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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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20th Jun 2017, 9:38 pm | #1 |
Heptode
Join Date: Feb 2007
Location: La Spezia, Italy
Posts: 834
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Cyrus two amp hum
Hi, I saved a Mission Cyrus Two amp from a skip
The amp works well with a slight background hum on line level inputs but when I select the phono input I get a really loud hum on both channels It was apparently stored in a damp place as it has a strong musty smell, however there are no traces of corrosion inside. The hum sounds muffled, not the typical high pitched open ground hum. Could it be a bad filter capacitor? |
21st Jun 2017, 11:29 am | #2 |
Heptode
Join Date: Dec 2016
Location: Sheffield, South Yorkshire, UK.
Posts: 539
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Re: Cyrus two amp hum
Almost for certain it's the capacitor.
The hum on the other might be caused by a bad earth connection. |
21st Jun 2017, 11:37 am | #3 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,059
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Re: Cyrus two amp hum
Less hum on line level input is a good sign, of course with a quality amp there should be none. It could be the main supply capasitors are bad, with the phono pre-amp being more suseptable to ripple - or there could be a different fault. Test capasitors for capasitance / ESR or, measure ripple with a scope - assuming you know how much ripple there should be under quiesent conditions!
dc PS: the musty smell could be a vented capasitor ... |
21st Jun 2017, 12:22 pm | #4 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,788
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Re: Cyrus two amp hum
Examine all electrolytics closely for signs of bulging and venting.
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21st Jun 2017, 3:59 pm | #5 |
Heptode
Join Date: Feb 2007
Location: La Spezia, Italy
Posts: 834
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Re: Cyrus two amp hum
There are four ROE branded 470mf capacitors which are slighly bulging, i'll replace them
Also there's a 100 ohm power resistor in place of D12 and R119 near it (also a 100 ohm power resistor) runs so hot you can light a cigarette on it, the print near R119 is quite scorched too |
21st Jun 2017, 4:49 pm | #6 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: Cyrus two amp hum
Not knowing the amp circuit, I wonder if excess current going somewhere (hot resistors) might be involved in the hum problem. It may be a symptom of something that's died in the power stages.......
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21st Jun 2017, 5:00 pm | #7 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,193
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Re: Cyrus two amp hum
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