View Single Post
Old 1st May 2018, 10:12 pm   #9
mark2collection
Hexode
 
mark2collection's Avatar
 
Join Date: Jul 2010
Location: Royal Berkshire, UK.
Posts: 470
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
Attached Thumbnails
Click image for larger version

Name:	Front.jpg
Views:	262
Size:	48.6 KB
ID:	162120   Click image for larger version

Name:	Rear.jpg
Views:	261
Size:	59.9 KB
ID:	162121  
mark2collection is offline