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K200B-6 [message #27505]

Mon, 16 September 2019 11:08

aeroboy47
Messages: 15
Registered: September 2019

Junior Member

Amp makes loud pop when powered on. Other than that it works fine. Have checked numerous voltages and all are good according to schematic. If anyone has had a similar problem corrected, I would love to hear about it.

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Re: K200B-6 [message #27506 is a reply to message #27505]

Tue, 17 September 2019 06:06

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Hello!
The number 1 main reason for the loud pop instead of the normal thump that the amp should make is due to the two big can type 4500 / 50 volt power supply rail filters needing to be replaced..
A secondary reason could be the many smaller electrolytic type caps on the other circuit boards that the amp has.

To nail down where the issue is coming from you can do this check out.

The blue wires from each front mounted PC board carry the audio signal to the amps rear wall mounted PC703 board.

If you cut open each blue wire one at a time and then power on the amp again to can then confirm if it's either of these two boards that are making the loud pop .

These wires can the easily be soldered back together and insulated.

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Re: K200B-6 [message #27507 is a reply to message #27506]

Tue, 17 September 2019 16:40

aeroboy47
Messages: 15
Registered: September 2019

Junior Member

Thanks. I will try your suggestion. I have also found that one of the output transistors has nearly the same resistance (12k/13k)between the base and emitter.

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Re: K200B-6 [message #27508 is a reply to message #27505]

Wed, 18 September 2019 06:37

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Well that may be leaking D.C. Voltage and making for that bang at power up.

For all of the well less then bucks that these 2N3055 output Transistors cost each to day I would get a pair of them like the NTE131MP and install a new one on each side of the amps thermal cut off switch that sits between them and see if that clears up the issue, as., there far cheaper then new computer grade main can filters !

This will help to keep the output stage balanced for lower distortion and low output hum levels.

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Re: K200B-6 [message #27511 is a reply to message #27508]

Wed, 18 September 2019 11:16

aeroboy47
Messages: 15
Registered: September 2019

Junior Member

I'm going to replace all four of these output transistors. Resistance readings are not exactly what I would expect from any of them. I'll let you know if this corrects the problem. Thanks for the input.

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Re: K200B-6 [message #27512 is a reply to message #27505]

Wed, 18 September 2019 12:01

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Are they all original RCA 36892 Transistors?

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Re: K200B-6 [message #27513 is a reply to message #27512]

Thu, 19 September 2019 13:25

aeroboy47
Messages: 15
Registered: September 2019

Junior Member

Yes, they are all the original ones

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Re: K200B-6 [message #27515 is a reply to message #27505]

Thu, 26 September 2019 16:08

rodak
Messages: 516
Registered: October 2001
Location: Georgia

Senior Member

Do the 2N3055s need to be "matched" in any way, or is that non-critical?

www.combo-organ.com

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Re: K200B-6 [message #27516 is a reply to message #27505]

Fri, 27 September 2019 05:43

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Ideally they should all be matched pretty close as that will allow the lowest amount of D.C. Voltage offset ( hum at idle) and the highest level of clean output wattage before clipping/ distortion.

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Re: K200B-6 [message #27655 is a reply to message #27516]

Mon, 10 February 2020 15:49

rodak
Messages: 516
Registered: October 2001
Location: Georgia

Senior Member

(Catching up on my reading here)

Steve, how do you go about matching a pair of 2N3055s for he output stage?

www.combo-organ.com

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Re: K200B-6 [message #27656 is a reply to message #27505]

Tue, 11 February 2020 08:58

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

3 ways to go about this without a Transistors matching / testing rig.
1) get a fist full of 2n3055 Transistors and install them one at a time while measuring and noting the D.C. Voltage on the speaker jack with no speaker hooked up .

The lowest D.C. Voltage seen will be a decent combination of matched Transistors.

The other way is to just buy 2 matched pairs of NTE130MP Transistors .

The 3rd way is that some folks offer a matching srervice that you can find online.

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Re: K200B-6 [message #27657 is a reply to message #27505]

Tue, 11 February 2020 10:36

rodak
Messages: 516
Registered: October 2001
Location: Georgia

Senior Member

Thanks, Steve. I think I actually have a matching rig I built a long time ago to test/match some power transistors in my Krossroad 500 head. I'll have to dig that out and post the schematic here to see if that's what you're talking about. Or do you have a link to a suitable rig I could compare it to?

www.combo-organ.com

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Re: K200B-6 [message #27659 is a reply to message #27505]

Wed, 12 February 2020 05:54

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

If you've made a rig that can do matching for the 500 then your set !

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