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Maycom em27 CB Radio

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large_ghostman

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I have a dead Maycom EM27, it dosnt switch on although the LED lights for the back lighting on it. It pulls around 6A when you turn it on so I am thinking its shorted somewhere, I am thinking the marked diode is a likely candidate for the problem. Anyone got any other ideas before i go screwdriver happy? I dont have access to a camera until Monday I will take some pics then.

diode.JPG


The full schematic is too big to post but can be seen here **broken link removed**
 
A good start.
JimB
 
Hi LG,
I think that diode is designed to blow the input fuse (Assuming one has been fitted in the supply to the unit.). I think L300 will have a VERY low resistance and if D103 was short circuit I think it would draw more than 6 amps. Check the resistance of L300 to see if it is high enough to limit the current to 6 amps with D103 short circuit. (If D103 has gone short circuit it is very likely to have a very low resistance.)

Les.
 
Ok cheers, I will have it apart later :D
 
I've just noticed that I did not finish the first sentence in my last post It should be "I think that diode is designed to blow the input fuse (Assuming one has been fitted in the supply to the unit.) if the power is connected with the polarity wrong."

Les.
 
There is no fuse, I have a feeling this has been reversed because there is plastic on the side of the metal socket like you get by forcing a plug!! Funny how further down the food chain you go in RF the more stupid the mistakes! I picked the diode because it seemed obvious, the radio might have had it but its good practice.
 
Hi LG,
It still might be worth checking the diode. If the polarity was reversed with it connected to a supply that could supply a lot of current then the diode could have first gone short circuit but then blown open circuit if the current was very large. A 1 amp diode is a bit small for this type of protection. If the diode is open circuit then the unit is probably not worth trying to fix. If it is short circuit (Or still behaves as a diode.) then it is probably worth looking further for the cause of the high current.

Les.
 
I am a bit puzzled why no fuse but a little diode?? Seems a bit odd, the radio guts are used in many different transceivers apparently, so if I can fix this then around 8 other models are basically the same insides :D. This is also the radio i am going to modify (well same model), I cant do anything to it until later as I am on little sister duty :D.
 
Generally speaking, on the CB radios that I have encountered, the fuse is in-line on the power cable.
Yes except this one didnt have one so i assume it was used without one.
 
There is no fuse, I have a feeling this has been reversed because there is plastic on the side of the metal socket like you get by forcing a plug!! Funny how further down the food chain you go in RF the more stupid the mistakes! I picked the diode because it seemed obvious, the radio might have had it but its good practice.

The fuse is in the lead feeding the radio - as is normal on car radio equipment.

If it's been removed (by some idiot) then you need to refit it.

However, assuming this is part of your ongoing effort to convert a CB to 10m, then it seems pretty pointless, as a CB radio won't provide the correct mode for 10m use, and it's specific special PLL chip won't work either.
 
The fuse is in the lead feeding the radio - as is normal on car radio equipment.

If it's been removed (by some idiot) then you need to refit it.

However, assuming this is part of your ongoing effort to convert a CB to 10m, then it seems pretty pointless, as a CB radio won't provide the correct mode for 10m use, and it's specific special PLL chip won't work either.
No these innerds were designed for europe and other uses, you can just about get into the 10m band with them, its well documented on the net with these particular units. There is a series of jumpers inside designed to give you more channels etc.
 
a CB radio won't provide the correct mode for 10m use
What is the correct mode?
You may not be aware, but FM is used on the 10 Metre amateur band.

it's specific special PLL chip won't work either
The original "UK legal" cb radios had a fixed synthesiser which could not be easily reconfigured.
However by using an add-on circuit board, they could be modified up into the 10m band.
If I remember correctly, the add on simply subtracted the appropriate number of cycles in the PLL and voila! the thing locked on the new required frequency.

However, things have moved on a bit since then and the newer generations of cb radios are configurable for various national standards.

JimB
 
This transceiver also has AM as an option on the jumpers, considering its age its fairly neat what it can do, apparently the same board is used inside the president Lincoln CB (woohoo).
 
Now it gets a bit more tricky! The diode was shorted and is now replaced, radio still dead but I found a transistor that had been desoldered and replaced the wrong way around, it is also dead but I have an equivalent in the junk box. I also noticed a few dry joints and the backup battery is dead and needs replacing.

Maybe not technically worth fixing except it is good learning if I can fix from the junk box, also if I get it fixed then I can do the modifications on this one first without risking the good one.
When I get home tomorrow I will sit down and have another go at it, things have just got interesting though :D, I will also try and grab the camera tomorrow to take some pictures.
 
If you look at the schematic you will see that there are several supplies, each with their own regulator.

Ensure that IC300 and IC303 are giving the correct outputs.

The radio is completely controlled by a microprocessor, so that must have power (Backup B+ from IC300) before the radio can be switched on.
The power switch is a digital input directly into the microprocessor.

You also say that the back-up battery is dead, please tell me where it is on the schematic as I cannot find it.

JimB
 
If you look at the schematic you will see that there are several supplies, each with their own regulator.

Ensure that IC300 and IC303 are giving the correct outputs.

The radio is completely controlled by a microprocessor, so that must have power (Backup B+ from IC300) before the radio can be switched on.
The power switch is a digital input directly into the microprocessor.

You also say that the back-up battery is dead, please tell me where it is on the schematic as I cannot find it.

JimB
W9w I was worried it was only me that couldnt find it!! The battery isnt on the schematic that I can see! But apparently the older orange scree ones like mine had a bad battery, this was changed to a better one or some like the green screens had a super cap. The net is full of places detailing how to change them but I havnt seen one on a schematic, I am wondering if the drawings are all later and the cap is shown.

I missed a regulator!! I will go back and check later when I get home again. That little processor sure does alot, shame the source code isnt around :D
 
I will check the radio when I get in but I think it might be the cap marked C211 coming off of pin10 IC 201 right hand side top schematic. I assume it connects to one of the IC pins somewhere, when I get back I will go through data sheets and see what the pins do.

I can still only see 2 regulators!! I need a better screen :p
 
Last edited:
C211 is just a decoupling capacitor.
JimB
 
I cant see anything on the drawing thats likely to be a back up source, I will see what value these are meant to be. The drawing says red led as well and none of the versions have a red led inside the radio :D.
 
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