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Old 10th Jan 2019, 10:01 pm   #21
Sean Williams
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Default Re: 160 /80 / 60 mtr AM TX project

That's a great result Tim, and impressive looking build quality as well.
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Old 18th Jan 2019, 3:48 pm   #22
g4wim_tim
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Default Re: 160 /80 / 60 mtr AM TX project

Hi Guys,

thought you might like to see a couple of photo's of the finished tx - had one or two small bugs to sort out, but all good now.

A nice easy 75 watts of carrier on all bands with 100% modulation, however I've just ordered a SL6270 VOGAD chip to help keep the modulation in check when needed.

The VOGAD will be external and switchable.

The VFO drifts some 400Hz LF in the first 30 minutes then settles on freq to within 10Hz (all at 1.9MHz the VFO fundamental).

Final job is a cage for the RF PA and a wrap around case.

73 Tim
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Old 18th Jan 2019, 4:00 pm   #23
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Default Re: 160 /80 / 60 mtr AM TX project

Lovely - in particular I like the 'ancient and modern' combining of valves and a LCD panel for the frequency-display.
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Old 18th Jan 2019, 4:58 pm   #24
Ian - G4JQT
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Default Re: 160 /80 / 60 mtr AM TX project

Beautiful workmanship Tim. Thanks for sharing. Hope to work you on 3615/25 or 5317 soon.

Ian

PS: FYI there is a facebook AM group "AM Amateur Radio Europe", although most members are in the UK. It promotes AM on some of the amateur bands using new or old, commercial or home-brew equipment.
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Old 18th Jan 2019, 6:53 pm   #25
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Default Re: 160 /80 / 60 mtr AM TX project

Thanks Ian,

just joined the FB AM group you mentioned.

Looks like my auto antenna tuner doesn't like AM as it sometimes tries to retune mid transmission - will have to get my hands on an old school manual tuner.

Regards Tim
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Old 18th Jan 2019, 8:24 pm   #26
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Default Re: 160 /80 / 60 mtr AM TX project

Most autotuners have a hold switch input.
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Old 18th Jan 2019, 8:56 pm   #27
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Default Re: 160 /80 / 60 mtr AM TX project

Tim, was it you published an article (1980s ? radcom, swm?) for an autotune 160m mobile antenna?
Rob
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Old 18th Jan 2019, 9:10 pm   #28
g4wim_tim
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Default Re: 160 /80 / 60 mtr AM TX project

Just been reading the instructions for my auto tuner and I think I've got it set to hold it's tuning - so we'll see how it goes !

Yes I've had a number of articles published over the years including several in RadCom, dual band 4/6 mtr tx/rx, 160mtr auto tuning vertical and most recently an auto tuning Mag loop.

How time flies !

Regards Tim
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Old 23rd Jan 2019, 1:16 pm   #29
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Default Re: 160 /80 / 60 mtr AM TX project

Results of air testing are promising but I was concerned about over modulation so I built a SL6270 VOGAD followed by diode limiter and low pass filter seemed to work ok but still wasn't happy.

Instead I installed a negative peak stay alive combined with a mic gain ALC based on this but with an LED / FET optocoupler rather than an LDR.

I used an H11F1 mosfet opto coupler as the control element, much faster response time than the 20mS or so from an LDR.

Seems to work very nicely once I had adjusted the component values.

However initially I was getting asymmetric modulation positive peaks of 60% and negative peaks of 80%. Changing the polarity of the modulation transformer secondary balanced the +ve and -ve peaks quite nicely.
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Old 23rd Jan 2019, 6:29 pm   #30
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Default Re: 160 /80 / 60 mtr AM TX project

Interesting... I like that ALC circuit.

One trick I've used in the past is "negative cycle loading" on the modulator transformer secondary. The thing that causes 'splatter' is that when the modulator pulls the PA anode voltage going to zero - at which point the modulator is working into an infinite impedance which causes nasty things to happen to the waveform.

"negative cycle loading' uses a diode and series resistor, wired across the modulation transformer, so there's always an appropriate load on the modulator, even on negative peaks, so there's less liklelihood of generating splatter when the modulation-level is high. It's actually possible to achieve over-100%-modulation !!
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Old 23rd Jan 2019, 7:13 pm   #31
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Default Re: 160 /80 / 60 mtr AM TX project

Yes negative cycle loading is required which is why there is a resistor in parallel with the diode to the PA such that is the transformer output goes below zero the diode is reverse biased and the resistor takes over.

However due to the ALC circuit I'm yet to detect the output going below zero and the RF spectrum always looks nice and clean.

Measuring the modulation distortion on my HP8901B is remains below 2.5% up to 75% mod above which point the ALC kicks in and the distortion rises to 8% at 95% mod - so not a bad trade off.

Regards Tim
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