Power supply components in mc240

[terry dactel]

Hi all hope someone can help a noob out with some mc240 power supply questions.I need to order some parts for my mc240 but before I can do that I need to know the correct name's of components that are in the power supply and where they are located in the picture below.
Thank you Paul

 

[dshoaf]

Here's a list of parts I used with an MC-240 I redid some years ago. This will include the coupling caps, too, so check your schematic with the references for details. Also do some searching for other recent discussions on MC-240 restorations. We seem to have a run on MC-30s and 240s of late.

The key thing to remember is that there were production changes made to amps so there _may_ be slight differences in reference numbers and parts values.

Cheers,

David

 

[terry dactel]

I think I've read just about everything on mc240 here at AK over last two weeks, David I've noticed your input in just about everyone of them. I'll take this opportunity to say thank you for your input on the McIntosh forum. You and Terry along many other regulars have been a great source of inspiration for me to take on the restoration of my MC240.

The trouble I'm having with the posts, is they don't cover the fundamentals for the novices like me, its assumed that everyone knows what a, B+ or selenium rectifier or a doubler cap looks like and where it is found from the get go.
In a nut shell, could you please name the the components out lined in red inthe pictures below.

Thanks once again from the greenest of greenhorns

 

[dshoaf]

I can understand why you can feel that way. I know it can be frustrating, for sure. I remember puzzling over the very same things when I was still in high school. In the little town I grew up in there wasn't anyone who could explain things to me either.

Ok, first things first, then. Tell us about what sort of mearuring equipment you have: a DVM? How about a variac? What else, if anything?

Next, to your picture:

- The 2 black cased caps are very likely the 2 voltage doubler caps. They are marked as C23 and C24 on the schematic. From the pic, yours appear to have been replaced, I think. They should read 250mf in value.

- The other 2 silver-cased caps are the filter caps. If you look at the schematic and trace the wiring, they should be C25 and C26. You will find that these have more than one terminal on them. This is because they are multiple caps stuffed into the same can. There cans will be stamped with the values and a shape that corresponds to the terminal on the end of the can with the wire(s) connected to it. Before you start _any_ work, you should make a drawing of all of those connections, with wire colors and where they go. Take lots of closeup pictures, too.

Next, the small board you have there contains the power supply diodes (there's 2 of them - those small little bullet things with a black band toward one end. Those are SR1 and SR2. They've been replaced, BTW.

Next from the left, that gray thing with the 2 legs sticking out is the selenium rectifier. The fat cap beside it is the filter cap for the bias supply, C27.

So, I'll ask a few more questions, here. How comfortable are you working with voltages in excess of 300V? Do you know some of the precautions you need to take to do so? Has anyone ever explained how to take high voltage reading with one hand in your pocket?

I ask as I am concerned about anyone choosing to work on old tube gear. We want you to be able to come back and tell us how well it works and not that you ended up hurt or, worse, killed!

Cheers,

David

 

[terry dactel]

Thank you for your detailed reply David.My over all plan for the amp is to replace the chrome top chassis, replace all the smaller resistors, on the tag board, and replace the larger bumble bees on the opposite side of the tag board that measure out of spec , whilst keeping the black beauty's.

Change over the RCA inputs, clean up the tube sockets, and replace the rivets that hold in the tube sockets with small blind pan head screws with washers and nuts, replace the original power supply cord.

I don't have any equipment apart from a soldering iron, and wouldn't have a clue how to take a volt measurement. But I do have a friend who has built and designed tube/ ss gear, who has all the electronic equipment to do the job.

He has offered to diagnose what's wrong with the amp, and watch over my shoulder whilst I do the work, I could wait around and ask him to answer all these questions I have. But that's lazy man's way out, because I fugue it's my responsibility to at least come up to speed, by finding what and where these components go and try to get a basic understanding of what these components do. and the best place to learn this on the McIntosh forum.
Thanks Paul

 

[dshoaf]

Ok, then, you've got someone local to help out - and with test gear. Cool. I guarantee that you'll learn a lot just going through the steps. There are some books you can purchase to help understand more about the use of resistors and other components.

To get more theory under your belt, I'd suggest you take a look at the book counter at www.tubesandmore.com for some examples. Don't overlook the guitar amp restoration books, either, as the same circuits are used there.

BTW, the term, B+, is an old radio reference back to when a typical radio had batteries inside - 1 for the high voltages, one for the bias circuit (known as B-) and another for the filaments. The B+ side, then, still refers to the high voltages in tube amps today. You'll also see the term used with Solid State gear, too, on occasion. It can be replaced with the term, voltage rail(s), as well.

Cheers,

David

 

[terry dactel]

Thank you David ,the project will probably take a few months, and will depend on when my mate can help me out I don't want to impose on him.

I've checked out Jim McShane's web site and will use his refrub kits ( for the smaller resistors and larger bumbles bees also the orange drops) , that he and Michael Samra have put together.Like you say the power supply has already been done so I'm hoping that it will be ok.

http://pages.prodigy.net/jimmcshane/citpage.htm

 

[dshoaf]

I have had very good success with Jim McShane as a supplier. He does know his stuff.

On preparing to rebuild, you can go ahead and check the power supply voltages now. As a matter of fact, it isn't a bad idea to go ahead and take a whole series of operating voltage measurements of the amp as it is. Same for the resistance measurements.

This will do 2 things:

- Voltages: Allow you to have something to refer to when you've rebuilt the amp and start taking measurements then.

- Resistance checks: This will tell you if you might have something mis wired before you apply power for the first time after the rebuild.

The more notes and pics you take now the better off you'll be when it comes time to redo it.

As me how I know this......

Cheers,

David

 

[terry dactel]

Don't worry about that David I'll be taking more photo's than Annie Leibovitz, do more measurements than Gordon Gow and sketch more diagrams than an architect.
I'm a carpenter by trade and work systematically.Cheers Paul.

 

[drillher]

Mike Samra wrote in a therad on Citation amps that using the McShane power supply and doubler cap values makes a big improvement to the Mc240.