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In Reply to: Re: McIntosh forum/servicing info posted by Michael Samra on April 10, 2007 at 22:36:05:
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?
Follow Ups:
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
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