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D-150A II Hum in speakers


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#1 P=IV

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Posted 28 September 2011 - 09:42 AM

I have two D-150A II amplifiers being used in a fixed PA application. Both are bridged in mono and used to drive one speaker cabinet per amp. Recently one of the amps developed a low level 60 Hz hum in the output with or without an input connected to the amp. I'm guessing a dry electrolytic or maybe a bad ground. Do these amps have a known common fix? Can someone perhaps point me to the components on the schematic? Thanks for your assistance!

#2 KAW

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Posted 29 September 2011 - 10:07 AM

QUOTE(P=IV @ Sep 28 2011, 10:42 AM) View Post
I have two D-150A II amplifiers being used in a fixed PA application. Both are bridged in mono and used to drive one speaker cabinet per amp. Recently one of the amps developed a low level 60 Hz hum in the output with or without an input connected to the amp. I'm guessing a dry electrolytic or maybe a bad ground. Do these amps have a known common fix? Can someone perhaps point me to the components on the schematic? Thanks for your assistance!



It could be cap related. Make sur ethe main supply caps the screws that mount them are tight. Over the years the fiber washers can dry causing the filter caps to become loose. Also put the amp back into stereo mode to see if this is on one channel or still both. Since you are in bridge mono both channels are linked together. The most common thing over the years would be C4 and C5 on the main board getting leaky. These supply the 10V to the input IC's and can cause noise problems when they go bad. So make sure filter caps are tight and replace C4 and C5.

Thanks,

Kip Whitehead
Crown Technical Suport

#3 jmann

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Posted 10 March 2012 - 08:49 AM

QUOTE(KAW @ Sep 29 2011, 10:07 AM) View Post
QUOTE(P=IV @ Sep 28 2011, 10:42 AM) View Post
I have two D-150A II amplifiers being used in a fixed PA application. Both are bridged in mono and used to drive one speaker cabinet per amp. Recently one of the amps developed a low level 60 Hz hum in the output with or without an input connected to the amp. I'm guessing a dry electrolytic or maybe a bad ground. Do these amps have a known common fix? Can someone perhaps point me to the components on the schematic? Thanks for your assistance!



It could be cap related. Make sur ethe main supply caps the screws that mount them are tight. Over the years the fiber washers can dry causing the filter caps to become loose. Also put the amp back into stereo mode to see if this is on one channel or still both. Since you are in bridge mono both channels are linked together. The most common thing over the years would be C4 and C5 on the main board getting leaky. These supply the 10V to the input IC's and can cause noise problems when they go bad. So make sure filter caps are tight and replace C4 and C5.

Thanks,

Kip Whitehead
Crown Technical Suport


Kip - Have a "twin" to the above I use for home hi fi. Tightening the PS caps fixed the problem initially, but it came back over time. Also makes a "crack" on power-up and is getting worse as is the hum. Both channels but worse on the right but is not related to gain setting. Then did the C4 and C5 R&R with no improvement. Main PS caps failing?  Ideas? Thanks.

#4 joust

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Posted 13 April 2012 - 11:18 AM

Some older 150s have mica insolators for the output devices. I've seen (heard!) really weird noises caused by leaky isolators. I replaced them with silicone type insolators and the noise is gone! If micas are in place, I would still change them...