AM car radio

[BeerBelly]

I've been thinking about restoring some vintage car radios. I have some of the old Sams books and the test equipment needed but I don't see much information about sensitivity or selectivity.
How many microvolts should I inject and what should the channel width be adjusted to.
Can anyone steer me in the right direction?

 

[MikeMl]

Sensitivity should be something better than 1uV (-107dbm). Typical bandwidth for an AM car radio is like 15 to 20Khz.
Use a test signal with 1000Hz 90% AM modulation for testing and alignment. Most car radios do not have a 50Ω input; it is a very high input impedance to interface with a very short whip...

 

[BeerBelly]

Thanks for the reference Mike. I haven't been seeing this kind of sensitivity yet out of these early 60's AM car radios.
There is typically
1 RF stage
1 Converter
1 IF stage
3 transistor Audio
All Germanium

Thanks again Mike.

 

[WTP Pepper]

Here in the UK, AM was used for local radio stations where quality was not an issue. Speech/telephone quality pleased most. I'd be looking at -80dBm sensitivity with about 5KHz bandwidth.
A radio from the 1960's maybe as simple as a TRF (Tuned Radio Frequency) which is nothing more than a tuned parallel LC circuit, amplifier and a diode detector and a further audio amp.
Modern car radios that support AM tend to be basic quadrature detectors. AM is dying out here.

 

[Nigel Goodwin]

A radio from the 1960's maybe as simple as a TRF (Tuned Radio Frequency) which is nothing more than a tuned parallel LC circuit, amplifier and a diode detector and a further audio amp.

I've NEVER seen, or heard of, a car radio that was only TRF - all have been superhets (and I've repaired hundreds over the decades, including ones back to the 50's and earlier - valve ones as well).

For repairing radios you don't need much in the way of test equipment, full alignment (as you're suggesting) is only required during initial manufacture.

However, I will point out - car radio's are an absolute pain the bum to work on (which is why we don't repair them any more).

 

[WTP Pepper]

My Father's old Ford had one circa 1975. It was rubbish.

 

[Nigel Goodwin]

My Father's old Ford had one circa 1975. It was rubbish.

How do you know?, I'm pretty sure it wasn't, and I've repaired numerous Ford car radio's in the past.

 

[BeerBelly]

car radio's are an absolute pain the bum to work on (which is why we don't repair them any more).

There are car collectors that need these original radios repaired.

 

[WTP Pepper]

I know because I'm an electronics engineer. I am sure you think it wasn't as I am sure you think it was. Let's just leave it at that.

 

[Nigel Goodwin]

I know because I'm an electronics engineer. I am sure you think it wasn't as I am sure you think it was. Let's just leave it at that.

OK - but that's not an at all useful 'explanation' (it says nothing).

Have you seen the circuit for that radio?, or seen inside one? - while I've not seen every Ford model from that period, I have seen a great many of them - and there are essentially only three makes (as I remember), Blaupunkt, Philips, and an unknown Brazilian manufacturer with a Ford badge.

As an 'electronics engineer' you will obviously be well aware of the failings, limitations and design problems with TRF radios, which is why they weren't ever used.

I get the impression that your only reason for believing it was TRF was because it was crap - a far more likely reason was that the aerial trimmer needed adjusting correctly (which makes a HUGE difference on an AM car radio), or it could have been faulty?.

Incidentally, I've recently been involved in repairing an old Radio mobile car radio (for a classic Jag) - which I did have a 'similar' circuit for. Two faults on it, first one of the AF117 transistors was S/C to the screen (a VERY, VERY common fault on that series of transistors), and secondly the ON/OFF switch was O/C - which unfortunately is a special multi-gang control (I shorted it out to make the radio work).

 

[audioguru]

In the late 60's I worked in a Philips factory fixing then designing car radios. Most transistors were silicon except the output complementary pair were germanium. They produced FM very well but their AM sounded as bad as other AM radios.

My first car radio was a Blaupunkt for my German car and I was surprised that its service manual had the Chinese (!) manufacturer's name crossed out.
I souped up its cassette tape recorder/player so it became truly hi-fi and sounded great.

 

[WTP Pepper]

OK - but that's not an at all useful 'explanation' (it says nothing).

It says alot. A dual gang cap with a ferrite rod across it with an aerial feed, an simple amp, a diode and an audio amp. Not an IC in sight. It was a crystal set with an amp.

 

[Nigel Goodwin]

It says alot. A dual gang cap with a ferrite rod across it with an aerial feed, an simple amp, a diode and an audio amp. Not an IC in sight. It was a crystal set with an amp.

Sorry, but I can only presume you're just making this rubbish up? - have you EVER seen inside a car radio?.

 

[WTP Pepper]

It does seem on this site if you disagree with a "super moderator" you are wrong and it's not open to any kind of flexibility that may fall outside their "expertise". Can't you just accept that it doesn't make you a "super engineer"?

Much the same on another thread I have been posting to on this site.

Let's just agree to disagree and move on. Same as the 24VAC detection thread.

 

[Nigel Goodwin]

It does seem on this site if you disagree with a "super moderator" you are wrong and it's not open to any kind of flexibility that may fall outside their "expertise". Can't you just accept that it doesn't make you a "super engineer"?

You can't just leave such incorrect statements on a forum unchallenged as they will be perceived to be 'true' by readers in the future, a forum is an historical record.

To make my point clearer, and why I suggested that you have never even seen inside a car radio - car radios don't have tuning capacitors (they are permeability tuned - variable inductors), they don't have ferrite rods (it would be useless inside the Faraday cage of a car, and the permeability tuners are the tuning inductors anyway). They are all also superhet designs, a TRF set would be unusable in a car.

As for 'super engineer', while I wouldn't suggest any such thing - I have won three national servicing awards, Grundig Satellite Engineer of the Year once, and Sharp TV/VCR/Audio Engineer of the Year twice.

If you truly want to start repairing old car radios, I may be able to help with service information for some of them?, particularly as we're in the process of clearing out most of the old paper manuals.

 

[WTP Pepper]

You can't just leave such incorrect statements on a forum unchallenged as they will be perceived to be 'true' by readers in the future, a forum is an historical record.

To make my point clearer, and why I suggested that you have never even seen inside a car radio - car radios don't have tuning capacitors (they are permeability tuned - variable inductors), they don't have ferrite rods (it would be useless inside the Faraday cage of a car, and the permeability tuners are the tuning inductors anyway). They are all also superhet designs, a TRF set would be unusable in a car.

As for 'super engineer', while I wouldn't suggest any such thing - I have won three national servicing awards, Grundig Satellite Engineer of the Year once, and Sharp TV/VCR/Audio Engineer of the Year twice.

If you truly want to start repairing old car radios, I may be able to help with service information for some of them?, particularly as we're in the process of clearing out most of the old paper manuals.

OK point made. I have no time to fix car radios but thanks for the offer. I didn't make it up. I have better things to do than troll forums. I am just saying what the old car had in it. I was too young at the time to see if it even worked, it just ended up on my bench years later before being binned.

 

[Nigel Goodwin]

I am just saying what the old car had in it. I was too young at the time to see if it even worked, it just ended up on my bench years later before being binned.

If so it wasn't a car radio - it 'may' have been a portable radio?, some of those years ago had external aerial sockets that accepted car aerial plugs. However, the radio wouldn't have fitted in the dash, it would have to have sat on one of the seats or somewhere (it would also have performed horribly - the internal ferrite rod made them useless in a car). But in any case, it still wouldn't have been a TRF, the dual gang capacitor is because it's a superhet, one half for the front end, the other for local oscillator.

 

[KeepItSimpleStupid]

I remember having one of these https://www.radiomuseum.org/r/zenith_650hd_hudson_terraplane_6.html to play with as a kid. It did get thrown out. Not sure if I wanted to throw it out or not. It looks like it was capacitor tuned, but back then the floor boards were wood. Not sure if I took down the antenna from the cellar. It was a long piece of wire under the floor.

There are still some old car radios in the house and most were easy to fix without major test equipment. Most were, indeed, variable permeability tuned.

Most of the car radios I remember repairing in my teens were Delco.

 

[Nigel Goodwin]

Jesus - now THAT'S an old car radio

Pre-dates my car radio experience, which goes back to valve versions and vibrators. Mind you, I notice even that antique is still superhet.

 

[Nigel Goodwin]

Not quite as old as the museum example above, but here's a scan of the circuit from the service manual I used a few weeks back, it's dated December 1964.

The first PDF is the two different RF stages used in different models, and the second the two different audio circuits used (single or double ended).

Pretty crude and simple really