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Biasing MOSFETS
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Author:  davidaker [ Fri Sep 28, 2018 11:49 am ]
Post subject:  Biasing MOSFETS

Hello all,

I've got a problem trying set the bias.

Wound all 4 pots CCW to read between 50 and 52 ohms from gnd to gate, hooked up leads. Powered up and when switch is on, DMM goes from 0 to 11mA. Turning VR1 does not increase mA. When powered down, the pot was reading 400 ohms on the gate. Any ideas please?

Author:  jimveatch [ Sat Sep 29, 2018 10:26 am ]
Post subject:  Re: Biasing MOSFETS

I suspect there's something wrong with the current measurement setup. If all the cables are connected properly you should see 0ma with the power switch off and the amp control circuits powered by a clip-lead.

When the amp is first powered on (by the switch) it draws about 70 mA for 1-2 seconds then settles down to ~45mA.

You should also test your meter, if you put a 1K ohm resister in series with your meter across your power supply, you should read the same number of mA as the power supply is putting out volts.

73,
Jim WA2EUJ

Author:  davidaker [ Sun Sep 30, 2018 12:07 pm ]
Post subject:  Re: Biasing MOSFETS

Hi Jim, thanks for the reply.

I tested the DMM with the 1k resistor and it tested okay. I checked the draw on the amp and it is about 48 mA although I could not see the 70 mA on startup as it too quickly went to 48 mA.

I double checked the lead setup for the biasing and it showed 0 mA with power switch off (if I turned on power switch on, the draw was 9mA) although I'm not sure I was to flip the amp switch to on.

On the ohmmeter test 1, the resistance went up to around 12 Meg Ohm but test 2 (pwr switch on) dropped resistance to around 1.9 kOhm.

Where do I go from here?

Thanks,
David

Author:  jimveatch [ Sun Sep 30, 2018 5:58 pm ]
Post subject:  Re: Biasing MOSFETS

You should be making the bias adjustments with the power switch in the 'OFF' position. The amplifier should be 'ON' because you are powering the control circuits via a clip-lead with power that does not go through the ammeter.

The next thing to check, with the setup as shown in the manual and power switch off, the ammeter should read '0'. check the voltage on the drains of Q1-Q4. It should be 12V. If it is, start advancing VR1 and monitor the voltage at the gate of Q1. It should increase linearly as you turn VR1. You'll see 0 current until you get to 3-4V if you get to 4.5V and still no current, and there's 13.8V on the drain, the MOSFET is bad.


73,
Jim WA2EUJ

Author:  davidaker [ Tue Oct 02, 2018 9:23 pm ]
Post subject:  Re: Biasing MOSFETS

Thanks Jim,
I desoldered rly15 to check if pin 1 was shorted (it wasn't) but in the process of removing it, I apparently made it inoperable. I've got a replacement on the way and will swap it out. I'll try again once it's in.
Thanks,
David

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