The lack of a RITis a serious shortcoming of the Heathkit HW-8
The following circuit has now been installed in both my HW-8, and seems to work quite well.
A capacitor CD1 is connected parallel with the main tuning capacitor, and a solid state relay Rl1 is now activated together with the antenna relay. During transmission the voltage at TP1 is approximately 5,87V. On reception the voltage at TP1 varies from 2,64V to 9,1V, depending on the position of P1. The circuit is fed with 9,1V taken at the cathode of ZD1.
Rl1 is a solid state relais, but for reason of simplicity its contact are represented as mechanical. Another relay (i.e. a reed relais) can be used, but there are some restrictions:
- The LBA120 uses mosfets for the contacts and this allows for current to flow in both directions. A relay with a bipolar transistor as switch will NOT work.
- Rl1 should need no more than a few mA drive for it to switch, so that the voltage drop over the antenna relay between dots or dashes does not get worse than it already is.
- It is important that Rl1 activates before and deactivates after the antenna relay (it must be more sensitive).
I soldered C1, and CD1 on the body of the main tuning capacitor (works pretty well with a 50 W soldering iron, after slightly scraping the surface of the tuning capacitor), and I fixed a small board containing the other components at the rear panel of the HW-8. An articulated knob was fixed on P1 so that the position is visible when leaning a bit over the transceiver.
- Connect the HW-8 to a dummy load. Let the signal from the HW-8 zero beat with the signal from a generator (listen to both signals with an external receiver in AM mode).
- Now listen to the signal from the signal generator with the HW-8 and adjust P1 to obtain the pitch you normally tune to.
- Fix the knob on P1 so that it is in the neutral position.
- Although potentiometer P1 has a switch, in practice I have the RIT permanently switched on. When not needed I place P1 in the neutral position.
- After installing the RIT, it is necessary to recalibrate the dial of the HW-8, preferably by means of the trimmer on the main tuning capacitor. It is best to use a varicap with little capacity so that the shift made by it is as small as possible. The capacity of the BA142 varies from 16 to 2,2pF for 3 to 28V. I do not know exactly how the capacity changes between 2,64V and 9,1V, but the RIT shifts the receiver frequency a bit more than 1,5 KHz to either side. Enough to switch to the opposite sideband if QRM demands for it. The dial error due to the modification was about 15 kHz. Compensation of the error was just possible with the trimmer.
This modification does not alter the original circuits. It only adds a few components and is therefore easily reversible, apart from the little hole in the rear panel. That the control of the RIT is on the back of the set does not bother me much.