TRX1 PA Board

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TRX1 PA Board

Postby WB2MSC » 21 Nov 2012 16:17

Greetings from New Jersey after the storm

The instructions for the PA board (TRX1) say to ensure the lowest resistance for the MOSFETS biases gate to ground.

Taking TR1 as an example, do I use an ohmeter at the gate of TR1 and the ground of R2? Or just from the gate to ground? What is the best way to take this measurement? You can see that I'm not an electronics expert. Maybe I can learn something today.

73, Marc - WB2MSC
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Re: TRX1 PA Board

Postby OH7SV » 21 Nov 2012 18:15

Marc,

The point is that you should turn the bias trimmers to minimum voltage before powering the PA. Pse measure the resistance from gates (TR1, TR2, TR3) to GND and adjust the corresponding bias trimmers until ohm meter reading is indicating lower than few hundred ohms. You can use any GND joint for the ohm meter GND. This way you can be sure that the bias voltage is low enough during the first power ON. After this proceed according to the instructions http://www.jumaradio.com/juma-trx1/trx1 ... ments.html

73 Matti OH7SV
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Re: TRX1 PA Board

Postby WB2MSC » 21 Nov 2012 19:18

Thanks, Matti

See, I did learn something today.

Kiitos. Ystävällisin terveisin, Marc
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Re: TRX1 PA Board

Postby WB2MSC » 22 Nov 2012 15:46

Matti,

I took the readings from the GATE-TO-Grnd of the 3 MOSFETS and come up with 700 - 800 ohms each. That seems to be the best I can do. Is that reasonable for the biases to start with?

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Re: TRX1 PA Board

Postby OH7SV » 22 Nov 2012 16:26

Marc,

They should go lower. Are you sure that you are measuring from gates?
Maybe it is easier to locate the trimmers center leg. Pse measure resistance directly from trimmers center leg to GND and turn the trimmers to minimum ohms reading.

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Re: TRX1 PA Board

Postby WB2MSC » 22 Nov 2012 18:15

Hey Matti

I assume that the center leg of the pot is the leg that is on its own side. If that is true, I'm getting the same range of readings. The resistance does change when I rotate the screws on the pots. I even tried a different multimeter.

Just to be certain, I am using the mounting screw holes as the ground although I tried other ground points with the same result.

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Re: TRX1 PA Board

Postby WB2MSC » 22 Nov 2012 18:31

Matti

I just remembered that the pots looked a bit old and the contacts weren't shiny. I put some liquid flux on the contacts before soldering. The joints seem ok as the pots didn't fall off but maybe there is still some oxidation which could make the resistance higher.

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Re: TRX1 PA Board Bias adjustment

Postby OH7SV » 22 Nov 2012 18:51

Marc,

There must be some very simple silly mistake in your measurement because this should be very easy to measure.
The bias trimmer pots are 10 kohm trimmers and typically they will go clearly below 100 ohms at both end when turning fully to corresponding direction.

Let's start from the beginning and check trimmer R2 first as follows. Connect ohm meter between center leg of trimmer R2 and GND. The center leg is the lower leg routed to resistor R1. See the layout picture here www.jumaradio.com/juma-trx1/trx1-pa-layout.pdf
You should get indication clearly below 100 ohms when turning the trimmer fully to corresponding direction.

Important note! The trimmers are multiturn trimmers, so keep turning until you get low ohmic indication. And try both directions.

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Re: TRX1 PA Board

Postby WB2MSC » 23 Nov 2012 01:04

Matti

I did as you suggested but nothing changed. I turned each trimmer many times and the ohm meter went up and down but never down enough. The highest values were about 2.5K. Please note that the trimmers I got were different from the ones pictured. Those pictured were large, white ones while the ones I got were small and just large enough for the screws. This didn't concern me as many of the parts I received didn't look like those pictured. Could this mean something? Also, the contacts of the ones that came with the kit had oxidation on them. Maybe I didn't clean them well enough.

If you are certain that I should have less than 100 ohms, I will order new trimmers according to the parts list for this stage.

Kiitos avustasi,

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Re: TRX1 PA Board

Postby OH7SV » 23 Nov 2012 07:16

Marc,

The 2.5k high value is ok because there are 3 trimmers and other components in parallel.

Pse check your ohmmeter by shorting the meter wires. It should show zero to few ohms. If you have got a low ohmic resistor e.g. 100 ohms, test your ohmmeter with that too.

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