COBRA 148GTL DX.......POWER PROBLEM.

[rickirst1]

I HAVE A COBRA 148GTLDX THAT WONT POWER ON.

THE PROBLEM CAME FROM A VARIABLE POWER MOD THAT WENT WRONG!
AFTER THE MOD THE RADIO SWITCHED ON, BUT WHEN THE OUTPUT POWER WAS INCREASED IT SHUT DOWN THE POWER SUPPLY AND WOULD NOT TURN BACK ON WHILE THE RADIO WAS STILL CONNECTED.

I HAVE TRIED ANOTHER POWER SUPPLY AND THE RADIO FLICKS ON FOR HALF A SECOND THEN OFF AGAIN.

WHEN POWER IS TO THE RADIO (POWER SWITCH OFF) THE VOLTAGE REGULATOR READS AS FOLLOWS:
LEAD 1=13.45v
LEAD 2=0
LEAD 3=13.93V . SAME VOLTAGE TO POWER SWITCH.

WHEN THE POWER SWITCH IS TURNED ON:REGULATOR READS:
LEAD 1=0.6v
LEAD 2=0
LEAD 3=1.2v SAME TO POWER SWITCH WHEN ON.

I HAVE CHANGED TR50 WHICH LOOKED BURNED ON THE LEADS.

CAN ANYONE HELP? IS IT THE REGULATOR?

THE BOARD IS A: PB010AA.

ALL HELP IS APPRECIATED.
THANKS

RICK.

[Rick]

Why the big text, dont shout we can hear you.
In sounds like the driver and final have gone short, try removing the test board at the back of the radio and just in front of the driver and final it is the plug in board connecting TP7, TP8 and TP9, then power the radio up and check the voltage at TR50. Let us know how you get on.

[rickirst1]

ive done as you said Rick, took out the test board and powered on no problem.
the voltage at TR50 reads:

lead 1= 5.49v
lead 2= 0
lead 3= 5.64v

is this as it should be? is it what you thought and if so is it just a matter of replacing the driver and final or should i do further checks before hand?

much appreciated
thanks

rick.

[Rick]

It look like TR50 and TR51 need to be replaced also. You may find it's just the final which as gone short. If I was doing the job I would replace Drive, Final, TR50 and TR51. But before you power it up make sure you do not have a short on the Drive and Final to Ground and reset the bias on the Driver and Final.

[rickirst1]

Hi rick im going to replace all four components as you recommended. however as you can probably sense, i have very limited experience so please be patient..
i can change the components but how do i reset the bias and how will i know if thers a short on the drive and final, what should i expect to see on the meter?? can you run me through the procedure please?

i appreciate your help.

thanks
rick.

[Rick]

You will need Heat Sink Compound also known as Heat Sink Grease

I will try to describe the procedure. If you do not understand then ask.
1. Remove one transistor at a time, and replace one at a time, doing this way will help you find any fault you have created.
2. The Final transistor is TR44 and should be a 2SC2312, you will find between the transistor and the chassis is insulation plate.
3. Before you replace the transistor you need to clean insulation plate and chassis removing all the old Heat Sink Compound and check that the insulation plate as no cracks and is in good condition also check the small insulation which go between the transistor and the nut which holds the transistor in place
4. Apply some new Heat Sink Compound to the insulation plate and the back of the transistor. Do not apply to much a light film is all you need.
5. When you have the transistor back in place, do not solder the legs back to the PCB board.
6. Get a Multimeter and set the meter for a resistance check, to check the Multimeter is working ok touch the Red and Black probes together and the Multimeter should read 00.0 if is then the Multimeter is ok.
7. With the Multimeter Red probe connected the centre leg of the transistor and with the Black probe connected the chassis you Multimeter should have the same reading as when the Multimeter was not connected. If the Multimeter shows a reading then you have a short.
8. If you have, a short repeat steps 3 to 7.
9. Use the same procedure for the Driver transistor TR45 (2SC2166) and TR51 (2SA1012).

Setting up the Driver and Final bias.
1. Set the radio to channel 20 MID band USB and TX mode, and the Mic Gain set to zero or minimum.
2. Get a Multimeter and set the meter for a Milliamps check.
3. Remove the test board at the back of the radio and just in front of the driver and final
4. Driver bias connects the Multimeter Red probe to TP9, connect the Multimeter Black probe to TP8, and adjust VR11 for 50ma on your Multimeter.
5. Final bias connects the Multimeter Red probe to TP9, connect the Multimeter Black probe to TP7, and adjust VR10 for 50ma on your Multimeter.

This CD may help in the future
http://www.geocities.com/open_channel/cobraproject.html

[captian_radio]

Hey Rick how you doing? To bad you didn't have a CD for the SS158EDX,I would be very interested it that.Happy new year my friend.
Bob

[rickirst1]

hi Rick, i changed the the regulator and the drive transistor and without the final in circuit i powered on the radio, immediately it blew the regulator in half (the transistor i used was a TIP32c which i was told this was an suitable replacement) the one it replaced was a BDX54C is this an higher powered transistor?
on the + the radio stays powered on now with the test board in place which it didn't before.

the replacement parts are:
2166 drive
2312 final
TIP32C regulator??? (R51)
b525 L9D (R50)

could this indicate another component failure ?

Thanks for your help

rick.

[Rick]

You must have a short circuit on the output side of the voltage regulator TR51.

Remove the voltage regulator TR51.
Set the meter for a resistance check.
With the Multimeter Red probe connected the centre solder pad where the TR51 was and with the Black probe connected the ground you Multimeter should have the same reading as when the Multimeter was not connected. If the Multimeter shows a reading then you have a short.
In would be best to test each solder pad, and let us know what readings you get.

[rickirst1]

hi Rick, there was a short on a capacitor just in front of TR51 which must have caused the problem so again i have changed all the components just to be sure... checking for shorts after every step,i weren't getting any readings on the meter, none were found!

The capacitor
TR50
TR51
DRIVE
FINAL
are all in place so i wired to my supply at the side of my other radio and im getting a carrier from the driver but no output? could the 2312 be suspect? what would you do next?

i thank you for your patience, it is appreciated.

thanks again
rick.

[Rick]

Have you replaced the Driver and Final?
Have you set the bias?
What was the bad capacitor ref number C?

[rickirst1]

hi Rick, the bad capacitor was C235 directly in front of TR51
i first changed TR51 then driver checking for shorts as i went. i reset the bias as you discribed...before i fitted the final i powered up the radio to see if the driver was functioning and it checked out with my other radio so i proceeded to fit the final after this the result was the same as in, it seemed to be working only on the driver?? although TR51 becomes extremely hot? not normal.

ive obviously caused a bigger problem than i thought!

thanks again.
rick.

[Rick]

Check R263 you will probably find it is open circuit.
Re-set the bias, if you replace R263.
The TR51 will get hot, but if you have used a non-standard transistor it maybe dissipating more heat.

[rickirst1]

hi Rick, sorry for the delay......i haven't had chance to do anything further with the radio as yet. as soon as i do, i will let you know how i get on.

thanks again.
rick.