what that translates into specifically. I think your suggestion of getting it fixed before it causes greater harm makes sense. I will have to check out the local "tech". He is on Craigslist but I havent found anyone who used him much less knew about him yet. I will need to see if have the service manual...
Mahalo for the input.
 
Do you have the ability to do the cleaning of the switches and pots? That is always the first step even as you put the amp on the test bench to observe what the distortion looks like on the scope. I always expect to do a contact cleaning for any unit when it comes in for a bench test.

Sometimes just manipulating the switch or pot will temporarily restore the function to let you know what might need cleaning.

There are some semiconductors that fail in a fashion that would duplicate your observations but so would a dirty contact.

I can do the cleaning. That pretty much maxes out my knowledge/capabilities there though. So, will do that and go from there. I am thinking that if I send it back to Terry or one of the guys I have heard about on the West Coast that the approach might best be change/update anything that is suspect. Not sure if there is rationale for a "maxed out update" ,i.e. change all the transistors, caps, resistors so that should carry me for another 20 years? It cost me about $250 round trip to ship my 5100 to and from Terry. Might be a little less to AZ, WA or CA but I know Terry is going to take care of me and knows his stuff. We will see. THanks for the input.
 
Where is the gain set? Definitely exercise the pots and the stereo / mono switch. Also clean the input jacks while you are it. I need to keep the gains 3/4 to full on my 250 as the right channel is scratchy after thorough cleaning. These are getting close to 50 years old after all! living in the islands does have some drawbacks :eek:Good luck!

Have run it at or near full gain on the amp as I used to use it with a Luminox Passive pre. I will do the cleaning as suggested. We are pretty humid out here and even though the LR has AC we only use it a few months a year. Yeah, the islands are nice but expensive and remote are not good for audiophiles. Thx for the response.
 
Well as I look out my deck door and see 12 inches of fresh snow on our deck table.........all I can think of is ..........road trip!
 
Have run it at or near full gain on the amp as I used to use it with a Luminox Passive pre. I will do the cleaning as suggested. We are pretty humid out here and even though the LR has AC we only use it a few months a year. Yeah, the islands are nice but expensive and remote are not good for audiophiles. Thx for the response.
Any update Joe?
 
I swapped out the preamp and left the amp on for a few days and the noise stopped. That was the last use before we left town for the holidays. Unplugged everything then so it will be high on my list of to dos to test it when we get back home next week. I haven't forgotten it though I was in a Mac dealers store last week and right now I have new Mac lust.
 
Well I finally got my hands on some generic Deoxit and lube from Radio Shack. It was not easy to find out here. I had to hit 5-6 places to get it. Had tried to buy it online and 2 places wouldn't ship to Hawaii. 3rd place would only do it by boat which would be slow. I went to an auto supply store to look and the guy I asked for help suggested the Radio Shack in Haleiwa, about 25 miles away. I thought all the RS stores were gone. Called the store and they had Deoxit and their private label. The guy I spoke to was knowledgeable and set me up. Cleaned the stereo mono switch, the two gain pots, worked them a bunch,then the inputs, blew out the inside. Let it dry a few days then put it inline. Sounds great! No noise at all. Turned it up to max with no media and it was so quiet I was shocked. Nothing! So, I need to find the posts regarding cleaning the chrome then I will finish it and put it back in the rack. THEN, I have to try the same with the MA5100 backup which is showing noise from the volume, treble and bass pots and the speaker switch. A little more involved but I have more confidence now.
Thx to everyone who helped me with this. It is awesome the way people helped. Much aloha and good karma wished for all.
 
Well I finally got my hands on some generic Deoxit and lube from Radio Shack. It was not easy to find out here. I had to hit 5-6 places to get it. Had tried to buy it online and 2 places wouldn't ship to Hawaii. 3rd place would only do it by boat which would be slow. I went to an auto supply store to look and the guy I asked for help suggested the Radio Shack in Haleiwa, about 25 miles away. I thought all the RS stores were gone. Called the store and they had Deoxit and their private label. The guy I spoke to was knowledgeable and set me up. Cleaned the stereo mono switch, the two gain pots, worked them a bunch,then the inputs, blew out the inside. Let it dry a few days then put it inline. Sounds great! No noise at all. Turned it up to max with no media and it was so quiet I was shocked. Nothing! So, I need to find the posts regarding cleaning the chrome then I will finish it and put it back in the rack. THEN, I have to try the same with the MA5100 backup which is showing noise from the volume, treble and bass pots and the speaker switch. A little more involved but I have more confidence now.
Thx to everyone who helped me with this. It is awesome the way people helped. Much aloha and good karma wished for all.

OK, new but potentially related issue? After I had put the amp back online, I had left it on plugged into a power strip. THis unit does not have an ON/OFF switch so that is the way I have handled it for the 7+ years I have owned it. Usually, I turn it off after each use but the strip was not easy to access. Turned it off a couple days ago then on again last night to listen. First time ever, there was like a loud thunk thru my speakers. That had not happened before in all the years I have had it...or at least I dont recall it?I did it three times to see if it was consistent and it was. Anyone have this issue with their unit and if so what causes it? Is this something I need to be concerned about?
 
OK, new but potentially related issue? After I had put the amp back online, I had left it on plugged into a power strip. THis unit does not have an ON/OFF switch so that is the way I have handled it for the 7+ years I have owned it. Usually, I turn it off after each use but the strip was not easy to access. Turned it off a couple days ago then on again last night to listen. First time ever, there was like a loud thunk thru my speakers. That had not happened before in all the years I have had it...or at least I dont recall it?I did it three times to see if it was consistent and it was. Anyone have this issue with their unit and if so what causes it? Is this something I need to be concerned about?
My Mc2100's have a 4 pole, double throw stereo/mono switch. (I'm thinking that you very well might have the same type of switch) I've Deox'd the crap out of it, and it still does not work as it should. I have hum, and distortion if the switch isn't 'sitting just right'
I had one of the amps on the bench today, and I noticed that Phenolic crap that houses the contacts has bowed outward down the middle of the switch. (12 contact points) I suspect that this is the source of the malfunction, and I've been looking over the Mouser catalog for a replacement.
 
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My Mc2100's have a 4 pole, double throw stereo/mono switch. (I'm thinking that you very well might have the same type of switch) I've Deox'd the crap out of it, and it still do not work as it should. I have hum, and distortion if the switch isn't 'sitting just right'
I had one of the amps on the bench today, and I noticed that Phenolic crap that houses the contacts has bowed outward down the middle of the switch. (12 contact points) I suspect that this is the source of the malfunction, and I've been looking over the Mouser catalog for a replacement.

I checked that switch. No issue. I had cleaned it up pretty well when I worked on the pots and switches. The thunk at startup could be an indicator of an issue but based on some other searches its not? Have to see what else I find.
 
My Mc2100's have a 4 pole, double throw stereo/mono switch. (I'm thinking that you very well might have the same type of switch) I've Deox'd the crap out of it, and it still does not work as it should. I have hum, and distortion if the switch isn't 'sitting just right'
I had one of the amps on the bench today, and I noticed that Phenolic crap that houses the contacts has bowed outward down the middle of the switch. (12 contact points) I suspect that this is the source of the malfunction, and I've been looking over the Mouser catalog for a replacement.
Just as an FYI: I ordered a couple of this particular switch:
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtNT9UGfLL4eEXUF0YHaQ0QGWpgisi1IJE=
 
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