Cobra 1000 dead, then alive, now dead

[dozerman]

motorboat sound means unregulated DC coming right off the bridge rectifier giving you about 23 volts fluctuating up and down (which is why you have the huge deadkey)

do not run the radio from that power supply anymore until you get it fixed.

disconnect its DC output from the main PC board and check the voltage regulator(s) on the power supply board.

that 23 volts might also be what blew that diode, but thats just a guess.

LC

It's on the plug in supply now. Last night I was listening in on the local channel and everything was fine with it. I went to another channel and checked my key and pep and all was fine. Turn it on a minute ago and my rx is gone. Well I say gone, but thru the speaker I can hear everything. But very very diminished. I can hear the volume control work from minimum to max. Rf gain works as well. Does it on both speaker outputs. Going to check that out in a few. Gotta get back on building the wife's cabinets. She's about to take my quarters away and I can't talk on the radio then lol

 

[LeapFrog]

Does public address or transmit audio work (now) on the external D.C. supply?
If you've accidentally over-volted the radio, the audio I.C. can be one of the first things to fail.

 

[loosecannon]

there's only one reason that you would have a 9 watt deadkey on the internal supply, and a 5 watt deadkey on the external supply (unless that external supply is 2 amps or less), and that is a greater voltage getting to the collectors of the driver and final.

also a motoboat sound is a clear indication of bad voltage regulation.

now it sounds like you have blown your audio amp or the audio transformer with that over voltage condition.

something doesn't add up here.
how sure are you that you are measuring the output of your internal supply correctly?
LC

 

[nomadradio]

If the internal power supply's regulator is "running away" and pumping 20-plus Volts into it the radio will cure itself of all its other problems before too long.

Cure, as in blowing out multiple parts on the main circuit board.

Failing electrolytic capacitors are know to fail, and then "heal" an internal short after the power is removed for a while. When a shorted capacitor shuts it down, the radio gets powered off. The problem will appear to have fixed itself when turned on the next time. But never for very long.

73

 

[dozerman]

Does public address or transmit audio work (now) on the external D.C. supply?
If you've accidentally over-volted the radio, the audio I.C. can be one of the first things to fail.

Yep, the PA works as well as everything on the radio with an external 13.8v supply. Except the clock. Over the air, all the locals say it sounds great.

 

[dozerman]

there's only one reason that you would have a 9 watt deadkey on the internal supply, and a 5 watt deadkey on the external supply (unless that external supply is 2 amps or less), and that is a greater voltage getting to the collectors of the driver and final.

also a motoboat sound is a clear indication of bad voltage regulation.

now it sounds like you have blown your audio amp or the audio transformer with that over voltage condition.

something doesn't add up here.
how sure are you that you are measuring the output of your internal supply correctly?
LC

Well, it may not be the right way but, I can measure voltage at the fuse and get about 22 volts. I can go from there and get 19-22 volts on the board that the internal supply goes to. The 4

there's only one reason that you would have a 9 watt deadkey on the internal supply, and a 5 watt deadkey on the external supply (unless that external supply is 2 amps or less), and that is a greater voltage getting to the collectors of the driver and final.

also a motoboat sound is a clear indication of bad voltage regulation.

now it sounds like you have blown your audio amp or the audio transformer with that over voltage condition.

something doesn't add up here.
how sure are you that you are measuring the output of your internal supply correctly?
LC

I can measure voltage at the fuse and it's 22.3v. On the board inside that the transformer feeds, I get 20.5 +\- on the two regulators as well as two of the diodes across the board from them.

 

[dozerman]

View attachment 22076

Well, it may not be the right way but, I can measure voltage at the fuse and get about 22 volts. I can go from there and get 19-22 volts on the board that the internal supply goes to. The 4


I can measure voltage at the fuse and it's 22.3v. On the board inside that the transformer feeds, I get 20.5 +\- on the two regulators as well as two of the diodes across the board from them.

Don't know why that first response is in there, supposed to have been deleted. I might also add, the VR for the internal voltage, will measure 9 volts on two of its legs

 

[loosecannon]

measure the voltage at the trace (on the power supply PC board) where TR201 (emitter leg) R204, and C206 all meet.

TR201 is the 2SC1061 transistor and it is a TO-220 package so it will look like your driver and final transistors in size.

if this measures up at the 20 or so volts that i think it will, then you have a bad regulator or support component on that board.

running the radio on the internal supply at that voltage level will destroy your radio.

also, this radio sends that voltage to the digital clock which im going to guess is disabled when using the external supply. so, don't plug the radio into the wall just to have the clock on either, as you will destroy it too.

obviously you can just unplug the AC cord and run the radio from the external supply if that is satisfactory to you.
LC

 

[dozerman]

measure the voltage at the trace (on the power supply PC board) where TR201 (emitter leg) R204, and C206 all meet.

TR201 is the 2SC1061 transistor and it is a TO-220 package so it will look like your driver and final transistors in size.

if this measures up at the 20 or so volts that i think it will, then you have a bad regulator or support component on that board.

running the radio on the internal supply at that voltage level will destroy your radio.

also, this radio sends that voltage to the digital clock which im going to guess is disabled when using the external supply. so, don't plug the radio into the wall just to have the clock on either, as you will destroy it too.

obviously you can just unplug the AC cord and run the radio from the external supply if that is satisfactory to you.
LC

It measured 18.9 volts at that point, LC. I've ordered new regulators as well as diodes for that board. It will sit on the shelf until those get here next week

 

[loosecannon]

good deal.

from what you posted, it sounds like you are fine to run it from the external supply as long as the AC cord from the radio is unplugged.

BTW, these are great sounding loud radios when set up properly. it is the same chassis as the 29GTL, which was the predecessor to the 29LTD, and if you compare the two chassis, you will see many similarities.

i found them to have an excellent receiver, and one of the best noise blankers i've come across.

a year or two ago i had an 89GTL which is the same as your 1000 without the clock, and looks like a 142GTL.

I changed out the power supply for a 12 amp switching supply and installed an RFX75 in it.
It took quite a bit of experimenting to get it right, but once done, i had a base station that would deadkey 15 watts and swing up to about 85 or so.

ended up selling it to someone...

LC

 

[dozerman]

good deal.

from what you posted, it sounds like you are fine to run it from the external supply as long as the AC cord from the radio is unplugged.

BTW, these are great sounding loud radios when set up properly. it is the same chassis as the 29GTL, which was the predecessor to the 29LTD, and if you compare the two chassis, you will see many similarities.

i found them to have an excellent receiver, and one of the best noise blankers i've come across.

a year or two ago i had an 89GTL which is the same as your 1000 without the clock, and looks like a 142GTL.

I changed out the power supply for a 12 amp switching supply and installed an RFX75 in it.
It took quite a bit of experimenting to get it right, but once done, i had a base station that would deadkey 15 watts and swing up to about 85 or so.

ended up selling it to someone...

LC

It's a cherry radio. The covers and face plates are in great shape. No hacking or adding crap like echo or noise toys has been done to it. It probably has been screwdrivered in the past. The rx isn't quite what I had heard it was so I'm guessing the IF cans have been tweaked. Certainly doesn't hear like my Dwight D anyway. Will post how things went after all the stuff comes in the mail. Thanks again everyone for the help this far!

 

[dozerman]

It's a cherry radio. The covers and face plates are in great shape. No hacking or adding crap like echo or noise toys has been done to it. It probably has been screwdrivered in the past. The rx isn't quite what I had heard it was so I'm guessing the IF cans have been tweaked. Certainly doesn't hear like my Dwight D anyway. Will post how things went after all the stuff comes in the mail. Thanks again everyone for the help this far!

just an update on the radio. i replaced the two transistors on the power supply board and my voltage dropped back to 14.87 volts. was the able to use the trimmer to bring it back to 13.8. i went ahead and replaced the smaller 945 transistor as well as the 2200uf cap on the board. i had already replaced it but it showed signs of bulging and discoloring on the bottom. the radio now does 3 watts with a 19 watt pep. on air reports were good. thanks again everyone for helping out on this project. all thats left to do is replace the sticky meters, but for $60, they can wait a bit. thanks again