Heathkit HW-7 project part 2

The Heathkit HW-7 CW transceiver I had recently acquired had a problem with the VFO, it wasn’t oscillating. I’m going to use these blog posts to record what I find so that it might help others trying to renovate or restore these transceivers.

I found two faults with the VFO. The 350 uH choke has a high resistance of 12 ohms so I replaced it with a modern 470 uH choke.

The 47k ohm 10% tolerance resistor between the VFO FET gate and ground measured high at 58 k ohm. I remember back to valve or tube equipment of this era that also used carbon resistors like these. The resistors usually failed with the resistance value drifting high and I always though it was due to the heat and high voltage in that equipment. It looks that carbon resistors aged around 45 years also start to fail that way.

I replaced it with another 47 k ohm resistor. In the equipment we manufacture we use 1% metal oxide resistors and as far as I know we have never had one fail in the many thousand resistors we have used. It’s one area which has really developed over the years. I might look at “blueprinting” the HW-7 by replacing all the carbon resistors with 1% metal oxide resistors.

Now the VFO was working I ran through the alignment of the set. I found three dry solder joints around the multiplier coils. I also found dry solder joints on the coils themselves where the coil wires were soldered to the mounting pins. Using a solder sucker to remove the old solder and flux and replacing it with new after checking the wires were tinned correctly fixed the problems.

Now it was working I started to go through the list of Heathkit service modifications. The first modification was to remove R1 100 k ohm and short C6 100 pF with a short piece of wire. This really reduced the noise produced by the 40673 mixer and I could hear a 0.5 uV signal from the signal generator clearly on all three bands.

I found that the alignment was quite critical to get the best performance on receive, maybe some more resistors have gone high.

The next service modification was to fit a 220 k ohm resistor and 100 nF capacitor in series between pin 4 and pin 5 of the CA3035 IC. A 1 uF capacitor was also fitted between pin 6 of the CA3035 IC and ground. I’ve shown this in the photo above as it’s quite difficult to work out which pin is which from underneath the PCB with an early round IC. This modification reduces the gain slightly but the background noise from the IC almost disappears. It is well worth doing this mod.

I decided to do my first modification. The sidetone level was too high and most other mods to the HW-7 just hack the board to add a 10 k ohm pot or similar to turn it down. In my mod I replaced C46 0.05 uF with a 10 k ohm resistor and R34 33 ohm with a 100nF capacitor. This now acts as an RC filter that filters the tone but also attenuates the sidetone level. This gives a comfortable level that also sounds a bit cleaner too.

It may be a lunchtime project but I spent most of the afternoon and evening listening on headphones to it. Now that it’s working it’s surprisingly good for such a simple direct conversion receiver. The audio filtering is more suited to SSB than CW so I’ll probably look at that next while I’m waiting for some replacement PA transistors to arrive. The 40673 mixer does suffer from intermods and you have to be careful with the preselector to peak the amateur band rather than the adjacent broadcast band. I’ll look at replacing the mixer with a modern balanced switching mixer. I’ve got some ideas for that that won’t take too many milliamps of current.

An interesting point is that a resonant antenna like a dipole acts as a reasonably sharp preselector so the 40673 mixer intermod tuning problem seems acceptable for casual use with one. When I took it home and used it on the wideband Terminated Inverted U there was no selectivity in the antenna and you can tune the preselector and easily note all the HF broadcast bands. It really does need a better mixer!

The VFO is very stable and is fine for CW and SSB reception. Heathkit did a good job with that.

So far so good, it’s a fun receiver to use…

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