Ekco A23

 

 

Reluctant inductance

 I have a theory that many of the old radios around today have survived because something really bad happened to them years ago. When they were quite newish they went wrong and were put on one side until they could get seen to. Judging by what I've seen some of them got seen to and probably never did get fixed. Maybe then, they just found their way to the loft, or if it was the local bike shop (where a lot of repairs were done) , under the counter, because they were too good to throw away. When the owner of the house or the bike shop proprietor, departed this world and his possessions disposed of, the radio emerged at a local saleroom or the local tip.

Now, years later original faults reappear like ghosts and I seem to to get more than my share of them. Take this old Ekco A23. The HT electrolytic boasts the date "Dec 47". Without the "T-cut" treatment the bakelite case looked really shabby, but hot soapy water and a quick rub and it looks as good as new.

Anyway, out came the chassis and the loudspeaker and onto the bench it went. Gently out with the valves. Each one then subject to a careful, upside-down clean with warm soapy water and a small paintbrush to avoid losing the red metallic finish and the legends.

Now brush all the accumulated dust and debris off the top of the chassis. No rust to speak of -so no need for the "Jenolite". Balanced upside down so its vital bits can be measured the only thing of note was the usual conundrum- why couldn't they make 47K resistors that stayed 47K? Still, nothing really outlandish as far as the resistors were concerned. Disconnect the audio coupling condenser and a check with the old AVO set to the 100Kohm scale. Well, I've never seen one as good as this, absolutely without a whisker of a leak.

Electrolytics look innocent enough- they also measure up to their labelling. No problems here. A squirt with switch cleaner on all the contacts- wavechange switch, five push buttons and the tone control switch.

A new mains lead to replace the foot or so of red, green and black bits of wire sticking out the back of the chassis, a couple of new dial lamps and we should be in business. Plug it in and gingerly switch on. Well the switch works cleanly- no signs of it having gone to sleep. A slight re-assuring hum and it springs into life for the first time in goodness knows how many years. A quick jiggle of the volume control clears a slight crackle and a check reveals long and medium waves are pretty good. A press of the push buttons and a twiddle of the cores of the preset coils indicates that all is well here.

Where are the short waves? I always have trouble with short waves. Band conditions aren't that bad- at least there ought to be something around 31 metres. Plug in the "2008" signal generator and sweep up and down a few times... nothing! The local oscillator's not working. There's a paxolin panel covering the coil ends and the trimmers... so off it comes. Judging from the pristine screw heads and the fact that they're all there says that it's never been off. All the coils and all the trimmers are liberally coated with wax and haven't been touched since they left the factory- nice to know they haven't been butchered. I wonder if one of those awkward to reach capacitors has gone off. I know; I'll make a list of likely candidates and do this logically: trace the circuit. The ECH35 triode grid circuit is connected to the feedback winding. The anode's connected to the tuned circuit. A coil tap carries the trimmer connection. There's a nice big 4700pF padder, a little 100pF in the grid circuit and a mate in the anode circuit. A second 47Kohm resistor's switched across the 47Kohm anode load for short waves. Check the continuity of everything to check the wavechange switch isn't acting up and the coils are OK. No problems. Condensers are taken out and measured for leakage and accuracy. They're all blameless. The extra 47K resistor's a bit high so just to be safe I'll bridge it. No joy there. Maybe the HT's low? Bridge the electrolytics- no difference. Change the AY31- up goes the HT. Still no short waves though. Let's melt the wax and have a twiddle. After some effort including splitting the ceramic washer on the trimmer I get some life- the set tunes the generator signal from 15 metres just short of 20 metres. Hopeless tracking though and the readings can't be made to fit the dial. Time to think again. Out with the coil. It looks like it must have done when it left the factory. Covered in wax and nothing to indicate a problem. Connect a condenser across it and a quick check with the GDO reveals it is a coil. Back in it goes taking care to get the connections right.

On with the computer. If I can't think of a solution I have a play on the computer. In with the homemade model of an old valve radio front end and in with the coil details. A fiddle here and a fiddle there tells me the coil and the dial should go together. The 4700 pF padder's just the job. A twiddle of the vitual slugs and a tweak of the virtual trimmer and Bob's your uncle- at least the computer model works even if the radio doesn't. Back to the bench. On with the generator and on with the scope. The RF stage succumbs to a bit of fine tuning and the ECH35's getting exactly what the set designer had in mind. Dial calibration's perfect. The oscillator coil seems to want a beefier slug but nothing I do seems to get me enough inductance. At 20 metres on the dial the oscillator's running at 18.

With some discomfort I dangle the capacitor meter in the wiring and twiddle the tuning condenser. Perfect, just like a 500pF tuning condenser with strays should read. Back to the drawing board. If the laws of physics are still operating in the workshop the coil should be around 0.8uH. Out it comes again. Across goes a 515pF condenser and on goes the GDO. Out with the slug and a quick twiddle and out with the calculator. Hold it up to the light so the solar cells get enough candlepower and 0.56uH is the result. Back in goes the slug and I can squeeze it up to 0.75uH. The Q's not very good and anyway the computer told me it needed around 1.3uH. In with the eyeglass and I slowly start to melt the wax off the coil. The bare wire's carefully laid in a groove so no chance of a shorted turn. There's no continuity between the reaction winding and the tuned circuit so there's no short between them. Wait a minute- that tap for the trimmer. The way it's made looks a bit suspicious- it's not soldered to the winding- its part of the inductance left loose and twisted together. The twists go right up to the coil and blow me squash up against adjacent turns. The coil winder must have been a bit heavy handed then concealed the evidence with melted wax.

Carefully scrape away surplus wax and manoeuvre the twists of the tap clear of the coil- apply some of the wax back, connect the 515 pF condenser and what do you know, the GDO tells me its 6.5 MHz- spot on.

Back in goes the coil- clean up the rat's nest and on with the 2008. Adjust the slug for 50 metres- superglue the ceramic washer and adjust the trimmer for 15 metres- twiddle to 50 metres, tweak again and then to 15 metres another tweak and stations are filling the band - for the first time since 1948??

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