FORD RADIO DISTORTION

I am working on a ford radio.I checked all the tubes and they are ok.I replaced all the paper caps and the electroletics.The radio plays but the sound bubbles.This is at any volume.Could it be a mica cpaacitor?Any ideas?Thanks

Possibly, but I would sub out the speaker for a known good one of similar impedance just to make sure the voice coil mount isn't dragging...or else disconnect and feed that old Ford speaker with a clean audio signal and see if it sounds OK then.

The sound "bubbles"? Maybe you are listening to Lawrence Welk music.

Regards

WC

I love to listen to L.W. but my problem is in the radio.I am using a speaker that is my bench speaker.It is 8 ohms.

Sorry for being a smart-aleck. I just could not resist.

Do you mean that the output sounds distorted? How are you powering this radio for servicing? If you are using a bench power supply, is its output sufficiently large for a car radio? Is this radio six or twelve volts? Could you provide a model number? This would help in our providing specific suggestions. Did you check the B+ voltage at the cathode of the rectifier and at the filter capacitors? If B+ is low, it can cause distorted output. Next check the voltage on the screen and the grid of the output tube. If the grid is driven positive, distortion can result. Does the distortion appear when the radio is turned on, or does it take awhile to develop? A gassy output tube or one with grid emissions is something to check for. Subbing the first audio tube and the output tube is a possible way to eliminate this possibility. Did you check the resistors in the set, particularly in the audio section, for any that may be out of tolerance? Resistors out of spec can cause several different conditions that will contribute to distortion.

Next, I would break out the signal tracer and find out where, if anywhere, the signal is clear. Start at the center terminal of the volume control. This will "break" the radio in half, seperating the front end from the audio section. It is possible that the voice coil is rubbing in the speaker you are using, so try a different one. Is the one original to the radio in usable condition? If so, try it. If the signal is clear at the volume control, then move toward the speaker first the grid, then the plate of all the tubes in the signal chain after the volume control. If the signal is distorted at the center terminal of the volume control, you need to move toward the front end of the radio in like manner only in reverse. First the plate, then the grid of all the tubes from the volume control to the antenna of the set. When you find where the distortion is and isn't, then you can check the individual stage for defective components and connections. Check all your solder joints and double-check wiring and lead dress. Good luck.

Regards

WC

It could be that you are using a speaker with double the impedance. Most vehicle radios use 4 ohm speakers. When you put an 8 ohm speaker on something that requires 4 ohms you will lose half of your output power. In a SS radio it usually wouldnt be too much of a problem besides cause low volume from losing power but in a tube radio it could cause problems with the output section of the radio. I would try changing the speaker to a 4 ohm one before doing anything else. If that doesnt improve the sound then i would start checking components.

Speaker impedence is why I suggested you try the original speaker if it is in working condition. Without more specific information on what exactly you are working on, we are shooting in the dark. I would really like a model number so I could consult SAM's and give you specific suggestions. I use an 8 ohm on my bench and it does not cause problems with car radios. My GUESS is that you have problems other than the speaker.

Regards

WC

Just a thought, I had a friend who used to junk old cars and decided to test the radios on a bench at his junk yard.  He complained to me about a similar problem with every radio he tested!  I checked out his setup and found he was using a battery charger to test the radios. Battery chargers don't have any filtering because a rectified pulse is better for charging. I built a pi type filter for him and all was well.  I'm not sure if your problem is this simple but it doesn't hurt to check.

Another good point! I asked what kind of power supply he was using. We just need more information to be able to help the OP.

Regards

WC

Dont overlook the antenna also as a bad connection can resort in distorted sound. Like WC said, we need more info on what the OP has for his workbench set-up.