Mc2300 distorted channel (continued)

[quartersaw]

I had a local tech examine my sick Mc2300 since I don't have the time to haul down to Audio Classics. This gentleman said the problem was with the Stereo/Mono switch. (!!) The amp ran with a nice clean sine wave on a scope. He said that he ran it for hours w/o issue. (I had cleaned the switch, and the pots with Deoxit previously)
Feeling like a jackass after the previous discussion about dirty/corroded switches on these pages, I picked the amp up, and set it up @ home.
The amp had a pretty loud 60Hz hum when I turned it on, which subsided after approx. 10 minutes of use. (Disappointing.) After a few days, the right volume pot ceased to work, unless turned up to full volume. (Measuring it with my multi tester indicates it is now an 'on/off' pot) The distortion in the left channel returned, as did the left channel bank of heat sinks heating up. Is it possible that the distortion is simply being cause by a bad Stereo/Mono switch, since the amp came back to life after yet another cleaning?
The hum is a different story. I have a replacement for the small can cap that I had made up a few years back by Hayseed Hamfest. After the original can caps were pronounced fit when the amp was serviced in Oct. 2014, I didn't bother installing the replacement, since it looks like some disassembly is required with the rail holding the can caps, and a few rivets to remove.
I'm thinking of going ahead with replacing the switch, small can cap, and the volume pot to see what happens. I think I have the correct value pot, and of course, I have the can cap. The Stereo/Mono switch looks to be a bit heftier and the correct value, and type of switch is not mentioned in the service manual. Does anyone know what the correct type, and value of that switch is?
- Does the hum issue sound like the big can caps have met the end of the road?
Any other advice is much appreciated!

 

[c_dk]

My concern would the amp going in and out of oscillation. The heating concerns me. What kind of load are you putting on it compared to the tech?

What outputs are in the unit? Have the driver boards been rebuilt to the 1980s spec? The switches are a pain but that is just to remind you this amp is over 40 years old.

 

[damacman]

My MC2300s are so temperamental at the moment - one needs the right gain control replaced, both need the meter range switches thoroughly cleaned (again), and both need a few bulbs. I use them every day and just futz with the controls a bit as necessary.

It is what it is. I'll be using them 25 years from now though, I guarantee it.

Good luck QSaw!

 

[quartersaw]

My concern would the amp going in and out of oscillation. The heating concerns me. What kind of load are you putting on it compared to the tech?

What outputs are in the unit? Have the driver boards been rebuilt to the 1980s spec? The switches are a pain but that is just to remind you this amp is over 40 years old.

 

[quartersaw]

I stopped using the amp when these new issues came about.
The boards were updated with the 'flying saucer' upgrade kits when I first bought the amp, and all of the small + - caps have been replaced.
I thought that it was odd that a volume pot, which is hardly ever used in an amp like this, would suddenly crap out like that.
I tried rocking the stereo/mono switch back, and forth w/o a signal going through the amp, and it did sound like a really brief squeal happening as I actuated the switch..With no input signal, the heat sinks are heating up, so there may in fact, be a high freq. oscillation.
As far as I know, the transistors are stock. The innards looked completely untouched when I took delivery of the amp.
I'm running a stock pair of CV XLS-215's, which measure about 6 ohms. I'm running them in the 8 ohm tap. Brand new 16 ga. speaker wire, with about an 8-10 foot run for each side.
The tech was running a signal generator into a load, and then he demo'd the output for me through a small speaker.
He is a 'tube guy', and I got the impression that this beast was outside of his comfort zone.
I've been enjoying one of my Mc2100's in the meantime, but I want to put the issues with the Mc2300 back on the front burner soon. I thought that I might take another crack at trying to fix it before I turn it over to someone else.

 

[quartersaw]

My MC2300s are so temperamental at the moment - one needs the right gain control replaced, both need the meter range switches thoroughly cleaned (again), and both need a few bulbs. I use them every day and just futz with the controls a bit as necessary.

It is what it is. I'll be using them 25 years from now though, I guarantee it.

Good luck QSaw!

Thanks, Tony! No doubt, the 2300's will be running like a fine watch in the decades to come. Just a bit louder.

 

[c_dk]

I would want to monitor the distortion and the current draw and be looking for either to start rapidly increasing in level.

Then you can start switching boards and such side for side.

It is not uncommon for large parts of the carbon track inside the volume pot, to fall off the phenolic substrate when the glue lets go

 

[twiiii]

Of all the big SS amps the 2300 is the simplest to repair. But that doesn't mean certain issues can't be a real pain to trace down and fix. Bad switches, pots, connectors, oxidized and polluted pin connectors, the list just goes on and on. I am sure Ryan and the guys will help you any way they can. They just saved my C-34 which had a compromised, record EQ switch. Of course they checked out and tested it thoroughly. I got very nice report card when they returned the unit , Its been working great since. Only highly experienced people could have gotten my problem fixed right the first time with out costing me a fortune. New parts were not an option.

 

[c_dk]

I thought over my morning coffee. Something to check for maybe two of your intermittant issues.

BUT FiRST, you must make sure they are discharged, I use a old Christmas lamp with a pair of test leads soldered to it. I use this to absolutely confirm the main filter caps are absolutely discharged!

Back doing amp clinics with DOB he would have me remove the bottom of Mac amps and loosen then snug back up the screws connecting the main filter caps. As he explained it corrosion could creep in and compromise the connection between the screw, leads and cap threads. Sometimes the connection had worked so loose the filter function would be greatly compromised causing a noticeable buzz and weak power output.

IT IS VERY TIGHT quarters in a MC2300 and I would use my 90 degree Philips screw driver. IF THE CONNECTION is loose the cap might not discharge completely thru the normal means, therefore the lightbulb.

You need to be careful, DC voltages there across your heart will kill you dead!

I now always check those connection on any amp that crosses my bench. Just had a Hafler 9505 being tested for resale that would barely put out 50 watts. The screws had loosened to barely finger tight.

 

[quartersaw]

I thought over my morning coffee. Something to check for maybe two of your intermittant issues.

BUT FiRST, you must make sure they are discharged, I use a old Christmas lamp with a pair of test leads soldered to it. I use this to absolutely confirm the main filter caps are absolutely discharged!

Back doing amp clinics with DOB he would have me remove the bottom of Mac amps and loosen then snug back up the screws connecting the main filter caps. As he explained it corrosion could creep in and compromise the connection between the screw, leads and cap threads. Sometimes the connection had worked so loose the filter function would be greatly compromised causing a noticeable buzz and weak power output.

IT IS VERY TIGHT quarters in a MC2300 and I would use my 90 degree Philips screw driver. IF THE CONNECTION is loose the cap might not discharge completely thru the normal means, therefore the lightbulb.

You need to be careful, DC voltages there across your heart will kill you dead!

I now always check those connection on any amp that crosses my bench. Just had a Hafler 9505 being tested for resale that would barely put out 50 watts. The screws had loosened to barely finger tight.

Thanks for the advice! I have seen what a small cap can do when it discharges. Getting bit by 39000uF is an experience that I will definitely avoid.
I will have time to look at the amp again on Friday. I will report back with my findings. Thanks again!

 

[quartersaw]

[QUOTE="c_dk, post:
IT IS VERY TIGHT quarters in a MC2300 and I would use my 90 degree Philips screw driver.
You need to be careful, DC voltages there across your heart will kill you dead!
[/QUOTE]

Just a quick thought: Is it possible to remove the entire large cap assembly with a nutdriver?

 

[quartersaw]

Of all the big SS amps the 2300 is the simplest to repair. But that doesn't mean certain issues can't be a real pain to trace down and fix. Bad switches, pots, connectors, oxidized and polluted pin connectors, the list just goes on and on. I am sure Ryan and the guys will help you any way they can. They just saved my C-34 which had a compromised, record EQ switch. Of course they checked out and tested it thoroughly. I got very nice report card when they returned the unit , Its been working great since. Only highly experienced people could have gotten my problem fixed right the first time with out costing me a fortune. New parts were not an option.

I'll use the advice that I've gotten on these pages, and see what I can do with it this weekend. If I can't make any progress, then out it goes to the Shop. (Fingers crossed)

 

[quartersaw]

I'll use the advice that I've gotten on these pages, and see what I can do with it this weekend. If I can't make any progress, then out it goes to the Shop. (Fingers crossed)

I thought over my morning coffee. Something to check for maybe two of your intermittant issues.

BUT FiRST, you must make sure they are discharged, I use a old Christmas lamp with a pair of test leads soldered to it. I use this to absolutely confirm the main filter caps are absolutely discharged!

Back doing amp clinics with DOB he would have me remove the bottom of Mac amps and loosen then snug back up the screws connecting the main filter caps. As he explained it corrosion could creep in and compromise the connection between the screw, leads and cap threads. Sometimes the connection had worked so loose the filter function would be greatly compromised causing a noticeable buzz and weak power output.

IT IS VERY TIGHT quarters in a MC2300 and I would use my 90 degree Philips screw driver. IF THE CONNECTION is loose the cap might not discharge completely thru the normal means, therefore the lightbulb.

You need to be careful, DC voltages there across your heart will kill you dead!

I now always check those connection on any amp that crosses my bench. Just had a Hafler 9505 being tested for resale that would barely put out 50 watts. The screws had loosened to barely finger tight.

I pulled the rail that the caps reside within. One (-) screw was the only one that broke free. I hit it with a quick spray of DEOXIT, and tightened it down as best that I could. The rest of the screws could not be moved w/o excessive force with a FAT screwdriver, so I left them alone.
I'm replacing the small can cap since I've come this far. I had a 'Hayseed Hamfest' can cap made up to fit in that space. I'm taking my sweet time with it. It's still a tight space. I'll report back....

 

[c_dk]

I always crack them loose and then resnug them up. You can always bypass the pot with a resistor to remove it's issue from the unit. You can easily hardwire the mode switch also.

 

[quartersaw]

I always crack them loose and then resnug them up. You can always bypass the pot with a resistor to remove it's issue from the unit. You can easily hardwire the mode switch also.

The amp is still open, so I will do that. I had to file down a big screwdriver to get a tight fit on those screws, so it makes perfect sense to check all of them. The space for the small can cap is still very tight to work in, in spite of the fact that the rail is raised. I may just install the correct individual caps to replace the can instead of wrestling with the new can cap itself, as it appears to be a slightly different size. My thinking is small can caps of that size tend to fail as a rule, (44 years on the clock) whereas, the big 39000uF. caps seem to be generally OK.
Interestingly, the wire for the .5 ohm Autoformer tap on the rouge channel simply fell off upon inspection.

 

[quartersaw]

The amp is still open, so I will do that. I had to file down a big screwdriver to get a tight fit on those screws, so it makes perfect sense to check all of them. The space for the small can cap is still very tight to work in, in spite of the fact that the rail is raised. I may just install the correct individual caps to replace the can instead of wrestling with the new can cap itself, as it appears to be a slightly different size. My thinking is small can caps of that size tend to fail as a rule, (44 years on the clock) whereas, the big 39000uF. caps seem to be generally OK.
Interestingly, the wire for the .5 ohm Autoformer tap on the rouge channel simply fell off upon inspection.

Once again, I tried to loosen the screws to the + - terminals on the bottom of the 39000 Uf caps, and they simply will not budge with a correctly sized flat headed screwdriver. I decided to leave them alone as I was concerned about the amount of force it would take to free them up with the loose/floating cap rail, and the adjacent 44 year old wiring in mind. I did replace the small can cap, and It took me quite a long time to do this. I removed the 4 small nuts holding down the rail that the big cans sit on, and this gave me just enough space to remove the old can cap, and replace it with the new.
I had a brand new 500K audio taper pot in my parts box, so I used it to replace the faulty pot which is of a smaller value.(250K?)
The good news, is the amp plays, and the distortion, AND the hum have gone away. There seems to be a difference in the *apparent* output of the two channels, but it is tough to verify due to the different value pots. The bad news is that one side of the banks of heat sinks (Left channel) still heats up to a disturbing level within a few minutes. This may be as far as I can go with this amp in terms of trying to fix the issue.

 

[quartersaw]

I always crack them loose and then resnug them up. You can always bypass the pot with a resistor to remove it's issue from the unit. You can easily hardwire the mode switch also.

I was looking at the wiring of the Stereo/Mono switch, and it looks like it would just be a matter of moving the wire from the 'stereo' position to the center soldering lug. Would that be correct? Interestingly, the 3 lugs on the other side of the switch are wired together