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Sunday, October 9, 2016

EF Johnson 242-5876 2-Way Radio Teardown

        Well, this teardown has been in the making for some time now. I'm not particularly big on RF stuff, but I had a hunch that this thing would be nicely engineered and built on the inside. I was expecting a lot of analog goodness mixed with some digital control.
And what do you know.....I was right.


      Now, I've searched high and low for a manual to this thing, but I wasn't able to find anything. And when I say "anything" I mean absolutely nothing relating to that model number, which kind of lead me to believe that these things (of which I have two in my possession) might be a custom  police or military issue (police most likely).

     If anyone has a user manual, or better yet, a service manual, please leave a comment. I'd really like to see how this thing ticks.

Most of the hand-held stuff I came across from EF Johnson seems to be operating on the 800-900 MHz range, but this thing  seems to work on the 80-100 MHz range. I did a quick FFT on my Rigol 1054 and there seems to be a nice big spike at about 88 MHz when I press the transmit button. Also, the FM receiver chip is spec'd for up to 200 MHz.

However, this should be taken with a grain of salt, because these things require a network infrastructure to work. So without that network, I can't say for sure that that spike I see isn't just a signalling band or something.

Ok, enough talk. Let's get into the meat of it.


This thing is ancient. How I know this? It uses Ni-Cd Batteries. 7.5V, 1.4 Ah. And yes, they still take a charge, but that might be because at some point in time someone sawed off the back of the battery pack and stuck new batteries in it.


Taking the back off, you can immediately see this thing resembles something that's MIL-spec. A nice thin cast magnesium (probably) shielding with a ton of screws.


Lifting up  the skirt, this thing really looks packed full of parts. (Yes, I know, I have to get a better camera.)
The two 14 pin chips are a Motorola MC14066B Quad Analog Switch (the one on the left) and a MC14094B 8 stage shift register with 3-state outputs.
The smaller 8 pin one is an MC939  divide by 6 counter (I think...)


The face together with the keypad came right off, revealing another shield, also cast magnesium, together with a boatload of adjustment pots and caps.


On the back of the face, there are two boards stacked together and which connect to the rest of the unit only through that pin header. Also, notice the nice o-ring  seal (OK, not really O-shaped, but you get the picture). A bit of it is peaking out in the lower left corner.
There are a few goodies on these boards, but let's leave it to the side for now and take a look at what;s under that other shield.


Et Voila! This is the good stuff. A whole myriad of analog and digital goodness, just waiting to be reverse-engineered. Not by me, mind you. I'm not into this kind of stuff, remember?
And, because I have a shitty camera, I tried to make things easier, hence the red markings.
So. from the top left corner, we have the MC3362DW  FM receiver chip. Now the datasheet says it works up to 200 MHz, and this is consistent with what I saw on my scope's FFT.

On the right, we have the MC145158-2 which is a serial input PLL sinthesizer.  The two  PCF8574  probably shift data to and from the two vertical boards. Unfortunately, I couldn't read the part numbers  off the ICs because there was no angle I found that allowed me to do so.

What I plan to do  is if and when I get a hold of some manuals for this specific model, I'll  do another teardown sometime and desolder these 2 boards and have a proper look at what's happening in here. Also, I want to poke around some stuff in there with the ol' 1054 scope. Some LO's, some I2C buses... stuff like that. Who knows, maybe I'll even get to learn more about the RF side of things.

Ok, so now let's get back to the boards on the faceplate.


     So, first thing to notice is the big chip to the left of the speaker. That's the PCF8576T driving the LCD on the front. And, of course, it's controlled via I2C. To the left of that, there is a keyboard  controller, the MM74C923. It can encode up to 20 keys, which is  5 more than what the faceplate has. "What a waste" I hear you say?
Well, yeah, but being the smart engineers that they are, they probably read the datasheet and it looks like if you're decoding a 5 by 4 button matrix, then the 923 is the way to go.

Here's a funny thought....maybe a Rigol engineer will somehow make his/her (everybody deserves to be an engineer) way to this blog and decide to turn over a new leaf and actually start reading some datasheets. They're free to download, people... just so you know.
OK, rant over.

Just above the 923 controller, there's a 14-pin X24C16  16Kbit (2048x8) EEPROM, with an I2C interface, as one would expect. Chalking this one also on the list to probe, for the next teardown.

Now, going to the other board, we have the big chip to the left. It's made by Motorola, just like  most of the other chips in these units, except my Google-Fu can't come up with a part number. Most likely it's a custom part...hence the label on it.

Also, the buttons on the side  seem to match the ruggedness of the unit. Despite how mucky the bottom transmit button looks, it's still got some life in long as you press it juuust right. It's one of those Goldylocks things. not too hard, not too light of a press....

And, after putting it all together again, what do you know....It still works

All in all. I'm pretty impressed how this thing was built. You gotta love the good ol' days. 
If and when I get hold of some more information about this radio, Ill try and play around with it a bit more, maybe even connect it up to a PC and see if I can modify something in the firmware. 

This link has some info on how the remote interface cable and about the PC software.

I'm also going to do a mini-teardown of the charger, just to see what's in there. I have 2 kinds of chargers: one that plugs in directly into an AC outlet and another that takes in 18 V DC, so stay tuned for that as well.

Full teardown pictures are available on my Picasa Google Photos page.

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