Old Radio Help

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In the past week, I've managed to fit a Bendix-branded AM-FM radio to my MG. This is an early 70s radio that's branded "British Leyland" and is the sort that would have been fitted to the car when new(none shipped with a radio, but this is one of the sort a dealer would have installed).

My "original" antenna(again, really dealer fitted) on the front passenger wing had its wire chopped off, so I swung by Autozone and picked up a 31" telescopic whip antenna and mounted it to the original mounting. As I understand it, 31" is a good compromise length for most of the FM band. I hooked all of this up, and it works perfectly. I can get all the local FM stations and plenty of more distant ones. In the evening, I can reliably pick up 840AM and 650AM, two clear channel stations of interest to me. The former is a ~200 mile line of sight, and the second probably more like 250 miles.

That's all good and well, but that's in the garage without the engine running. It all goes to heck when I start the engine. With the engine running, at best I can pick up strong local stations through a lot of static and never really very clear.

I know that, first of all, the ignition can wreak havoc on the radio reception. My antenna is a few inches above the ignition coil, and not too far from the distributor. I don't have resistor plugs fitted(I used the now-discontinued NGK BP6ES, which I fortunately have a good stock of) but do have NGK carbon core wires which are supposed to be suppressed. The rule of thumb I've always heard is that either resistor plugs or resistor wires are fine, but that using both can affect ignition energy too much. I can fit NGK BPR6-ES plugs if needed, but I'd prefer to not.

The radio did come to me with a big inductor looking thing that was tied into the radio +12V at one end with a big spade lug at the other end that was under the grounding post on rear of the radio. I didn't wire this in(although it's still there) as the hot side insulation is dry-rotted and flaking. I'm wondering if this is the issue, and if so if replacements or a modern equivalent are available(I didn't see an obvious way to just replace the wire).

Otherwise, I'm GUESSING maybe the issue is in something "dirty" in the car power supply. It's on what should be a "clean" circuit that has the headlights and brake lights. The alternator is new(~18 months old) and hasn't given me any reason to think the rectifier is bad.

Is there anything else I should be checking, or if not I'm wondering if there is a filter I should be using somewhere on this to "clean up" the power supply. Can anyone offer any advice?
 
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Three things I can think of right off. One, Bad capacitors in the radio. That old, chances are they are dried up and leaky. this alone will cause great deal of funky things. Two, bad ground. three, needs a filter into the radio power supply. Keep also in mind if it is an older point-driven ignition, resistor plugs are a must.
 
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MG was positive grounded electrics thru '67, then negative grounded chassis afterwards.
Are you positive the radio is also a negative ground unit proper to a 1970 MGB?
 

bunnspecial

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MG was positive grounded electrics thru '67, then negative grounded chassis afterwards.
Are you positive the radio is also a negative ground unit proper to a 1970 MGB?

It's a pull from a '71, I think. If it were a positive ground radio, I don't think it would have worked at all(or I would have killed it already). Not sure if I've ever even seen a positive ground AM/FM.
 
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You didn't mention if you have noise in the radio from the ignition system (RFI). You could have interference from the alternator (generator) but I suspect your problem is with the spark plugs. You need resistor spark plus AND wires. Not one or the other. Also, make sure your new antenna has a good electrical connection to the body where it is mounted. If the antenna mount is not well grounded, the outside shield of the coax going to the radio is acting as an antenna picking up noise (common mode noise).
 
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It's a pull from a '71, I think. If it were a positive ground radio, I don't think it would have worked at all(or I would have killed it already). Not sure if I've ever even seen a positive ground AM/FM.
I don't think your MG is positive ground. My '62 sprite was positive ground and when I tried hooking up a radio I knew right away. Let the sparks fly!
 

bunnspecial

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Three things I can think of right off. One, Bad capacitors in the radio. That old, chances are they are dried up and leaky. this alone will cause great deal of funky things. Two, bad ground. three, needs a filter into the radio power supply. Keep also in mind if it is an older point-driven ignition, resistor plugs are a must.

I know I should do caps, but passed on it since this bench tested out just fine(and works fine there again with the ignition off.)

When I installed it, I was aware of the potential for bad grounds. For that reason, I took a wire that I'd terminated in a pair of ring terminals. One is under one of the radio cases, and the other under a screw into the body(which I scuffed up good, put the ring on it, and then put dielectric around it). Hopefully that is a solid ground. Just to check, when reception goes to heck, I've used another loose wire to give an additional ground to the case. There's no discrete ground wire on this.

What filter would that be? Is it the inductor I mentioned? If not, what should I use? These did presumably work in the day just connected directly to the electric system.

I have points in the car, but it's a capacitive discharge(Winterburn) ignition system that just uses the points as an on/off signal for the external CDI. In any case, the "conventional wisdom" I've always heard is resistor plugs OR resistor wires, but that both can be detrimental to the ignition energy. Should I be using both?
 

bunnspecial

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I don't think your MG is positive ground. My '62 sprite was positive ground and when I tried hooking up a radio I knew right away. Let the sparks fly!

68 and newer are negative from the factory. Mine couldn't possibly be positive considering that it has an alternator.
 

bunnspecial

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You didn't mention if you have noise in the radio from the ignition system (RFI). You could have interference from the alternator (generator) but I suspect your problem is with the spark plugs. You need resistor spark plus AND wires. Not one or the other. Also, make sure your new antenna has a good electrical connection to the body where it is mounted. If the antenna mount is not well grounded, the outside shield of the coax going to the radio is acting as an antenna picking up noise (common mode noise).

I'll grab a set of plugs and try-in the worst case I'm out $8. I'll see what happens.
 
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Kevil,Ky
You need the input power filter and a capacitor on the output of your alternator and be sure you have bare metal under the base of the antenna. I agree that the resistor wire should be enough for the ignition noise. Please verify that the case of your coil for ignition also has a good ground connection.
 

bunnspecial

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The antenna is a standard 1" mounting. It has the collapsible "claws" that clamp onto the underside of the hole when the nut is tightened. Does this sound like a reasonable ground for it? Would it be beneficial to solder or otherwise attach a wire to the "claws" and then ground it under a screw?

Also, any recommendations for a filter capacitor?(value and type of cap) Should it be inline with the +12V line, parallel to it, or from the line to ground?

Is the big metal thing that attaches by a spade to the back of the radio that I've been calling an inductor actually a capacitor?
 
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4,476
Location
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I know I should do caps, but passed on it since this bench tested out just fine(and works fine there again with the ignition off.)

When I installed it, I was aware of the potential for bad grounds. For that reason, I took a wire that I'd terminated in a pair of ring terminals. One is under one of the radio cases, and the other under a screw into the body(which I scuffed up good, put the ring on it, and then put dielectric around it). Hopefully that is a solid ground. Just to check, when reception goes to heck, I've used another loose wire to give an additional ground to the case. There's no discrete ground wire on this.

What filter would that be? Is it the inductor I mentioned? If not, what should I use? These did presumably work in the day just connected directly to the electric system.

I have points in the car, but it's a capacitive discharge(Winterburn) ignition system that just uses the points as an on/off signal for the external CDI. In any case, the "conventional wisdom" I've always heard is resistor plugs OR resistor wires, but that both can be detrimental to the ignition energy. Should I be using both?
Usually it is a line filter. Used to be able to get them everywhere, now, gotta look for them. Something like this:

There are, of course, better ones around, but this was for example.
 

bunnspecial

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Usually it is a line filter. Used to be able to get them everywhere, now, gotta look for them. Something like this:

There are, of course, better ones around, but this was for example.

Thanks! I ABOUT to order that one, but before I do, I'm wondering if that's in fact what this is

This is on a separate AM-only that was in the trunk of my Marina when I got it, and saved when I sold it(the Marina had an AM-FM in the dash identical to the one I'm putting in the MG). I'm talking about the metal can with the big lug on one end and the black wire going to the splice connector on the other end.

IMG_2155.jpeg


Also, I completely cut off a harness from an old radio install when I put this in. It had a small "black box" that IIRC from looking in it in the past had a blade fuse(which I wasn't worried about dumping since this one had an inline glass fuse) but I'm wondering if there was a supressor in there also. I need to pop it opn and look, because that one might get wired back in.
 
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It appears that is what the can is. But I would replace it, nevertheless. It looks like a ground loop interrupter. One more thing, are their florescent light bulbs anywhere near this auto when you are running it? Those bulbs will kill reception and introduce all kinds of noise. I used to restore old 40's style Am radios and those florescent bench lights caused me alot of unneeded frustration!
 
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Aftermarket ignition probably had no consideration for radio interference. Anyway since it is CDI there is plenty of extra energy, resistor plugs and wires will work fine. Make sure the hood is grounded to the body with copper braid straps crossing both hinges. Also it's important that the base of the antenna (which is attached to the shield of the antenna cable) is grounded to the car. Test with ohmmeter from the outside part of the plug at the radio end to body ground.

The radio usually has a trimmer capacitor to match the antenna to the radio for AM. It can be reached without opening the case and adjustment is a normal part of installation. Proper adjustment optimizes reception of weak AM stations but it may not help much on interference.
 

bunnspecial

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Thanks for the suggestions...

So, as plans go:

1. I'll order and fit the line filter, since it sounds like that's a practical necessity anyway

2. I'm pretty sure the antenna is ground well, but I'll double check and do what I need to

3. I can switch the ignition to Kettering easily(the CDI amplifier I use has a switch on top to do just exactly that) so will see if eliminating that makes a difference. If it does, I'll stick resistor plugs in there

4. If things persist, I'll unplug the alternator.

For some of the other things:

I have an OLD fluorescent fixture in the garage, but the radio works fine with that on. With the engine running, it sounds terrible even when out driving.

I'm hoping the line filter solves everything-since there's not one now it seems a likely culprit.
 

bunnspecial

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I still have a new filter on the way as linked above on Amazon, but none the less I wired in the one pictured above. It's a night and day difference, and everything is perfect now with the engine running.
 
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