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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 22nd Apr 2018, 6:06 pm   #1
Biggles
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Default Basic stereo FM signal generator

There must be quite a few of those little gadgets you used to plug into an MP3 player or whatever to transmit a low power FM radio signal to your car radio lying around unused in drawers etc nowadays, as technology has made them virtually redundant. What you have in that little box of tricks is a complete stereo FM signal generator with a simple audio interface for providing a test signal into your FM broadcast radio. Granted there is no output attenuation but I would say either by distance or circuit output modification a basic cheap useful generator could be made.
Alan.
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Old 23rd Apr 2018, 4:43 pm   #2
'LIVEWIRE?'
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Default Re: Basic stereo FM signal generator

I have one of those, bought from W H Smith a few years ago for 15. Mainly I've used it to relay signals from my DAB radio to a FM portable in my kitchen, but it has sometimes served as a signal generator for alignment purposes. Not tried to modify it in any way, simply used it 'out of the box'.
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Old 23rd Apr 2018, 5:02 pm   #3
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Default Re: Basic stereo FM signal generator

I dare say that by injecting two different frequency audio tones it would be quite easy to check whether the stereo decoder was doing it's job, and also you could check the stereo broadcast beacon was working in the presence of a pilot tone. The unit I have has a digital readout of the transmit frequency, and is frequency selectable so is quite a useful bit of gear. I have a decent signal generator but no way of producing a stereo broadcast for alignment. Wrapping the gadget in some tin foil may be a cheap and cheerful way to attenuate the signal, which I think is pretty low anyway on mine.
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Old 24th Apr 2018, 2:38 am   #4
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Default Re: Basic stereo FM signal generator

I built my own using a BH1415 Rohm FM Tx ic back in 2007 (the BH1417 [dip switch selectable frequency] was a common chip in those little transmitters back then).

Fully frequency agile within the FM band, lcd display.

Lent it to a friend and haven't seen it since.
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Old 28th Apr 2018, 3:57 pm   #5
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Default Re: Basic stereo FM signal generator

The Belkin Tunecast range of devices are reasonably useful for quick and dirty testing of VHF/FM stereo multiplex receiver-decoders. The 'auto-off after n seconds of loss of audio input' feature is a bit of a nuisance since it resets your output frequency preference back to tbe default however, there is a smple and effective modification that can be done to prevent this. The search engine of your choice should track down the details fairly quickly.

I keep thinking that I ought to try and find an ex-BBC chum with access to one of the Designs & Equipment Department 'reference-standard' receivers, against which the performance of one of the above could be compared ...

"'Allo, 'allo, Evesham ... zis is Night-'awk calling ... are you receiving us? - ovair ..."

Guy
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Old 28th Apr 2018, 6:22 pm   #6
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Default Re: Basic stereo FM signal generator

My little transmitter/S.G. is actually a 'Logic 3'. This also has the 'auto off' feature, but I've never found it too much of a problem. T.B.H. I rarely use it, either as a 'pantry transmitter' or a signal generator, and, on the rare occasions I use it for the latter purpose, I make sure there's an audio signal present.
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Old 29th Apr 2018, 9:01 pm   #7
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Default Re: Basic stereo FM signal generator

I picked this up a couple of years ago, Im not sure of the age (80/90s?). The instructions show it connected to a record player and rxed on an FM radio but the range is short so I dont see the point, anyway I can connect it to my phone and tx to my FM radios.

John
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Old 30th Apr 2018, 8:15 am   #8
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Default Re: Basic stereo FM signal generator

If one could be otherwise bothered I suppose one could fit such a stereo transmitter to a shielded box and add the appropriate 2-tone input and an output attenuator(s) to make a (probably) useful piece of test equipment?
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Old 1st May 2018, 10:12 pm   #9
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Default Re: Basic stereo FM signal generator

Hmmm, a bit like this? (Pictures below). I made this myself back in 2010.

It started out as a device for the car. To be inserted into the cigar lighter socket (you can probably see this by the size/shape of the display diffuser) with a trailing lead terminated at a 3.5mm jack plug for your intended source.

Two problems with it:-

1. The classic car I tried this in, by todays standards, is enormous. With the radio antenna mounted far-out on the rear wing, & the power socket/cigar lighter socket right at the front of the car. The radio would keep flicking from stereo to mono ... hopeless ... distracted from the pleasure of the source.

2. The music I listen to has a good number of quiet passages, not a problem in the car as even at motorway speeds, she's whisper quiet, but this device would go into 'stand-by' even with my source volume ramped up ... hopeless ... the device couldn't 'hear' my music.

Sooooo, I ripped the device apart, fitted it inside a Maplin enclosure, made my own 'gubbins', fitted phono sockets, an I.E.C power socket, coaxial socket & a 47K potentiometer in series with a couple (Left - Right) 10uF 25 volt caps, oh and a little telescopic antenna plus some buttons to operate the device.

The stand-by feature was nothing more than a bipolar transistor being switched on to pull one of the uPC lines to ground. Simply lifting one end of its bias resistor, it never goes into stand-by

The device performs great now, extending the telescopic antenna, the device will transmit around my workshop, but not extend beyond that. I use my Marconi TF995 for alignment & the transmitter for spot checks on frequency vs dial reading. Also came in handy when a Leak 2000 arrived which had a stereo decoder issue ... and has proven very useful on many occasions when helping with building/testing external stereo decoders for other vintage HiFi tuners.

With the telescopic antenna retracted, & sitting the device next to my Ferranti U1003, she'll pick out the transmitter no problem, but across the other side of the room, we need to fully extend the devices antenna.

The devices original antenna was the lead which had the 3.5mm jack plug fitted, which was around 2 feet long, my telescopic is about a foot long.

For ages I thought my silly little homebrew projects were just that, silly, & who'd be interested, but the more time I spend within the forum & speaking with friends/colleagues, it appears I should devote some time to writing up my previous endeavours, even those dating back a moon or two ago ...

The buttons on the front of the device, above the display, from left to right, power on/off, open memory/store, frequency/preset down, frequency/preset up.

The LED's below the display, from left to right, on-air, on/off (bi-colour LED, red for stand-by, green for on), audio 'clip' & the last LED was linked to the original 'stand-by' line.

I trialled a modification to the input of the device by lowering resistor values, trouble was, anything above 400mV & all you had was distortion. I left it as nature intended, but installed a potentiometer on the back to trim down 2.2V P-P line levels & lifted the trannys bias resistor ... jobs a good'n ...

Mark
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Old 1st May 2018, 10:16 pm   #10
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Default Re: Basic stereo FM signal generator

I found a circuit many moons ago but never had time to build it, I even laid out a strip board for it though be warned I have not fully checked it out. Maybe one day I shall build it and see if it works though getting hold of the special 6m8 choke might be a small problem as also the 4.864MHz crystal
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Old 2nd May 2018, 10:04 am   #11
kellys_eye
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Default Re: Basic stereo FM signal generator

Quote:
Originally Posted by mark2collection View Post
Hmmm, a bit like this? (Pictures below). I made this myself back in 2010.
Veeerrrry nice indeed - idea 'stolen' for further use!
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Old 2nd May 2018, 11:57 am   #12
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Default Re: Basic stereo FM signal generator

John (stitch1) That device is similar in principle to the so-called 'pantry' transmitters commonly available today. I didn't know they were made back in the 80s. The range is intended to be short, not least because of legal restrictions on the use of transmitters - it should be capable of sending a signal from your phone to any FM radio within your property. Incidentally the other day I saw in my local ALDI, a similar device which can send transmit DAB, DAB+, and other signals to FM radios. Price is about 30, IIRC.
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Old 9th May 2018, 8:18 am   #13
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Default Re: Basic stereo FM signal generator

A search for tunecast on the forum brings up plenty of results. One of the benefits of this device is you can alter the modulation level and set it to mono. It shows up just how badly most FM stations are compressed.
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Old 9th May 2018, 9:53 am   #14
stitch1
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Default Re: Basic stereo FM signal generator

Quote:
Originally Posted by 'LIVEWIRE?' View Post
John (stitch1) That device is similar in principle to the so-called 'pantry' transmitters commonly available today. I didn't know they were made back in the 80s. The range is intended to be short, not least because of legal restrictions on the use of transmitters - it should be capable of sending a signal from your phone to any FM radio within your property. Incidentally the other day I saw in my local ALDI, a similar device which can send transmit DAB, DAB+, and other signals to FM radios. Price is about 30, IIRC.
Yes I use it a pantry transmitter, quite handy, the quality is good. The specifications don't list the power output. Still not sure about the date, I have the instructions and it mentions Walkman's and the option to use with a FR-119 FM stereo headphone receiver, there's a model number but no manufacturers name and I've not found anything on the web.

John
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Old 15th May 2018, 11:25 am   #15
Nymrod121
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Default Re: Basic stereo FM signal generator

Belkin Tunecast

I've previously modified a Belkin Tunecast so that its RF output is now presented via a short length of 50 ohm coaxial cable with a bulkhead 'N' socket on the end (the latter being a re-purposed adaptor lead from some long-range WiFi experiments).

Just for the heck of it, yesterday I checked the Tunecast's output on an analyser (no baseband audio applied):

-32 dBm @ 88.1MHz
-24 dBm @ 107.9MHz

(NB: I noticed significant - i.e. 2-4dB - amplitude 'steps' at certain spot frequencies when going up/down the band by holding down the relevant button - I didn't note the actual frequencies at which this happened but if anyone needs that info I can repeat the test and post later)

Harmonics/sprogs/other misc. digital 'shash' generally < -45 dB w.r.t. carrier
19 kHz pilot @ -14 dB w.r.t. carrier
38 kHz subcarrier @ -32 dB w.r.t. carrier.

( @John_BS, russell-wb, swb 18 et al ... nowhere near CD2/11 / TM4/6 standard!! )

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Last edited by Nymrod121; 15th May 2018 at 11:28 am. Reason: typo
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