Do crystal radios still pick anything up?

[tabbypurr]

Last time I went down there the red light stuff had taken over :( I used to like Proops etc.

NT

[Syd Rumpo]

Until they started filling them with opaque blue gel, same as was used for some if not all OC44s etc.

Cheers

[tabbypurr]

Last time I looked I found new ones at Rapid. That was over a decade ago though. Not sure what anyone does with them any more.

NT

[News]

In message , [e-mail] writes

OC71s still available via eBay. Well, they would be, I suppose.

[Dave Plowman (News)]

PNP can be useful, though.

[Johnny B Good]

Lack of selectivity in the MW tuning filter. A suitably strong enough MW broadcast is likely to be heard "Right across the dial".

[Johnny B Good]

Is that "third paragraph" a reference to the one I've just sent to Pamela in reply to her question? :-)

[The Natural Philosopher]

Nope. inside was blue opaque gunk after the first few years

[Terry Casey]

Currently there is a discussion going on here which might be of interest:

There's an impressive list of stations received in Kilmarnock in post #14.

[Chris Hogg]

In other words after Mullard found that they weren't selling many OCP71's and realised what everyone was doing.

[Terry Casey]

That was what most people believed at the time but there was another reason:

If the paint on a transistor got accidentally scratched and artificial light fell on it - though a ventilation grille, for example - the transistor would respond to the 50Hz light.

I would imagine that the first engineer to have a battery operated transistor radio turn up on his work bench with a mains hum fault was very, very, surprised!

Especially when he took the back off and the fault got worse ...!

So, Mullard started adding the opaque gunk ...

[Syd Rumpo]

When I was young, I worked for the Beeb as a technician. One day, I had an outside broadcast mixer to check out.

While I was 'lining it up' - checking signal levels at various points - I noticed a really bad mains hum. So I unleashed the oscilloscope. The hum was definitely there audibly, but seemed to disappear whenever I connected the scope probe.

To cut a very long story short, whenever I leaned over to connect a scope probe, I put the PCB in the shadow of a fluorescent lamp and the mains noise went away.

The mixer was all Germanium transistors. Of course, in normal use, with the cover on, there was no problem, but when the cover was off, small scratches in the black paint on the transistors was enough to make it pick up the fluorescent lights, and hum.

It took a long time to find that. Nowadays, I'd just assume I'd gone totally fishkettle aardvark alumni.

Cheers

[Graham.]

There are some Raspberry Pi boards that crash if you photograph them with a xenon flash.

When my daughter was little we bought her a VHS tape of cartoons. The top-loading Hitachi machine we had would not play this tape. Later we discovered it would play if the curtains were drawn. The cassette body was made of a translucent coloured plastic instead of the usual opaque black, allowing light to reach the optical tape end sensor that detected the transparent leaders.

[pamela]

I've never had a crystal set where a station is spread over an excessively wide part of the dial. My crystal sets picked up MW stations quite well (or as well as a crystal set can be expected). Radio 3 used to be on 464m and it came storming in nicely even on the crummiest tuning coil but didn't spread itself across the band. Admittedly, those stations at the shorter wavelength end of the band didn't come through so well.

I'm not sure how your, albeit tentative, design (which you point out may be somewhat flawed) would help with poor selectivity. Wouldn't the most effective thing be an improvement to the "power host" tuning circuit rather than the second diode arrangement you mention?

To be honest, I can't really visualise your suggested circuit but that may be because I don't really understand all the necessary considerations when designing a circuit. Maybe you're trying to overcome the problem of such a parasitic design providing power only momentarily (when the radio signal is carrying loud audio).

[Dan S. MacAbre]

Just been having another read of it this morning. Why do they not make books like this any more? I've a six-year old boy, and we try to find interesting educational books for him, but there's nothing now that pulls you in like those beautiful Ladybird illustrations.

I wonder how many people tried the spring of wire and piece of coke thing? I never did.

[Dave Plowman (News)]

When I was at school in Aberdeen in the '50s, schools broadcasts were recorded on a Ferrograph using a Truvox radio jack - basically a crystal set. It had two preset frequencies selected by a switch. Now in Aberdeen in those days all you'd get during daylight was the local BBC transmissions. Home Light and Third. And you could just hear another burbling away under the wanted one on this Truvox device.

[Dave Plowman (News)]

Snag is kids see others playing with phones and computers. So that's what interests them.

In the Ladybird days, radio might well have been the state of the art technology.

[Ian Jackson]

The selectivity of a crystal set is greatly affected by how much the aerial impedance damps the basic selectivity of the tuned circuit.

The selectivity might be sufficiently narrow if you have a short-ish aerial (essentially a high capacitive impedance) connected right at the top end of the coil - but being short, the signal it picks up might be rather weak.

A longer aerial will pick up more signal, but will have a lower impedance. It will have more capacitance (which will change the tuning point of the tuned circuit) - and in particular, the resistive part of the impedance will be lower, and this will damp (broaden) the selectivity of the tuned circuit.

To help overcome the broadening of the selectivity, the coupling of the aerial to the tuned circuit can be reduced either by connecting it to the top end of the coil through a suitably low-value capacitor, or (maybe better) by connecting it to a tapping point lower down the coil. Either may reduce the signal somewhat, and it is a question of experimentation to get the best results.

The other thing which broadens the selectivity is the damping effect of the headphone(s). For best sensitivity and selectivity, these must be high impedance, and again it may be desirable to move where the detector diode connects to the coil to a tapping point lower down.

Those who are aficionados of crystal sets (I'm not one, BTW) may go much further with circuit variations and complexity in order to optimise their performance. For those interested, I'm sure that Google will reveal all.

[pamela]

I never heard of a Truvox Radio Jack until you mentioned it but it looks lovely.

It reminds me of a crystal set a friend showed me when I was young. From memory, they had essentially wound a coil around what I think was a capacitor and attached a crystal earpiece. There was a long wire aerial feeding it.

I still can't understand how that could work as a crystal set but maybe the crystal earpiece was somehow acting as a diode detector.

By some total fluke, this contraption actually received Radio 3. (Which I should really be calling the Third Programme).

The whole device was smaller than that little Truvox Radio Jack.

[Dave Plowman (News)]

That's the one. I was hoping it was an Ebay ad showing they were now worth thousands. As I've got one somewhere. ;-)

There's not much inside them. Just a large enough case to make handling easy - and made from metal. These days you could probably make a plastic one much smaller but strong enough.