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Author Topic: Heathkit DX35 controlled carrier modulation  (Read 23822 times)
HAMHOCK75
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Posts: 621




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« Reply #30 on: April 12, 2016, 11:19:26 PM »

After your last comment about the meter, I decided to test the meter's accuracy (500 ohm/3 mA full scale ). It was way off. The DX35 has a 12 ohm sense resistor in the cathode of the 6146. When 1.2 VDC appears there, the current through the 12 ohms will be 100 mA.

I disconnected the meter and used an HP6205B power supply to provide exactly 1.2 VDC to the meter with my Fluke DMM was in series with the Heathkit meter. The Fluke read 2.32 mA when the Heathkit meter was at full scale reading 150 mA. when it should have been reading 100 mA. I added a 220 ohm resistor in series with the meter leads to drop the reading to 100 mA.

If the DX35 loses drive, the final loses negative bias so it essentially is absorbing 65 watts on its plate. If the DX35 loses drive and breaks into oscillation because it is unstable, most of the 65 watts still goes out into the dummy load as RF. Curious that the instability can actually save the final from excess plate dissipation.
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QRP4U2
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Posts: 262




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« Reply #31 on: April 13, 2016, 10:14:06 AM »

You might check the accuracy of the 12 ohm metering resistor as well as I have found them out of spec.

I use at least a 5%.

Also recall the cathode current is the sum of the plate, grid and screen currents, so subtract at least 10 mA from the meter reading when determining actual plate current. 

Phil

--------------------------------------------------------------------------------
AC0OB - A Place where Thermionic Emitters Rule!

Besides, when you're a Ham, you experiment with and improve boat anchors - that's what you do. Smiley
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AC0OB - A Place Where Thermionic Emitters Rule!
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Besides, when you're a Ham, you experiment with and improve boat anchors - that's what you do!. Smiley
HAMHOCK75
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Posts: 621




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« Reply #32 on: April 13, 2016, 02:52:37 PM »

The 12 ohm resistor I did check early on. It is a 1% part which is better than the 5% parts used in the DX60. I did assume that the real plate current would be less but the manual says to tune for a meter reading of 100 to 125 mA. If the real plate current is about 10 mA less then the input power  will be about 600 VDC x 110 mA = 69 watts which is close to the specified 65 watts input. If I actually adjust the meter to reflect true plate current and someone tunes as the manual says for 125 mA then the input power would be 600 VDC x 125 mA = 75 watts.

Given the ability of the transformer, I prefer to err on the side of lower input power. I think this pretty much concludes the work on the DX35. Thanks for all your inputs. Obviously, you have been there and done that, lol.
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KC3NG
Member

Posts: 122




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« Reply #33 on: June 24, 2016, 01:52:36 PM »

Well said!!! John K4WVJ used to run a DX40 with CCM and it actually sounded good. It can be done!
Renee KC3NG
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