DX-100 Transmitter

IMG_2825.JPG (726795 bytes)

Time to restore a wonderful Heathkit DX-100. All the tubes test good on an emission tester. And all the transformers test good too. There are some unidentified modifications. But that is expected with Ham gear. That is one way us hams learn, by trying things out.

The HV and filament transformers are checked with a 12 volt ac wall wart. That holds down the secondary voltages and provides a little magnetic coupling for an "active" test. Just one more test beyond a DC ohm meter windings check.

I am gathering documentation so the circuit can be put back to original. A parts inventory will be taken from the documents and a parts order placed. I measured on of the 1 watt carbon composition resistors. It was quite high out of tolerance. So the whole chassis will get fresh resistors along with a compliment of electrolytic and paper caps replaced.

	Cooing fan mod on RF cage has no power cord attached.
	Mods found.	5V4 socket. 360vdc	From the brown mod resistor.	From Brown mod resistor.
Goes to new 1/4" TRS mic jack.	Drives the 1625 grids P-P. Connected directly to mic jack. Stancor WF-20. See below text.	Things got a bit HOT (high SWR) in the RF cage! Looks like a shorted turn. Application of the heat gun should fix this.	Initial resistance checks found this open 10mh choke. Both choke wires were snapped off of the lead wires. There was a too tight radius on the lead wire.	A small bit of magnetic wire extended the one side. I did not want to unwind the choke to reattach to the lead wires. This open killed the B+ to the VFO and buffer stage.
Plate supply filter caps. The one can was cut off on the chassis the other in a vise.	The previously removed insulators are put back for looks. There is 700 volts on the outer case of one of these cans.	They look original.	There are two carbon comp resistors in the VFO. These resistors will be replaces with new carbon comps. No paper caps to be replaced.	Replacement of the parasitic harmonic suppressor resistors. I ordered up a bunch of 47 ohm 2 watt resistors of different construction. The smallest are metal oxides.
The loading coil had some heat. There was a shorted turn. A bit of heat applied to separate the windings and some airplane glue.	This is an alternative to drilling a hole for the Heathkit VCO Grid Block modification. Down position acts like the push button switch specified in the mod.	Two 6146s pumping out more than the specified 100 watts! Git-R-Done!	This knob has warped and will move the inner red driver knob. I will look for a replacement.	Until a replacement outer knob is found the red inner knob has been positioned so it is unaffected by the outer knob.
The original power cord has a broken plug that exposes the fuse metal. A new grounded power cord and an internal line fuse was used to replace the broken two wire origonal.

AM testing went well with an HP 239A audio signal generator input. 100% modulation was achieved with about 1 to 2 volts input from the generator. There is a Stancor WF-20 input transformer driving the 1625 modulation tubes with a frequency response of 30-20khz HiFi. It has several input impedances available from 50 to 600 ohms. The output impedance is 50k ohms. The original transformer needs to supply around 93kohms impedance to the 12BY7 audio stage. Therefore, this Stancor transformer needs a hot line level input and frequency shaping to keep in the ham allocated bandwidth.

Bench testing into a 50 ohm dummy load.

MHz	Drive	Amp	Load (outer.inner knob)	Min Watts Output Drive at min.	Max Watts Output
1.95	45	55	10.0	100	180
3.7	30	52	1.7	50	100
7.18	50	42	802	70	160
14.225	30	30	8.3	1	160
21.275	38	17	3.2	1	70
28.48	52	15	10.2	1	125
27	72	7	0.4	1	100

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