http://www.nostalgiaair.org/PagesByModel/224/M0009224.pdf
I don't remember having this trouble with the 1st one I did last year though.
As soon as you turn it on it had a serious noticeable irritating hum.
...even before the filaments seemed to be heated.
I was dead arriving hear ( bad filter caps)so no evidence when it came in.
This hum now was after everything was re-capped by the way..
At first I tried a few fast or quickie shot-gun trouble shooting maneuvers.
First I tried increasing the filter caps to double each and made no difference.
Scoped the B+ and ripple is insignificant.
Swapped the output tube and each of others to eliminate possible inter element problems. No dice.
Finally decided to get serious.
Determined that volume had no effect on the hum.
Since I did not disturb the wire dress ( and it was very neat) I saw nothing was needed there. Everything shielded too.
The 6D6 tube was properly shielded.
Grounded the grid of the 75 tube... no difference.
Then I just yanked out the 42 final audio tube completely.
No difference!
Yanked out ALL tubes.... no difference !!!!!!!!!
WHAT? !!!!!!!!
Yep that speaker was still humming!
I felt it and was actually able to feel the hum in my hand a bit on the basket.
Felt the transformer and it didn't really seem to be vibrating too bad. ( but I was wrong...lol)
So I un-bolted and removed the speaker from the chassis and it got much better but still something was wrong.
I decided to try to tighten down the transformer mounting and case bolts and found them to be loose as a goose.
As I cranked them down I could hear that hum fade away.
But then after starting to bolt the speaker back I accidentally pulled a wire and ripped the paper on the field coil paper tape a bit and tried to tape it back in place.
This started my next headache. A field coil (hair wire broke)
I re soldered it to a newer more flexible lead wire for both ends.
But while testing it.. the radio seemed to work but I could smell something burning and hear arching in the radio.
Yep... one of those hair wire leads under the paper in the field coil was still broken and arching and smoking and what a mess.
Had to find a new end and really make some good solid repairs to the leads this time. Shrink wrap on the connections too.
Finally got those darn field coil wires back and everything else back together and tried the radio.... OMG... lol.
The good news is that the radio worked.
The bad news is:
.. the darn loud hum is back ...as loud as ever ...even with no tubes!!!!
Holy crap.
So I un-soldered the four speaker lead wires and re-wired the radio to a different but identical E.D. speaker on my bench.
Silence! The radio was quiet.... playing beautifully.
Great... but why?
What's the difference between these two speakers I ask myself.?? ... Myself does not reply!...lol
So I decide to remove the Howard speaker and put it on the bench to test IT.... and THAT's when I found that while I was re-installing it ... (at the time I broke the wires)... I didn't really finish the install and one of the mounting screws was real loose and the other was half out....lol
Put all the tubes back in... turned on the radio again and as I cranked down that speaker mounting screws again (properly this time)... the hum finally disappeared!
It was all mechanical.
Now let that be a lesson to me!...lol
:I've been restoring my second Howard-256 this week.
:
:http://www.nostalgiaair.org/PagesByModel/224/M0009224.pdf
:
:I don't remember having this trouble with the 1st one I did last year though.
:
:As soon as you turn it on it had a serious noticeable irritating hum.
:...even before the filaments seemed to be heated.
:
:I was dead arriving hear ( bad filter caps)so no evidence when it came in.
:
:This hum now was after everything was re-capped by the way..
:
:At first I tried a few fast or quickie shot-gun trouble shooting maneuvers.
:
:First I tried increasing the filter caps to double each and made no difference.
:
:Scoped the B+ and ripple is insignificant.
:
:Swapped the output tube and each of others to eliminate possible inter element problems. No dice.
:
:Finally decided to get serious.
:
:Determined that volume had no effect on the hum.
:
:Since I did not disturb the wire dress ( and it was very neat) I saw nothing was needed there. Everything shielded too.
:
:The 6D6 tube was properly shielded.
:
:Grounded the grid of the 75 tube... no difference.
:
:Then I just yanked out the 42 final audio tube completely.
:No difference!
:
:Yanked out ALL tubes.... no difference !!!!!!!!!
:
:
:WHAT? !!!!!!!!
:
:Yep that speaker was still humming!
:
:I felt it and was actually able to feel the hum in my hand a bit on the basket.
:
:Felt the transformer and it didn't really seem to be vibrating too bad. ( but I was wrong...lol)
:
:So I un-bolted and removed the speaker from the chassis and it got much better but still something was wrong.
:
:I decided to try to tighten down the transformer mounting and case bolts and found them to be loose as a goose.
:As I cranked them down I could hear that hum fade away.
:
:But then after starting to bolt the speaker back I accidentally pulled a wire and ripped the paper on the field coil paper tape a bit and tried to tape it back in place.
:
:This started my next headache. A field coil (hair wire broke)
:I re soldered it to a newer more flexible lead wire for both ends.
:But while testing it.. the radio seemed to work but I could smell something burning and hear arching in the radio.
:Yep... one of those hair wire leads under the paper in the field coil was still broken and arching and smoking and what a mess.
:Had to find a new end and really make some good solid repairs to the leads this time. Shrink wrap on the connections too.
:
:Finally got those darn field coil wires back and everything else back together and tried the radio.... OMG... lol.
:The good news is that the radio worked.
:The bad news is:
:.. the darn loud hum is back ...as loud as ever ...even with no tubes!!!!
:
:Holy crap.
:
:So I un-soldered the four speaker lead wires and re-wired the radio to a different but identical E.D. speaker on my bench.
:Silence! The radio was quiet.... playing beautifully.
:Great... but why?
:
:What's the difference between these two speakers I ask myself.?? ... Myself does not reply!...lol
:
:So I decide to remove the Howard speaker and put it on the bench to test IT.... and THAT's when I found that while I was re-installing it ... (at the time I broke the wires)... I didn't really finish the install and one of the mounting screws was real loose and the other was half out....lol
:
:Put all the tubes back in... turned on the radio again and as I cranked down that speaker mounting screws again (properly this time)... the hum finally disappeared!
:
:It was all mechanical.
:
:Now let that be a lesson to me!...lol
:
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After trying every conceivable electronic alternative and swapping this and swapping that etc etc ... I finally solved the problem but not by myself.
It was suggested to me that perhaps the cone itself had a "resonant" frequency at 60hz and that adding mass to the cone may shift the resonant frequency.
That was it.
So I glued on two little flat washers each the size of a dime. One at 12 o'clock the other at 6 o'clock.
The hum disappeared!!!!!
...here's the whole thread and solution:
http://antiqueradios.com/forums/viewtopic.php?t=90080&postdays=0&postorder=asc&start=20 at ARF.
So I used smaller ones about 3/4 the size of a dime... and I relocated them nearer to the inner area about a little more than 1-1/2" out from center.
One at 12 O'clock the other at 6 O'clock.