HW-12, modification for AM?

[K1MVP]

Was wondering if anyone might know of possible mods for the HW-12,
22, or 32 SSB Heathkit monobanders to put on AM?
I know have seen mods available for the SB-101, and HW-101 and
have heard some on the air on AM es they sound good.
Appreciate any info, and thanks.
                                   73, Rene, K1MVP                                         
 

[w3jn]

Rene, I've done it but I need to refine it a bit more.  What I did was remove the crystals in the filter circuit and replace the 2 legs of the ladder with 100 pf caps.  I fed the audio at the cathode of the cathode follower.  I ended up getting all kinds of RF in the audio so I set it aside for a while.

After the crapstals are removed the receiver is essentially worthless as it now has about a 100KC bandwidth.

What needs to be done:

1)  disable ALC circuit
2)  rewire RF gain pot to adjust bias on grid of first IF amp (between -120 V bias supply and where ALC used to go) - this will end up being a drive adjust
3)  Audio should go to top of audio input pot, not at the cathode of the follower where I put it.
4)  .47 uF cap from cathode of cathode follower should be fattened up so it passes some low end
5)  Bring aux relay output to jack to control receiver

I converted a HX-20 (the little mobile SSB xmitter) to AM in much the same fashion and the audio is flat from 30 hz to > 20KHz.  Only problem with it is the carrier balance drifts around.

These rigs are dirt cheap - I've gotten a couple for $5 each - and should make a good AM exciter.  Once I get this more or less straightened out I'll post the mods.

73 John

[K1MVP]

Ok John,
Sounds good and will be looking forward to the mods when you post
them at some time in the future.
I have a couple of them I just recently acquired for 75 and 20 meters,
and was thinking,--they might make a "neat" little am exciter for my
SB-200 linear.
                                 73, and thanks, Rene/K1MVP
 

[w3jn]

Rene, I got the modified HW-12 on the air this past weekend and it seems pretty decent.  Good for about 40W carrier @ >100% mudulation.  Sweeps flat 30 Hz to > 10KHz.

Here's what I did.  Dig the skizmatic out and go to town:

1)  Remove mike audio from first audio toob and re-route to top of audio gain pot.  Disconnect the wire that was there (going thru a .02 cap to the plate of the first audio).  Ground the bottom leg (disconnect the parallel resistor/cap that was there).

2)  Clip out the 500 pF cap at the grid of the cathode follower.

3)  Parallel (or replace) the .47 uF cap at the cathode of the cathode follower with a 20 uF/50V electrolytic.

4)  Remove the crystals associated with the crystal filter.  Short the two that were going from the coil to the grid of V3, the IF amp toobr.  Connect two 200 pF caps from the common point of the first two crystals to the top and bottom terminals of the coil in the filter circuit.  Essentially you're bridging the filter with 200 pF caps, but the crystals MUST be removed.  You could probably just remove all the crystals and the coil and just use a single 200 pF cap to the grid of V3.

5)  Clip the .02 uF cap from the grid circuit of the finals going to the ALC detector diodes.  Clip out the 2.2 M resistor in the ALC circuit as well as the .47 uF cap.

6)  Remove the wires on the RF attenuation pot, run the top leg of that pot thru a 10K resistor to the top leg of the bias pot (puts about -80V bias on the RF attenuation pot).  The bottom leg goes to the relay contact that has two brown wires (this contact is grounded thru a 330 ohm resistor in xmit), the middle leg goes to the ALC line.  The RF attenuation pot is now a drive control, but the bias rises to about -30 V or so to cut off the 12BY7 and the xmitter IF tube during xmitter standby.

7)  Remove the wires from the ALC jack.  Clip the yel/white wire from the 11-pin power plug (this is the aux relay out, pin 11 on the power plug) and connect to the ALC jack.  This jack is now a dry relay contact for keying your amplifier or for RX standby.

  Remove the coax lines going from the Receiver Output jack to the circuit board (but dont remove the coax going to the relay).  Because the receiver is now worthless (it has about a 100KHZ bandwidth) this jack now provides a switched path to the antenna for your receiver.

So, feed line audio to the mike jack, connect up your scope, and tune it up.  Adjust the carrier balance for a null, adjust the bottom slug of the balanced mod xformer for a null, repeat until the carrier is completely gone.  Now crank the carrier balance pot all the way to one side (full carrier out) with the drive control full output.  Adjust the carrier balance for about 40W carrier, adjust the mike gain for a good pattern on the scope (no flat topping), peak all the IF xformers (except the bottom of the balanced modulator xformer) and you're done.  You can adjust the output from about nothing to about 40W carrier.

This post was done from memory, so if you run into trouble gimme a shout.

[K1MVP]

Ok John,
 Thanks,-- I downloaded your final details for the mods to the HW-12.
 I am supposed to pick up a schematic from a local ham for the rig this
 weekend and will start to "scope" out the changes you described.
 Thanks again for the "info" and will let you know how I make out.

                         73, es have a good holiday, Rene, K1MVP       

[w3jn]

Any success on this one, Rene?

[K1MVP]

Any success on this one, Rene?

John,
I had to put the mod to the HW-12 on the "backburner" for now, as have
had other "projects" to get to,(honey do`s etc.) but I do plan to get to
it as soon as I can.
I have also since, we last spoke, obtained another HW-series monobander,
a real nice HW-22A for 40 meters, and am now considering modifying it
as a 40 meter AM exciter.
Not sure which one I will do first.
In any case, thanks again for the "info" you provided, es will let you know
how I make out.
                                     73,--Happy New Year,
                                           Rene, K1MVP 

[w3jn]

I don't know about the HW-22, but the 20 meter version has some differences in the ALC circuit from the HW-12.  In any event trace out the ALC circuit, disable it, and put bias on the 12BY7 and the IF tube controled thru the RF attenuation pot and the relay.  The relay will put cutoff voltage on the tubes when its in receive and controlled bias thru the RF atten pot in xmit.

73 John