Mc240 Rebuild - Question and Progress

I'm finally able to start focusing on my MC240 that I picked up a month or so back. I'm good with part for part swap and A-B wiring. The rest of it I need a bit of help with.

Progress so far:

Audiodon helped me a few weeks back. He tested the tubes and found all to be good with 2 of them being on the low side of good, the rest testing strong.

He brought it up on a variac and he found AC voltage but no DC voltage. The amp did not pass sound, with known source. Tubes glow.

My buddy at work took a look and he feels the caps in the power supply are shot, will bring a tester in tomorrow for the caps themselves. We have 115VAC at the plug. 155VAC at the transformer, measured on the red wires between Sr1 and Sr2 / between c23 and c24.

My buddy suspects the caps are gone but the transformer (T3) is good. Will know more tomorrow.

I plan on and have planned to from the start, replacing all the caps, testing the resistors to see if they are in spec and replacing any that are not. My concern is the transformers. There is no "I let the smoke out" smell in the amp.

Questions

From above, does our logic / findings seem sound?

Do I have enough info to be 90% or even 80% confident in the transformers?

I have searched but have not found a parts list / kit recommended on AK. I suspect there are likely threads with posts in them with that info but no specific thread has popped up. Can someone point me to a source of kits?

And finally, any recommendations?

Thanks everyone.

PJ

Here's one kit source:
http://www.mcshanedesign.net/citpage.htm#parts.htm
Scroll down the page for Mac info.
There are some kits available on ebay also but I've no experience with the people selling them.
Mr Mcshane can accommodate you with probably any type of kit you'd like, from basic rebuild to heavily modified and he's very helpful with any info you may need and very reasonably priced.

Most common failure for the MC240 resulting in low or no audio caused by low or no B+ are the 2 doubler capacitors. A quick check for this is to bridge them with another known good capacitor of 100 uf at 250V minimum and see if the B+ comes up, an increase of 20 V or more says the can is bad and needs replacing, if you replace 1 you should replace the other with the same value.

dewickt said:

Most common failure for the MC240 resulting in low or no audio caused by low or no B+ are the 2 doubler capacitors. A quick check for this is to bridge them with another known good capacitor of 100 uf at 250V minimum and see if the B+ comes up, an increase of 20 V or more says the can is bad and needs replacing, if you replace 1 you should replace the other with the same value.

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Thanks Terry,

I plan on recapping the entire amp. I also have a MX110z that I will be sending to you as soon as you start accepting work again. Cosmetically it is near mint. A small scratch on one of the aluminum caps that frame the face is the only thing keeping it from looking new. I'll ship it as soon as I know you are ready to receive it.

PJ

mibag6 said:

Here's one kit source:
http://www.mcshanedesign.net/citpage.htm#parts.htm
Scroll down the page for Mac info.
There are some kits available on ebay also but I've no experience with the people selling them.
Mr Mcshane can accommodate you with probably any type of kit you'd like, from basic rebuild to heavily modified and he's very helpful with any info you may need and very reasonably priced.

Click to expand...

Thank you. Exactly the sort of thing I'm looking for.

Related question to you and anyone else who cares to chime in. The 240 will be completely recapped. Should I be worrying about the value of the amp fluctuating due to the caps I choose to use? I'm not looking to 'cheep out' the project, I'd actually be willing to spend more money for higher quality. But I worry that people may look at it as 'non-vintage' if modern caps are used? Then again, I really don't expect to ever sell it so in the grand scheme, it doesn't matter.

sxty8goats said:

Thank you. Exactly the sort of thing I'm looking for.

Related question to you and anyone else who cares to chime in. The 240 will be completely recapped. Should I be worrying about the value of the amp fluctuating due to the caps I choose to use? I'm not looking to 'cheep out' the project, I'd actually be willing to spend more money for higher quality. But I worry that people may look at it as 'non-vintage' if modern caps are used? Then again, I really don't expect to ever sell it so in the grand scheme, it doesn't matter.

Click to expand...

Jim supplies a nice rebuild selection - resistors too. I've used his kits for a pair of MC30s I rebuilt and they sound pretty amazing. Dead quiet, very open with great bass, mids and highs.

As to the decrease in "vintage value", yes, it will decrease, but if you are concerned about that sell it now before you do anything. If you want a great sounding amp, modern spec parts will augment that great circuit design. You won't regret it.

x3workshop said:

Jim supplies a nice rebuild selection - resistors too. I've used his kits for a pair of MC30s I rebuilt and they sound pretty amazing. Dead quiet, very open with great bass, mids and highs.

As to the decrease in "vintage value", yes, it will decrease, but if you are concerned about that sell it now before you do anything. If you want a great sounding amp, modern spec parts will augment that great circuit design. You won't regret it.

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That is pretty much where I'm at right now. I have no intention of flipping this amp. It is going to run my analog rig. I've talked to Jim via email and will be purchasing his kits as soon as I get the pp invoice from him.

I hope that your 240 restoration goes well.As others have posted here,Jim McShane is a good source of components and a really nice guy to work with.I know that coupling caps influence the quality of music reproduction.Do the type of resistors used to replace the original carbon comp. influence the sound as well?

Looking forward to some pictures.

drillher said:

I hope that your 240 restoration goes well.As others have posted here,Jim McShane is a good source of components and a really nice guy to work with.I know that coupling caps influence the quality of music reproduction.Do the type of resistors used to replace the original carbon comp. influence the sound as well?

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Good quality metal film resistors are quieter (less hiss) than the original carbon comp resistors. When you replace your caps, might as well take care of the resistors too since you're working those terminals anyway. They are more than likely off spec given their age.

I replaced the coupling caps and 99% of the resistors.Other than hiss noise,the type of resistor will not chance the sound qualities?

x3workshop said:

Good quality metal film resistors are quieter (less hiss) than the original carbon comp resistors. When you replace your caps, might as well take care of the resistors too since you're working those terminals anyway. They are more than likely off spec given their age.

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I had planed on testing the resistors to see if they were still in spec and leaving them if they were. didn't realize that there was a possible improvement by replacing.

When Jim sent me his parts lists the cost on the resistors was @ $30. For that money I ordered them as well, just in case. I guess I'll be doing them as well then.

There will be plenty of pics once I get going. I started setting up the work area this weekend. Small apartment, had to turn the art studio to a tech bench. This is probably going to be a slow moving project. I can decipher schematics. To say I can 'read' them is a bit of a stretch.

Have you worked on a Mac tube amp before?

drillher said:

Have you worked on a Mac tube amp before?

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I have not. But I have built a couple solid state pre-amps, wired a few tone arms, rebuilt a couple SL1200Mk2's and have a pretty good head for learning what I need to know if I set my sights on a goal. I wouldn't touch the Mx110z. There is way more going on there. Most of what I've read seems to indicate that a complete cap / resistor swap in a MC240 is within my abilities. I'll be sending the Mx110 to Terry.

The most difficult aspect for me was de-soldering the original leads from the lugs on the board.McIntosh really built their tube amps well and firmly twisted the leads on the lugs before they soldered them.I used a solder sucker and needle nose pliers.I took it slow...

drillher said:

The most difficult aspect for me was de-soldering the original leads from the lugs on the board.McIntosh really built their tube amps well and firmly twisted the leads on the lugs before they soldered them.I used a solder sucker and needle nose pliers.I took it slow...

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It's a slow process. I used a desoldering gun, solder sucker and solder wick at various points. Just don't be too rough as the terminals can be brittle. It is easier if you snip the old leads first and then remove them.

I cannot stress enough how taking photos of every bit of the amp & board, even as you work, will help you in your rebuild. It's a good way to double check your work when you are finished.

drillher said:

The most difficult aspect for me was de-soldering the original leads from the lugs on the board.McIntosh really built their tube amps well and firmly twisted the leads on the lugs before they soldered them.I used a solder sucker and needle nose pliers.I took it slow...

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Seems to be a running theme with my projects. I just redid a guitar for my cousin that had a bad jack. For a POS/ bottom of the line Epiphone, it was wired well. Crap components, wired well...

Just ordered a book on tube amps. Figured it would be a good read before I got into it.

Are you going to order all of the components from Jim McShane?

drillher said:

Are you going to order all of the components from Jim McShane?

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Already did. They shipped Monday. The whole kit&kaboodle was under $200. I realize that the appearance of the amp won't really resemble a stock amp but from what I've read and heard, it should sound the same and with possibly less hiss / blacker background. I'm good with that. I have no plans on ever selling it.

The mx110 is shipping out to Terry as soon as McIntosh delivers the box I ordered yesterday. There is way more going on there than my limited skill set can address.

Take pictures of everything before you start.You can also make a drawing of the resistor/cap group that you are starting.I did one set of resistors at a time to reduce the chance of mistakes.You need to be patient removing the leads of the old resistors and caps from the lugs so as not to break off a park of the lug.Jim set me mostly Russian K40 PIO coupling caps to replace the bumblebee caps.
Dave