Mc2300 issues revisited

quartersaw

Super Member
I finally had some time to look yet again at my ailing Mc2300.
The main problem remaining is the overheating left channel transistors. However, there is also some detectable background 60Hz. hum in spite of the fact that all of the + - caps have been replaced except the 39000uF. can caps.
Anyway, I was playing around with the stereo/mono switch (Deox'd several times) and the amp is again running cool!
My question is: Does anyone know what is the type, and current handling spec. for that slider switch? That information does not appear in the service manual. I was going to pick up the correct 250K volume Pot for the amp today. (I installed a 500K pot that I had in my parts box) I thought it would be a good idea to replace that switch, since it is now a suspect in the heating issue.
Any information would be much appreciated!
 
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Hmmm . . . if the amps output channels were tied together and the switch not actually switching the amp into MONO, that could be the source of the left channel overheating. Was the left channel also running hot with the channels disconnected as they would be for stereo use? I forget . . .

One of mine has a bad pot on the right channel and both need to have the meter range switches taken apart and totally cleaned. It is what it is . . .
 
The channel was overheating with a signal being passed through it, but I think it was getting hot at idle as well.
I went to Radio Shack, and picked up an 'appropriate sized' DPDT switch, (A few dollars) and Guitar Center had the 250K Audio Taper pot in stock. ($7)
All of the really good electronics stores are closed up for good around here, and the Radio Shack that I visited was having a going out of business sale. :-(
I'll see what happens after installing that switch. -Can't believe Rat Shack actually had what I needed for the Mc2300. :)
I could not make a visual judgement of the output via the meters, due to the newly installed 500K pot, vs. the original 250K pot on the right channel.The 250K replacement should at least give an approximate idea of what is going on with the power output.
This will be interesting.........
 
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Hmmm . . . if the amps output channels were tied together and the switch not actually switching the amp into MONO, that could be the source of the left channel overheating. Was the left channel also running hot with the channels disconnected as they would be for stereo use? I forget . . .

One of mine has a bad pot on the right channel and both need to have the meter range switches taken apart and totally cleaned. It is what it is . . .
The left channel pot failed suddenly. I thought for sure it had something to do with the hum/overheating. Replacing the small can cap seemed to cure the loud hum. I'm going to re-examine the wiring for any possible proximity issue that might cause hum. The wiring leading to the small can cap had a very neat trail to the connectors. It was a tight squeeze working in there with my bad eyesight, and the newly installed cap did not have QUITE the same neat trail to the connections. (I'm using 4 diopter 'readers' for close up work these days)
 
...Installed the new Stereo/Mono switch, and the 250K volume pot, and only the left channel worked. I stupidly reversed the right channel connections on the pot. Switch works correctly, and the annoying background hum is now completely absent! I had found several wire connections that were hanging by a thread. The shielded signal wire connections with the added loop to ground (Mc2300 owners will be familiar with these connections) was the problem in every case. That wire is pretty brittle after 44 years. I also replaced C9, C10, as the caps that I had installed a few years back were from Radio Shack, and I recalled that the package that they came in was a bit on the yellow side. I suspected that they were not 'fresh'. I used 10uF Nichicon Muse caps to replace those Rat Shack 1.5uF caps. The earliest Mc2300's used 10uF non polarized caps for C9,C10. I have been impressed by the bass definition of the knarly looking Mc2100 that I have been using for some weeks now. I don't know if those 10uF (polarized) caps have any influence on the sound of the amp, but this is about as good as this 2300 has sounded.
The left bank of transistors still feels a little bit warmer than the right side, so I am still going to have the amp inspected by a tech.
I have the amp blasting upstairs right now, and I am very happy. :)
 
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I didn't see a part number on the plastic package, (which I tore open) but it looks like it is Radio Shack DPDT Heavy Duty Slide Switch #275-0403.
https://www.radioshack.com/products/pk2-slide-dpdt-sw
The 6 amp rating (@ 125v.) is higher than the 4 amp rating of the original switch. (Two to a package) It is also a perfect fit! Just to be certain, there is another part number on the side of the switch: R13-602
I destroyed the old switch when I pried it off of its rivets. It was a PITA fitting a screw, and nut in the tight space inside the amp for the top of the switch.
 
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Nice work, you must be relieved to have it all working as it should....

Sounds like the Bias resistor value may have drifted on the channel which is heating up.....
 
Nice work, you must be relieved to have it all working as it should....
Sounds like the Bias resistor value may have drifted on the channel which is heating up.....

Thanks, Kev! Thanks for the advice!
I ran the amp hard with the top cover in place for about an hour (The house was in a near state of thunder), thinking that it may be possible that the cooling air (This is a 1st. gen. Mc2300) might be directed more efficiently with the top in place. There wasn't much in the way of warming on the left channel after the repair beyond the fact that it was noticable. I was right. The heat sinks were uniform in temp with the top in place (to the touch, that is) on both sides of the amp.
I thought about tempting fate, and disconnecting the fans to see what those heat sinks are really up to, but in the meantime, I think that I will enjoy the amp for a while.
Interestingly, the volume is not uniform between the two channels. Initially, the left channel had distortion, and loss of power that led me to believe I was in for a very expensive repair. Now, It is the RIGHT channel that is somewhat weaker compared to the left. This is a head scratcher...
I don't think I will strap the amps until I know what is going on with the weaker channel, but that is for another day.
 
QS - did you replace one or both gain pots in this amp? You should have identical pots for each channel.
 
This CTS audio taper pot is generally used for electric guitars. It has a longer 'circle' compared to the orginal pot, which means that I can turn it up to '12'. (sorry) :)
 
Heat tests must be done at 1/3 power continuos to mean anything relative to the amps proper working. This means a big load box and the correct testing facilities.

That said, I have found so many bias resistors with drifting values of I find myself changing them almost all the time.
 
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Heat tests must be done at 1/3 power continuos to mean anything relative to the amps proper working. This means a big load box and the correct testing facilities.

That said, I have found so many bias resistors with drifting values of I find myself changing them almost all the time.

My testing method is faulty, for sure. :)
Where are the bias resistors located on the Mc2300?
 
I'd say that is the ticket to the issue right there....
Point taken, but the right channel pot is cranked up more than I would expect in spite of the difference in circular travel.
I'll pick up a matching pot this week, or try to find a pair that will match the Mcintosh specification.
 
You guys are correct. I didn't install a new pot (yet) but I adjusted the new pot with my multi tester to (closely) match the resistance with the right channel.
To my ears, (my only real test equipment) the two channels are pretty balanced.
 
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