160m 20w AM transmitter – Walford Cam

160m 20w AM transmitter project – Walford ‘Cam’

Walford Cam - AM transmitter for 160m - front view

The ‘Cam’ project was built as a companion transmitter for my Yaesu FRG-7 receiver, now around 30 years old and previously belonging to my late friend Tony G0DLX.

FRG-7 companion transmitter

Originally I planned to do a complete transceiver mod to the FRG-7 as described by Ian Keyser G3ROO in 1980 but sadly many of the IC components (SL610, SL630, SL640 – 2 of and SL641) are now obsolete. So in an attempt to get more use out of the FRG-7, the ‘Cam’ was built.

Walford ‘Cam’ project

Tim Walford has now discontinued the Walford Electronics Cam AM transmitter kit; however, he has a number of DSB, SSB and CW kits available.

The Cam’s design is a 0.4w carrier, 1.5w peak AM transmitter for 80m or 160m. Living in the UK with our small gardens and neighbouring domestic QRM too close for comfort, there’s a need for a bit more “umph” to get heard these days. Consequently, I decided I needed around 15w output and would achieve this by having a linear amplifier stage supplied by a higher voltage and significant cooling for the amplifier’s transistors. The linear amplifier is another of Tim’s kits and uses two IRF510s in push-pull mounted on good sized heat-sinks.

The resulting rig has turned out rather better than expected and happily delivers 20-25w peaks for long duty cycles of 15 minutes and more. Maximum output is 35 watts peak but that drives the transmitter PA’s BS170s rather hard and, with no heat-sinks, I don’t think they would last a minute or two like that.

Higher output from the Walford Linear Amplifier

An earlier project of mine was the Sutton, a multi-band double-sideband transceiver, which I’ve used on 160m and also had to increase its output to above 15 watts peak to make the design practical at this QTH.  To achieve that, I have provided an option to feed a higher voltage supply to their linear amplifier boards using a PSU adjustable from around 21v to 32v at 6A. The same approach has been used with the Cam. To protect the IRF510 MOSFETs in the amplifier I have mounted a (ex-PC) fan above their heat-sinks and run it at 12v or less. A good compromise between expelling enough hot air and not being too noisy seems to be 9 or 10 volts so a series resistor is used to reduce voltage.

Transmit-receive switching

The ‘Cam’ project being a transmitter without a corresponding integral receiver, I’ve used relay switching triggered by the microphone PTT switch to both mute the receiver when transmitting and switch the antenna routing between receiver and transmitter. There wasn’t a great deal of room for the relay so I mounted it on its back using sticky Velcro and it just fits on top of one of the solder joints between the transmitter and amplifier back-planes.

Walford Cam - AM transmitter for 160m - rear view

In the photo below, the front-panel is to the left and rear to the right. The relay is where the ‘mess’ of wires are connecting to in the upper middle of the photo.

Walford Cam - AM transmitter for 160m - top view

Controls and Features

The Cam came with a trimmer capacitor and tuned between around 1.983 and 2.005 MHz. I replaced this with a front panel variable capacitor (Polyvaricon) and with some experimentation with other capacitors in the oscillator tuned circuit, I now have tuning between 1.963 and 1.993 Mhz which is more usable.

I also wanted PA drive control to be on the front panel. It was a 10k trimmer resistor but I found a 10k linear variable resistor to have all the ‘action’ within about 10% of its travel so I used a 4.7k linear variable with another 4.7k fixed resistor in series. The control is still tight within 25% of its travel but it is usable.

Frequency stability is impressive with negligible drift using a 74HC02 IC (quad NOR gate array) as the oscillator and a 2 MHz ceramic resonator pulled down a bit in frequency, as described above. I’ve fitted a front-panel phono socket connected to the oscillator output so that I can more precisely measure frequency with my meter although now the tuning dial is calibrated it’s not so vital.

The oscillator is off during reception and so to net the frequency, I’ve fitted a mini non-latching push switch which switches to oscillator on so that you can tune for zero beat on the receiver.


Walford Cam - AM transmitter for 160m - inside viewThe enclosure is thanks to Tony G0DLX who built the enclosure for another project some time ago. It turned out to be a perfect size and of excellent design. It has a side wall with lips at top and bottom with the PCBs mounted on the lower lip. There are top and bottom removable covers which are excellent for maintenance.  The front control area is accessible without removing the metal covers and has a push on plastic cowl which was fashioned with heat and softening adhesive.

My workshop metalwork facilities are poor and thankfully all I needed to do was put a few more holes in for extra controls/sockets. I also needed to cut out a section from the upper cover so that the fan could be mounted there.

Further construction projects of this nature need a workshop upgrade however!

Front Panel Graphics

The front panel was designed in Serif Page Plus which I use for my business Squirrelhouse Media. It was printed on photo quality paper and laminated and the controls and sockets then cut out using a craft knife.

Cam AM Transmitter front panel graphics

Construction and Issues

In general the construction was straight-forward and went very well. I had a few snags early on and burnt out the two BS170 driver transistors in the transmitter section. With Tim’s advice I was able to pin this down to poor grounding in the Low Pass Filter stage which was presenting the wrong impedance to the transmitter output.

I monitored the output on my oscilloscope as well as with power meters but in fairness the scope didn’t really make much difference to me getting the alignment right.

Probably the most difficult part of the construction was the coil winding for the linear amplifier transformers. There aren’t many turns although they are a little complicated if you haven’t seen this sort of thing before and it was seven years since I last had. My frustration is always in ensuring I have clean and tinned ends to the coil windings that I can solder. I find burning off / scraping the enamel a bit of a challenge sometimes although get there in the end by… (i) scrape with a knife (ii) burn the enamel with a 25w soldering iron and then (iii) rub with wire wool before tinning the ends.