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pulsating loud bass static sound [message #20620]

Thu, 15 May 2014 15:11

vintagefan
Messages: 166
Registered: March 2014
Location: Dallas

Senior Member

Any ideas on this problem?

One of my K100C-8 4x10 amps started having a pulsating bassy static sound when I
turnd it on. It is quite louder than the guitar 's sound is. It started out less loud lasting 10 seconds, then louder and lasting up to a minute before it faded out.... But it began lasting longer. It pulsates even with teles and my Gibson
ES135 with humbuckers.....I originally just played a tele with a lot of treble 1meg tone pot with no problems from the amp. But when I played my Gibson ES135 with humbuckers, the amp began a loud pulsating bassy static sound which occurs everytime I turn the amp on and lasts a minute, even when playing a tele now. The pulsating bassy static sound occurs on powering on even with no guitar plugged in.

I took it to a tech 2 weeks ago. He's a tech who has worked on these back in the 70's. He powered it up today and he says it sounds quiet now with no pulsating bassy static sound. Could not being powered up for two weeks affect the quieting down of the pulsating bassy static sound? Any ideas?

Thanks.

[Updated on: Thu, 15 May 2014 15:13]

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Re: pulsating loud bass static sound [message #20621 is a reply to message #20620]

Thu, 15 May 2014 15:43

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

I know that we started to discuss this amp a few weeks back. Leaving the amp off for two weeks may have allowed something to happen, but I will guess that the problem will come back.

From your earlier descriptions, the problem is probably in the power supply or the power amp circuit. The only way to find it would be to monitor the voltages through the amp when the problem occurs. This way the problem can be isolated to a specific circuit.

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Re: pulsating loud bass static sound [message #20622 is a reply to message #20621]

Thu, 15 May 2014 15:49

vintagefan
Messages: 166
Registered: March 2014
Location: Dallas

Senior Member

Not sure if it makes any difference, but the pulsating bassy static sound occurs loudly even with the volume knob on zero. The tech is going to spend more time with it and install a 3 prong cord, so I'll let you know what transpires.

thanks.

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Re: pulsating loud bass static sound [message #20623 is a reply to message #20622]

Thu, 15 May 2014 16:20

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

vintagefan wrote on Thu, 15 May 2014 14:49

Not sure if it makes any difference, but the pulsating bassy static sound occurs loudly even with the volume knob on zero.

That is why I suggested that the problem is in the power supply or power amp. If it was in any of the preamp circuits, the controls would have an effect on the noise.

A three wire cord is always a good thing.

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Re: pulsating loud bass static sound [message #20625 is a reply to message #20620]

Fri, 16 May 2014 08:03

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Tell the tech that if he disconnects the red and green wires from the power supply filters that feed into each preamp and the problem still takes place then the issue is narrowed down to the power supply and or driver / output stage.
There are very few Tantalum type caps on the driver board and I would start by changing these out for all of the 3 dollar total parts cost that
it would be.
Also when the amp is acting up if a voltmeter is placed across the speaker jack and you see any level of DC voltage higher than .070, you`ve got electrolytic cap problems more than likely and those Tantalum caps can make for that, as can the two big main power supply rail filters.
I would print my reply here off as pass it along to the tech you use.

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Re: pulsating loud bass static sound [message #20629 is a reply to message #20625]

Sat, 17 May 2014 08:07

vintagefan
Messages: 166
Registered: March 2014
Location: Dallas

Senior Member

Thanks a million! Much appreciated.

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pulsating sound fixed! success... [message #20670 is a reply to message #20629]

Wed, 28 May 2014 20:49

vintagefan
Messages: 166
Registered: March 2014
Location: Dallas

Senior Member

Success! The bassy pulsating sound is fixed..... Tech replaced 2 transisters in the output section. Sounds like new now and quiet too. I found a good tech who has worked on these back in the 70's. He still has some NOS Kustom parts in his supplies.

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