I got cobra 89gtl excellent condition it was recapped etc.After using it for a few days it blew the final(Hg1969)On checking this radio is a Cobra 29..However I replace.the final and as I keyed it shorted..radio went dim..I had a C2029 which is the original final as soon as I put it in the radio I got really hot. any ideas. where to look.
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Cobra 89gtl/ cobra 29 Keeps Blowing finals
- Thread starter BAGEBOY
- Start date
Might want to check the power supply voltage. If the regulator fails, you could get 20 Volts or more onto the final. This is know to get them too hot.
Or just plug it into an external regulated supply and see if it behaves better?
73
Or just plug it into an external regulated supply and see if it behaves better?
73
I tried that I am getting 13.8 at the final.. but the final still is very hot even if I don't ey
Do you have a mica insulator and heat compound applied when you replaced the part?
Sounds like this transistor and this circuit is getting bias voltage to turn it on and conduct in receive mode. Have you looked for a bridged solder joint, other bad components, and compared this circuit to the schematic to be sure it is all correct - yet?
Just wondering . . .
Sounds like this transistor and this circuit is getting bias voltage to turn it on and conduct in receive mode. Have you looked for a bridged solder joint, other bad components, and compared this circuit to the schematic to be sure it is all correct - yet?
Just wondering . . .
I did not see the Driver side of this but I see you highlighted the R48 as well as C61
Gotta be careful with these.
When your part is now getting hot, follow @Robb advice - verify the tab is not shorting to that cage - mounting - for that is bolted to foil ground.
It may already be toast - the 2029 - they blow pretty easy from neglect.
The 1969 you put in there, if you changed any of the parts highlighted, did you change them back when installing the 2029?
This sounds like an assembly (put back together) problem - is the soldering job caused a short, R48 POTENTIALLY being the bad part - if it's wrong values (S/B 22 ohm) , or shorted by blobbing of solder - Final will go poof.
Gotta be careful with these.
When your part is now getting hot, follow @Robb advice - verify the tab is not shorting to that cage - mounting - for that is bolted to foil ground.
It may already be toast - the 2029 - they blow pretty easy from neglect.
The 1969 you put in there, if you changed any of the parts highlighted, did you change them back when installing the 2029?
This sounds like an assembly (put back together) problem - is the soldering job caused a short, R48 POTENTIALLY being the bad part - if it's wrong values (S/B 22 ohm) , or shorted by blobbing of solder - Final will go poof.
I checked the 22 ohm it was reading 25 I changed it the 1ohm was reading ,5 ohms I change as well..still no difference
Does the 2029 - show shorted?
You might need to re-check the Driver too.
This is continuity check time - make sure the 2029 doesn't show a short - or see a short from it's middle leg (Collector) to foil ground (Emitter) - something is triggering it on - could be the Base shorted to emitter - taking out the collector.
The two resistors R47 and R48 - complete a BASE to ground circuit - should not be any less than 22 ohms - more higher reading is ok, but NOT less - can force the transistor to latch because the collector to base path to ground is too low to do you any good.
So do a continuity check make sure those resistors are NOT totaling less than 22 ohms. Lift the Base leg to do this take the base out of circuit - then check.
While you're at it, is the part still gets hot - check your "tab" to ground NO SHORT - if it is a short it's cooking it,.
You might need to re-check the Driver too.
This is continuity check time - make sure the 2029 doesn't show a short - or see a short from it's middle leg (Collector) to foil ground (Emitter) - something is triggering it on - could be the Base shorted to emitter - taking out the collector.
The two resistors R47 and R48 - complete a BASE to ground circuit - should not be any less than 22 ohms - more higher reading is ok, but NOT less - can force the transistor to latch because the collector to base path to ground is too low to do you any good.
So do a continuity check make sure those resistors are NOT totaling less than 22 ohms. Lift the Base leg to do this take the base out of circuit - then check.
While you're at it, is the part still gets hot - check your "tab" to ground NO SHORT - if it is a short it's cooking it,.
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I will follow ur instructions and I will provide an update any more ideas feel free to share
Is your PCB Board number 407?
Go Here...
Cobra 89GTL (cbtricks.com)
Or do you have that "flat face" 23 channel Base Radio?
My (more) Thoughts on this are with the PreDriver circuit "triggering" your mess.
IF there is an IF signal not not two of them - the radio could use a good strong tune up and hope L24 and L25 just need to better the alignment.
The radio is ancient - so are the caps fresh or are they the old OEM ones?
If there would be any concerns - it would be between TR17 (2SA733 PNP) and TR18 .??? (945?) NPN - which may be leaky - easily triggered.
Go Here...
Cobra 89GTL (cbtricks.com)
Or do you have that "flat face" 23 channel Base Radio?
My (more) Thoughts on this are with the PreDriver circuit "triggering" your mess.
IF there is an IF signal not not two of them - the radio could use a good strong tune up and hope L24 and L25 just need to better the alignment.
The radio is ancient - so are the caps fresh or are they the old OEM ones?
- The TX line that runs thru to the Predriver is what I'm getting at
- if the CB PA switch is ok, but you use the LED lamps for the Meter - this changes power presence
- this can upset the apple cart like how the Cobra 2000 has for it's own power supply service.
Power on the TX line can float "up" triggering the TX mode on because the Pre-Driver is getting enough power to work, turn on and amplify with the leaky IF signal that comes out of L25 and L24 (Past TX mixer) .
It later models, like the 25 and 29 they simplified the TX circuit - to ground when you wanted it to - to Pin 3 and forcing the Pre-Driver to ground to TX at the same time - turning it on.- if the CB PA switch is ok, but you use the LED lamps for the Meter - this changes power presence
- this can upset the apple cart like how the Cobra 2000 has for it's own power supply service.
Power on the TX line can float "up" triggering the TX mode on because the Pre-Driver is getting enough power to work, turn on and amplify with the leaky IF signal that comes out of L25 and L24 (Past TX mixer) .
If there would be any concerns - it would be between TR17 (2SA733 PNP) and TR18 .??? (945?) NPN - which may be leaky - easily triggered.
The radio was recapped and they are no Leds I will check.Tr 17 and 18. I have attached a picture of the radio.
What I recently found a blue wire to be loose it comes from the ground trace b y the audio ic and goes to the back to the ground of a capacitor. I was able to see in another cobra 89 from a user on Facebook and his radio has the Identical blue wire . Will also post pic ..what's ur thoughts
Attachments
I bought an 89 brand new in 1978. Liked it, added channels (that I was too scared to use back then) but never touched the power. Took a while to save up for it in 1978 $.
But is the Board Number a 407 or something else?
I can only guess...there are so many people trying to take working chassis out of older sets and putting them in better looking sets trying to make a fast $$$ or two off of the turnabout.
That radio is in really good shape for the use and year...I was thinking it was more of the XLR - but yeppers that is an 89 but NOT the "cam" like the older Crystal set - can see that this is not the older Crystal Synthesis one.
This is the GTL one - 89 GTL - 2816 PLL - not the 858 like the XLR would have - unless you know something you're not telling...
Hint - CAM Did come with 89's but they have Black Front panels. You show Chrome and can only guess they tried to use a Chrome front Face. The GTL and XLR versions used Chrome.
In the Cam 89 Xtal - TR20 is the Predriver which uses 100 ohm to ground thru Pin 3 to TX and turn on TR20 to make TR19 (Driver) and TR18 (Final) to work.
You show the 2029 Final and 2028 Driver schematically...
The older CAM 89 uses this...
A much different setup from a vastly different era.
This radio - what you have, is more like the GTL - if it uses a PLL it is more than likely the 2816 used in 29's since they dropped the Relay and Xtals and went to PLL.
XLR has the 858 chip in it.
Looks like prior work has been done. You'll need to clean up the soldering.
But as far as Blue wire, that is a "ground loop" - it is done on purpose to "shelter" sections that would otherwise have had a much longer path to shield and ground (Foil return) - so that will need to be soldered.
It's uses LAMPS for the RX and TX MOD meter lights however in your photo - RX and TX modes they appear to be LED types.
So this radio looks more towards the 2816 GTL version - the CAM 89 (Black Face) one used a "first time we tried this" Variable pot (VR8) for Delta Tune and showed " - 0 + " on the faceplate to show which side of the channel you had to shift to - to hear them.
So if it's got a PLL and 2028/2029 parings - that previous link I provided is about the best were going to get to get the board I think it has.
The older CAM one directly TX's (grounds directly thru a 100 ohm resistor to Pin 3) so to see the differences, the Chrome front 89 (XLR/GTL with Electronic Switching) board is the one that we have to troubleshoot.
It still uses a 100 ohm resistor - but IT IS POWERED by TR17 - a 2SA733 - PNP - like TR21 in Cobra 29's.
Locate D20 - still intact? This is the TX side for the Delta Tune that is "fixed" on TX mode. If mods have been done - suspect this area to be potentially the culprit.
If intact and seems to Key up on it's own? Usually that TX is switched thru PIN 3 MIC - which Grounds D17 the PLL's SHIFT at Pin 9 - D15 forces PLL's Pin 15 to go high - power goes to IC3 and the TX switch TR18 - IC 3 can't produce anything unless it gets power, which it's only supposed to get in TX mode - if it's on all the time - suspect a blown power steering Diode - a Shorted D20 can do this - backfeeds into TX side from Delta Tune - you'd know it by turning the Delta tune in one direction or another and it's start keying - won't show TX yet, but it can make IC 3 work from this new power source.
No TX light - suspect D25/D24 and RED LED - as needing to be checked.
So if changes were made to the Pre Driver - then you start there.
Why? Because this radio has NO way (NONE) to "unkey" once the oscillations start - which this thing is prone to because? several reasons but mostly of a mistuned IC3 - (L20 Input) - Mixer peaking is L24 and Output is L25 to Pre-driver - WHICH IS ON when in TX, so something is throwing off the Pre Driver - so diode work (Good/Bad) and continuity checks - to make sure the Pin 3 of the Mike key isn't causing this condition.
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THE BOARD IS PC 407 AcBut is the Board Number a 407 or something else?
I can only guess...there are so many people trying to take working chassis out of older sets and putting them in better looking sets trying to make a fast $$$ or two off of the turnabout.
That radio is in really good shape for the use and year...I was thinking it was more of the XLR - but yeppers that is an 89 but NOT the "cam" like the older Crystal set - can see that this is not the older Crystal Synthesis one.
This is the GTL one - 89 GTL - 2816 PLL - not the 858 like the XLR would have - unless you know something you're not telling...
Hint - CAM Did come with 89's but they have Black Front panels. You show Chrome and can only guess they tried to use a Chrome front Face. The GTL and XLR versions used Chrome.
In the Cam 89 Xtal - TR20 is the Predriver which uses 100 ohm to ground thru Pin 3 to TX and turn on TR20 to make TR19 (Driver) and TR18 (Final) to work.
You show the 2029 Final and 2028 Driver schematically...
The older CAM 89 uses this...
View attachment 46457
A much different setup from a vastly different era.
This radio - what you have, is more like the GTL - if it uses a PLL it is more than likely the 2816 used in 29's since they dropped the Relay and Xtals and went to PLL.
XLR has the 858 chip in it.
Looks like prior work has been done. You'll need to clean up the soldering.
But as far as Blue wire, that is a "ground loop" - it is done on purpose to "shelter" sections that would otherwise have had a much longer path to shield and ground (Foil return) - so that will need to be soldered.
It's uses LAMPS for the RX and TX MOD meter lights however in your photo - RX and TX modes they appear to be LED types.
So this radio looks more towards the 2816 GTL version - the CAM 89 (Black Face) one used a "first time we tried this" Variable pot (VR8) for Delta Tune and showed " - 0 + " on the faceplate to show which side of the channel you had to shift to - to hear them.
So if it's got a PLL and 2028/2029 parings - that previous link I provided is about the best were going to get to get the board I think it has.
The older CAM one directly TX's (grounds directly thru a 100 ohm resistor to Pin 3) so to see the differences, the Chrome front 89 (XLR/GTL with Electronic Switching) board is the one that we have to troubleshoot.
It still uses a 100 ohm resistor - but IT IS POWERED by TR18 - a 2SA733 - PNP - like TR21 in Cobra 29's.
Locate D20 - still intact? This is the TX side for the Delta Tune that is "fixed" on TX mode. If mods have been done - suspect this area to be potentially the culprit.
If intact and seems to Key up on it's own? Usually that TX is switched thru PIN 3 MIC - which Grounds D17 the PLL's SHIFT at Pin 9 - D15 forces PLL's Pin 15 to go high - power goes to IC3 and the TX switch TR18 - IC 3 can't produce anything unless it gets power, which it's only supposed to get in TX mode - if it's on all the time - suspect a blown power steering Diode - a Shorted D20 can do this - backfeeds into TX side from Delta Tune - you'd know it by turning the Delta tune in one direction or another and it's start keying - won't show TX yet, but it can make IC 3 work from this new power source.
No TX light - suspect D25/D24 and RED LED - as needing to be checked.
So if changes were made to the Pre Driver - then you start there.
Why? Because this radio has NO way (NONE) to "unkey" once the oscillations start - which this thing is prone to because? several reasons but mostly of a mistuned IC3 - (L20 Input) - Mixer peaking is L24 and Output is L25 to Pre-driver - WHICH IS ON when in TX, so something is throwing off the Pre Driver - so diode work (Good/Bad) and continuity checks - to make sure the Pin 3 of the Mike key isn't causing this condition.
Attachments
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