Heathkit HW-7 part 4 – receiver filter

Now that the Heathkit HW-7 is working close to the original specification with all the 45 year old carbon resistors changed for 2019 metal film resistors I can now look at some modifications to bring it up to modern standards.

The first thing I want to look at is the receiver filter. The HW-7 has a simple LC low pass filter immediately after the 40673 mixer so lets see how it performs.

To do this I fed a wideband noise source into the antenna input suitably attenuated so that it didn’t overload the receiver. I then fed the audio from the headphone socket into an external sound card connected to my Windows laptop. I then opened the Spectrum Lab audio analysis program to look at the filter bandwidth and response.

The existing filter is fairly flat from 200 to 1200 Hz at the -6 dB points with the peak at 1000 Hz. It’s not a bad filter for such a simple design as the stopband from 3500 Hz is 50 dB down.

It’s probably a bit wide by modern standards and later I want to modify the transceiver so that it has a correct 750 Hz offset between transmit and receive so can the filter be modified to sharpen it up?

I modeled the existing filter in the Elsie filter program and came up with some capacitor value changes that reduced the loss between the mixer and audio amplifier by 5 dB and sharpened the filter. The peak of the filter is now at 750 Hz and the -6 dB points are at 400 and 1000 Hz. The stopband figure remains the same at 50 dB down.

The changes look very subtle on the display as the 5 dB reduction in loss tends to hide the changes but it does sound a lot better listening to CW on air.

Only three capacitors were changed, C5 is now 0.47 uF from 0.22 uF, C9 is now 0.47 uF from 0.1 uF and the coupling capacitor C52 is now 0.22 uF from 0.05 uF.

I did try experimenting by changing the inductor bypass capacitor C7 from 0.01 uF to 0.1 uF. This gave a really sharp cutoff above 1000 Hz but the stopband figure reduced from 50 dB to only 30 dB so while it would sound great working weak signals at sunspot minimum it would fail listening to a band full of strong contest signals.

With simple filters like this it’s tricky to get the right balance but you can get close with modern computer modeling and measurement tools.

Heathkit HW-7 part 3 – resistors

Before I start doing the fun stuff like adding a new mixer or tuning the audio filter I thought that I had better get down to basics.

Earlier I mentioned that vintage carbon resistors tend to fail by slowly increasing their resistance value and the only resistor I had removed so far showed signs of that. It also had an intermittent fault on the receiver where the mixer appeared to cut off when the PCB was pressed.

Today I removed all the old carbon 10% tolerance resistors and replaced them with modern metal oxide 1% resistors. I measured the value of each old resistor to compare it to it’s nominal value.

It took a bit longer than my lunch time to do it, it felt like I was building the kit from new!

Only two low value resistors remained within the 10% tolerance rating. All the others were 20 to 30% higher in value than what they should have been.

It was worth doing as testing after the work revealed that the receiver sensitivity had increased considerably and the VFO stability was improved especially immediately from switch on. The intermittent fault had gone so it was probably a dry joint on one of the old resistors.

With the correct resistor values in circuit it’s probably now working as the Heathkit design engineers intended back in 1972.

Old radio ideas revisited

bulgin_radio_service_manual

One of our local radio clubs (South Manchester Radio and Computing Club) has a very extensive library of old technical books donated by the members. Looking through it one evening I came across the Bulgin Radio Service Manual 1944-45 edition.

A few pages in I found an interesting article called “Aerial Efficiency” This described how to make a Doublet antenna for short wave listening using a selection of Bulgin radio components.aerial_efficiency

One line in the article stood out…”Varying forms of lead-in are employed, one of the simplest is twin-twisted rubber-covered wire, and very satisfactory indeed this is for experimental purposes.” It suggested that the rubber covered wire wasn’t good for outdoor use so ladder line was used for permanent installations.

Many years ago back in the 1970s I started my working life as an apprentice engineer for the local Electricity Board and I spend a few weeks with the house rewiring electricians removing the “twin-twisted rubber-covered wire” from a few houses. It didn’t last very long indoors and was fairly lethal with 240V AC applied to it as over time the rubber perished and started to crack up.

However the article got me thinking… What if I tried using modern materials for the “twin-twisted wire”. In the workshop I keep a few drums of 0.5mm and 0.8mm tri-rated cable from RS Components. This is a flexible copper wire of typically 16 or 32 strands but covered in a thick PVC insulation. It’s designed for use in industrial machinery and can withstand oil and other chemicals and a fair amount of heat. It’s called tri-rated as it’s meets the wiring regulations in Europe, USA and Canada and as it’s made in vast quantity it’s relatively cheap.

Twisting two wires together with a battery drill was a very easy and quick way of making a twin feeder. Entering the wire dimensions in an online calculator gave a characteristic impedance of 108 ohms for the 0.5mm wire so it should be possible to use it with a doublet antenna as the book suggested.twisted_pair_doublet

I soon whipped up a temporary doublet antenna to fit on the 9m Hilomast I use for antenna testing. The doublet elements are 8m long and the twisted pair feeder is 12m long to run down to the workshop. Two 20m long lengths of wire were used to make the feeder and the wire elements. The photo shows it on the mast just ty-wrapped on for testing. It certainly looks a lot better than ladder line and a lot cheaper too!

Using a Z Match tuning unit it could be tuned for an efficient match on all bands from 60m to 10m and would tune and match on 80m but it’s very short in terms of wavelength.

The most interesting observation is that because the majority of the electric field between the two wires in the feeder is within the PVC insulation the feeder can be run through a metal framed window closed onto the wire with no noticeable effect. I also tried running it on top of a metal cabinet again with no noticeable change to the tuner match or signal level. Try that with ladder line and it’s badly affected.

Testing with 150W on all bands through the tuner showed no adverse effects even when the metal window is closed on the feeder.

Of course any temporary antenna that works well becomes semi-permanent so it’s still up on the mast several weeks after the first test!