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Hiraga "Le Monstre"
Hiraga "Le Monstre"
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Old 19th September 2018, 08:04 PM   #1511
vishalk is offline vishalk  United Kingdom
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OK this is where I get confused now and need help!
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Old 19th September 2018, 08:23 PM   #1512
vishalk is offline vishalk  United Kingdom
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Ok so what do i have to do? Clearly 4.7v is to high. My PSU is giving out 17.2V without load.

I'm just not clued up about offset and bias adjustment. I thought i measure across the 1R resistor which gives me my bias?

How do i change the bias value if i need to?

arrrggghh i tried sourcing information on this but to no avail.
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Old 19th September 2018, 08:32 PM   #1513
apolon34 is offline apolon34  France
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4.7V across 1R gives 4.7amps of bias, way too much.

Can you replace R2 and R3 on your schematic with 100R and measure again ?
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Old 19th September 2018, 08:38 PM   #1514
e_fortier is online now e_fortier  Canada
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Hiraga "Le Monstre"
Or instead or removing and replacing R2 , R3 you can add a resistor in parallel to each or a potentiometer so you can adjust your bias. This is what I did.
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Old 19th September 2018, 11:27 PM   #1515
vishalk is offline vishalk  United Kingdom
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Default 4.7Amps!!!

hiraga8w.jpg

ok thats the board, i have used 20w arcol resistors (lucky i did!)

i gathered 4.7A is ridiculous, but why? Thats what i want to know, i thought it would be just plug and play, measure a voltage, turn a screw and bobs your uncle.

I would like to get this down to an adjustable value, i would need to get the circuit diagram to figure out which resistor i need to change.

Or if someone can just tell me where on the board i should check, change, replace to get it right.

Thanks guys.
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Old 19th September 2018, 11:37 PM   #1516
danieljw is offline danieljw  Australia
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Hi Vishalk,

There are two fixed resistors which set the bias.
Send me the schematic you used and I will help you out.

-Dan
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Old 19th September 2018, 11:57 PM   #1517
e_fortier is online now e_fortier  Canada
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I believe it’s the 1K resistors marked 1001F. They are each located slightly under the 1 ohm power resistor.

Last edited by e_fortier; 20th September 2018 at 12:01 AM.
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Old 20th September 2018, 07:21 AM   #1518
vishalk is offline vishalk  United Kingdom
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Screenshot_20180920-081943.png


Thanks guys.
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Old 20th September 2018, 08:41 AM   #1519
danieljw is offline danieljw  Australia
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R2 and R3 in the schematic you sent

-Dan
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Old 20th September 2018, 08:51 AM   #1520
gaborbela is offline gaborbela  Canada
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For R2 & R3 use 2pc 1K multiturn trimmer pot as an adjustable resistor. (the two lead -one in the middle with the other- connected together) You will set up somewhere 560R 600R both, depends on your rail voltage and JFet Idss. After you set up your bias let it warm up and readjust let say 1A total bias. You can replace those trimmers or just leave it like that.
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