|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
63.249.84.6
In Reply to: McIntosh forum/servicing info posted by ketchup on April 10, 2007 at 17:20:49:
You can search here and in the Vintage Asylum for many posts by a number of folks, including me - search on Doodlebug and you'll get a number of them. Like a number of members here, I've redone a number of Mac tube amps including a couple of MC-225s, which I still have.Also AudioKarma.org has a Mac forum where you'll find much the same information. Check their Hints and Kinks for little details, too.
Specifically, what technical info are you looking for?
Cheers,
Follow Ups:
Thanks, Dave. Much appreciated.I bought this MC-225 and it is in VERY good cosmetic shape. Just a few letters missing here and there, shiny chrome, nice black paint, & original tubes (except one).
Anyhow, I brought it up on a variac and then tried it out with my Jolida JD-100 straight into it. It sounded delicious. Buttery smooth. I was in awe, really. After a few hours I noticed that either the right channel was getting louder or the left channel was getting quieter. The left channel gain had to be raised significantly to get things centered.
I want to find the cause of this. I think I read somewhere that this might be caused by resistors that change value as they warm up. I'm interesting in determining which are the offending resistors and replace them. I also imagine that there are some caps that should be replaced. I'm not sure I want to replace them all right now, but maybe just some of the more crucial ones. I'm not sure how I feel about the possibility of changing the sound of the amp by switching too many caps, but if it's necessary I will. I'm willing to do what it takes to turn this into a reliable, good sounding amplifier that functions as it should.
Thanks again for the info, David. I'll check those places out.
David
You hit it on the nose..I do a lot of mac rebuilds and the biggest thing is out of tolerance resistors and the .047 caps in the phase splitter are always bad if they are original..The original caps throughout the amp are paper type and the resistors are carbon comp.I replace with new carbon comps or carbon films but I go the next wattage up.If its a half watt i use a 1 watt and so on.The 12k 2 watt resistors I change with a 4.7w wire wound type.The coupling caps I use vitaman q paper in oil and it keeps a similar sound only its better and silkier and detail and depth come to new heights in this amp.
Filter caps are another big issue in these amps and I like to use a low esr filter cap like jim mcshane sells such as the F&T or the JJ but you can also use the twist locs but they arent as good.You can put the caps in with rtv and that way you can go back to the original twist loc type if you want to sell it later but those amps are soooo good you would crazy to sell it.
Thanks for all the info!
So I'm not imagining this change in level in the one channel? Out of curiousity, as the resistors warm up, do they loose or gain resistance? To find the offending resistors, should I measure them when cold and measure them again when warm?
1) Safety first
- Replace the power supply caps - all of them - *and* the recitifer in the bias circuit. You'll need a variac to reset the bias voltage once you've rebuilt the bias circuit. I normally swap the filter cap and the 2 resistors while I at it. It too easy to do it right the first time.
- Consider this insurance so that you don't have a problem with power transformers getting too hot.2) Improve the audio chain
- Swap out all the "bumble bee' caps - again, no discussion - just do it. They weren't that good to begin with and they are guaranteed to fail.
- Next, bring up the amp and let it cook for a number of hours. Watch the bias voltage and the plate voltages on the output tubes. Monitor the heat of the power transformer.
- Then take measurments of the plate, grids and cathodes on all the tubes. There's a table of the values in the MC225 service manual. Copy it and pencil them in.
- Suspect any resistors around voltages that do not fall within spec.
- Power down the amp and test the resistors separately. Replace any that are out of spec. Also, while you're in there, use a small hand reading glass (magnefying glass) and look for hairline cracks in all resistors. These will cause noise and will fail at some time in the future.
- Bring up the amp again and use it for about a week, if all voltages still check out.
- Repeat the above procedure again in about a month.This is the way you suss out wayward resistors in these amps. Both of my MC225s have had about 33%-50% of the resistors replaced now but they're rock-solid stable. I also did what Mike S did and went with the higher wattage where possible. On the large resistors, I stood them further off the boards to prevent heat damage.
Hope that helps.
Cheers,
Thanks for all the great info so far!
After some looking around, I found this guy on Ebay selling a set of PS caps. Does this look like a good way to go? It also includes the rectifier and the bias filter cap.http://cgi.ebay.com/Power-Supply-Capacitor-Refurbish-Kit-McIntosh-MC-225_W0QQitemZ280099208350QQihZ018QQcategoryZ71573QQrdZ1QQssPageNameZWD1VQQcmdZViewItem
David, when you said that you usually replace the two resistors, which ones do you mean? Another question... Are the bumble bee caps all of the black ones? It looks like I have 6 in my amp. If you guys think they should go then they go.
While poking around in the amp, I noticed a metal "disc" stuck between the chassis and the big filter cap (C29). Well, it used to be part of the thermistor. How was this originally attached to the thermistor? I *guess* I should put it back on!
Thanks again for all the help. You guys are a wealth of information.
ditto
This post is made possible by the generous support of people like you and our sponsors: