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Old 4th Jan 2019, 11:36 pm   #15
g4wim_tim
Pentode
 
Join Date: Nov 2018
Location: Freckleton, Lancashire, UK.
Posts: 232
Default Re: 160 /80 / 60 mtr AM TX project

Hi Ian and others,

This project was conceived initially as a Codar AT5 copy but grew / evolved into this.

I always plan things out with cardboard cut outs first - can't be bothered with a full 3D CAD package, but seems to work out ok.

Just been doing some tests on the modulator and I think I have a problem in either the output valves or transformer as I can't achieve more than about 70% modulation before clipping sets in.

I notice the DC resistance on the UM2 transformer primaries are different, one side measures 71 ohms (pins 3 to 5) and the other side measures 92 ohms (pins 4 - 2). This might be just down to the way it's wound as under AC conditions the ratios seem the same.

Full winding 3 to 6 measures 114 ohm while the other side 4 to 1 measures 155 ohms .

None the less when running for real the 3 - 5 side (lower resistance side) seems to create a distorted sine wave on the anode whereas the other side seems ok.

Hope its not shorted turns !

Bias is balanced as is the drive signal.

I'll try switch valves over and / or different taps to see what happens - hope its nothing serious !

I wonder what the DC resistances should be for UM2 ? I've not been able to find them on line so far.

Any suggestions ?

Regards Tim
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Building and tinkering for over 50 years, from 807's to digital amateur TV.
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