Re: Bias current problems


Hey gang,

I just finished assembling a CS-40 yesterday and went through the TX alignment.  After reading about all the destroyed IRF510's, I was a bit nervous when setting the bias current.

Here's how it's done:

1. Connect ammeter in the transceiver's power supply lead.  Keep in mind that the current can approach 1.5 amps during the entire alignment process (including TX power peaking), so pick an appropriate range on your ammeter.

2. Make double (triple) sure the SET BIAS trimmer (pot) on the main PCB is fully counter clockwise.

3. IF you've already fiddled with the TX IFT coils, I'd suggest returning them to their factory setting by screwing them back fully clockwise, gently.

4. Apply power to the transceiver.  Take note of the idle receive current draw.  My CS-40 shows about 0.07 amps at 12.7v input power.

5. Key up the transceiver via the PTT switch but DO NOT TALK into the mic.  Write down the current shown on the ammeter for reference.  My CS-40 showed 0.54 amps.

6. With the PTT switch still pressed, turn the SET BIAS trimmer clockwise VERY VERY VERY slowly while carefully watching the ammeter for an increase in current.

7. Somewhere beyond the middle travel of the SET BIAS trimmer's rotation the power supply current will begin to increase.  Occasionally pause between SLOWLY turning the SET BIAS trimmer to let the supply current stabilize.

8. Once the supply current has increased (and stabilized) to approx. 60mA (0.06 amps) beyond the current you started with, STOP.  In the case of my CS-40, I stopped at 0.59 amps total.  That works out as 0.54 amps beginning + 0.05 amps increase of bias current.

9. Unkey the PTT switch.

10. Now you can peak the TX band pass filters (IFT coils) for maximum TX power following the instructions in the assembly manual.

My CS-40 wound up with approx. 8 watts of TX out at full modulation with 12.7v input power.

Hope this helps clear things up for someone.



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