Hi everyone,I own an s-40a that I have been debugging for some time now.I have managed to cure all it's problems but one has reared its ugly head that I just don't have the experience to conquer.I have recently found that if I turn up the volume on a strong station the receiver begins to growl or exibit feedback if you will.I never noticed this problem until now because I have a tendency to listen at low volume seeing as though the speaker is only about a foot from my ear.I have found however that if I loosen the speaker mounting bolts and let it hang in the rubber grommets,the growling stops.Therefore I assume the problem involves vibration from the speaker.I replaced all the tubes to see if a microphonic one was the problem but it still growls.I checked underneath to see if applying pressure to any of the components would help.Still growls.
Just for giggles I took some resistance readings at the first and second audio tubes.I found that the grid resistor on the first audio tube(6sq7) was way out of spec.It is supposed to be a 15 meg,but measures just under 24.Would this cause the feedback problem by altering the gain of the first audio tube?I read somewhere that this can cause the tube to become some sort of oscillator under the right conditions.Maybe I'm just crazy.Would love to get this problem straightened out and finally call this radio done.Any suggestions would be greatly appreciated.Thanx.

I am betting on that out of spec resistor. Increased value of the grid resistor can cause increased response at lower frequencies. It has set up an oscillation as a result.

Thank you for the reply Jim.I was hoping that someone would tell me I was on the right track.I have just plain run out of ideas.I took the speaker out today and experimented with different mounting methods to help dampen the vibrations,but it became obvious that I was going to have to cure the problem,not the symptom.Thanks again.

Mike,
Had a similar problem with a radio/phono console, where the tilt-out phono tray also contained the speaker. It was like damped oscillations during certain music passages at medium volume. It happened in both radio and phono mode, so knew it wasn't accoustic feedback to the cartridge. Since the tilt out aimed the speaker downward, I always had it pushed in during tests, and thats when the phono mounting frame was hitting the cabinet. Since the original phono was no where to be found (Philco "Beam of Light"), some minor mods were made to accomodate a later version, and thats when a minor measurement error crept in. Took my Dremel tool and cut a small relief in an inconspicious area of the cabinet crossmember. Presto!! No more damped oscillations or should I say cabinet vibrations.

marv

:Hi everyone,I own an s-40a that I have been debugging for some time now.I have managed to cure all it's problems but one has reared its ugly head that I just don't have the experience to conquer.I have recently found that if I turn up the volume on a strong station the receiver begins to growl or exibit feedback if you will.I never noticed this problem until now because I have a tendency to listen at low volume seeing as though the speaker is only about a foot from my ear.I have found however that if I loosen the speaker mounting bolts and let it hang in the rubber grommets,the growling stops.Therefore I assume the problem involves vibration from the speaker.I replaced all the tubes to see if a microphonic one was the problem but it still growls.I checked underneath to see if applying pressure to any of the components would help.Still growls.
: Just for giggles I took some resistance readings at the first and second audio tubes.I found that the grid resistor on the first audio tube(6sq7) was way out of spec.It is supposed to be a 15 meg,but measures just under 24.Would this cause the feedback problem by altering the gain of the first audio tube?I read somewhere that this can cause the tube to become some sort of oscillator under the right conditions.Maybe I'm just crazy.Would love to get this problem straightened out and finally call this radio done.Any suggestions would be greatly appreciated.Thanx.

Thank you Marv for the comments.I had the same ideas about speaker placement and so forth and how to dampen the vibrations.I exaggerated a tad when I said that hanging the speaker loosely from the grommets stopped the feedback.What I should have said is that it lessened it a bit.Holding the speaker in my hand inside the cabinet still produces the feedback as long as the sound waves can contact it.I have checked it over very thoroughly,there are no loose parts anywhere rattling.I feel that Jim's suspicion of the grid load resistor is the culprit.I had to go to the local tv repair shop to get one of such high value.I paid over two bucks for that resistor,that shows you how much I would like to finish this rig.He didn't offer me any Vaseline to go with it,unfortunately.Thanx.

Mike,
I'm not familar with the mounting arrangement of your speaker, but have found most larger units (10-12 inch) and some smaller ones are semi-isolated with rubber grommets and no direct contact at the cabinet or frame. I've found garden variety grommets of today much harder than the older latex items (when new), and probably offer far less acoustic isolation, as was the case I described in my earlier post.

marv

:Thank you Marv for the comments.I had the same ideas about speaker placement and so forth and how to dampen the vibrations.I exaggerated a tad when I said that hanging the speaker loosely from the grommets stopped the feedback.What I should have said is that it lessened it a bit.Holding the speaker in my hand inside the cabinet still produces the feedback as long as the sound waves can contact it.I have checked it over very thoroughly,there are no loose parts anywhere rattling.I feel that Jim's suspicion of the grid load resistor is the culprit.I had to go to the local tv repair shop to get one of such high value.I paid over two bucks for that resistor,that shows you how much I would like to finish this rig.He didn't offer me any Vaseline to go with it,unfortunately.Thanx.

:Hi everyone,I own an s-40a that I have been debugging for some time now.I have managed to cure all it's problems but one has reared its ugly head that I just don't have the experience to conquer.I have recently found that if I turn up the volume on a strong station the receiver begins to growl or exibit feedback if you will.I never noticed this problem until now because I have a tendency to listen at low volume seeing as though the speaker is only about a foot from my ear.I have found however that if I loosen the speaker mounting bolts and let it hang in the rubber grommets,the growling stops.Therefore I assume the problem involves vibration from the speaker.I replaced all the tubes to see if a microphonic one was the problem but it still growls.I checked underneath to see if applying pressure to any of the components would help.Still growls.
: Just for giggles I took some resistance readings at the first and second audio tubes.I found that the grid resistor on the first audio tube(6sq7) was way out of spec.It is supposed to be a 15 meg,but measures just under 24.Would this cause the feedback problem by altering the gain of the first audio tube?I read somewhere that this can cause the tube to become some sort of oscillator under the right conditions.Maybe I'm just crazy.Would love to get this problem straightened out and finally call this radio done.Any suggestions would be greatly appreciated.Thanx.

Make sure the tuning capacitor isolation grommets have not lost their resiliency. My S40 howled until I replaced all of the isolation rubber. Ordinary grommets may be too hard. It has been a while but I think that I used some soft grommets from a defunct CD player.

Hi Terry,I think you have hit the nail squarely on the head.I installed my two-dollar resistor in the first audio grid and it didn't amount to squat.I then isolated the audio stage and fed a signal directly from an outside souce.It quickly became obvious that that stage wasn't the problem.I then worked backwards one tube at a time.I cleared the I.F. stages with no feedback issues.But when I hooked the generator to the signal grid of the converter tube,it howled like mad as soon as I turned up the volume.This was with the R.F. amp tube removed.I did find that the lower two bands will not howl no matter how strong the signal is.The next band up will with an extremely strong signal.The uppermost band,starting at 15mc,acts up very easily.I checked all voltages,resistors etc. and did not find anything.I did notice that bass notes aren't necessarily what causes the feedback,it doesn't really seem to make a difference.Istarted suspecting the tuning capacitor grommets,but was hoping to find a more easily fixed problem.You jarred my memory by mentioning the cd player grommets.I have a portable cd player support that screws to the dash of a car.The mounting platform for the player is isolated from the mount by some soft grommets at the four corners.I may be able to use those.Man I hate the thought of taking the front of this radio off again.I had to restring both dials and swore like a truck driver through it all.Thanks for your help.

Was up to about 1:30 a.m this morning replacing the grommets under the tuning cap.Found some excellent soft rubber ones in,of all places, an old Baby Ben alarm clock.The main tuning dial string broke(I hadn't replaced that one as I had thought)so that was taken care of at the same time.Cleaned the top of the chassis up nice with some Windex and now looks super as it has no rust, like most I've seen.
Put the whole thing back together,hit the switch,and it still howls to beat the band.'Tis truly a mystery if I ever saw one.