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Old 8th July 2002, 06:09 AM   #1
jleaman is offline jleaman  Belgium
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Default N-channel Problem HELPPPPPP

hello all there i have finaly taken the amp and Some what powered it up.. now here are the problems..

1st there was a flaw in anthonys schematic that he personaly sent to me he drew the 15 v zener diode in the wrong place and when i powered it up with the var-ac it blew the fuse and the diode so my dad and i looked over it and he seen that he drew it on the wrong spot. i bought myboardabout 2years ago when i got it he had drawn on the pcb overlay where the parts go..


Now i fixed that problem but there is another problem still. with or with out a input or grounded load h-f osicalations occours.. can any one please help me i am in need of help i email anthony and no responce so i am stuck..

if need the over lay let me know and ill scann it..

Please help

jleaman@citytel.net
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Old 8th July 2002, 06:16 AM   #2
jleaman is offline jleaman  Belgium
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here is the pic of the error. and in pencil you will see the corection..
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File Type: jpg anthony.jpg (64.5 KB, 2288 views)
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Old 8th July 2002, 06:17 AM   #3
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Jason,

Which zener are you referring to - is it the one that is in the shunt bias section. In any case, a scan of the component overlay and the exact part will help.

I have assembled four N-channel amplifiers, on self-designed PCBs and I have never encountered oscillation problems. Check whether the 0.33ohm resistors for the output devices are wirewound (inductive types). If so, replace them with 3x1ohm 1 watt MFR/CFR resistors. Add a Bouchoret network(Zobel) at the output consisting of 4.7ohm 5watt resistor and 33nF 400V capacitor. The network is not part of Anthony's schematic.

Waiting for your response,
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Old 8th July 2002, 06:19 AM   #4
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Jason,

I see that you posted the image a minute earlier than my post. I will go through the schematic and reply again.
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Old 8th July 2002, 06:26 AM   #5
jleaman is offline jleaman  Belgium
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what .33ohm resisters.. are there suppose to be some mabey on the bottem.. he didnt tell me at all..

do you see the error that was made..?
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Old 8th July 2002, 07:55 AM   #6
jleaman is offline jleaman  Belgium
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ttt

Please i need help
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Old 8th July 2002, 09:45 AM   #7
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Default What? No source resistors??

The last time I looked at the cct for this, each upper and lower fet had a low value resistor from it's source lead to where it is commoned with the other two. Have a look at the pic of the current pcb and see if there is a difference between it and yours.
http://www.aussieamplifiers.com/images/n-chn3.jpg
It doesn't show provision for source resistors but I seem to remember there was on the schematic...

GP.
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Old 8th July 2002, 10:58 AM   #8
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Jason,

Ciclotron is right. On the positive side, the Drains of the output Mosfets are connected to the output via 0.33 ohm 3 watt resistors and on the negative side the Source of the ouput Mosfets are similarly connected. The temperature sensing device, which is part of the shunt bias circuit is now an IRF610 and not a BD181 as earlier. This device should be mounted in thermal contact with one of the output devices and not directly on the heatsink. Also note that three zener diodes are used in the circuit and all have been lately changed to 9.1V 1 watt from 15V.

Have you checked Anthony's site for notes on the construction of the N-channel amp?
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Old 8th July 2002, 06:55 PM   #9
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Hi Jason

You need to download the n-channel manual from my website and read it very closely. Concerning the source resistors. You need to remove the source pins from the pcb on each mosfet and insert a 0.33 Ohm 5watt resistor in each hole, with the resistor lead pointing into the air.
Then bend the the resistor lead over so you can solder the lead to the Source pin of each mosfet.
The manual says to cut tracks and connect the resistors in a different mana for the bottom side or the negative voltage side of the o/p stage. In your case I would suggest what I have told you above as it is much easier to done and it makes no difference to the performance.
Remove the 10pf cap across the 22k resistor, I think you will find that the oscillation will stop.
But do the source resistor mods before you go any further, This is a must!!
When your get to adjust the bias for the o/p stage. Measure across one of the source resistors for a measurement of 33mv, and then check the rest of them...

I Hope this helps

Regards

Anthony
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Old 9th July 2002, 11:15 AM   #10
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Anthony,

Finding you is so very difficult, hence, while you are at it, what about the mods to increase the power of the N-channel amplifier. I am thinking of using perhaps, MPSA92/42 for the input section (lower Hfe types, I know), IRF710 for the splitter and IRFP460s (may be 6 in each bank) and raise the voltage to around 90-95volts.

Any comments? Thanks.

Jason,

Oops! I am sorry. My earlier post was made in a haste - I should have actually said that the 0.33ohm resistors are connected to the Source of the Mosfets in the positive bank and the Drains of the Mosfets in the negative bank; I said the opposite thing earlier. All the best.
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