Radiomobile 50TM

[Homersimpson]

Hi,

I was hoping that someone may be able to help me with a problem I have with an old Radiomobile 50TM from circa 1950's/60's in a classic car.

Before I fitted it in 2018 it worked ok on the bench using its original speaker, as this was a bit battered I fitted two new ones in the car, however, ever since it hasn't worked and I haven't really bothered with it.

As we are all currently locked down it seems and idea opportunity to have a look and see if I can get it working.

The radio switches on and makes a hissing noise as you scan up and down the bands and at one point I could faintly hear something but it was clear at all.

The radio has two speakers connected to it in parallel, both are stamped 8 ohms which I understand from reading about this probably aren't right for the unit, i've tried running just one with the same result but also get a funny echoey noise as well.

The radio is positive earth and the car is still positive earth, the aerial seems ok and if I unplug it with it on you can hear the output change.

Sadly the original speaker is long gone so I can't test it with that but if I wanted to buy a new one what would I need to buy (assuming the 8 ohm units can't work with this radio), would these speakers give nothing at all not even on their own or is it just a case that the output should work but won't be optimum?

The output from the radio to the speakers also has three pins, i've used the top two but i'm unsure what the other one is for? I've tried swapping them around but just get the strange echoey sound.

Thanks for any help you can offer.

[Herald1360]

The original speaker is likely to have been 3 or 4 ohms so two 8 ohm ones connected in parallel should be fine.

You say it worked fine on the bench- was this with the same aerial and lead as in the car? First thing I would check is that the aerial trimmer is set correctly. This should be accessible through a small hole somewhere in the front panel- you may need to remove the knobs or even the trim panel to find it. When you do, it'll look like a small screw. Tune the radio near the high frequency (short wavelength) end of the medium wave band and tweak the trimmer for best signal (or noise!).

If this doesn't improve things it could get more "interesting". Come back for more help!

[G6ONEDave]

Hi and welcome,
The Radiomobile 50 series (similar to 40 series in the Newnes Radio and TV Servicing book 1959-1960) appears to be designed for a speaker load of 3.5ohms, so 2 x 8ohm speakers wired in parallel should be near enough not to cause any issue. However things to check are that both speakers are correctly phased, otherwise weird sound may result. Basically conect both '+' terminals together and both '-' terminals together. Also since the output transformer winding is connected to the chassis, make sure that you don't use the chassis as a speaker return, since any high resistance in the chassis return path may also cause sound issues. Simply put, just take a 2 core wire to each loudspeaker. Another thing to check is that the speakers don't have any internal connections to their frames.

The 3 pole connector is possibly wired for a 1.75ohm and 3.5ohm speaker loading using a common chassis ground. To check that you are on the correct 2 terminals, with the power off to the radio, using a test meter on the low ohms setting, measure between each terminal and vehicle chassis. the 0ohm reading will be the earthy common '-', the other 2 will have low ohm readings. Take your speaker '+' connection to the highest reading. You could also do this measuring by taking the reading between any 2 of the terminals and keep going until you find the highest reading, then connect your speakers to it. Hope this helps. Dave

[Homersimpson]

Hi,

Thanks for the response, as you may have gather I'm a complete novice when it comes to radios, I turn them on, twiddle the knobs and the music comes out. That's about the limit of my knowledge.

I've been out and had a look and by turning the aerial trimmer I have now got a few stations on one of the wavebands (not sure which one) but they still have a very strong background hissing noise.

The otherwave band just hisses (which changes as you move the dial) but I can't find any stations. I recall that this is what it did before I fitted it (only one waveband had anything on it) but I don't recall the loud background hiss.

The speakers are correctly phased and don't seem to have either of their terminals connected to the chassis of the speaker.

They seem to be connected to the right output terminals from the radio on the basis of the above testing description and to be fair the otherway around the sound is awful.

Any idea what I might be able to try?

Thanks for your help with this.

[Homersimpson]

I've been out today and had a bit more of a fiddle with the trimmer and the sound is better but still has a background hiss.

There are a couple of things i've noticed:

With the cover off (which I had to do as lost one of the speaker connection pins inside the unit) there is another screw below the trimmer screw, any idea what this would be for? It looks exactly the same.

When I screw the trimmer screw out to get the best signal the washer behind it is loose, is this meant to be like this? I am concerned that when in motion the vibration of the car would make it turn.

The radio is currently only earthed by the heater control cables as the dash panel is out of the dash so I can work on it, I added another earth temporarily but depending on what I connect it to the signal either stays the same or gets much worse, any idea what would cause this? One of the temporary earth points I tried is one of the das panel mounting bolts so it will be connected to this when its back in.

Thanks for any help you can offer.

[Herald1360]

If the trimmer is "loose" it's probably at minimum capacity. Is the screened cable from the car aerial extra long or perhaps just ordinary coax? This could make its capacitance higher than the set's trimmer is able to cope with.


When the set has a decent earth connected is the trimmer lack of range still a problem?

[Homersimpson]

I've managed to make it work much better by isolating the earth from the aerial at the plug, this seems very strange but without this corrected it works really well (until I start the engine and the interference kicks in but thats another problem once i've got it working correctly.

Any ideas what this might signify?

[Herald1360]

That sounds consistent with too much capacitance in the aerial coax lead. You need the right sort of low capacitance screened lead. Might be harder to find nowadays, there's not much call for it in a modern car radio setup with active anteenae and the like.

How long is the aerial lead? An old set like this would probably expect no more than about a metre- long enough to reach from the set to the nearest front wing. There used to be special "low loss" extension cables if you wanted a rear mounted aerial.

[Homersimpson]

Hi,

The aerial lead isn't much longer than 1m, will the way I have it configured at the moment (with the aerial screen isoalted from earth) cause any problems in use?

I've had a look on ebay and all the aerials of the correct type seem very similar to the one I have, do you have any idea where I could obtain the right sort of aerial from?

Is there a specification for the cable that I should look for?

Also is there any reason why only one of the wavebands work? Is there anything simple I can check?

Thanks for your help with this.

[Herald1360]

Hmmm.... only one waveband working could point to a problem in the aerial tuning circuit or switching which might also explain the aerial trimming problem rather than it being an aerial lead problem.

Trying to use the set "for real" with no earth to the aerial cable screen isn't really a runner- the ignition interference problems would be dire.

Other than a close look for any connection problems in the aerial tuning and switching circuits and a dose of switch cleaner and some exercising for the switch contacts I don't have any more simple suggestions to try.

Classic cars and vintage wireless both float my boat and you're not that far away either. It's another casualty of the present situation that a hands on look at the set isn't possible just now.

Any of the ex-professional ICE members got any ideas?

[Chris55000]

Hi!

It really needs the circuit diagram of the set plus knowledge and experience to be able to fault–find car radios, as these are usually permeability tuned (the tuning control moves the cores in and out of the aerial and oscillator tuned coils) and the circuits look unconventional to engineers used to capacity tuned portables, and the mixer–oscillator is sometimes common–base configuration!

Transistors of the "AF" type can fail partially between screen and collector to cause noisy or crackly reception before the short becomes more permanent and silences the set!

Accidentally connecting the set reverse polarity due to uncertainty about the polarity selector switch or changeover links, etc., or the earth polarity of a classic car can also partially damage transistors that cause very weak or noisy reception as well!

Car Radios MUST be securely and efficiently earthed to the body and battery earth terminal – heater Bowden Cables are not sufficiently low enough resistance for this purpose and the added impedance in the aerial tuned circuit will upset it!

Leaky or o/c decoupling electrolytics in the a.g.c. line can also cause weak or distorted reproduction – there's usually only one in most sets so it shouldn't be too difficult to locate it!

Chris Williams

[Herald1360]

A little bit of circuit help- this set is a 12V HT valve transistor hybrid. It uses ECH83 "H" as RF amp, another ECH83 as mixer/oscillator, EBF89 as IF/detector, ECH83 "C" as audio preamp, EF98 as audio driver and 2N178 as audio output. It's supposed to be very similar to the 40T which is listed "up top".


Should be a good performer when it's sorted.

[G6ONEDave]

Given that the TR suffix was a specific design for Rover cars, I wonder if the op's TM was designed specifically for a Morris car. It is possible that the metal case of the radio is not actually battery positive earth but is actually connected to the positive earth via an internal capacitor. This would most likely be done so that variants of the radio can be used in negative earth vehicles, assuming that there is a polarity change plug / socket. The radio positive earth ( possibly a wire coming out of the radio casing) needs to to go the the vehicle metal bodywork or chassis. There should be a hole in the vehicles metalwork somewhere under the dashboard. The radio earth wire should have a ring terminal fitted, which should go to a set screw, which in turn needs to go through the hole in the vehicle metalwork. There should also be a star washer between the ring terminal and vehicle metalwork, of course making sure that the vehicle metalwork contact area is clean before tightening up the set screw and fixing nut. The aerial socket may or may not be directly connected to the radio metal casing, but the coax does need to be earthed at both ends. Once the supply is connected correctly, see if the engine interference still exists.
If the engine interference is still there you need to figure out if it is coming from the ignition or the voltage regulator. Traditional methods used for the former were to either use resistive HT leads or resistive spark plug caps. The regulator interference would have been filtered with an inline choke / capacitance in the negative (in this case) supply wire.
Many of these radios were an after market accessory and came with a plastic mounting pod, so were not actually radio metal case to vehicle metalwork directly connected. Not having any photos of the particular installation or a Radiomobile manual make this a bit harder to advise.

[sparkymike]

I have the original Smiths Radiomobile service manual for the 50 T/51T/52T and 52TC. Its around 30 pages. I could possibly scan it and convert to a pdf if you need it. PM me if you do.
Speaker can be 3.5 or 1.75 ohms, confirming above post.
50T was positive earth only. 51T and 52T were reversible polarity.
The aerial trimmer is located on the right side of the receiver.
If this is same set as above, then it is valve with power transistor output.
Mike.

[Chris55000]

Hi!

The Manual offered by Mike is most definitely recommended as in my experience, car radios of this type are tricky to work on because of the densely clustered components needed to fit the relatively limited space in a car radio!

Remember that "low voltage valves" of the '83 series are necessarily much less efficient than their 250V mains–operated counterparts, so decoupling, screening and earthing of individual circuits must be beyond reproach if the gain and sensitivity isn't going to suffer – there must, of necessity, be no voltage dropping or decoupling resistors in a 12V hybrid valve circuit!

Chris Williams

[sparkymike]

Circuit of the 50T.
I might be able to take a better photo with my other camera. Will see.
Mike.

[sparkymike]

Some better photos and picture of the radio to identify if that is the same set as original poster has.

[radiorescue]

The 50TM is the last version of the 50T. Made in 1961. The main difference is that the last stage driver was changed from an EF98 to a transistor driver, OC28 or something like that. The B7G valve base for the EF98 is a real trouble spot on this radio and the only permanent cure is to replace it.

There is another version, the 50TC, which has a tone control. This had to be specially ordered.

I have never seen a circuit for the 50TM. I have one in for repair at the moment. I may sketch out the output stage if I have time.

Eric