From Randy Hardengrader, WJ4P

HW-7 Mods

My Heath HW-7 has been through quite a few changes over the years. Right from the start I noticed a few(!) things lacking and dove right in! Some of the earliest changes were pretty basic. I highly suggest these for your HW-7 if you haven't done them yet...

None of this is done as a step-by-step instruction. Rather, it is a list of modifications that I did to mine to improve this radio to "usability". These mods are presented with the idea that they can be improved upon by another. The schematics for all the modifications listed here are available for the asking.


The very first thing I added to the HW-7 was a RIT. It came in very handy to help with QRM because I could tune to the other sideband with the hopes of eliminating a station (or two). The NA5N's Data Book has an example of a RIT circuit. My circuit uses a transistor to "sample" the keying line so that regardless of how poorly the key contacts conduct I still had an "absolute" switching of the tuning voltage to the varicap. My circuit was designed using three transistors to control the switching from RX to TX. Remember, you want the RIT to de-activate on TX!! I noticed that I have a different "zero" on the front panel RIT for at least two of the bands. This is because a different tuning cap is used by the vfo depending on which band you are operating.

VFO Voltage Regulator

The second thing I did to the HW7 was add a voltage regulator to the VFO circuit. I used a three terminal variable regulator W7 wset its output voltage to 10VDC. This keeps the VFO from drifting with supply voltages changes, especially during transmit. (otherwise known as chirp) It is very easily added. Mount the regulator wherever you want. To isolate W7 wrun the voltages to Q2(vfo) W7 wQ3(buffer) just pull up one e7 wof R24 W7 wR25 W7 wconnect the regulated voltage to them flying connection style.

T/R switch

I added a solid state T/R switch to replace the klunkywsemi-break-in antenna relay. I now have full-break-in with the HW-7. The circuit is built on a piece of perf board with leads extended to meet the old relay holes a7 wsuspend the perf board vertically above the old relay location. Transistor Q9 is replaced with an NPN transistor like a PN2222 W7 wcapacitor C19 is removed. The finals are always connected to the antenna W7 wthe Rcvr input is switched onto the antenna during periods of receive. One fine point- I actually get the antenna connection at the final transistors' collectors so that the output network can help filter out some of the BCI. Also it is good practice to place a diode based T/R switch before the output network to keep any spurious signals down. Those non-linear devices like to make harmonics in the presence of RF energy.

New Receiver

I designed a Direct Conversion receiver board based on the NE602, an LM3900 opamp W7 wan LM386 audio amp. The circuit includes a FET mute switch W7 sidetone oscillator. The LM3900 is primarily used to provide gain and some audio filtering. Most of the design has been "lifted" from other gear schematics and massaged to work in the HW-7 environment. The board mounts on the side panel nearest the rcvr section of the HW7. I installe wan audio gain control on the back of the case W7 wthis control has the on/off power switch. In place of the original front panel AF gain control I installe wa 1K ohm pot. This is used as an RF gain control by controlling the amount of antenna signal presented to the preselector circuit. The VFO signal is connected via coax to the new rcvr board through a 100pF cap W7 wit is taken from the main board at the collector side of C51. A small hole needs to be drille win the HW7 board right into the grou7 wlW7 warea for a grou7 wconnection for the VFO coupling coax. The Preselector circuit is connected to pins 1 W7 w2 of the NE602 via a toriod transformer constructed of an ft-37-43 core using 10 turns of #30 wire for both the primary W7 wthe secondary. The primary (pick one!) is connected to "G2" of the removed dual mosfet Q1. The other lead is connected to grou7 wat the grou7 ed e7 wof R2 on the main PCB. The idea here is to present a balW7ced a7 wDC isolated antenna connection to reject BCI W7 wto cure a feedback condition that results in "howling"wat certain preselector settings. There is a lot of gain at play here! The headphone connections are pretty straightforward. I highly recomme7 wcommunication headphones (8 ohms) for best audio. IMPORTANT! If you are using my rev0 board, be sure to connect the grou7 wconnection to the board at the lm3900. The board shows some instability otherwise.


Some operating notes. The transmitter output tuning has little effect on the receiver sensitivity. Tuning the receiver preselector carefully will eliminate most BCI, W7 wreducing the RF gain will also help reduce BCI. Remember that the RIT "center" will be different depending on what band you are operating.

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