Suddenly un-loved Sudden

From an unknown circuit board ripped from a radio in a car park to working 80mtr receiver.

At the Kempton Park Radio Rally of 2010 when asking a stall holder if he had any Toko coils, the reply was that he’d just sold a junked radio that had some. He commented that the guy who’d bought it had said he’d no interest in the circuit board within the home made radio instead had bought it for the hardware on the case. If your quick and find him, he’ll probably let you have the bits he doesn’t want. He had headed off into the car park I wast told.

Without further ado, we rushed out into the car park and found the man. He was glad to hand over the circuit board to me, if we could only get it out. Together with little more than a pen knife we sat at one of the open air benches and dismantled the radio.

This is the unwanted part, the suddenly unloved Sudden – but at the time I did not know this was what it was. The three silver can’s are the Toko coils and I believe they are no longer made which is why they’re getting hard to find which was all I wanted.

The pcb with Toko Coils

The pcb with Toko Coils

On the back of the board it bears the name PW Publishing Ltd and the board number WR283 together with a date, 1991. This puts it just outside any ‘digital age’ of Practical Wireless magazines online information and I had no further knowledge of what it was.

Having sat in my junk box for almost a year the usual sources of Ebay was used to try and locate Practical Wireless magazines that would have described the project. Nothing came up, probably more due to a lack of effort than that it would not have eventually given a result.

Anyhow, when taking a further look it should have been more obvious. The two chips used are an LM386, which is an Audio Amplifier, and an NE602, a mixer / oscillator. These two components suggest this is some sort of Direct Conversion receiver. The KANK333 coils find application around the 3.5Mhz amateur band. At this point a decision was made that it would not be disassembled for its coils, a process that is not going to be easy any how, but rather I would try and work out what the missing bits were and see if we could get it going.

Eventually the passage of time with searching gave way to this circuit diagram of the Sudden, a design by the infamous Rev George Dobbs.

Circuit of the Rev G Dobbs Sudden Receiver

Circuit of the Rev G Dobbs Sudden Receiver

Caught ya!

The circuit shows three coils, T1, T2 and the vfo T3. T1 and T2 are KANK3333 types. The two chips, NE602 and LM386. The finger print seems to fit. Now I knew what I had, it only needed the collecting together of the missing parts and a few hours putting it together.

I chose a tuning capacitor from the junk box with a slow motion drive. I’ve no idea of its value but its probably the usual 200 to 500pf type, and it did have another three vane section which is actually what I used. However this sounded like it was only able to tune almost between two stations so this was disconnected and the larger section used one the radio was receiving. A 4k7 pot was found for the volume control, a linear type, while a second was added to the front end for strong signal attenuation control. Other bits as usual from the junk box. With no headphones to hand some other designs were consulted that showed the LM386 was sometimes used to drive an 8 ohm speaker. This is how I set it up.

No short circuits to success

Having achieved the together stage, power was applied and nothing happened. There was a slight spark from power lead as I touched it to the power source, and that didn’t look very good. Its a low power device. I felt the chips and to my surprise neither of them were warm to the touch. Measuring the current consumption showed 180 milliamps. Opps! I measured the DC resistance into the power leads which read 22.8 ohms. This gives a an input voltage of only 4 volts and the circuit is meant to powered from 9v. The power pack used was rated at 11.4 volts but the measured terminal voltage was only 7.4 volts. It was what I had to hand. Clearly the voltage was falling away to deliver the current while at the few 10’s of milliamps I expected the circuit to draw the terminal voltage should have held up better. It was however time to use something else and the only thing to hand I could find was the camera battery rated at 7.8 volts. So this was used but it was far from easy to keep connected.

Expecting one of the chips to be faulty, in particular the LM386, this chip was removed and power applied. The power consumption was still the same so this clearly wasn’t the problem at this stage. Part of my troubles were going to be that I had no spares of either chip to hand.

Removing the NE602 and applying power the consumption was the same with no change. At this point there should have been no current consumption since the power comes in on two resistors, the 1.8k and the 22R resistor and goes directly to the now disconnected chip pins. The only other things are the two decoupling capacitors to earth. Hang on a minute! Whats that 22R Resistor? Thats what I measured its DC resistance as. How can that be?? There is something short there.

Checking the board for a short from the 22R resistor revealed nothing, while measuring the Tantalum capacitor revealed a short circuit. This was the culprit. With this removed temporarily the radio powered up but was extremely quiet from the camera battery.

As the night drew in stations from France, Germany and possibly even the USA were tuned in on the 80mtr band. The LM386 seems to be having to work flat out to drive a small 8R speaker. Headphones would be better. This is my first meeting with a Direct Conversion receiver. The tuning needs to be very precise to resolve SSB transmissions. A fine tuning control would help greatly. The two front end tuning coils could probably do with a tweak but at the moment I do not have the correct tool for this and no I am not going to use a screwdriver because this will almost certainly crack the ferrite slug.

I never did ask the guy who gave it to me what he was going to build in the remains of the case. Possibly a Sudden?? From a ripped out circuit board in a car park to a working radio once more. Surprisingly this radio is already 20 years old.

The Sudden receiver kit is still available from Kanga Products either as the original version, as I have here, or as their updated version.

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