A fellow club member recently decided to tackle the restoration of an old Astor DL 4-valve receiver for a friend of his. He began by cleaning the set and then replacing any parts that, based on experience, were likely to be faulty or which might give trouble later on. The replacement parts were fitted in exactly the same places as the originals, to conform to the original layout. This is always a good idea unless you are very experienced. Deviating from the original layout can lead to instability and other problems.
However, despite his careful work, my friend ran into all sorts of problems with this set, particularly with microphonics and instability. We’ll take a closer look at the work he did and the steps we took to overcome the problems later on. First though, let’s take a look at the circuit.
Fig.1: the circuit is quite simple and consists of a converter (6A8G), a 455kHz IF (intermediate frequency) amplifier and detector (6B8G), an audio output valve (6V6G) and a rectifier (5Y3G).
The Astor DL is one of the few sets not featured in the Australian Official Radio Service Manuals (AORSM) and so few restorers have any information on this set. However, I was fortunate enough to have a copy of “Those ‘Missing’ Radio Services Manuals”, 1939-1942, which was compiled by the late Ray Kelly. It also has some service data on a few 1947 receivers that were missed in the AORSMs and the Astor DL is included. Without the circuit, I could have been chasing my tail sorting out the problems in this set for quite some time.
Fig.1 shows the circuit details. As can be seen, the antenna circuit uses both capacitive top-coupling and inductive coupling to the tuned secondary winding. The tuned signal is applied to the signal grid of a 6A8G valve which functions as the converter (ie, mixer and local oscillator). The oscillator tuned circuit (bottom left of Fig.1) operates 455kHz higher than the tuned signal and mixing these signals produces the 455kHz IF (intermediate frequency) signal (along with others).
This 455kHz IF signal is applied to the first IF transformer (37) which is tuned to 455kHz. It is then applied to a 6B8G IF amplifier stage after which it is fed to the detector and AGC diodes in the 6B8G envelope. The detected (or demodulated) audio signal appears across the associated 0.5MΩ resistor (23).
This view shows the underside of the chassis minus the metal shield for the 6B8G IF stage. Note the proximity of the mains connections to the edge of the chassis. By the way, we don't recommend using a cable gland to secure the mains lead, nor do we recommend soldering the mains Earth lead directly to the chassis (see panel for more details).
From there, the audio signal is fed via a 0.05µF (50nF) capacitor (6) to a 6V6G audio output valve, then to the speaker transformer and finally to the loudspeaker. Astor were renowned for their complex and quite effective tone-cum-negative feedback circuits but in this receiver, this circuitry is missing and has been replaced with a very simple negative feedback system.
This consists of taking the cathode return of the 6V6G to earth via the voice coil of the speaker. It gives around 3dB of negative feedback which is probably all that could be tolerated in a set with only one audio stage. Any more and the volume would have been unacceptably low on weaker stations.
It’s puzzling as to why Astor didn’t take the 300Ω cathode resistor (31) straight to the chassis and simply connect the 25µF electrolytic capacitor (12) to the voice coil. The feedback would have been almost the same but with no DC current through the voice coil. This would mean that the cone would rest in its natural position instead of being biased away from this neutral point.
In theory, the DC offset in the Astor DL will restrict the volume that can be reached without undue distortion. However, since there’s only one audio stage and the volume is limited anyway, it really did not matter in this receiver.
The volume is controlled by using a 25kΩ potentiometer (34) to vary the cathode resistance on the 6A8G converter valve. This effectively varies the bias to the signal grid of the valve. The oscillator section is not affected, as its bias is developed across an oscillator grid resistor (24) of 50kΩ and is not reliant on the cathode-to-earth voltage.