BBC Radio 5 MW-FM Adaptor
This is from a twitter thread from August 2022
I’ve left it pretty much as the twitter thread, just removing some comments that don’t belong to me.
Getting re-acquainted with the deeper, more spider inhabited recesses of my shed, I found this:
Sun Aug 21 13:59:33 +0000 2022
Replying to @DTL
The topside looks like this:
Sun Aug 21 13:59:35 +0000 2022
Replying to @DTL
The underneath has a stylised BBC Radio 5 logo
Sun Aug 21 13:59:37 +0000 2022
Replying to @DTL
Back of the unit.
Sun Aug 21 13:59:39 +0000 2022
Replying to @DTL
Four screw holes, but only three screws, I suspect someone has been inside this before.
Sun Aug 21 14:03:55 +0000 2022
Replying to @DTL
The circuit board.
A Medium Wave ferrite rod antenna, a TEA 5570 AM/FM Radio IC from Phillips, an NE612 Mixer / Oscillator and a 68.5 MHz Crystal make up most of the interesting parts. IC Date code are mid-late 1990.
Sun Aug 21 14:08:47 +0000 2022
Replying to @DTL
This looks to be a very specific purpose item, to let you receive BBC Radio5 on a FM radio that doesn’t have medium wave capability.
Sun Aug 21 14:10:08 +0000 2022
Replying to @DTL
I wonder what was broadcast on BBC Radio5 in the early 90s that was so important the BBC would make a device like this to frequency shift and re-modulate the AM Medium wave signal to FM Band.
Sun Aug 21 14:11:56 +0000 2022
Replying to @DTL
Rear view of the PCB. Single sided, looks like the type of PCBs some old DOS software I had would produce.
A curled inductor trace on the lower part of the board connects to the red jumper wire on the top side.
Sun Aug 21 14:18:15 +0000 2022
Replying to @DTL
A ferrite bead on a diode. I’ve never seen that before.
Sun Aug 21 14:21:27 +0000 2022
Replying to @synx508
Aha! Thank you!
Sun Aug 21 14:23:17 +0000 2022
Replying to @DTL
An answer to what was being broadcast:
https://twitter.com/synx508/status/1561357744103428096
Sun Aug 21 14:24:32 +0000 2022
Replying to @DTL
Powering it up. No magic smoke escapes. An LED comes on and a relay clicks.
Sun Aug 21 14:48:13 +0000 2022
Replying to @Hookean1
Interesting - it fits with the time I was at school myself, but I never saw anything like this.
Sun Aug 21 14:48:57 +0000 2022
Replying to @DTL
The relay shorts the input sockets and output socket together when the power is off. This makes the device act as a pass-though with no effect on the signals (except a tiny bit of attenuation)
Sun Aug 21 14:51:08 +0000 2022
Replying to @DTL
A bit of a lash up to see if I can get a signal from the device into an FM receiver. I’m just using what I have to hand, the receiver is also on the bench for repair.
Sun Aug 21 15:01:04 +0000 2022
Replying to @DTL
I can’t get to the oscilloscope at the moment else I’d have a probe around and see if the oscillator is running.
Sun Aug 21 15:02:17 +0000 2022
Replying to @DTL
Taking a break for ice cream
Sun Aug 21 15:02:49 +0000 2022
Replying to @DTL
The lash up was far too crude, at best I could hear a slight raising of the noise in the radio when the device was tuned on. Let’s try with something a bit more sensible. Enter the tinySA Spectrum analyser.
Sun Aug 21 15:21:17 +0000 2022
Replying to @DTL
A slightly less crude set up to see if the device is working now. The output is capacitively coupled through the white wire loosely wrapped around the input antenna of the tiny SA. With the device off we see the general background of the FM band on the Tony SA.
Sun Aug 21 15:29:52 +0000 2022
Replying to @DTL
Powering up the device with the spectrum analyser attached to see if there’s any output.
Sun Aug 21 15:35:49 +0000 2022
Replying to @DTL
This variable capacitor lets you change the output frequency.
Sun Aug 21 15:40:38 +0000 2022
Replying to @DTL
But the far end of the capacitors travel, we can get the output frequency to 89. 13. Megahertz.
Sun Aug 21 15:41:46 +0000 2022
Replying to @DTL
I’ve not modified my TinySA to add demodulation, so to see if I’m just getting a carrier or any modulation too, I’ll just connect up the Pye radio again and see if I can hear anything now the signal should be in range.
Sun Aug 21 15:43:37 +0000 2022
Replying to @DTL
It works!
Without the device on, I hear Abba on radio 2 I think. With it on, we get some football rubbish from radio 5.
Success!
Sun Aug 21 15:49:19 +0000 2022
TL;DR for this whole thread about some long obsolete kit I found in my shed.
It works. It converts Radio 5 on the MW AM band, to an FM signal at about 88 MHz. https://twitter.com/DTL/status/1561379930348429317
Sun Aug 21 15:53:43 +0000 2022
https://twitter.com/DTL/status/1561435671096705024)
@BBCRD Don’t suppose you have a schematic for this in the archives do you? https://twitter.com/DTL/status/1561352307484381185
Sun Aug 21 19:36:12 +0000 2022