McIntosh A129 Monoblock

a129

You never know what you might get, I bought a weird looking McIntosh at a garage sale 20 years ago, I thought it was a piece of test equipment, dont know where it ended up, when I started working at McIntosh I realized it was a 10W-50!

I am off to CEDIA now, so anybody near Indy should come see the McIntosh booth , we have a 3,700 watt theater with 118 drivers!

as for opening the transformer, I am not a restorer, so I dont really know. Ron-C gave me a clue for where to find history on that a129, so I will look some more next week,

play 'em loud,
 
Update

Well, since the amp's not doing anyone any good in its present state, I decided to begin work. First thing I had to do is to get the transformer covers off and see what damage was done inside.

After drilling out the rivets holding the cover to the amp, I discovered that I didn't need to drill them out at all :mad: . The covers come off with the bolts from the bottom. Not a big deal though, as I can match the rivets up from the body shop at work.

I get the cover off, and the transformer is potted in about 5 pounds of black tarry goo :mad:

I tried applying some heat with a heat gun, and that got some of it, but it would have taken forever. So, brought it to work, and fired up the oxy-acetylene torch up on low heat and went over the outside case a few times. In about 15 seconds, the tar holding the transformer in place loosened up, and pop! Out came the transformer! :) . Getting the tar off after that was simple. I'm going to save it and reheat it to seal it up when I'm done.

My only problem is that, since it seemed far easier to snip, disconnect, number, and then reconnect wires than to desolder them and fish them around and out (I have a far beter chance of getting it right personally doing it that way, trust me on this one), I labelled all of the wires by number, and of course two of my number tags got burned up :mad:

So anyway, here's where I'm at......the cover is going to the autobody shop at work tomorrow.
 
Tim,

Good for you. I'm glad to see you tackling this project. :) Have fun.

Shouldn't be too hard to figure out what wires those are that lost the ID tags with an ohmmeter. What one was it?
 
Actually I don't think it's going to be hard either, there seems to be enough color left in the fabric wires to match them up.

So the next question is, what would be the best way to test the transformer to see if it's any good? How do I go about it?
 
First thing will be to ohm it out, DO NOT apply ac power to it yet. There will be two wires that are the input to the primary windings on the amp. This is where the 120vAC would go from the wall. Now there will be several secondary wires. These will ohm out to different ohms, as they supply different AC voltages to the amp. Without a schematic I can't tell you what the ohms are or what the voltages would be if power was applied, but you should get some ohm readings. It should not read open or no ohms.
 
Transformers can only go bad if they get shorts between turns or to the case, or if windings go open. It is extremely unlikely that they are bad unless the dents in the outer case mashed the winding stack. If the inerds look physically OK I'd assume they are good and proceed with restoring the amp. You could do an ohmeter check of the windings. This is when you need that service manual I steered you to on ebay with the schematic. It would likely have had the winding color codes identified. Mc used cathode feedback on their output xfmers so there is an extra winding or two to be found from conventional output transformers.
 
Update

Well, we've got the dents and holes in the one said transformer can all patched up and in primer. I'm going to wait until the transformer is tested and repotted back in the case with all of the original tarry stuff to apply a coat of paint.

I set the case still in primer on top of the chassis to show how much of a difference from A to B :)
 
All of the holes were spot welded and ground before the glass went on, just a skim coat as they hammered out the dents too :)
 
Metric rant

OK, I gotta question. I see your new signature line has misleading temperature information. How can it possibly be so many degrees C in Buffalo? That's like my Canadian weather channel reporting that "they had so many centimeters of snow fall in Buffalo". Can't happen. Buffalo news services know how much snow falls in Buffalo and they will tell you "so many inches of snow fell".

Same reason Canadians don't have to worry about Americans taking our water. Canadian water boils at 100 degrees. Your water boils at 212 degrees. You wouldn't be able to use our water because your coffee made from it would be too cold, your clothes wouldn't come clean in a hot water wash, etc.

Actually, modifying the American tempertaure (an American commodity in this case) reading to appear like a Canadian commodity is obviously something done for the purpose of exporting a service to Canada. That being the case aren't there issues like export and import duties and tarriffs to contend with by using your new signature? Is this even covered under NAFTA? :D
 
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I'm somewhat disappointed. I thought my rant would have gotten a response from someone. Everybody sleeping out there or what? :D

If y'all were worried that I'd lost my sanity and were politely abstaining from commenting, don't worry I was just kidding. Everyone pretty much knows the Americans would love to get their hands on clean Canadian water, ...despite the fact that it now boils at a much lower temperature! ;)
 
Update

Well, I've decided now I may have a little time to invest in this amp once again, so I placed it back on the workbench.

Let's get some input from you guys. Removing the transofrmers is pretty necessary to get all of the dings and dents out of the cases, so even if they're okay, the effort is well worth it. But after examining the amp in detail, I'm thinking, what went wrong here???

So what I've done is take a few pics and I'm going to give a little explanation to what I see, and see if anyone here can make a perspective call. Now mind you I have no idea where a cord for this amp might even be attached, or if it runs off 120 VAC at all, since I don't know what this amp is. But, it looks like everything that did happen to lead to its demise happened all at once and for one reason only, at least that's my guess.

Here's the first pic, which is a burned unknown resistor. It goes from pin 4 of the one KT88 to the junction board, to what appears to be a wirewound resistor, to pin 3 of the next KT88.
 
This is a 21 ohm resistor, 10% tolerance, from pin 4 of the 12AU7 to pin 3 of the 12BH7. You can also see a couple other burned resistors on the board in this shot.
 
Shot of the amp's underside with the the cover in place. Note that it is a perforated cover, and why the cutout????
 
Kamahiri said: what appears to be a wirewound resistor, to pin 3 of the next KT88

Digging way back into my tube amp design and building days, I think those "wire-over' resistors are not wire wound resistors. I think they are chokes wound over carbon resistors and are used to supress transients - usually in the cathode or screen feeds. It's been a long time but do recall actually winding several of these by hand when I couldn't aford to purchase new components.

Anyone else recall this product?

Paul

PS: Nice project! Makes me a bit sad that I gave away my last home-built amp last year.
 
OK, you wanted input.

First, have you metered the transformer as per Wardsweb's suggestion? That is a required first step. Because determining transformer windings can be a bit hit-and-miss unless you have the lead colors from a schematic.

Next, it would be best if there was a schematic of this puppy. Any chance at all? If not available, can you draw one out from the physical amp? It would help a lot. I have some typical circuits around here somewhere and we can do some matching once we can see what's what.

Paul
 
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