Car radio aerial issue

[ITimmins]

Firstly I must apologise as I only have a basic knowledge of electronics so here goes;
I have acquired a Radiomobile 900T transistor car radio from the mid 60s. It is set up for negative earth (correctly per wiring diagram on inside of cover lid). It powers up fine but the weird thing is the reception strength varies from almost zero with casing fully assembled to ‘reasonable’ when the case lid is removed!

I thought the aerial flying lead was either broken or not earthed correctly to the case but it tests fine for continuity. It’s as though the aerial lead is doing nothing as when I plug an aerial into the socket the reception is unchanged. Also, why would removing the cover help? I assume it must be creating a shield of some sort but why??

Does anyone have any ideas or further tests I can do with a basic multimeter?

Thank you,
Ian

[Herald1360]

What age car are you trying to use it in? If it's contemporary with the radio have you tried tweaking the aerial trimmer on the set? This is usually done with a small screwdriver inserted in a 3mm ish hole in the front panel somewhere- possibly behind one of the knob skirts. If it's modern, it may have an active aerial which won't work with this set without a power feed adaptor in the aerial line.

Beware that this set uses the (in)famous AF11x transistors which go short eventually from tin whiskers growing inside the can.

Improved reception with the case open is caused by signals getting directly into the circuits rather than through the aerial connector. In a car there's a lot of local interference from the ignition and other electrical equipment which would also get in if used like that. With the case on, only signals from the aerial ( mounted outside the car body which helps to screen the noise) get in.

[ITimmins]

Thanks Chris.

It’s going in a 1960 Rover so should be compatible. So far my tests have been done on the bench and not yet in the car until I’m happy it’s working correctly. In the past this has worked with other car radios of a similar age so I assumed it wouldn’t make a difference with this one.

I have tried the trim screw but no appreciable change.

The radio has a short flying lead with its core wire soldered to a pin inside the radio and the screen soldered to two brass clamps which are screwed to the case (chassis) in two different locations. My assumption is that this lead is somehow dead and the poor reception is from the internal components?

Can this be the case? Would the AF11X transistor failure cause this mode of failure?

Many thanks, Ian.

[julie_m]

It almost sounds as though the socket is not connected to anything, and the set is picking everything up on the short internal connection from the socket to the frequency changer. This of course is shielded by the metal case on the set is properly assembled. If this is indeed the problem, then you won't see continuity from the tip of the aerial to wherever on the printed circuit board the connection from the socket goes to.

Another, slightly less improbable cause of the same symptoms would be the aerial trimmer just being really, terribly mis-set enough for the aerial to make the better part of no difference. That's an easy enough fix; there should be a diagram showing where to find it and how to adjust it, somewhere on the outside of the set. It's best to use a non-metallic tool for this, because any metal object coming near the sensitive RF circuits can disturb them -- the practical upshot being that removing the screwdriver used for adjusting the trimmer afterwards will throw the adjustment off again. which makes it much harder. A suitable tool can be fashioned easily enough from a plastic chopstick (mount it in the chuck of a cordless drill and use like an improvised lathe), obtained from your nearest Chinese take-away (apologies if I've just made you hungry .....)

['LIVEWIRE?']

The aerial fly-lead may be faulty. They sometimes develop short- circuits between the inner core and screen, or occasionally the connection between the inner core and either the socket end or 'radio' end breaks, as Julie suggests. There should also be a small R.F. choke connected from the pin inside the set to the input tuned circuit. If this, for some reason is missing, or is open circuit this would also give rise to the symptoms you mention, Ian. Incidentally, the 900T circuit diagram/service notes are available for £1.99 from the V.R.S.D. link at the top RH corner of this section. This should help you locate the cause of the problem.

[ITimmins]

Thank you; a good point on the trim as I did try it with a metal screwdriver.

It does however seem that the aerial lead connection inside the radio may be broken down somewhere. If the trim doesn’t fix it I’ll have to open it up again to see if there’s an obvious break or component failure. Hopefully there’s something obvious to see/test.
Oh well, I’m learning all the time!

[Herald1360]

Sounds like a good use for that basic multimeter. Check for end to end continuity between the inner socket of the aerial connector and the place that the inner conductor from it goes to.

[tigger449]

When did transistor car radios first appear? I had a 1957 MG and the radio was a valve set with most of the works in a big case under the passenger seat.

[Panrock]

I too have a 1960 Rover. It has the original radio and this uses valves (with 12v HT) for the RF and IF stages and a single transistor for the output.

Steve

[ITimmins]

Latest update:
1. Trim checked again with sophisticated insulated tool (pen top) and makes no difference to reception or background noise. Can I measure the trim function some other way eg resistance??
2. Checked aerial socket continuity of inner wire and outer screen, also for any short bewtween the two, and all seems perfect.
3. Checked resistance of the in-line choke (which is fitted between the pin which inner wire is soldered to and trim unit) and measures @1 Ohm (about right from what I can tell).
4. Checked between wire from trim to circuit board and continuity is perfect.
5. Followed track on circuit board to first post of the first tuning barrel (not sure what it is but itÂ’s like a piston which moves in and out with tuning knob movement) and continuity is good.
6. With the boards removed from the casing the reception was loud and clear and again, as I fitted the top and bottom case it becomes quieter as I tighten each fixing screw.

Bottom line, from all I can tell thereÂ’s nothing obviously wrong unless itÂ’s internal in a component.
Can a tin whisker transistor issue create this problem IÂ’m seeing or is there some other potential cause?

Any ideas are appreciated!

IÂ’m still bewildered as to the issu

[ITimmins]

For those interested, the Rover has been converted to negative earth at some point and the original radio is unable to be converted unless I get an expensive conversion.
The radio I have acquired has the correct period appearance and dual polarity so should be very nice if it will work correctly. I know it’s only MW and LW but while there are a few stations available it will be good enough.

['LIVEWIRE?']

The readings you get in the aerial circuit show that the aerial fly lead, choke and connecting leads are OK. What you describe as the first 'tuning barrel' is part of the permeability tuner. These were used in most car radios for many years, the frequency in use being determined by the position of the rod which moves in and out of the coil as the tuning knob is rotated. From memory the coil in that first 'barrel' has 4 terminals, and a check with a meter set to a low ohms range should show that there are two windings, each of which should be low resistance, that connected to the aerial being lower resistance than the other. A faulty AF117 could cause the symptoms you describe, though I doubt if you'd notice much difference in sensitivity with the cover on or off the radio. By the way, if both covers are removed, a jumper lead should be used to link the chassis of the main unit to that with the audio amplifier, otherwise the earthy sides of the two circuit boards will not be common.

[ITimmins]

Thank you for the further explanation. When I removed the two units from the cover the reception and volume were as I would expect were it functioning normally, so that would rule out a transistor problem I assume? I did have both halves earthed with a common wire so all set up as you suggested.

The one interesting point is that the cover screw nearest the aerial lead exit point seems to have most effect on reducing volume.
Could there be a related fault which is affecting the volume as opposed to hindering reception??

If there are no other obvious causes the only thing I can think of is to somehow insulate the covers so as to avoid a continuous ‘cage’ around the internals to reduce the apparent blocking of the signal. That goes against any good practice for reliable earthing or any static etc but it could be another useful test in the experiment.

Thoughts and suggestions welcome!

[John10b]

Did you say you have checked the aerial and lead? I have found many car aerials and leads wet and corroded. Get a couple of feet of wire and and stick one end into the aerial socket and let the other end of wire dangle out of window and note what happens.
Cheers
John

[crackle]

Can I just clarify something. You do have a length of wire fitted into the inner conductor on the aerial socket?
Can you also confirm that when the set is working with the cover off you can tune the radio fully up and down the dial and receive all the stations.
When you put the cover back on, have you tried retuning the radio to see if the problem is simply that the cover effects the tuning.

Mike

[ITimmins]

Thanks for all responses; to cover the points mentioned I can confirm that I am currently testing the set indoors, using a 12v car battery. I have tried with a length of wire connected to the inner wire socket of the aerial flying lead (where you would normally connect the car aerial) but it makes no difference to reception. I haven’t dangled it out of the window due to current weather but I’ve had it next to it indoors.

My first assumption was that the aerial lead was either shorting or broken but it’s all tested continuous.

I can also confirm that it tunes nicely to the locally available stations with the cover removed but with cover back on it doesn’t improve by re-tuning. The stations are in the same positions as with cover off but just very, very quiet.

[emeritus]

re #8, possibly 1957 for the UK, although early models were only partly transistorised: see attached Pye car radio leaflet.

[crackle]

Have you got the service information, it is available from Paul's website, see link at top of this page.
In the R&TVS service information are details of the resistances of the coils in the RF part of the receiver. Can I suggest you follow the schematic diagram for the RF front end and follow the path for the aerial signal as far as the first transistor and check the resistance of all the coils and the operation of the wave change switches..
Then report back anything odd that you find.

Mike

[ITimmins]

Thanks for the suggestion Mike; as yet I haven’t got a circuit diagram and I’m not too sure of my ability to follow it all and identify the correct components in the radio. I’m fine with basics but I’ve seen some components which are unfamiliar.
I was rather naively thinking it would be one of the text book failures that can create the fault I have but so far it does seem to check out ok up to the point of connection to the AE circuit on the board. Oh well, feint heart and all that....

[crackle]

My next suggestion would be to post an image of the circuit on page 4 showing the front end of the radio from the aerial to, and including, at least the first transistor. (It is not allowed to post the whole circuit).
Then you can discuss individual components with members on here, and everyone knows what you are talking about.
Mike