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COBRA 2000 GTL AM carrier issue
- Thread starter Unit 75
- Start date
Ok....I have a question. I think I have located the intermittent problem around IC5 (mixer). When I test IC5 with my DMM, I have correct voltages on all pins (as per the service manual). Pin 2 is the output and even though I have .469v rx and 7.93v tx off pin 2, I don't have the RF level of .357v coming off pin2 using the RF probe and I do not have the 85mv tx RF level off the one leg of C163 and on down the line towards the base of TR39 (pre-driver). Does this mean that IC5 is defective?
IC5 never goes bad.
Well, only about two of them in my 40 years' experience with that part.
Close enough to never.
It's time to see what's feeding into IC5. The inputs are both 'differential". Means that both pins on one input are "hot" with RF feeding in.
Pins 7 and 8 should have the 7.8 MHz carrier while transmitting in AM, nothing on receive.
Pins 11 and 13 should have the 34+ MHz PLL output all the time, transmit and receive.
If either of these is missing, the fault is upstream from IC5.
73
Well, only about two of them in my 40 years' experience with that part.
Close enough to never.
It's time to see what's feeding into IC5. The inputs are both 'differential". Means that both pins on one input are "hot" with RF feeding in.
Pins 7 and 8 should have the 7.8 MHz carrier while transmitting in AM, nothing on receive.
Pins 11 and 13 should have the 34+ MHz PLL output all the time, transmit and receive.
If either of these is missing, the fault is upstream from IC5.
73
Sorry, I had to shut my equipment down as we were under a severe weather warning with lots of lightning yesterday afternoon.
This morning I looked at the mixer input frequencies, pins 11 and 13 had the required 34+ MHz frequencies. Pins 7 and 8 did not have the 7.800 MHz frequency. So I looked upstream. Went to TR32 and checked for my 7.800 MHz from the carrier oscillator and it was there. I traced downstream from there and found I had my 7.800 MHz all the way up to the filter input but no 7.800 MHz coming out of the filter. Installing a new crystal filter now
This morning I looked at the mixer input frequencies, pins 11 and 13 had the required 34+ MHz frequencies. Pins 7 and 8 did not have the 7.800 MHz frequency. So I looked upstream. Went to TR32 and checked for my 7.800 MHz from the carrier oscillator and it was there. I traced downstream from there and found I had my 7.800 MHz all the way up to the filter input but no 7.800 MHz coming out of the filter. Installing a new crystal filter now
Thank you very much for reporting this here.... and I hope it fixes the other one as well. It's always good to have stories/results like this in your "utility belt"!
Well that seem to fix the one radio. Thank you nomadradio for the push. Going to do the other one now
Yep, Good info for those that have the same problem...they can follow the thread along and hopefully fix their unit as well.
Update. The problem was the same. I had 34+ MHz going to IC5 but did not have the 7.800 MHz frequency signals to IC5. First thing I did was check the 7.800 MHz crystal filter. Looked good, I had both input and output....so on down the line I went. C54 checked out and R63 checked out as the 7.800 MHz signal was there. When I got to R191, I lost the 7.800 MHz. Guess what I found..... (check the picture out) I had to look up the numbers to figure this out. Someone had put a LA 160 peaking coil (which is used as L48) in the L47 location. The correct one should have been a LA 219. Back to the bench.......
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Ok guys...This is important. I was doing some research due to the above post. You Cobra peeps make sure you write this down.
Even though the 148GTL and 2000GTL are alike in layout....I noticed this.
Cobra 148GTL
L47 is the 7.800 MHz peaking coil
L48 is the 34 MHz peaking coil
Cobra 2000GTL
L47 is the 34 MHz peaking coil
L48 is the 7.800 MHz peaking coil
So even though the board layout is the same...CHECK THE ACTUAL SILK SCREENING on the board because those two peaking coils are marked opposite of one another !!
Looks like someone had removed a peaking coil from a 148GTL and installed it in this 2000 GTL. That is what messed it all up.
-75
Even though the 148GTL and 2000GTL are alike in layout....I noticed this.
Cobra 148GTL
L47 is the 7.800 MHz peaking coil
L48 is the 34 MHz peaking coil
Cobra 2000GTL
L47 is the 34 MHz peaking coil
L48 is the 7.800 MHz peaking coil
So even though the board layout is the same...CHECK THE ACTUAL SILK SCREENING on the board because those two peaking coils are marked opposite of one another !!
Looks like someone had removed a peaking coil from a 148GTL and installed it in this 2000 GTL. That is what messed it all up.
-75
I don't think it matters about board number. I think it matters if it is a 148GTL or 2000GTL. I compared front mic 148's and side mic 148's and it seems the silk screening (L47 and L48) is the same for those 148's...even though they have different board numbers.
All the 2000's I have..Malaysia, Taiwan, Philippines all have the same silk screening for L47 and L48.
So the difference seems to be if it is a 148GTL or Cobra 2000GTL board.
All the 2000's I have..Malaysia, Taiwan, Philippines all have the same silk screening for L47 and L48.
So the difference seems to be if it is a 148GTL or Cobra 2000GTL board.
Low_Boy....Board number EPT014813Z (Cobra 148F) is the one that had the bad 7.800MHz crystal filter.