|The BC342 was manufactured in the USA during WWII and, in general design and construction, it's similar to a number of models of radios made at the same time; some for vehicle mounting and others for use in aircraft. In both these applications there was a requirement to make the sets really rugged so that their stability would be as good as the current state of art would permit. In those days this meant a heavy chassis and case with plenty of screening so that drift and oscillator instability due to temperature changes and vibration were minimised.|
Because the sets were used in a military environment they had to be well documented and relatively easy to maintain. However, apart from battle damage, I would hope that most of the servicing was restricted to changing valves rather than fault-finding down to component level, as this BC342 model is decidedly difficult to work on. Most of the small components are buried in the chassis and deep in metal screening boxes.
Alignment would have been straightforward as the labelling of the various trimmers is really clear.
The example passed to me had a number of faults still remaining after the owner had changed numerous components. Basically sensitivity was pretty poor and, especially with weak stations, there was an annoying rough hum superimposed on the audio.
After the IF strip had been aligned to 470KHz sensitivity was slightly improved. Because the set uses a simple crystal filter for narrowband reception, the correct intermediate frequency to which the set should be tuned has to be in accordance with the basic crystal frequency, rather than exactly 470KHz. Fortunately this example had a crystal close to the IF design frequency as it may have been tricky ensuring that good dial calibration was maintained if it had been otherwise.
To obtain good image rejection there are three fully tunable RF sections, each in its own sealed box and carrying six air spaced trimmers which are accessible after removing a blanking panel at the rear of the set. The oscillator section, with its tuning capacitor, is separate from the RF amplifiers and also has six trimmers. As far as carrying out full alignment is concerned there is no provision for changing coil inductance other than the provision of a detached turn of wire in each coil which can be physically moved to alter the overall inductance. Fortunately, because of the excellent RF design, it was not found necessary to alter the original inductance settings on any of the wavebands.
The method of aligning the tuned circuits is fairly standard. The tuning dial is first set to a specific frequency at the HF end of the scale for each selected waveband then the associated trimmers are adjusted for maximum output. After setting the HF end of each waveband the LF end of each scale is set and checked for frequency accuracy. I found that all six bands were no more than a few tens of KHz from the marked frequencies. Certainly just as accurate as the dial could be set by eye.
During RF alignment I found that two of the eighteen RF amplifier tuned circuits would not peak. The first was found to be due to a nut being loose. Some of the coil ends are connected via screws that pass through the base of their coil former to a solder tag which is wired to the associated trimmer. Getting at the scews is almost impossible without major dismantling so I drilled a small hole through the side of the coil former and through it passed a new wire from the end of the coil to the trimmer.
In another instance I found that one of the air spaced trimmers had come apart. The rotor should be held in place by a collar pressed onto the shaft but one hadn't been sufficiently pressed into place and had worked loose allowing the rotor to jam against the stator, shorting out the coil. It was next to impossible to remove the broken trimmer so the connection to the stator was cut and a beehive type trimmer was fitted next to the coil and soldered into place. This can only be adjusted by removing the top of the screened box.
Because of the amount of RF screening I found that absolutely nothing can be received with nothing plugged into the aerial socket. The immunity of the set to extraneous signals is therefore very good indeed, and because of the number of tuned circuits, and the quality of the screening between them, even 250mV of 470KHz injected into the aerial socket was not detectable.
After dealing with the lack of sensitivity I looked at the problem of hum. Because headphones are used with the BC342 even a small amount of hum can be objectionable. In this example the AF gain control had been fitted with a mains on/off switch and mains wiring was therefore pretty close to low level audio signals and, because the input impedance of the following valve (6R7 dd-triode) was extremely high, hum was being introduced (the grid leak resistor is 250Mohm). Once the mains wiring had been disconnected and cut back to the metal sceened box carrying the power supply the hum vanished
The picture shows the top and rear of the chassis with the trimmer cover removed and the 18 trimmers visible. Below these can be seen six screw-on covers hiding the oscillator trimmers. In the centre is the 3-gang tuning condenser, coupled through ashaft to the oscillator tuning condenser which is inside the lower screened box. Even the aerial trimmer is enclosed.. this is the cylindrical can in front of the lower RF amplifier valve.
The BFO is contained in the upper screened box. Also visible are three screened IF transformers. All the valves, in keeping with the quality of screening, are metal types.