Hi all! I am a complete novice to restoring radios. I have a 39-7 that I just opened up, today. I can see that several wires need re-insulating, a new power cord is a must and I plan to recap the radio. My first concern is for the speaker. It appears to me to need to be reconed, for sure, but I am also wondering about the piece (not sure what it is called) above the speaker that is very deteriorated. Will it need to be replaced as well? Am I better off finding a replacement speaker for this unit? Thanks for any help!
Hi And Welcome,
The part in question is called the output transformer. It has four wires. The important part is that two wires on the back side ( these are called the primary) that go to the 41 tube socket show some resistance (around 200-600 ohms roughly). If they show a much higher resistance or open it's kaput. Would be helpful to see a side view of the spkr too.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
I've seen many small output transformers that looked like this, that worked just fine; all they needed is a good blow from a canned air duster.
Re-coning - yes, kinda painfully obvious.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Thanks for your answers. I feel more hopeful, now. I will post sideview pics of the speaker. I am touch with a guy who recones vintage speakers who said he can probably do this one. He is concerned about the size of the voice coil if it is 1/2 inch or less in size. I am not sure how or where to separate the speaker from the transformer. Should I separate them or send the whole unit - speaker, transformer and magnet as a whole? It looks like the speaker is permanently attached to the magnet. Thanks!
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Well, as to how to do that - very simple, just take an ohm meter (your DMM set to Ohms), and measure across the field coil.
You should see 1,700 ohm or close. If it is open (shows hunderds of k's, or just open) - it is bad. It is almost never shorted.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
With help from Radioroslyn I checked the field coil and it is shot, along with the output transformer. I plan to replace with a modern speaker and update the capacitors.
With a modern speaker, you will still need a similar type output transformer and a modification to the power supply:
The field coil will need to be replaced with a similar ohms value power resistor--perhaps 1.8K ohms and at least 5 watts--in a suitably ventilated place in the chassis. C25 and C26 should probably be upsized to 8 and 16 microfarads, respectively. This last part is to compensate for the fact that a resistor is not as efficient as a field coil in filtering out the power supply "hum."
Rob
(This post was last modified: 09-21-2017, 11:20 PM by robsradioactive.)
Thanks, Rob! I believe all of the parts you recommended are in the pipeline as we speak. I have received great advice from the folks here at the phorum. If I can put all of the knowledge offered to me here at this phorum to good use we will have a restored radio, someday. I am currently reading Electronics for Dummies and Elements of Radio Servicing to build my basic electronic and radio skills.
I am currently reading Electronics for Dummies and Elements of Radio Servicing to build my basic electronic and radio skills.
