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 Post subject: Philco 37-38
PostPosted: Mar Sat 23, 2019 8:00 pm 
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Joined: Nov Sun 20, 2011 1:10 am
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Location: South Carolina
I am starting a new project Philco 37-38. The electrolytic cap is a 4-2-2 multi section can. What should I replace this with?

Any tips about battery powered farm radios. I plan on powering it with a pine board power supply.


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 Post subject: Re: Philco 37-38
PostPosted: Mar Sat 23, 2019 10:25 pm 
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Location: Austin, Texas
The A voltage of 2 volts needs to be correct and with very low ripple. It's best to have a series pass type regulator for that voltage.

Jay


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 Post subject: Re: Philco 37-38
PostPosted: Mar Sat 30, 2019 9:49 pm 
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Joined: Nov Sun 20, 2011 1:10 am
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Location: South Carolina
I am trying to wire this radio to a ces pine board power supply. I understand the A power
will have to be reduced to 2 volts. This supply has A+,A-, B-, (B+ in +22,+45,+67,+90,+135) and
C-,C+. My question is how can one wire go to C+ and B-, and Two different wires for
C- 3v and C- 9v. How do I wire that to a single C- on the power supply.

I used one of these power supplies on a old Atwater Kent. Battery radio and everything
came out fine.

I attached the schematic.


Thanks


Attachments:
M0013204.pdf [218.41 KiB]
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 Post subject: Re: Philco 37-38
PostPosted: Mar Sat 30, 2019 10:53 pm 
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Location: Mountains of Mourne. Ireland.
Two ways to solve the combined B+/C- bias supply...
viewtopic.php?p=647633#p647633
Image
Greg.


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 Post subject: Re: Philco 37-38
PostPosted: Apr Mon 01, 2019 11:48 pm 
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Location: South Carolina
How about the C+/B- black with a white trace?


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 Post subject: Re: Philco 37-38
PostPosted: Apr Tue 02, 2019 12:47 am 
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Image
Someone will have to verify this!
:) Greg.


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 Post subject: Re: Philco 37-38
PostPosted: Apr Tue 02, 2019 2:11 am 
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Current loads...
https://www.antiqueradios.com/forums/vi ... 7#p2399337

Greg.


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 Post subject: Re: Philco 37-38
PostPosted: Apr Fri 05, 2019 9:49 pm 
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Location: South Carolina
If I understand this right I can use my power supply for the A+,A- (2v) and
The +90 (B+), +135 (B+). With nothing connected to the B- on the power
Supply. Then use 3v batteries in series as shown above for the 3 and 9 volts (c)?


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 Post subject: Re: Philco 37-38
PostPosted: Apr Sat 06, 2019 12:06 am 
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Would I have to connect a wire to the B- of the power supply to the B negative of the batteries?


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 Post subject: Re: Philco 37-38
PostPosted: Apr Sat 06, 2019 3:03 am 
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Read this thread for Philco 38-38 and see the schematic.
Take note of Burdawg's comment about the 900ohm resistor part number 38 on his schematic - - part number 40 on your schematic.

Post subject: philco 38/38 p/s hook-up viewtopic.php?p=261456#p261456

See if you can understand and follow his modification instructions for the AES power supply.

Ask lots of questions. Someone more knowledgeable than me may answer them.

Greg.

Philco 38-38→ http://www.nostalgiaair.org/pagesbymode ... 013248.pdf


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 Post subject: Re: Philco 37-38
PostPosted: Apr Sat 06, 2019 3:23 am 
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For the benefit of others...
Image
Image
Image

Greg.


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 Post subject: Re: Philco 37-38
PostPosted: Apr Sat 06, 2019 9:01 pm 
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Connect B- and C+ to this blue line shown on the schematic.
Image

------------

Burdawg wrote: Just replace the 50k (R7) pot with a voltage divider network of 6.8k, 13k, and 30k (in that order) wired in series from the c+ terminal to the low side of the c supply. The network will still be about 50k so the load on the c supply will be about the same. Connect the c+ terminal to b- to reference the c supply to b-. Then you will have -3v at the junction of the 6.8k and 13k resistors, and -9v at the junction of the 13k and 30k resistors, referenced to c+. I said earlier that I also had -12v but forgot that I eliminated that to make it simpler. You could always play around with the values to get other c voltages if needed.<BR>Remember to disconnect that 900ohm resistor or it will load the supply and the voltages won't be right.
viewtopic.php?p=261458#p261458


Greg.


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 Post subject: Re: Philco 37-38
PostPosted: Apr Wed 10, 2019 2:12 am 
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Location: South Carolina
I tried using the CES power supply for A-,A+ (set at 2 volts),B+ 135v and 90v, C- (set at 3 volts), a 9v battery for
The -9v connected to C+,B- (black wire, white trace) and C- (Green wire). I jumpered C+ And B-.
So far I can’t even get the tubes to light up. I read somewhere on this site that this radio has
To have a good electrolytic so I started recapping tonight.

Thanks for the help.


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 Post subject: Re: Philco 37-38
PostPosted: Apr Wed 10, 2019 4:40 am 
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Location: Lexington, KY USA
The tubes lighting up should be pretty independent of everything else. Note that they normally don't get real bright. (Turn out the room lights in order to be sure of seeing the filaments.)

What was your A voltage at the tubes? You do want to set that before connecting the radio to the power supply, since the A supply can be set far too high for the tube filaments.

If your power supply is to be used only with this radio, it might be good to add a fixed resistor across the A voltage adjustment pot to limit the maximum to something like 2.25V.

Another mod to the A supply might be a dropping resistor ahead of the filter capacitor. This would reduce the power the regulator has to handle. An ohm or so might be about right.

It might be a good idea to add series string of diodes across the A terminals in the radio to limit overvoltage. Four regular 1A or 3A diodes in series, or three regular plus one shottky diode.

If I understand the radio schematic, the 9V C minus input is loaded by the 900 ohm resistor, so a high impedance source won't work for that voltage.

I have to ask, why did Philco do that? They even provide switch contacts to turn off the load resistor when the set is off. The only reason that comes to mind is it would increase sales of C batteries. I wonder if the 900 ohm resistor might safely be disconnected.

You will want to verify all the "battery" voltages at points inside the radio.

Ted


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 Post subject: Re: Philco 37-38
PostPosted: Apr Wed 10, 2019 11:58 am 
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Did you Remove the 900Ω resistor?
as mentioned here→ viewtopic.php?p=261456#p261456

You need to remove R7 (the 50K pot) completely, and replace it with the three resistors in series, as recommended by Burdawg, and pick up your -3V and -9V from their connections.
Image

Greg.
oh... the 3V and 45V batteries shown earlier where for illustrative purposes only.


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 Post subject: Re: Philco 37-38
PostPosted: Apr Wed 10, 2019 9:22 pm 
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Location: South Carolina
What is the wattage on these resistors?


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 Post subject: Re: Philco 37-38
PostPosted: Apr Wed 10, 2019 9:41 pm 
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As noted on the schematic - - Use 1/2W resistors.

Greg.


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 Post subject: Re: Philco 37-38
PostPosted: Apr Wed 10, 2019 10:09 pm 
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There should be about 10mW dissipated by all three resistors together. So you could use 50mV, 30V parts, that come in the 0201 package.

However, it might be better to choose something with leads that's big enough to see, rather than just big enough electrically. Something like 1/4 or 1/2 watt would be fine.

Ted


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 Post subject: Re: Philco 37-38
PostPosted: Apr Wed 10, 2019 11:31 pm 
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Location: South Carolina
I guess 1/4 watts won’t work. I have box full of them in all size. I don’t have 1/2 watts.
I may have to order some. Can a 15k be used in place of a 13k?


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 Post subject: Re: Philco 37-38
PostPosted: Apr Wed 10, 2019 11:58 pm 
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Just clip them together and take a few measurements. The 1/4W will be OK.
The value of the resistor determines the Voltage drop across it.

The values given are probably on-the-money.
Experiment and see what you get... :)

Greg.


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