Advice on Philco 70A basket case
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I am starting on a Philco 70A (serial number: 152093, no letter B). Circuit has been messed with but circuit layout and tube layout seems to match 70-A Below B-22000.
I have a couple of questions to get me started:
1. Original speaker is trashed. Shredded field coil, bad output transformer. I'm thinking I'm going to have to use an 8" PM speaker that I have along with a roughly 3000 ohm 10-20 watt resistor to replace the field coil. Perhaps eventually I'll find a Philco 70 speaker.
2. The power transfomer for a 70-A is supposed to be for 25-40 cycle current. Do I have to worry about using it with our 60 cycle current?
Thanks for you help in getting me started on this project.
Mark
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You'll do fine on both counts.
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I think if you do a little looking you can find someone that can rewind your field coil. A replacement transformer shouldn't be a problem to find. If you do go with a PM speaker and a resistor instead of the field coil you will find that you will need to up the value of the filters to reduce the hum. The field coil or a choke will help filter the hum out much better than a resistor with less of a voltage drop.
Terry
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!
Terry
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The big problem with a Philco 70 speaker is that you can't remove the field coil to get it rewound. It is held in place with a very tight factory pressed fitting. I don't know of anyone who has been able to remove one without destroying the speaker. The Philco 20 speaker, which is similar, has a field coil held in place by a large nut which makes removing it easier.
Both speakers are becoming quite expensive when you find them, especially when you are restoring a basket case with a very damaged cabinet that is never going to be a beauty queen....
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Between you and me the original speaker doesn't sound very good. I've got a model 370 (that's the chairside version of the 70) and between the stiff paper and spider it very little low frequency response.I did recone a speaker for a model 20 which did make an improvement but it still has a rather stiff spider.
Terry
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!
Terry
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retiredinprescott Wrote:2. The power transfomer for a 70-A is supposed to be for 25-40 cycle current. Do I have to worry about using it with our 60 cycle current?
It is lowering the frequency you have to worry about; if the frequency goes up (and the core is capable of handling it, and with old power xfmrs those usually OK with it) you have an oversized XFMR, but this is it - it will do just fine at 60Hz.
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.
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Yup, a 25 cycle power transformer will do just fine at 60 cycles. The reverse is not true, but I don't think any part of the USA or Canada has 25 cycle AC anymore.
Terry - are you saying your 370 speaker has a solid spider like a 20 speaker? I've never seen an original 70, 90 or 370 that had a speaker with a solid spider. Every one I've seen has the open spider that sort of resembles an old 45 rpm adapter in appearance. This was true of all June 1931 and newer Philcos.
The 70 and 90 speakers also have the pressed-in pole piece, as Mark has already pointed out; while the 20 speakers have a nut that holds the pole piece in place. The speakers with the nut holding the pole piece are the ones that have solid spiders. At least, that has held true for every type K speaker I have seen. The K-2 and above have open spiders and pressed-in pole pieces.
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Ron Ramirez
Ferdinand IN
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No the 370 doesn't have a solid spider just stiff.
Terry
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!
Terry
Posts: 57
Threads: 12
Joined: Feb 2009
City: Prescott AZ
Ron Ramirez Wrote:The 70 and 90 speakers also have the pressed-in pole piece, as Mark has already pointed out; while the 20 speakers have a nut that holds the pole piece in place. The speakers with the nut holding the pole piece are the ones that have solid spiders. At least, that has held true for every type K speaker I have seen. The K-2 and above have open spiders and pressed-in pole pieces.
Ron,
Do you know anyone who can rewind the field coil in a model 70 speaker by somehow removing the pressed in pole piece, rewinding the coil and then resetting the pressed-in pole piece? I believe Hank Brazeal could rewind the field coil but I don't think he can deal with the pressed in pole piece. It's certainly beyond my abilities to remove it. That's why I'm currently looking at a PM speaker and a large power resistor to replace it...
Thanks,
Mark
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Mark
No, I don't.
A machine shop should be able to press the pole piece out, and then press it back again when the field coil is repaired or rewound. I made some inquiries some years ago about this, and that is how I found out about it. I can't say all, but at least some machine shops have the equipment to do this. It would be worth making some local inquiries...
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Ron Ramirez
Ferdinand IN
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Hi Mark
Getting the pin out can be done. I had a 90 that I brute force drove out. It was either replace the speaker or give it a try. What was there to lose. I took the six screws out and unsoldered the wires from the transformer and removed the speaker cone. I then placed the frame on small pieces of 2x4s under the pin and used a ball pin hammer and also some key stock ( I think, it's been a while) to drive it out. Took some work to get it to move but it finally started to move a little at a time. Removed the old wire and wound a 3/4 pound of # 36 gauge enamel wire. A pound is a better bet. Should get you pretty close to the correct resistance. I cleaned up the pin and housing with emory cloth and it was kind of loose so I drilled a hole for a 1/4" bolt and tapped it on the back side. Put the coil on, inserted the pin and put the bolt and washers. Put on the cone and hooked it up. Worked great.
You may be able to rig something up with one of the old hyd floor jacks that has the round stock in them to press out the pin.
It takes time and you talk to yourself a lot but it worked for me. If you plan your moves out you will have no problems. I have a number of pictures of the whole process. Don't know if I can post them or not. Never done it before. Honestly what have you got to loose?
Terry
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Terry
If you can't post the photos, send them to me, and I will.
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Ron Ramirez
Ferdinand IN
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That photo is a big help. My field coil is more shredded than the one pictured. I've been pounding away on the field coil pin from the rear of the speaker but so far nothing has budged at all..... I did manage to smash my finger, though......
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Hi Mark,
It should come out with a lot of assistance! Did'nt think I would ever get it to move. Here is a very crude winder I setup. I am sure you can do better.
[Image: http://i220.photobucket.com/albums/dd137...ure046.jpg]
I used a variac to regulate my drill speed. When it got to one side I just pushed it the other way. I did it very slowly. Took about 45 minutes.
Not profesional but it worked.
Terry
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