Motorola CM200 Blows Fuses

[anniegirl]

New to forum here, I have a CM200 that blows fuses as soon as pos/neg is connected. Ohm meter shows a complete short across the positive and negative. Would this be from a protection diode issue. The board has a VR500 diode and I am thinking if a previous owner connected the leads incorrectly the diode got toasted. It looks impossible to change out? Should I look else where ...help needed.

[Bill_G]

I haven't been in a CM200, but generally Motorola used axial lead diodes across the power mains. So, if you don't think you can unsolder it from the board, you might be able to cut its leads.

[Satelite]

Hello:
Im not familiar with the CM200 radios circuit bd but have repaired many m radios with the issue you describe.
If in fact the protective diode is blown it normaly does short out if not completely blows up if short was that severe.
The purpose of the protective diode is to absorb problem voltage issues and blow the inline fuse and hopefully protect the radios components.
This doesn't always happen in all cases but it is a huge help in doing so most cases.
If by chance the protective diode is shorted then id ohm across it from each side and you shouldn't see a direct short but if you do then read next and then ill give you an idea as test and possibly get radio to power up and working again.
The protective diode when good and in circuit does not transfer any power through it and only when a voltage issue occurs to help blow inline fuse to drop power so no firther damage or fire from hot wires etc.
So if p diode is indeed bad then and im assuming the p diode is a black round hard bakelite plastic material here in the CM200 radio and if it is id take a needle nose plier and crush the round material and remove all the crushed p diodes bakelite as its now conductive causing the short from the nd and do clean the dust out as well leaving the two wires that were in the p diode clear of each other not touching anything.
This will return radio to operating operation IF THE LOGIC BD FUSE ISNT ALSO GONE BAD.
If radio still doesn't power up but stops blowing fuses then take a look at the cm200 logic bds fuse and replace it or for testing only jumper it to see if radio powers up.
Or better yet with radio turned on take a volt meter and see if 12v is on each side of the fuse which means fuse is good if not replace fuse.
Id replace it with a 3 amp pico fuse which you can easily buy and find on ebay.
Id also replace the P Diode as well to protect radio just in case something happens again.
Im not familiar with the cm200 radios p diode specs but if you replace it with one from a maxtrac 300 id think youd be good to go without looking up the specs on the cm200 radio to compare with a maxtracs p diode but I cant since I do not have a cm200 repair manual.
Ill say this but im not recommending you do this = You can run the cm200 radio without the p diode BUT IF YOU HAVE A SHORT OR REVERSED POLARITY AGAIN AND NO P DIODE TO PROTECT THE RADIO > Then youll be very unhappy with the next repair if and I do mean if you can repair the bd from being fire black torched from heat and or find lord knows what else got fried in the process.
So yeah test it without p diode but id strongly recommend replacing it.
Good Luck And Let Us Know How You Turn Out With This.
Satelite

[SteveC0625]

CM200/300 & PM400 Series UHF detailed service manual: https://drive.google.com/file/d/0B3k529 ... sp=sharing

CM200/300 & PM400 Series VHF detailed service manual: https://drive.google.com/file/d/0B3k529 ... sp=sharing

CM200/300 & PM400 Series Basic Service Manual: https://drive.google.com/file/d/0B3k529 ... sp=sharing

[anniegirl]

Thank you so much. I got refunded the money I paid for the radio and was told to keep it so I haven't nothing to loose here. Now that you explain it for sure that's what is going on. I had already researched the manual and learned of the VR500 Diode. I have a two bad CM300's but they powers up so I have some to tinker with. I just pulled the diode off the board and now it too has the direct short. I will try your suggestion on the other one to see if it works out. I still can't understand how it could even be replaced. Its older brother the M1225 had simple protection diodes that could be disordered from the board. I WILL TRY YOUR SUGGESTION TONIGHT AND LET YOU KNOW, Thank you so much my friend 73's KD9DIA

[anniegirl]

This VR500 Protection Diode is nothing like the ones in other Moto's like the M1225. I had another radio lying around for parts because it had no TX power but it powered on After I pulled the diode off the board it to now has the same issue showing a short between the positive and negative, This is not like a diode that I have seen, I can't even see how it could be replaced. The manual describes this as the protection diode for reverse power connections problems BUT HOW IS IT REPLACED?

[anniegirl]

With all the Motorola CM products being used I can't believe there are not any posts regarding replacing the VR500 Protection Diodes. The detailed manual describes it as a protection when the polarity is connected backwards. The diode will ground and make the fuse inline blow BUT when there is no fuse in line the diode is destroyed leaving the circuit grounded. The pos and neg terminals are then shorted. I understand the theory and hopefully it has saved this radio. The BIG QUESTION is how does one replace the VR500 Protection Diode. It appears to be soldered on top the board and then a pin underneath the diode appears to be soldered? Has anyone replaced one of these?

[anniegirl]

I have had a chance to get inside other Motorola Mobiles like the CM200. CM300 MCS2000. They all have the VR500 protection diode. I have discovered this diode will go to ground if reverse polarity is applied to the power leads. If there is a fuse inline it will blow the fuse no harm no foul. However when a knuckle head applies reverse polarity without a fuse inline the VR500 goes to ground permanently therefore needing to be replaced. This diode DOES NOT HAVE axial leads. I have on order 5 of them but it looks like a regular soldering iron won't do the job. I found a site that suggest an air soldering gun to flow the solder contacts. The diode is solder on top which is easy to get at but the contact underneath it is hard to reach. I was hoping to get help here on how to replace it. I now have on the bench a couple parts radios to experiment with but I have on order the suggested hot air soldering equipment for which I have no experience with. No, I am not blond

[anniegirl]

I finally got this radio going after I removed the protection diode that was grounding out the positive. The VR500 Diode is in many Motorola radios. In my travels I picked up several dead radios and never figured out why they would not power up. And here was the answer a blown protection diode. I used a mini soldering iron and a hot air soldering machine to get this VR500 off the board, it is not easy to remove. If you just want to get the radio going you can break the grounding by using a Drexel and cut the two ears off that are on the board. Doing this tho will also defeat the protection should someone mix the positive and negative power leads. They only cost $4.00 and it saves a radio. The next time I run into this I think that I will try installing a protection diode that has axial leads. Hmmm?

[Satelite]

Hello anniegirl
your idea of replacing the VR500 protection diode wirth a axial leads with proper rating on the specs isn't a hmmmm ?
Its a heck yeah its the kiss rule - keep it simple stupid
It will work just fine.
Do cut the leads leaving them available for the new diode if it saves you time from having to disassemble and its quicker anyway with out heating the circuit bd conductive pads twice so you only quickly solder once so you don't risk burning loose the circuit path by heating too much .
Now that you have accomplished a radio repair youll get bit buy the bug and want to do more and more as youll likely enjoy it and eventualy wont be asking for much info if any at all down the road with the gained experience and knowledge of repair that follows many.
How Ever Im Just Cautioning You !
THERES NO HELP IF YOU GET BIT BY THE RADIO BUG CUZ THERES NO ANTIDOTE SO YOU GRAB AND FIX EVERY RADIO YOU CAN THEN NEXT YOULL BE WANTING TO DO MODS AND THEN OUT BAND FIXES AND THEN ITS HEX EDITING BURNING EPROMS !
Never ends you just want more = RADIO JUNKIE ADDICTION.
Its more radios or lotsa BUD LIGHT or drop dead to get over it
Just be prepared to admit eventualy I AM A RADIO NUT
Satelite