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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: McIntosh forum/servicing info posted by ketchup on April 11, 2007 at 15:39:20:
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,
David
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Follow Ups
- Here's your priorities....... - doodlebug 20:53:03 04/11/07 (2)
- Re: Here's your priorities....... - ketchup 19:57:40 04/12/07 (0)
- Re: Here's your priorities....... - Michael Samra 05:02:01 04/12/07 (0)
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