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Testing my modulator

PostPosted: Tue Apr 03, 2012 5:22 pm
by Bronek M0DAF
I have just completed my PWM modulator using Steve's boards. Before applying HT I thought I would check whether all seemed OK with just the LT supplies. The PWM board is all fine, and the overload board does what it should in the initial set up condition. However, with the modulator on standby (with LT supplies to the two boards and to the output section on) I find that I get 16.8 V on the output from the PWM filter (with no load). Is this OK? I am a little nervous about applying the HT to the MOSFETS unless all is OK. Your advice would be welome.

Thank you

With best wishes

Bronek (M0DAF)

Re: Testing my modulator

PostPosted: Wed Apr 04, 2012 11:11 am
by kf1z
Yes, that is normal.

Another test is to short the output to ground.
This allows the charge-pump to operate.


Now you can test for proper PWM signals at the gates of the mosfets, and see that the carrier control on the PWM board adjusts
the pulse-width as it should.

After you're done, leave the carrier control at very low setting, for initial test with HV applied.

Good idea to pre-set the overload board as in instructions before testing with HV applied.


Re: Testing my modulator

PostPosted: Wed Apr 04, 2012 11:27 am
by Bronek M0DAF
Dear Bruce,

Many thanks for your advice. I did, in fact, discover some of this myself by placing a small 4.7 ohm resistor across the output. At the moment the whole thing is on a soak test.

Time to get the RF deck completed!

With best wishes

Bronek M0DAF