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K200 B1 crackling sound [message #12833]

Tue, 02 February 2010 01:51

real_to_reel
Messages: 11
Registered: September 2007
Location: Los Angeles

Junior Member

The problem started in the 70's and, repair shop never could fix. The amp is possessed with crackling sound that starts when powered up. Polarity and tone/volume controls do not affect it. I would like to fix this as I am playing again. If anyone has a schematic for the B200 it would be greatly appreciated as I'm quite sure I'll be asking many questions and need to refer to it.

The best description is that it sounds like a bad guitar cord that makes a crackling sound when moved while attached to a guitar and amp. The B200 crackles for a minute - stops - starts again for 2 minutes - stops for 5 minutes - starts again. Nothing is plugged into the head.

The only other known issues are: bulb missing - needs to be replaced, the original AC cord should be upgraded and, I noticed a spark as I plugged the speaker cable into the head.

Thanks in advance for any suggestions to solve this.

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Re: K200 B1 crackling sound [message #12834 is a reply to message #12833]

Tue, 02 February 2010 06:26

stevem
Messages: 4754
Registered: June 2004
Location: NY

Senior Member

Hi.
The first thing to do is to look at the top of the two big mian filter cans that are clamped to the floor of the amp.
on each can you will see a little round area that when all is well will look like a dimple.
If it looks puffed up like a pimple than these are the first two items on the list to get replaced before any more testing can take place, and in fact there is a chance that bad filters could be adding to the problem.
If all looks well, continue on with what I have posted below.

Your amp is made up of five main sections.
Main power supply
preamp power supply regulator
preamp channel #1
preamp channel #2
driver/output stage.
We can go about using two different ways of at least removing the preamp channels from the problem list.

One way will involve unsoldering two wires, the other way will invole clipping two wires and butt splicing them back together.
Option #2 will not require you to dismount eitheir preamp board( left channel or right) off of the inside face of the amp.

Each preamp board has a three wires goind to it.
red
green
blue
The blue wire is the signal output which carrys the signal to the driver/output board mounted on the rear wall of the amp.

By clipping each blue wire somewhere along its lenght where it will be easy to incert a butt splice to get it hooked back up we can check to see if the noise stops.
If after clipping each blue wire you still fine you have the noise then each preamp board can be concidered ok.

Another easy test to make if you have a volt meterwould be to test the regulated 8 volt supply that feeds each preamp board.
This voltage is feed to each preamp thru the green and red wires.
The red wire carrys a +8 volts DC, and the green carrys a -8 volts DC, so these must be tested across each wire to ground not by placing the meter across the red and green wire.
These test should show a voltage of around 8 volts. It could be a spec higher or lower but it should be stable with the meters reading changing only down in the .001 descimal point area.
any big jump in the reading( a higher voltage) in time with the noise would mean a problem in the regulator circuit on the rear mounted circuit board.

Try these test and post your findings and we will continue with the search for the root of the problem if these things have not narrowed it down.

[Updated on: Tue, 02 February 2010 06:32]

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Re: K200 B1 crackling sound [message #12842 is a reply to message #12833]

Tue, 02 February 2010 12:19

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Welcome to the place.

It sounds like a power supply or power amp problem. As Steve mentioned, there are many places that the problem could be located.

I'm not familiar with the B200 model, maybe you have a K200B? If that is the case, you will need to tell us which model K200B that you have so we can help you get a schematic.

Again Welcome.

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Re: K200 B1 crackling sound [message #12846 is a reply to message #12842]

Tue, 02 February 2010 16:58

real_to_reel
Messages: 11
Registered: September 2007
Location: Los Angeles

Junior Member

Confirming - It is a K200 B1, serial 57115.

Any recommendation on a part number for the connectors used when splicing after the cut of the blue wire?

I asume the procedure is to clip the first blue wire, power up, if ok do not clip the other channel blue wire.

Also, when making the power cable modification, do I completely remove the capacitor at the switch?

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Re: K200 B1 crackling sound [message #12865 is a reply to message #12833]

Thu, 04 February 2010 06:02

stevem
Messages: 4754
Registered: June 2004
Location: NY

Senior Member

Use a insulated butt splice connector.
These are sold in three different colors( red, blue,yellow) which relate to wire gauge.
To splice the blue wire back together you will need the red color butt splice and a butt splice crimper tool, which most times come with a stripper built in also
.
If you have a local radio shack they may have both.
You can cut the wire with the amp on if you care to as it will not hurt the circuit, just watch what you are doing around the wires that carry the 120 volts ac to the power switch.

When adding a grounded power cord you can leave the cap on the swicth since you now have a lesser resistance path to ground with the new cable in place.

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Re: K200 B1 crackling sound [message #13000 is a reply to message #12865]

Tue, 16 February 2010 06:39

real_to_reel
Messages: 11
Registered: September 2007
Location: Los Angeles

Junior Member

Thanks for the assistance.

Blue Mallory Filter caps look ok.

Meter readings for high voltage DC power supply: -38.9, +39.4

I replaced the original Kustom speaker cord. Cleaned all cord ends, pots and input/output jacks with Deoxit. Replaced the pilot light bulb.

The crackling is gone.

I now have:

Back panel and bottom of head is blazing hot - the naugahyde was stuck to the speaker cabinet’s T&R after being on for a few hours.

Both channels: continuous low frequency hum at -0- volume.

Normal channel:
Buzzing sound when volume is at 3 o’clock position, and “white noise” sound on the treble knob when turned to 11 o’clock position.

Bright channel
Volume at max is quiet, treble at 11 o’clock is noisy.
Bright switch adds noise when switched on.

Regarding: “…test the regulated 8 volt supply that feeds each preamp board. This voltage is feed to each preamp thru the green and red wires. The red wire carrys a +8 volts DC, and the green carrys a -8 volts DC, so these must be tested across each wire to ground not by placing the meter across the red and green wire.”

Where exactly do you place the tip of the multimeter probe (for the green or red wires) for this measurement?

I have PC303, PC703 and PC102 or PC105 inside this amp.

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Re: K200 B1 crackling sound [message #13005 is a reply to message #12833]

Tue, 16 February 2010 12:45

stevem
Messages: 4754
Registered: June 2004
Location: NY

Senior Member

The back and bottom of the chassie should not be blazing hot if the the amps is just sitting and idling.
This hot conditon would indicate a bias problem in the output stage.
Are all 4 output transistors on the bottom of the amp the same RCA brand part number, or at least all the same part number if not being a RCA brand.
Clicking a brite swicth to the on position will always add more hiss noise with any year or model kustom amp.
The + and - 8 volts can be tested from eitheir end of the red or green wire to ground, most times its easyer to pick of the red or green wire voltage point more back on the board where you can use a componet lead to touch a meter to.
Preamp problems can not be adressed until you output stage is working normal.

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Re: K200 B1 crackling sound [message #13009 is a reply to message #13005]

Tue, 16 February 2010 13:24

real_to_reel
Messages: 11
Registered: September 2007
Location: Los Angeles

Junior Member

The 4 output transistors are RCA 36892. There are also 2 RCA 36892 on the board. Can they be tested to determine if one is bad?

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