Why does newly installed (vintage) radio hum???

TWJ · TWJ's Garage

Need a little help from any electrical engineers out there...

I just installed a period Blaupunkt radio in my '72 T after removing a 90's Blaupunkt. The period radio works great when the car is not running but with the key turned on, however when the engine is running it makes a very loud hum almost drowning out the music. Why would this be??? Grounded incorrectly? But why does it hum only when the engine is running?

When I installed it I connected the ground to a wire that connects with many other wires then to a ground. This is the same ground that the newer radio had used. I connected the positive lead to a yellow wire with an inline fuse that is controlled by the ignition switch. This to was used by the newer radio although it also had a positive lead that went directly to the battery.

How do I stop it from humming?

Why did the newer radio, using the same wires, not hum???

Any help would be greatly appreciated!

ossiblue · 08-23-2009, 04:21 PM

Sounds like the vintage radiois affected by the ignition components--CDI and spark wiring. The early cars had shielded ignition wires, does yours still have them? Also, there were some noise suppressors often located in the engine fuse panel that might have been removed. If you post a picture of your engine compartment and electrical panel (in engine compartment) we can see if the devices are present.

I believe you can buy suppressors to install along your radio wiring, but real experts will have to verify how it is done.

Grady Clay · 08-23-2009, 05:10 PM

TWJ,

L.J. is correct, it is either the CDI ignition or the alternator. It could also be the fresh air fan or A/C fans if on. Testing for that is easy – simply turn them off. If the ‘hum’ frequency varies with rpm, then it is most likely the alternator or high voltage ignition.

There is supposed to be a radio noise suppression device plugged into your voltage regulator. Confirm that it is in place and in the circuit between the engine connector and the regulator.

Do you have solid core ignition wires? Are the plug connectors radio suppression resistor type? There may be six radio suppression resistors plugged into the cap terminals.

Is there the small ground strap from the back of the alternator body to the engine crankcase top perimeter stud & nut?

Is there the braided ground strap from the electrical panel to the chassis ground behind the MFI fuel filter console?

There are aftermarket radio noise devices that can go in the power lead to the radio. Do not use a long power wire with the excess coiled up.

You might put a small (and short as possible) braided ground strap from the radio chassis to the car chassis.

You might add a small braided ground strap from the left chain cover to the electrical panel at the voltage regulator.

It is also possible that there is a defective electrolytic capacitor in the vintage radio power circuit. After all it is now 35+ years old.

Best,
Grady

(EE Carnegie Mellon ’68)

joeym · 08-23-2009, 05:25 PM

+1 on the noise suppressor or ground loop , you can buy one from best buy , fry's etc. It's a pretty straight forward install. The newer radios probably have it built in.
The hum you're hearing is most likely coming from the alternator.

Hope this helps

TWJ · TWJ's Garage

Thanks much!

Think I'll try the after market noise suppressor and add a ground loop, sounds like the easiest route. I'll also look into the items that Grady has listed, for my own knowledge if nothing else.

Grady, my brother is a fellow alum of Carnegie Mellon, Chemical Engineering ('64?).

TWJ · TWJ's Garage

Hey,

Would this noise suppressor possibly do the trick? --->

http://www.google.com/products/catalog?q=car+radio+noise+suppressor&oe=utf-8&cid=7860793359075972032#p

OR...

There looks like there are a bunch of them. How do I know which one is correct?

Rob G · 08-23-2009, 06:56 PM

Are the noise suppressors only on the earlier cars? I have a 89 911 that has a humming sound even with everything off and the radio on. As i rev the motor the noise increases.

Rob G

TWJ · TWJ's Garage

Another question...

If I "hot wired" the radio directly to the battery (with an inline fuse) and have a good ground would that effectively by-pass the noise issue or does it not work that way???

nineball · 08-23-2009, 07:09 PM

rob

if the humming is in sync with the rmp levels chances are you have a bad ground somewhere. what brand radio do you have? do you use an external amplifier?

twj

while i am no expert on early 911 electrical systems i do know quite a bit about mobile audio. if i were in your situation i would try running new power and ground as you suggested. this will be the cheapest and easiest way to test for a bad ground. you could even go so far as to just run both lines to the battery terminals to limit any other potential causes. if that solves the problem then you can properly run new wires as needed. if it persists i would try to temporarily connect the new/old blau into another vehicle to see if the whine is present.

ischmitz · ischmitz's Garage

Well, the longer and thinner a wire is between the radio and GND or power the more noise you will pick up. Best to get a thick braided ground strap between the radio and the chasis. And then get a fairly thick (at least 12 gauge) wire to the battery. Then get the supressor and you should be fine.

It could also be that you alternator has an issue. For example I had one blown diode on my original 74 alternator and it caused a very faintly glowing alternator light (only visible in the dark). The car was still charging the battery fine. However, with an oscilloscope I found a very pronounced ripple on my board voltage. And my radio sounded like a turbo-charger. The hum changed with engine speed.

Ingo

Rob G · 08-23-2009, 07:36 PM

nineball,

I have a Alpine CDA 9856. And there is a AMP up front. I am not a stereo guy. The previous owner had all the stuff put in. It looks very well done. I would love to listen to the radio without that hum. It didn't always do this. One day it just showed up. Go figure. Thanks for your help.

nineball · 08-23-2009, 07:47 PM

takin it to pm rob.

911pcars · 08-23-2009, 11:58 PM

Why does it hum?

Doesn't know the words?

sorry, couldn't resist.

I hope you found the solution.

Sherwood

Rob G · 08-24-2009, 06:19 AM

Thanks for the good morning laugh.

Quote:

Originally Posted by 911pcars

Why does it hum?

Doesn't know the words?

sorry, couldn't resist.

I hope you found the solution.

Sherwood

TWJ · TWJ's Garage

The hum doesn't change with engine RPM, it's just a constant noise/hum.

Does that help?

Grady Clay · 08-24-2009, 08:10 AM

Quote:

Originally Posted by TWJ

The hum doesn't change with engine RPM, it's just a constant noise/hum.

Does that help?

Then it is either internal to the radio (unlikely) or it is coming from the inverter circuitry in the CDI.

In addition to all the above good suggestions, try improving the grounding of the CDI and the electrical panel. Put a noise suppressor in the power lead to the CDI.

Try a different CDI.

Best,
Grady

Rob G · 08-24-2009, 08:15 AM

What is the CDI? Were can it be found?

TWJ · TWJ's Garage

Quote:

Originally Posted by Rob G

What is the CDI? Were can it be found?

Good Question! I looked through our host's parts and now know what the CDI (what does this acronym stand for?) looks like and have seen it in the engine compartment of my car.

Can anyone give a brief description of it's function and associated problems / fixes / maintenance?

I understand coil / condenser / points but not this component of the ignition system.

Thanks!

ischmitz · ischmitz's Garage

CDI stands for Capacitive Discharge Ignition. It is the silver box with the cooling fins on the electric panel in the engine bay.

It uses a DC/DC converter that runs with 3.5kHz. To check if that is the culprit simply turn the ignition to ON but do not start the engine. If the hum appears it is likely the CDI unit. If the hum only appears when the engine is running it still could be.

The fact that the hum is NOT changing pitch with engine RPM makes me think you could have a ground loop. Are you using an amplifier or only the head unit driving the speakers?

Ingo

Rob G · 08-24-2009, 03:19 PM

I do have a AMP. ALPINE PDX 4.150. I also checked if the hum was present with the ignition on but car not running. It was NOT there. It is only there when car is running. I also check in the engine bay for a CDI. I didn't see one? Could it be located somewhere else on a 89? Im in the process of checking all the grounds on the car. How many are there? I found the one on the AMP and there are 2 on the left and right hinges of the spoiler.

Rob G

Quote:

Originally Posted by ischmitz

CDI stands for Capacitive Discharge Ignition. It is the silver box with the cooling fins on the electric panel in the engine bay.

It uses a DC/DC converter that runs with 3.5kHz. To check if that is the culprit simply turn the ignition to ON but do not start the engine. If the hum appears it is likely the CDI unit. If the hum only appears when the engine is running it still could be.

The fact that the hum is NOT changing pitch with engine RPM makes me think you could have a ground loop. Are you using an amplifier or only the head unit driving the speakers?

Ingo