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
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??