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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 20th Jun 2017, 9:38 pm   #1
Jimmyhaflinger
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Default 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?
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Old 21st Jun 2017, 11:29 am   #2
Grubhead
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Default 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.
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Old 21st Jun 2017, 11:37 am   #3
dave cox
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Default 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 ...
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Old 21st Jun 2017, 12:22 pm   #4
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Default Re: Cyrus two amp hum

Examine all electrolytics closely for signs of bulging and venting.
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Old 21st Jun 2017, 3:59 pm   #5
Jimmyhaflinger
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Default 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
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Old 21st Jun 2017, 4:49 pm   #6
Herald1360
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Default 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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Old 21st Jun 2017, 5:00 pm   #7
Station X
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Default Re: Cyrus two amp hum

The circuit can be found here:-

https://www.vintage-radio.net/forum/...0&postcount=10
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