We now turn our attention to the various circuits lurking in the die-cast labyrinth that is the chassis. This is the 2nd Mixer and 37.5MHz Amplifier which is deceptively tricky to work on. I had already started to remove a few components before I remembered to take the above photograph, hence the loose wire and missing resistor. It was on completion of this circuit that I had my second ‘Oops!’ of the project. (The first being the shattered tag-board in the 2nd VFO). This time round it was a bit more serious and produced some smoke! After replacing all the Rs and Cs I replaced the cover and switched on the RX . . . Nothing. I removed the cover to the 2nd mixer and a cloud of smoke greeted me . . . Not good. The 1K resistor (R66) at the bottom of the photograph was completely fried. The cold end of the resistor was intermittently shorting to ground which made tracing the fault particularly frustrating. The culprit turned out to be minute flakes of solder that had infiltrated the vanes of C108 in the 37.5MHz amplifier. Throughout this project I have deliberately avoided disturbing any tuned circuits. A squint at C108 showed that the movable vanes were not positioned exactly mid-way (vertically) between the static vanes, and unlike in large variable capacitors, there is no adjustment. I rotated the vanes and three tiny shards of solder were ejected. However once the vanes were fully unmeshed and I continued the rotation, they clashed due to the imperfect construction. I moved the vanes back to roughly where they had been set. If I moved them too far, they touched again, so I backed off a bit and left C108 at that. Fortunately the circuit was easily retuned by way of the adjacent L50 which is in parallel with C108. I have had no further problem with this circuit, although if I get the opportunity I will replace C108 if and when I can lay my hands on a better variable capacitor.
