Decca Intercontinental 1125

 As the name implies this radio was designed to receive worldwide stations. It dates from around 1961 and of course uses transistors.

 
 The dial is very clear and calibrated in Kc/s and Mc/ instead of the customary metres used in the UK. You can see from the markings that it's frequency range, at least from 525Kc/s, is continuous to about 30MHz. There's a gap between long and medium waves because there's not much of interest tre and of course this area includes the receiver's IF. Long wave coverage is 150 to 290Kc/s. You'll note a special band spread scale.

 

 Not much to see inside the rear cover. Some power supply modifications are visible including the addition of that third transformer behind the loudspeaker.

 

 Below, some views under the rear cover. A three gang tuning condenser instead of the usual twin gang implies aerial tuning as well as RF amplifier and oscillator tuning. In fact that three gang tuning condenser is purely for fine tuning. It's driven by one of those small circular brass slow motion drives. When I pulled off the knob the brass spindle popped out. To fix it I had to remove the slow motion drive, prise off its cover then dismantle it. I could then re-insert the spindle and after reassembly and reclipping the cover it worked OK. The main tuning condenser is under the chassis as you can see further down this page.

Across the top of the tuning gang is a ferrite rod aerial to supplement the whip aerial on top of the case.

 

 

 
 Left, the gram input terminals.
 

 

 

 Below a guide to adjusting and aligning the receiver, view from above the chassis.

Below this is the circuit diagram... click it to see a full size PDF version.

 
 

 The set uses the following line-up of transistors:

AF114 RF amp, AF115 RF/Mixer; AF117 local oscillator, AF117 IF amp, OC71 AF amp, OC81D output driver and 2 x OC81 audio output.

Unfortunately the early Mullard AF series of transistors has only a limited lifetime due to the growth of whiskers within the metal cases. Sometimes the transistors can be made serviceable by disconnecting the can earthing wire, sometimes not. When I get time I shall investigate....

Removing the chassis was hard work. Removing the four securing screws was straightforward but getting the knobs off their spindles was difficult. Because a set of matching knobs is always important you have to very careful getting them off. Using a wide screwdriver is not worth trying because you'll damage the fascia and probably break the knobs. The best way is to pass a length of string around the knob, then repeat using a second length in the opposite direction and arrange the ends so you have four strings equally spaced around the knob, then pull all four equally away from the spindle and keeping exactly in line with the spindle. You may have to pull a few degrees off line backwards and forwards until the knob comes off. With this Decca, all came off except one which took with it a brass spindle from the slow-motion drive. Hopefully I can replace this later.

Below, on the left are the IF amplifier and audio circuit boards and on the right, the front-end with an enlarged view following.

 

 

 Not one, but two ferrite rods, one for medium waves and the other for long waves. Short waves use the built-in whip aerial.

 

 Front view of the chassis. The white background plate needs cleaning.

 

 Before I went any further I connected the receiver to a home-made power supply and found much to my surprise it worked. I might check out its alignment next to see if it needs tweaking before I put back the chassis into its case. I need also to repair the slow motion drive that came adrift when trying to remove the knob and I notice the rear mounted Belling Lee aerial socket is missing its centre pin. A second check of the receiver, once I'd fixed the damaged slow motion drive, resulted in a fizzing crackly sound so I guess one of the AF transistors needs attention. At least these are easily accessible unlike the ones found in Roberts radios! A quick check revealed the resistance between base and ground of the AF114 RF amplifier was only 28 ohms. Snipping the black wire emerging from the transistor body restored Radio 4 on long waves, but still with a crackly backround, so there's another duff AF transistor still lurking, or maybe a whisker between collector and base in the same device. Because all the circuitry is accessible in this receiver I might change the AF devices and fit alternatives.

 
 Above is the set after some work. I fitted two new transistors. Many years ago I bought a huge bag of transistors from a local government surplus radio shop. They'd been cut off circuit boards and had something like quarter inch leads. Many were marked "711" and I'd put these aside because they were germanium PNP types. Over the intervening years I'd never considered them for use in Roberts radios because I thought they were low speed switching transistors. A few days ago I checked the spec of the 2N711 and was surprised to discover they have a rating up to 100MHz and are therefore ideal replacements for the Mullard AF series. I fitted two in place of the 1125 RF amp and oscillator and they worked perfectly, not even upsetting the shortwave band dial settings.

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