| 250 hiss [message #8821] |
Wed, 22 August 2007 12:53  |
timrsimpson
Messages: 11
Registered: August 2007
Location: Portland or
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Junior Member |
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I have a Kustom 250 that has a bad hiss on the effects channel. I have replaced a lot of the caps and Q1,Q2 and I1 (ecg725). With the board unplugged it is quiet.
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| Re: 250 hiss [message #8831 is a reply to message #8826] |
Mon, 27 August 2007 11:18  |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Hey Tim, welcome to the board.
If you have the fuzz circuit, I assume that you have a model 250-4. That means that the bottom pre-amp board is a pc5066.
You will find that there are two plugs attached to that board. One with 6 wires, and one with 3 wires. The plug with three wires sends the pre-amp signal to and from the fuzz circuit.
If you want to see if it is the fuzz circuit that is causing your hiss, unplug the cable and jumper pins 1 and 2 on the board (where the two blue wires go). This will completely remove the fuzz from the signal path.
Bill
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| Re: 250 hiss [message #8837 is a reply to message #8836] |
Mon, 27 August 2007 17:20 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Well, you've proven that the fuzz circuit is the problem.
Which diodes did you heat up to make the noise go away? There are two clipping diodes (CR10 & 11) and there are two diodes in the switching circuit (CR8 & 9).
When the fuzz is turned off with the Effect control, does the hiss go away? If it does then, the fet switch transistors are ok, if not then you should check them.
Does the noise go away when you turn down the Level control? If it does, then the check both of the transistors (Q11 & 12).
The fuzz circuit is a very high gain design and some hiss is normal, but with the right transistors, you can probably minimize the noise.
Bill
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| Re: 250 hiss [message #8839 is a reply to message #8838] |
Mon, 27 August 2007 18:28 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The fets are Kustom part number 007-0024-00.
They are an N-Channel mosfet, used for fx switching.
The RCA trade number is 40841, which crosses to an nte221.
Bill
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| Re: 250 hiss [message #8879 is a reply to message #8873] |
Mon, 10 September 2007 12:10 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Tim:
The 200 ohm dropping resistor, is only for the pilot lamp supply.
The rest of your primary wiring sounds perfect. Some people recommend the removal of the ground cap, because if it should short, it could place one side of the line on the chassis.
As for your hiss, now comes the hard part. If you've replaced all of the active components, the only thing left are the passives. In my experience, I've found that hiss and noise can be caused by so many different things, that it can be a very frustrating thing to troubleshoot.
Most of the time it is caused by an active component, a transistor or an IC, but some times a resistor or cap will contribute to the noise factor. Kustom used 1% metal film resistors (very expensive parts back in the day) in certain sections of their designs to help reduce some of the noise.
Start by isolating the source. Which part of the fuzz circuit is noisiest? When you turn off the fuzz switch is the noise quieter? Is the noise affected by the effect control?, by the level control? Isolate the noise source as best you can and then troubleshoot.
Earlier you said that you heated the clipping diodes and the noise went away. Then I would think that the noise source is in front of these diodes, as heating them will cause them to conduct more and shunt more signal to ground. What happened to the signal when you heated the diodes? The first fuzz transistor Q11, is set for really high gain. Install a socket and try a few different transistors here, to find the one with the best noise specs.
If all else fails you can modify the circuit to perform the way that you want it to.
Hope that this helps.
Bill
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