This radio had an open speaker field coil. I subbed a choke from my junk pile in place of the coil and used a PM magnet speaker. I have no idea what the specs on the OEM field coil were or those of the choke for that matter. I just grabbed something to try. The choke is about half the size of a baseball if that makes any sense. The choke came out of something God only knows what and how long ago and measures about 40 ohms. The filter, a three section twenty UF, two at 450V and one at 25 volts was replaced at some point and seems to be OK. I mention this because there is a very slight hum out of the speaker and I'm thinking that perhaps the elimination of the hum bucking coil in the OEM speaker is the cause of this. The hum isn't that bad. as soon as you turn the volume up a bit it's really not noticeable but without using another electro dynamic speaker can I do anything about this? I'm also wondering if I sized the choke properly. Does anyone have any thoughts on these two concerns? Thanks, Lenny

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This radio had an open speaker field coil. I subbed a choke from my junk pile in place of the coil and used a PM magnet speaker. I have no idea what the specs on the OEM field coil were or those of the choke for that matter. I just grabbed something to try. The choke is about half the size of a baseball if that makes any sense.
The choke came out of something God only knows what and how long ago and measures about 40 ohms. The filter, a three section twenty UF, two at 450V and one at 25 volts was replaced at some point and seems to be OK. I mention this because there is a very slight hum out of the speaker and I'm thinking that perhaps the elimination of
the hum bucking coil in the OEM speaker is the cause of this. The hum isn't that bad. as soon as you turn the volume up a bit it's really not noticeable but without using another electro dynamic speaker can I do anything about this? I'm also wondering if I sized the choke properly. Does anyone have any thoughts on these two
concerns? Thanks, Lenny



Did it have a slight hum before the coil opened up ? Could be the choke
is the wrong size, or unless the filter capacitors have been changed
lately, they are bad, weak, or whatever you want to call it. It could
even be some heater to cathode leakage in the tubes.

wrote:


This radio had an open speaker field coil.
I subbed a choke from my junk pile in place of the coil
and used a PM magnet speaker.


** Doing this eliminates the hum producing mechanism present in field coil "electro-dynamic" speakers - ie the field and voice coils acting like a transformer so the voice coil is supplied with double supply frequency hum.

Hum heard with a PM speaker set up is usually due to inadequate supply filtering (not enough L or C ) and the use of singled ended triode or pentode amplifiers.

Maybe try adding a series resistor with that 40 ohms choke - say 220ohms for a start. Should only result in a few volts loss of HT.

http://www.nostalgiaair.org/PagesByM...9/M0018529.pdf


..... Phil

On Monday, 23 July 2018 22:43:07 UTC+1, wrote:

This radio had an open speaker field coil. I subbed a choke from my junk pile in place of the coil and used a PM magnet speaker. I have no idea what the specs on the OEM field coil were or those of the choke for that matter.. I just grabbed something to try. The choke is about half the size of a baseball if that makes any sense. The choke came out of something God only knows what and how long ago and measures about 40 ohms. The filter, a three section twenty UF, two at 450V and one at 25 volts was replaced at some point and seems to be OK. I mention this because there is a very slight hum out of the speaker and I'm thinking that perhaps the elimination of the hum bucking coil in the OEM speaker is the cause of this. The hum isn't that bad. as soon as you turn the volume up a bit it's really not noticeable but without using another electro dynamic speaker can I do anything about this? I'm also wondering if I sized the choke properly. Does anyone have any thoughts on these two concerns? Thanks, Lenny

Check out the thread 2017 sci.electronics.repair ۼ
Ampeg (?) 15" bass speaker 5815026
and
https://www.radiomuseum.org/forum/ba...peakers.ht ml


NT

Did you toss the old speaker? If not, obtain the largest electrolytic capacitor at the highest voltage to which you can charge it fully.

Discharge it across the open coil. 2/3 of the time, that discharge will be enough to re-weld the broken ends of the coil inside.

OR:

Unwind the field coil if possible. 80% of the failures are at the connections.

Otherwise, live with the hum - you are correct in that the hum-buck does not exist on the choke.

Best of luck with it.

Peter Wieck
Melrose Park, PA

On Monday, July 23, 2018 at 5:43:07 PM UTC-4, wrote:
This radio had an open speaker field coil. I subbed a choke from my junk pile in place of the coil and used a PM magnet speaker. I have no idea what the specs on the OEM field coil were or those of the choke for that matter.. I just grabbed something to try. The choke is about half the size of a baseball if that makes any sense. The choke came out of something God only knows what and how long ago and measures about 40 ohms. The filter, a three section twenty UF, two at 450V and one at 25 volts was replaced at some point and seems to be OK. I mention this because there is a very slight hum out of the speaker and I'm thinking that perhaps the elimination of the hum bucking coil in the OEM speaker is the cause of this. The hum isn't that bad. as soon as you turn the volume up a bit it's really not noticeable but without using another electro dynamic speaker can I do anything about this? I'm also wondering if I sized the choke properly. Does anyone have any thoughts on these two concerns? Thanks, Lenny

Thanks for all the great ideas guys. I did get it going but it needs an alignment particularly on the short wave band. Lenny