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MG MGB Technical - British Leyland (Bendix)radio wiring?

I recently acquired a refurbished BL Radio made by Bendix Automotive Electronics Division. Model number is 2B FM BL, if that happens to mean anything to anyone. This is an AM/FM MONO unit. Four wires come out of the back. The wires were marked with masking tape with the following:

TAN wire - speaker ground
WHITE wire - Speaker
ORANGE Wire - Speaker
BLACK wire - Power

I am not sure how all this comes together for an old time mono unit going into a 73 MGB. I wired the two negative wires from the two speakers to each other and then to the TAN wire from the radio (should these also be grounded to the chassis???). The WHITE and ORANGE wires were spliced individually into the positive wire from each speaker. The BLACK went to the Green/White wire from the ignition switch. The BLACK wire is inline fused with a 2-amp fuse. The only other connection I made is to ground the radio chassis to the body of the car. The antenna is, of course, connected.

The radio works meaning I can receive stations but the quality and reception quality are poor. Sound comes from only one of the two speakers. Seems to me that I wired this unit incorrectly. Can anyone put me straight on this?
BOB in Atlanta
RES Schultz

Hi Bob,

I don't know the model, but from first principals, to me, are that the radio originally was for a positive earth vehicle, from just the wire colours and descriptions, which has been converted to negative earth. How I would interpret the info is that, as its a mono radio, you can only have one speaker, all wired thus, BLACK - power (+12V), BROWN - chassis (earth), WHITE & ORANGE to the speaker. Make sure that the BLACK wire is NOT connected to the case of the radio, by measuring with an ohmmeter, or seeing if you can get a light to light up through the BLACK wire and case.

Herb
Herb Adler

Thank you for your comment. Don't believe British Leyland ever produced positive earth MG's. In any case the radio comes on it's just that the speakers do not work properly. If the black wire coming out of the radio was grounded to the case then nothing would work.
RegardsBOB
RES Schultz

It's pretty rare for any radio equipment to earth via the case, & in this instance I think Herb is right. As it's a mono radio it was only ever meant to drive one speaker via the white and orange. I would guess that you are driving the amplifier into very low impedance, therefore reducing it's efficiency. try the unit with just one of the speakers via white & orange. To prove the speakers you can briefly connect a 1.5v battery across the terminals which should produce a rustling sound from them. Whether you can drive both speakers from the one output is dependant on their type and impedance. You need to match the output from the unit to the speakers, and use some fancy ohm's law stuff to match them up.

regards

Pat
P M Gregory

These radios usually had an aerial trimmer capacitor which makes a huge difference to reception. With the output power of a radio like that speaker matching isn't going to be a big issue, most had a range of impedances they would drive.
Paul Hunt 2010

Hi,

Bob
MGs were positive earth, until alternators were fitted instead of dynamos, around '68, '69. Check the BBS for threads on converting. The reason I suspect that the radio is a positive earth version is that the TAN wire is marked "SPEAKER EARTH", and reading between the lines suggests that this is really power earth, ie neg 12V.
TAN wires are the non earth power feed, whether +ve or -ve earth. The BLACK wire being marked as POWER would be correct colouring for the earth, but BLACK is also battery common, and for a positive earth car this would be chassis, or earth.
If the radio were being tested on the floor, then there's a good chance that the case wouldn't touch the chassis and all would work, until properly installed. I have just tested a car radio I have lying around and the BLACK wire is definitely connected to case. I know that about the time of the Bs there were a mix of positive and negative earth cars around (not just MGs) and radios had a way of setting the earth polarity.

Pat
What I reckon is happening is that the speakers are each being driven by half the cycle in alternate phase, as the speaker output doesn't seem to be referred to earth, by the fact that there are two wires marked SPEAKER. This would definitely give distorted sound. Also because only one speaker seemed to be working, I would test both speaker wires to make sure that one of them is not connected to earth.

Paul
Just connecting up speakers willy nilly could wind up cooking the output of the radio. Amplifiers were designed to work into specific loads and increasing the load, by wiring up two speakers in parallel would operate them outside their design parameters. I would carefully check if there is any indication of the speaker impedance on the radio itself. If not then I would use one 8 ohm speaker, or two 16 ohm speakers in parallel. Many automotive speakers today have an impedance of 2 or 4 ohms, which would be too great a load on 60s or 70s radios.

Herb
Herb Adler

"radios had a way of setting the earth polarity"

Not automatically, as I'm sure you are aware, but (typically) by a 4-pin plug the user had to orientate correctly.

I stand by my comment that on a radio of this age and typical output power, and the fact that typically they quoted a range for speaker impedance, the chances of damaging it other than at full blast (four watts of pure distortion) are minimal.
Paul Hunt 2010

This is a 1973 negative earth MGB. To the best of my knowledge and research all British Leyland radios were negative earth units. The black wire is not connected to the case. The radio comes on and generates music but only to one speaker. This proves beyond a doubt that the black wire is hot. My issue is simply how to wire the speakers so both of them work. The wires are marked with masking tape by the guy who rebuilt the unit. Not sure what he meant by "speaker ground". When I rewire with the orange and white going to one speaker I get nothing but the radio light. Grounding the tan "speaker wire" to the body does nothing. I'm stumped. Even tried other working car speakers but get the same result. Funny, when I was a kid I knew this stuff. The older I get the dumber I get (just like my kids said).

RegardsBOB
RES Schultz

It's illogical that a radio marked 'MONO' would drive two separate speakers, unless it was rebuilt with modern stereo electronics, I know there are people doing this. If a single speaker doesn't work at all between the orange and the white, but does between one of them (you don't say which) and the brown 'speaker ground' wire then maybe the other wire is for something else (night time illumination?). But if you know (or know of) the bloke who rebuilt it then ask him. Have you found and tried adjusting the aerial trimmer yet? Maybe it's just knackered.
Paul Hunt 2010

Speaker ground usually refers to the phasing of stereo speakers. You have a positive and negative wire running to each speaker. When connected correctly, this allows the speakers to move in and out together. If the negative and positive wires are reversed on one speaker, in a stereo application, one will be moving forward while the other will be moving rearward. This reduces the amount of the sound level being produced as one speaker is cancelling out the output of the other. RAY
rjm RAY

Hi,

Further to previous posts, the following link gives the standard colour scheme used on MGs and other British cars.

http://www.mgexperience.net/article/lucas-colours.html

The main points for this discussion are that BROWN is battery HOT and BLACK is earth, whether positive or negative earth car.

Attached are photos of a Bendix radio offered on ebay, at

http://cgi.ebay.com/ebaymotors/British-Leyland-AM-Pushbutton-Radio-Triumph-MG-B-Healey-/200482102959

showing the wires coming out of the back. What I can determine is that the wire out of the left corner, of the first photo, is BROWN, the figure 8 wire is GREEN and GREEN / BLACK.
What all this suggests is that the BROWN is battery, the figure 8 wires are the speaker wires, note the two opposite sex spade connectors. This leaves the case as the connection to earth.

Bob's radio does not conform to this, so might have anything inside.
Would it be possible for Bob to post some photos from inside the case, both from the top and underside, and if possible close ups, with adjacent area, of where the wires connect to the circuit board.

Herb

Herb Adler

The Lucas colours are for Lucas *harnesses*, you can't take those to be the same for a Bendix or any other manufacturer of radio even though it may have BL on the front. That single wire looks black to me, not brown. I'd have said it was more likely to be mono than stereo, so it doesn't matter which way round the speaker is connected. In any case if Bob has the radio working, albeit poorly, then he must have the polarity correct regardless of whether the radio is in the car or on the bench. As his his mono there will only be one way of connecting a speaker, so two speakers would have to be connected in series or parallel (depending on what gives the best match/results) to have sound coming out of both of them, and phased correctly.
Paul Hunt 2010

This thread was discussed between 13/06/2010 and 17/06/2010

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