The customer declared the set
to be "not working" and there was little chance of
it working as there was no mains lead
The radio is an expensive model made in the latter half of 1945 or early 1946 and uses many good quality military type components no doubt surplus to requirements of the day. The chassis is, unusually, chrome plated and is made in three sections which allowed different models to be fabricated from common sections. The better models in the series used a larger centre section and utilised additional valves (noted by the blanking panels fitted to this chassis). The cabinet rear panel is made of glass instead of the usual material so that the interior can be viewed and this explains why the chassis is finished in chrome plate and is mounted at an angle allowing one to see the construction.
The valve line-up is standard
being 6K8 frequency changer, 6K7 IF amplifier, 6Q7 detector,
AGC and audio amplifier, 6V6 audio output and 5Z4 rectifier.
The overall design probably dates back
Interestingly, certain safety aspects
are considered pretty horrific by today's standards, notably:-
The design of the antenna/earth connectors are useful in not requiring plugs although care must be taken not to short the end of the aerial wire to chassis.
The tuning control is not functioning
In summary, I reported that the radio is considered to be quite rare and despite the problem with the tuning should command a good price if offered to a collector. If restoration is carried out on the cabinet this should be done sympathetically to preserve as much as possible of the old finish.
Before connecting power to the set the
following tests were made:-
Mains was then connected to the set. The rectifier valve was observed to be sparking and glowing purple, a sign of internal gas. No sound was heard from the speaker. The primary winding of the audio output transformer was found to be open circuit. One dial lamp was found to be u/s and two others were of the wrong type. The mains transformer was observed to be very noisy due to loose laminations.
As the dial pointer was very loose and
inoperative it was tightened but when the tuning mechanism was
operated the pointer was found to hit the ends of its travel
when the tuning capacitor was still part meshed. Because of this
the frequency coverage was considerably restricted. In addition
because of the mechanical arrangement the pointer became loose
after hitting the end of the range and fell to one end of the
The plastic tuning scale is distorted and slightly damaged through ageing.
See notes 1-4 for other faulty items needing replacement.
The major item requiring repair, the dial mechanism, was carefully examined. The solution adopted, because of the non-availability of a new component, was to revise the coupling between the tuning capacitor spindle and the pointer. A new mechanical assembly having a mechanical advantage of about 3:4 was made which links the two. Because the coupling is extremely critical the new part is adjustable (see note 5). The design of the new assembly requires a tensioning spring to ensure its correct operation and this is adjusted by trial and error to achieve proper operation (see note 5). The new coupling unfortunately results in a slight mis-calibration of the dial originally calibrated using a linearising device which had to be discarded in the new design. By altering the tracking of the tuned circuits the scale misalignment has been minimised.
The loudspeaker output transformer has been replaced with a pre-war universal type and the correct tapping found by trial and error.
The rectifier valve has been replaced with a GZ34 which is superior, drawing less heater current and having a better HT rating.
A burned out resistor in the RF gain circuitry has been replaced with one having a value surmised to be appropriate.
A resistor disconnected from the RF gain switch has been re-connected.
Two replacement decoupling electrolytics have been substituted for the faulty items.
New dial lamps have been fitted.
The coil pack has been re-aligned and tracked as closely as possible to the scale (see note 6).
A replacement mains on/off switch has been fitted. The original was missing and would have been unobtainable. The replacement is fitted to a contemporary potentiometer which has been coupled to the tone control via a brass bush. The coupling is fairly critical and may require resetting in the future.
A new mains lead has been fitted together with a blanking plate fitted over the hole for the original (missing) connector.
The tuning scale was reset by gluing transparent plastic pieces between its edges and the backing plate. This results in a reasonable degree of flatness and allows the illuminating bulbs to backlight the scale.
Smoothing electrolytic, 8uF also measured at 8.5uF
Additional smoothing electrolytic, 4uF measured OK