Motorola 10T28M Radio

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I own a Motorola 10T28M radio. I’ve had it for a couple of years. I might have paid $10 for it a junk sale somewhere in town.

It’s been working pretty well, but the tuner cord broke a few weeks ago and I finally got around to fixing it. Rather than using cord, a previous owner used fishing line, that’s not going to last long.

While I was in there repairing the tuner cord, I also re-soldered some dry connections.

After I got it working again, I decided to do a little research about it.

I found a website RadioMuseum.org with some details about the Motorola 10T28M here.

Apparently this AM/FM 10-tube radio was made in the late 1950s. Mostly likely it was built in 1957 or 1958.

It’s not particularly valuable, but it’s a nice addition to my home audio system.

My setup works like this.

From my studio in the basement I broadcast via FM whatever is going through my mixer.

So rather than trying to blast the music all of the way up to the second floor, I have vintage radios scattered around the house tuned to my FM frequency.

Most of these older radios are just that – radios. They aren’t stereo and they usually always lack good bass, at least the cheap ones I’ve bought do.

In my house, the music playing from my studio in the basement reverberates through the house and helps fill out at the low frequencies.

My Crossover Design is Complete

Today I finished designing my crossover. Here’s what I have come up with.

I have to give a special thanks to DIY Audio and Video.

I’m building a speaker for an old radio that, in addition to a new loudspeaker, I’m creating a cool new case. It’s not a stereo, so I just need one loudspeaker.

Here’s what I’m using for speakers.

A Dayton 8-inch 100-watt woofer. You can check it out here. The specs for this woofer are here.

The tweeter is a 100-watt Goldwood GT-324. You can check it out here. You can read the specs for this tweeter here.

The woofer is an 8 ohm speaker with a frequency range from 29 Hz to 3,000 Hz. The tweeter is also 8 ohm, but with a range of 2,000 Hz to 20,000 Hz.

The woofer’s maximum decibels is 88 db and the tweeter’s is 96 db. Using an l-pad circuit, I plan to pull that tweeter down to 88 db to match the woofer.

And to calculate a proper Series Notch Filter, I need the free air resonance of the tweeter, which 1,800 Hz.

The Crossovers

So based on this tutorial, I came up with the following two crossovers.

For the tweeter, I’m going to create a 3rd order Butterworth 800 Hz crossover. For the woofer, I’m going to build 1st order Butterworth 3,000 Hz.

Those are the crossovers, now I need a Loss Pad circuit. A l-pad circuit, or driver attenuation circuit, is supposed to decrease the output levels of the louder speakers to match the output of the more quiet speaker.

In my case, the tweeter outputs up to 96 db and the woofer is 88 db, so I need to pull the tweeter output down 6 db.

A l-pad is two resistors, one in series and one in parallel.

Using the fabulous DIY Audio and Video l-pad calculator, here’s the circuit for the tweeter. I entered the values 6 db, 100 watts and 8 ohms.

Parts List


Resistors
R1 = 3.99 Ohms   49.88 Watts
R2 = 8.04 Ohms   25 Watts

 

The last thing to do is design the Series Notch Filter.

I’ll be honest, I’m not sure exactly what this one is for, but the purpose of this filter is to block frequencies within a certain range, letting through all others.

To build the series notch filter, I’m using the free air resonance (Fs) of the tweeter, which is 1.8 kHz.

Using the awesome Series Notch Filter Calculator at DIY Audio and Video, I was able to determine the necessary components.

Re = 8 Ohms

fs = 1800 Hz

Parts List


Capacitor
C = 16.68 uF
Inductor
L = 0.42 mH
Resistor
Rc = 8 Ohms

The last step to do, which I haven’t done yet, is to combine all of these circuits into one. I’m going to sketch it out on paper. Then I’ll pull my parts and prototype it on breadboard.

Stay tuned.

Parts List

  • 3rd Order Butterworth (Tweeter)
    • C1 16.58 uF
    • C2 49.73 uF
    • C3 33.16 uF
    • L1 1.19 mH
    • L2 2.39 mH
    • L3 .8 mH
  • 1st Order Butterworth (Woofer)
    • C4 6.63 uF
    • L4 0.42 mH
  • L-Pad
    • R1 3.99 Ohm 49.88 watts
    • R2 8.04 Ohm 25 watts
  • Notch
    • C5 16.68 uF
    • L5 0.42 mH
    • R3 8 Ohms

Tip Jar

If you think what I’m publishing here is worth more than nothing and you want to see me keep publishing, leave a tip.

When I bartend, I get a buck for pouring someone a beer, the shit I’m doing here is worth at least a dollar.

And no, I don’t want to sell ads.

Great Site to Learn About Loudspeaker Crossover Design

For the past week or so I’ve been tinkering around with designing a crossover for a two driver loudspeaker. I’ve never done this before, so the learning curve is steep for me.

The first step was to figure why I needed to make a crossover and what it’s supposed to do.

The best place I found to get your brain wet with crossover knowledge, is right here with the Speaker Crossover Wiring FAQ.

Basically, every speaker is rated to work at certain frequencies. If you push higher or lower frequencies than the driver can handle, you’ll blow it up.

Once I finished the FAQ, I found this really useful example here.

This example, I believe will get me 90 percent of the way to where I need to get with my crossover circuit. The rest will be up to me.

My drivers are an 8-inch Dayton woofer. This is an 8 ohm speaker with a frequency range from 29 – 3,000 Hz, Fs (free air resonance) 88Hz, and max 88 db.

The tweeter is also 8 ohm with a frequency range from 2,000 Hz – 20,000 Hz. But I need to learn more about this tweeter. I need to figure out Fs and the max db.

So my crossover point is from 2,000 Hz (tweeter) to 3,000 Hz (woofer).

The max db output will help me build the l-pad of the circuit and the Fs on the tweeter will guide me to the correct series notch filter configuration.

I’ll keep posting my progress. Once I figure out these last pieces of the puzzle, I’ll make a parts list and gather it all. I should have all of the capacitors and resistors, I just will need to make some inductors, but that should be easy.

Tip Jar

If you think what I’m publishing here is worth more than nothing and you want to see me keep publishing, leave a tip.

When I bartend, I get a buck for pouring someone a beer, the shit I’m doing here is worth at least a dollar.

And no, I don’t want to sell ads.

More useful links that I’ve found are:

Air Core Inductor Inductance Calculator

I don’t plan on using this loudspeaker test box, but it looks really interesting.

There’s another introduction to crosssovers here.