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Nicko

Fri Sept 03 2010, 08:36PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

...that has an FM tuner in it and re-broadcasts on a given fixed AM frequency (MW or LW)? Cheap & cheerful, hopefully. Maybe even with MP3 & line-in support too!

The reason? In my infinite stupidity, I have just spent a couple of days fully restoring/rebuilding a nice little 4 valve mains/battery portable radio. Very pretty little thing. Couldn't resist it. A present for SWMBO. Works a treat now (the radio, that is).

One (slight) problem is that there is not much around on AM any more, unless you like faint whistling German and Russian stations late at night... so I want to get an FM tuner to rebroadcast very (very) locally (a few meters would be good) on AM at say 500M or so, so I can use my (wife's!) nice restored radio.

For those who care about such things, its a Vidor "Lady Anne" from 1955.

Surely someone, somewhere, must produce something for this!

Cheers

Avalanche

Fri Sept 03 2010, 09:02PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I don't know of any commercial product, but it's very common for radio restorers to build their own 'micropower' transmitters for this exact purpose, for example see

I'm actually building one myself, using a much simpler 'off the shelf' approach - a 4Mhz crystal oscillator divided down to 1Mhz (exact 50% duty cycle square wave) driving a resonant LC circuit through a small amplifier stage. Amplitude modulation is done using high level modulation (modulating the DC rail of the class c stage). I'm doing this so I can listen to whatever I want on my recently restored GEC radio, and a couple of others I have.

Not quite what you're after, but I can post the schematic if interested once I have ironed out the bugs?

Nicko

Fri Sept 03 2010, 09:26PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Ah! That's you is it! - I'd seen that. Its my "Lady Anne" in the "Vintage Radio (domestic)" forum of the same site that I want to drive... I rather like restoring old radios now... got the bug...

I was looking at the "iTx - Tube AM Broadcast Transmitter Kit" from the Vintage Component Company as well as their "Spitfire - AM Medium Wave & Top Band (160M) Transmitter". Both look good - the iTx because its tube-based, and the Spitfire because its comprehensive and fully boxed already. Another is the AMT3000 from SSTRAN

I'd certainly be interested in your project as it progresses...

Cheers

Adam Munich

Fri Sept 03 2010, 10:28PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

You can use a 1MHz crystal oscillator to broadcast on AM. (at 1000kHz of course)

Link to buy oscillator.

Conundrum

Sun Sept 05 2010, 12:49AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

an "old school" trick is to use a homemade lambda diode using J310 and BC212 transistors and a tuning diode made of red LEDs in reverse bias.

-A
"Bother" said Pooh, as Nature rejected his paper on 281k superconductivity...

Mark-H

Thu Sept 09 2010, 10:52AM

Registered Member #607 Joined: Tue Mar 27 2007, 10:39AM
Location:
Posts: 64

Nick.
The 1000KHz osc and "recovered internals" of a cheap pocket FM radio is the way I've done it to over 30! Vidor Lady Margaret and Lady Catherine radios over the last 10 years.
As of late, all the pocket stuff has come from Freecycle.
A very cheap and compact mod that I build into the B battery (These two models are battery portables from Vidor in Erith.)
The B should be 90V but these will work down to 45V or even 27V so I can reduce the PP3s in the box.
I power this from the filament A battery.
Cheers.
Mark.

radiotech

Thu Sept 09 2010, 03:36PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

an "old school" trick is to use a homemade lambda diode using J310 and BC212 transistors and a tuning diode made of red LEDs in reverse bias.

Never used led's- old top hat rectifiers would shift enough to sweep the AM band. I do have a Heath 2240 4-point LR bridge
that allows using externnal DC bias on capacitors to test some some new LEDs on. Wonder if they would work as photo-capacitors like the vidicon screen coating does?

Conundrum

Thu Sept 09 2010, 08:05PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

interesting..

someone on the amateur radio forums suggested that often "old" germanium diodes can somehow be persuaded to oscillate by careful cooking (forward bias under substantial current until it starts to drop, then allow it to cool), which ruins the diode for conventional use but gives it a weak negative resistance zone due to electromigration or something ?

supposedly some LEDs behave the same, although most of the time the ball bond goes OMGWTFLEDBBQ and opens.

can someone investigate this "effect" please?
regards, -A

"Bother" said Pooh, as his cortical node malfunctioned...

Adam Munich

Thu Sept 09 2010, 08:47PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

"supposedly some LEDs behave the same, although most of the time the ball bond goes OMGWTFLEDBBQ and opens."

You could try and attempt it with a cree led. They solder the chip.

radiotech

Fri Sept 10 2010, 02:05AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Back in dim dark past there was a transistor with an alpha >1
called a hook transistor. They seem to have stumbled on them by screwing up a point contact transistor during manufacture.Maybe cooking the diodes created extra layers like a diac. Just think of the fun of altering leds with a laser or microwaves.

Years ago there was a great plentitude of Osram type R valves to play with and their characteristics could be altered by carefully melting depressions in the envelope with a bunsen burner.

If anyone knows why these appeared by the thousands in Montreal 50 years ago please tell.

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