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K200B-2 Hissing [message #25659]

Sun, 23 October 2016 11:32

slk854
Messages: 192
Registered: January 2015

Senior Member

I have one of my 200's that has a hiss when you power up the amp. All knobs can be turned down to nothing but still the hiss. When you start playing the sound does drown out the hiss, but it is still there. I can flip the power switch left or right and no difference. It is not unbearable but it is annoying to hear. Any suggestions to try???

Steve

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Re: K200B-2 Hissing [message #25660 is a reply to message #25659]

Mon, 24 October 2016 06:07

stevem
Messages: 4774
Registered: June 2004
Location: NY

Senior Member

Hiss is like white noise as when your TV has no antenna hooked up, and its frequency is around 8kz.
The polarity switch will only reduce hum which is 60hz based , so as you found out it does nothing for hiss reduction!
Hiss is caused by gain and sometimes the transistors in our amps get noisy and make even more hiss.

On top of that certain types of caps in the amp drift in value as they age and make a transistor apply even more gain which in turn makes more hiss!
one test you can make to try and narrow down which section of the amp the hiss is coming from is to take the signal off if the RCA Jack on the rear of the amp and pump it into another amp.
If with both amps playing at the same volume you find the hiss to be less on the amp fed by the RCA Jack , then your hiss issue lies in the power amp section of your amp.
If not then there is a lot of other sections in the amp that can make for excessive hiss the the troubleshooting to find that is far more intense!

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Re: K200B-2 Hissing [message #25661 is a reply to message #25660]

Mon, 24 October 2016 09:00

slk854
Messages: 192
Registered: January 2015

Senior Member

So do I connect a cable from the rca jack to the rca jack on the other amp or to the input of the other amp? If to the input then I will need to make up an adapter to go from rca to 1/4 phone...right??

Steve

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Re: K200B-2 Hissing [message #25662 is a reply to message #25661]

Mon, 24 October 2016 14:29

slk854
Messages: 192
Registered: January 2015

Senior Member

Ok I plugged into the rca jack on the back of the hissing amp, and the other end went into the input of another amp. I did not get any hiss at all through the second amp that I plugged into. Now keep in mind that the hissing occurs with the volume all the way off. Turning the volume up on the hissing amp DOES NOT increase the volume of the hiss/white noise in the slightest.

Steve

[Updated on: Mon, 24 October 2016 14:33]

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Re: K200B-2 Hissing [message #25663 is a reply to message #25659]

Mon, 24 October 2016 16:44

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

You have just proven that the hiss is being generated in the power amp. Which would make sense as the controls don't change the hiss level.

In my experience, hiss can be caused by multiple things. The two most common causes are transistors and carbon composition resistors. Finding the problem will not be easy. Sometimes heating or cooling parts will cause the problem to get better or worse.

Sometimes the only way to fix it is to rebuild the entire circuit.

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Re: K200B-2 Hissing [message #25664 is a reply to message #25663]

Mon, 24 October 2016 20:16

slk854
Messages: 192
Registered: January 2015

Senior Member

So could I try turning it on for several hours and see if the level diminishes any?

Steve

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Re: K200B-2 Hissing [message #25665 is a reply to message #25659]

Tue, 25 October 2016 06:07

stevem
Messages: 4774
Registered: June 2004
Location: NY

Senior Member

You can leave it on for days if you care to without harm, if it will help I can't say!
You need a Scope and good trouble shooting ability to pin this one down.

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Re: K200B-2 Hissing [message #27101 is a reply to message #25664]

Mon, 12 November 2018 13:44

JefCon4
Messages: 1
Registered: November 2018

Junior Member

Did you ever figure this out? I am having the same problem with my K200B-5.

I have confirmed it doesn't live anywhere in the preamp or reverb board.

Any advice?

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Re: K200B-2 Hissing [message #27102 is a reply to message #27101]

Tue, 13 November 2018 06:07

stevem
Messages: 4774
Registered: June 2004
Location: NY

Senior Member

like Bill posted on this question back in 2016 it could be many things , and even some in concert with each other.
I would start off by replacing Q701 and Q702, or at least take voltage measurements and compare them to the PC703 schematic to see how Q700 thru Q703 are biased.

If you do such then note the test settings on the lower right of the schematic.

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