1. Adding coverage for Top Band (1.8-2.0MHz)
This is an overview of the steps required - more details will be available as a download in due course.
Rather than lose an existing band position I elected to add an extra position to the bandswitch which requires some major surgery so this is not a task for the faint-hearted.
The extra band segment requires the provision of an extra band position, band crystal, oscillator tuned circuit and loading capacitors and trimmers in the RF and Mixer sections. The extra crystal should be 10.395MHz for 1.5-2MHz or 10.695MHz for 1.8-2.3MHz. I had the former crystal in stock in an HC6/U case.
To provide full band coverage for 1.8-2.0MHz on the preselector tuning when using loading capacitors, the LC ratio of the 3.5-4.0MHz tuned circuits must be increased. If you prefer you can use additional tuned circuits for 1.8MHz and avoid the need for loading capacitors and changes to the 3.5MHz tuned circuits - your choice which will be determined by the availability of suitable coil formers, trimmers etc. that will fit into the available space.
An additional inductor is required in the PA compartment to resonate the PA tank circuit on 1.8MHz and an additional loading capacitor is required under the chassis to match to 50ohms.
You will need to remove all of the printed circuit band boards attached to the bandswitch and remove all of the switch wafers from those PCBs so a solder sucker and fine desoldering braid are essential.
Adding extra positions to each wafer requires access to either a switch kit or unused switch wafers from which spare contacts can be removed - the latter also requires a small stock of eyelets or very small (10BA or similar) nuts and bolts to fix the new switch contacts to the existing wafers. There is a specialist eyelet suppler in USA.
Most of the switch wafers carry out more than one function and in some cases the shorting mechanism must be removed and one position cut back to ensure that the switch behaves correctly in the 1.8MHz position - this is one of the most difficult tasks in the whole process and requires good planning, very small tools and good eyesight (or good glasses).
The bandswitch click unit must be removed and modified to move the anti-clockwise stop back by one position.
More to follow but you get the general idea. The modification works very well and is well worth the effort but you need lots of patience and a clear understanding of each switch wafer function.
2. Transmit Audio Quality and Gain
Measurements showed that the screen voltage of the microphone preamplifier V1A was significantly higher than the anode voltage and the value of resistors R1 and R2 were within specification. Changing valve V1 made no real difference so I changed the value of R1 to 220K and R2 to 1Meg. This resulted in the anode voltage being higher than the screen voltage, an increase in voltage gain and a significant improvement in the distortion of the high level audio signal at the anode of V1.