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DX-60 Enhancements




 
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DMOD
AC0OB - A Place where Thermionic Emitters Rule!
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« on: October 23, 2015, 02:33:36 PM »

The Heathkit DX-60 is generally a solid rig in terms of reliability, but due to the economy of kit design, it does have some areas that can be improved.

Two of the major improvements involve screen grid voltages that are too high in the original design: one is the Buffer-Driver screen voltage and the other is in the area of RF final Screen Grid modulation.

The Buffer-Driver screen grid voltage is set by R6 and R7, both of which needs changing. R7 is too low in value and should be increased to 18-22k for two reasons: the original value of R7 allows too much current to flow through R6, causing overheating and failure, and secondly, the original circuit values allow too high a voltage at the Buffer-Driver screen grid.  In addition, a 22 uF capacitor needs to be placed at the junction of R6/R7 to both decrease hum and to stabilize the DC voltage. R6 is now a 25k, 5 Watt Wire-wound potentiometer for reliability.

R6 should now be used to set the 6146 grid current only. After modifications, initially set the Drive control to about 25% of full rotation and then adjust Drive control for a final grid current of 2.5 mA.

Power output is now controlled by the last stage of the 6DE7/6EW7's grid voltage. After dipping the final plate current and loading, set power output to ~ 17 Watts by the 250k pot. Of course, R6 and final plate current and loading may have to be readjusted for final power output and grid current values.

To obtain the best modulation waveform, use a scope and set Microphone gain R26 until just before "flattopping" at Pins 2,3 of the 6DE7/6EW7. Set the negative modulation clipping by adjusting the 5k trimpot to obtain 95% negative modulation.


Phil - AC0OB



* DX-60 Modification Schematics NG V3.pdf (328.58 KB - downloaded 160 times.)
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