Adding an FM radio to a valve/tube car radio

Seemed a neat idea but ... There is space inside this 1950s dashboard radio for one of those keyfob/ matchbox size FM radios, this one based on a TDA7088T. The original AM radio works fine, well minimal hash from the buzzer mechanism of the vibrator 12V to HV supply and the FM works fine with a proper linear mains derived HV unit and the 12V to 3V ps. But try the FM off the original HV supply and the hash from the buzzer mechanism makes it unlistenable to. Whatever I try in the way of screening/ wire routing makes little difference. Any ideas before rejecting the project?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N_Cook
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Most mp3 players derive their radio reception from the headphones. Isolate and bring out that wireto an external aerial and you will be good for the strong FM stations. Next, get some filtering caps and put them across the B+ and grounds of your player. Your audio output ground will need to be shielded properly. Can you make a Faraday box inside that radio?

Adam Kb2jpd

Reply to
Adam Kb2jpd

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I've replaced the switches with a 10 turn pot for dial tuning of the FM via the varicap. The 3V is derived from the 12V which is positive ground which I'm wondering if that may be part of the problem, grounding the + rail of the TDA. The audio out buffered through a transistor, I've fed in via a switch , at the volume control prior to the EL42.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

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YOu need to figure out if the Hash is getting conducted or radiated into the receiver. It should be posible to put the FM unit into an aluminum foil shielded compartment and then filter the heck out of the input voltage.

Bob Hofmann

Reply to
hrhofmann

YOu need to figure out if the Hash is getting conducted or radiated into the receiver. It should be posible to put the FM unit into an aluminum foil shielded compartment and then filter the heck out of the input voltage.

Bob Hofmann

Yeppers, that's exactly what I was going to suggest. I'll bet that shielding will significantly reduce the hash. Probably need to use a shielded wire for the antenna as well. The vibrator is a nasty source of hash noise, and it might take a lot of shielding to get it under control. I don't think the positive ground system is necessarily a show stopper. You'll just have to experiment to get the right configuration... what and how to shield and where to connect it.

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Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
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Reply to
DaveM

is

Or put in a modern little isolated switcher followed by a LM317 linear regulator. OTOH this sounds like a vintage car and one isn't supposed to have modern fluff such as FM, let alone integrated circuits in there ;-)

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Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

My gut feeling is the buzz is getting into the FM through the power supply

- your voltage-reduction circuit isn't filtering it out.

What kind of filtering is on the 12=>3v ps?

Reply to
Doug Smith W9WI

As many on this newsgroup know, I'm converting car radios to modern AM/FM stereo.

The Stereo PCB's are made by Antique Automobile Radio of Palm Harbor, Florida. They also make solid state vibrators, voltage boosters/inverters, and they used to make an "FM Module."

The FM Module was a neat little device, in my opinion. It was a very small PCB with an FM tuner, that used the AM oscillator to tune the FM. Connections were -- one wire to the LO coil, a wire to the antenna, a wire to the 12v supply (or a small voltage booster could be added if 6 volts and/or positive ground), and two wires to the volume control -- where you would tap in at the top of the control. This module worked very well; you could tune FM with the tuning knob and even set pushbuttons. You switched bands by turning the radio off and back on. Aside from the FM tuner, the radio worked normally.

These modules are no longer available, and it's a complicated story, but overall its popularity diminished when the stereo PCB's were perfected -- when they were discontinued I was one of only two dealers in the world who were still offering them.

Now -- as for vibrator hash, you are right -- vibrator power supplies are very noisy. For this reason, car radios always used shielding to keep the power supply isolated from the tuner. On some radios there is a metal partition between the vibrator power supply and the rest of the radio. On others, the power supply (and often the output tubes) were mounted on the back of the radio, outside the rest of the casing. On still other models the power supply and output was mounted on a second chassis beside the radio itself, sometimes with the speaker mounted to the power unit. But in ALL cases there was isolation to prevent interference from vibrator noise.

The FM modules seldom had problems with this noise, as they were mounted near the AM tuner which was already isolated from the power unit. However to prevent the AM from interfering with the FM, a 6.8uH choke was added to the AM antenna wire (if the radio didn't have one already). In extreme cases (rare) a relay was placed at the antenna socket, to disconnect the antenna from whichever band was not being used. On occasional instances where the vibrator noise would find its way into the FM, a hash choke in the DC line would take care of it.

Now -- one place where noise WAS a problem was with Stereo Conversions. Although the vibrator power supply gets removed entirely, on 6 volt or positive ground cars a voltage booster/inverter is added. Yes, with earlier versions THESE were noisy (new ones are better) and they would interfere with the AM reception. The fix: mount the voltage booster as far away from the PCB as possible, then install a 1000uF electrolytic at the voltage booster output, then a choke. Although not specified by AAR, I used a second electrolytic to make a pi filter -- why not? This eliminated the noise.

As for using a little FM tuner, does it make the noise when you're using the batteries? If not, probably a pi filter would do the trick. If this runs on a lower voltage than 12v, you might try a resistor and a zener, and then an electrolytic (probably doesn't have to be very big) and maybe even bypass this with something like a .001. Of course you'll want to mount the filtering near the power supply, and mount the FM tuner in a place that's partitioned off.

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Gary E. Tayman/Tayman Electrical
Collector Car Audio
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Reply to
Gary Tayman

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_________________ There used to be this FM tuner, back 30-40 years ago when most cars came only with AM radio standard; FM and/or cassette cost extra. You would mount this paperback-book sized unit below the dash, hook it into power and add an antenna to a fender to pick up signal. On the back was a switch that allowed you to select one of three AM frequencies to which this FM Tuner's AM transmitter would broadcast the audio to. You would then turn on the existing factory AM radio and tune it to the AM frequency that matches that selected on the FM tuner box. Voila - FM - mono! - but FM for sure! I don't know if this is still manufactured or if most of them are in wrecking yards or in peoples' garages gathering dust.

Basically it contained an AM transmitter similar to the FM transmitter you'd plug a MP3 player into, nowadays, to transmit the MP3's audio over a selected FM freq, IE 88.7 or 108.1 .

-CC

Reply to
ChrisCoaster

New AM/FM car radios are cheap.

Barry ===== Home page

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Reply to
Barry OGrady

This is true, but the OP, and many classic car buffs (most, in fact) do not want to have a modern radio in the dash. They want the original, or one that looks exactly like the original.

Reply to
Brenda Ann

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How about an MP3 player:
http://wps.com/projects/MP3-system/radio-player.html
Reply to
MikeS

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Upped the 3V rail cap from 100uF to 1000uF and upped the 50 ohm resistor (replacing the earpiece) to 330 ohm to give more signal and the FM was not too bad, about equal to the AM. That was as far as I was going. AM/FM switch and small 10 turn tuning pot knob tucked out of sight under the radio dashboard mounting frame next to the built in ashtray. At least with this arrangement, rather than down-converter, there is always a station on either side of the waveband switch without having to tune into a spot on the AM dial.

So it works in principle but a few more chokes,screening or something is required, also the bass is lacking on the FM. I'd replaced the 0.1uF AF out coupler with 2uF but no noticeable difference. No mention in specsheet about output audio other than the mW but maybe TDA7088 is designed only for earphones.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N_Cook

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