:The output on the Bendix is distorted (garbled) across the full frequency range. I've replaced the audio amplifier tubes (14B6 & 35A5), checked the resistance values of the cathode bias resistors and plate load resistors, and replaced the coupling capacitor between the audio stages --- all have no impact. The distortion is minor when the volume control is very low, but increases significantly when volume is increased slightly. Any ideas? THANKS.
:How are the plate and screen voltages?
:Greg
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::The output on the Bendix is distorted (garbled) across the full frequency range. I've replaced the audio amplifier tubes (14B6 & 35A5), checked the resistance values of the cathode bias resistors and plate load resistors, and replaced the coupling capacitor between the audio stages --- all have no impact. The distortion is minor when the volume control is very low, but increases significantly when volume is increased slightly. Any ideas? THANKS.
Looks like degenerative feedback....Reduces distortion. Notice it is applied to the plate of the audio tube and also to the cathode. C-14 and R9 cause a phase shift in the audio signal which slightly reduces gain but also reduces distortion.
There are all sorts of things you can do with this circuit. With my original set-up, the audio is cleaner, and tends to distort less. If you're looking for maximum efficiency, though (which most manufacturers were), you want to keep that harsh middle range, as this is what gives the audio the illusion of power. It takes a lot more power to drive the high and low frequencies, especially since our ears respond less to them, and because of the difficulties encountered with transmitting them through the air.
Thomas
Thomas
:They are indeed negative feedback. I don't know what kind of results they produce. When I incorporate negative feedback into any of my amplifiers, I tune it so that it nails the frequencies which annoy me. I usually use a value of .01 MFD for C14. The resistor is anything from 50K to 470K. Some other things I do is I place a 50K resistor prior to the plate resistor (R6). Where the two join I inject the negative feedback. I also place a .001 to .002 MFD condenser from this point to B-. What this does is cancel out negative feedback at the super high frequencies. It almost accentuates these frequencies. Without the negative feedback, these frequencies get lost with normal unbalanced amplification. I specially chose the .01 MFD condenser and the series resistor for my original application, and have altered the series resistor slightly for various other radios. It all depends upon the impedances found in the various circuits. I chose the two components because I noticed a very harsh mid-range in the original receiver which I used this circuit in. The condenser passes frequencies down to a certain point, but not so efficiently the super low ones in the circuit which it is used in (if used in a several million ohm impedance circuit, it'll pass low frequencies with ease). The resistor doesn't pass high frequencies easily. The two meet in the middle and pass the frequencies which I want to cancel. If you wanted to cancel bass notes, you could choose a much larger condenser and a larger value resistor (or perhaps not a larger value resistor). Change the .001 MFD condenser to a larger value so that negative feedback at the higher frequencies is cut off, and the circuit will only cut out bass.
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:There are all sorts of things you can do with this circuit. With my original set-up, the audio is cleaner, and tends to distort less. If you're looking for maximum efficiency, though (which most manufacturers were), you want to keep that harsh middle range, as this is what gives the audio the illusion of power. It takes a lot more power to drive the high and low frequencies, especially since our ears respond less to them, and because of the difficulties encountered with transmitting them through the air.
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:Thomas
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:Thomas
By the way... is the B+ clean?
:I mean have the power supply capacitors been replaced yet?
:You haven't mentioned anything about "hum" so perhaps the B+ is ok.
:Did this radio ever play clean since you've had it?
Check around the top and underside of the speaker cone. Move it gently. Is it well connected to the voice coil? If not, drizzle a very fine amount of Elmer's white glue around where the cone joins to the voice coil. Use a tooth pick if necessary. Check to see that the spider is well glued to the frame and the voice coil. If loose from either, this'll cause rattling, and will allow the cone to go off center. Again use a tooth pick to distribute glue into this very confined area--the spider is the small circular flexible piece under the cone. On more modern speakers like this one it's usually cloth impregnated with varnish or lacquer. It is corrugated in construction--wavy.
Move the cone up and down and listen. Place your fingers at the 3 and 9 o'clock positions (180 degrees apart) and move cone evenly. If the voice coil is rubbing against the magnet, you can re-center it. What you do is remove the dust button from the center of the cone. This is either paper or felt. Obtain a sheet of plastic that is slightly thinner than an index card. Cut into three strips which are each about a quarter inch wide and about two inches long. You can probably find such plastic at a hobby shop. If not, improvise. Some Christmas ornaments come in clear plastic boxes at Walgreens. Can't think of any other examples. The plastic should be perhaps .010 to .014 inches thick. You can also use cardboard of the same thickness, but it can bend at the corners, and doesn't like moisture. Place your three shims between the voice coil and the center magnet piece so that they are evenly spaced from eachother. Mist the cone with water until it is damp. Do not soak it but make it obviously damp. Allow to dry in an oven or overnight on the table. The oven will dry the cone much more rapidly. To dry in an oven, turn the oven up to about 150 degrees. Allow it to cycle once. Once the flame or element has shut off, turn the oven off. The residual heat will safely dry the speaker. Check on the speaker in about 15 minutes. You can also use a hair dryer. Once the cone is dry, remove the shims and check for voice coil rubbing again. Also try playing the speaker to see how it sounds.
Thomas
:Could be an output transformer issue. Sounds more like you have speaker issues. Try temporarily replacing the speaker. If this cures the problem, then try this with the original speaker: Check to see that both of the flexible leads are connected properly. If they have any breaks, the speaker will operate intermittantly. This may be either an obvious cutting out of the music on loud notes, or a fuzzy sound if the break is small and the wires are still touching fairly well. Check the speaker's resistance while moving the cone. If it changes, then the wires probably have small breaks. The wires coming off of the cone are tinsel wire. If you have such a problem, let me know, as repair can be difficult, depending upon how the wire is broken. More often than not it is fairly easy--usually the wire breaks right at the binding post. Since tinsel wire has strands of copper tinsel wound around fibers, the wire may look complete, but the electrical connection may be broken. The fibers will hold the wire together, making it look unbroken.
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:Check around the top and underside of the speaker cone. Move it gently. Is it well connected to the voice coil? If not, drizzle a very fine amount of Elmer's white glue around where the cone joins to the voice coil. Use a tooth pick if necessary. Check to see that the spider is well glued to the frame and the voice coil. If loose from either, this'll cause rattling, and will allow the cone to go off center. Again use a tooth pick to distribute glue into this very confined area--the spider is the small circular flexible piece under the cone. On more modern speakers like this one it's usually cloth impregnated with varnish or lacquer. It is corrugated in construction--wavy.
:
:Move the cone up and down and listen. Place your fingers at the 3 and 9 o'clock positions (180 degrees apart) and move cone evenly. If the voice coil is rubbing against the magnet, you can re-center it. What you do is remove the dust button from the center of the cone. This is either paper or felt. Obtain a sheet of plastic that is slightly thinner than an index card. Cut into three strips which are each about a quarter inch wide and about two inches long. You can probably find such plastic at a hobby shop. If not, improvise. Some Christmas ornaments come in clear plastic boxes at Walgreens. Can't think of any other examples. The plastic should be perhaps .010 to .014 inches thick. You can also use cardboard of the same thickness, but it can bend at the corners, and doesn't like moisture. Place your three shims between the voice coil and the center magnet piece so that they are evenly spaced from eachother. Mist the cone with water until it is damp. Do not soak it but make it obviously damp. Allow to dry in an oven or overnight on the table. The oven will dry the cone much more rapidly. To dry in an oven, turn the oven up to about 150 degrees. Allow it to cycle once. Once the flame or element has shut off, turn the oven off. The residual heat will safely dry the speaker. Check on the speaker in about 15 minutes. You can also use a hair dryer. Once the cone is dry, remove the shims and check for voice coil rubbing again. Also try playing the speaker to see how it sounds.
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:Thomas
Dave
:It has been my personal experience where I have had distorted sound, esp when vol is increased, that after checking everthing else... I found the speaker itself was the cause. If the speaker has torn sections or has been repaired prior. Sometimes it has simply been caused by foriegn matter stuck insde by the voice coil area where the speaker travels freely over the magnet. If there is no dust cover felt, sometimes things get stuck in there and need to be blown free.
:It is vbey easy to make this determination by simply swapping another speaker by unsoldering the leads right where it goes out to the speaker and trying another.
Also, I had a speaker re-coned last year. It now has a wonderful RED cone! That's the only one they had available. Unfortunately they put a small burn mark in it, and they also cut the leads too short. This caused obnoxious distortion on loud bass notes--the cone would tug on the wire and would pull out of shape, causing irregular waves and extra waves not normally part of the musical notes. I added some wire to the lead--regular wire, but this allowed the tinsel wire more slack. Now the speaker plays beautifully.
Thomas
I have had fair luck with attaching the new cone to the center of the old cone (rest of old cone removed). Sometimes it sounds poor. There's a problem with matching angles. When the old cone is at one angle and the new one is at another, the old cone will tend to flex on its own somewhat, transferring the music to the new cone less efficiently. When a cone is mounted properly to the voice coil, it moves as a unit and transfers music to the air with perfect efficiency. You will also notice the efficiency and fidelity problem with speakers that don't have enough of an angle to the cone (regardless whether they've been re-coned or not). If the cone is rather flat, multiple waves will be set up along the cones surface from the center to the outside instead of the entire cone moving as a unit.
You can stiffen the old cone where it meets the new cone with a liberal application of Elmer's white glue to the old cone. However, this increases the entire unit's mass, and makes it more difficult for the unit (new cone, old cone, and voice coil) to reproduce treble notes faithfully. I think that a better stiffener would be super glue. It is lighter and stiffer for the amount you use. Soak the old cone's paper with super glue. Be very careful in doing this. If any gets down to the voice coil, it can freeze the voice coil between the magnet.
Thomas
Thomas