Home Up DX-100 SB-610 SB-620



DX-100 Transmitter


IMG_2824.JPG (586726 bytes)

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. 


IMG_2591.JPG (1474333 bytes) IMG_2594.JPG (1029747 bytes)Cooing fan mod on RF cage has no power cord attached. IMG_2595.JPG (1080988 bytes) IMG_2596.JPG (1583005 bytes) IMG_2597.JPG (1674970 bytes)
IMG_2598.JPG (1816532 bytes) IMG_2598.JPG (557191 bytes) Mods found. IMG_2608.JPG (1707651 bytes)5V4 socket. 360vdc IMG_2610.JPG (1592648 bytes)From the brown mod resistor. IMG_2611.JPG (1558699 bytes) From Brown mod resistor.
IMG_2607.JPG (1519990 bytes) Goes to new 1/4" TRS mic jack. IMG_2614.JPG (1251668 bytes)Drives the 1625 grids P-P.  Connected directly to mic jack.  Stancor WF-20.  See below text.  IMG_2606.JPG (1500386 bytes)Things got a bit HOT (high SWR)  in the RF cage!  Looks like a shorted turn. Application of the heat gun should fix this.  IMG_2671.JPG (130502 bytes) 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.   IMG_2672.JPG (148056 bytes)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.
IMG_2661.JPG (177706 bytes) Plate supply filter caps.  The one can was cut off on the chassis the other in a vise.  IMG_2665.JPG (229305 bytes)The previously removed insulators are put back for looks. There is 700 volts on the outer case of one of these cans. IMG_2666.JPG (222762 bytes)They look original.  IMG_2617.JPG (215578 bytes)There are two carbon comp resistors in the VFO.  These resistors will be replaces with new carbon comps.  No paper caps to be replaced.   IMG_2773.JPG (1021446 bytes)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.  
IMG_2780.JPG (316571 bytes) The loading coil had some heat. There was a shorted turn.  A bit of heat applied to separate the windings and some airplane glue.   IMG_2782.JPG (55587 bytes)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.   IMG_2785.JPG (1634072 bytes)Two 6146s pumping out more than the specified 100 watts!  Git-R-Done!IMG_2786.JPG (619007 bytes) IMG_2788.JPG (22769 bytes)This knob has warped and will move the inner red driver knob. I will look for a replacement. IMG_2787.JPG (21928 bytes)Until a replacement outer knob is found the red inner knob has been positioned so it is unaffected by the outer knob. 
IMG_2789.JPG (170413 bytes)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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