| Author |
Message |
    
Kevster
| | Posted on Friday, April 19, 2002 - 5:42 pm: | 
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Does anyone know where the final RF transistor is in a Cobra 29 LTD or Classic? Do you know what it is marked on the board? Thanks! Kev ~ N8KKP |
Taz
| | Posted on Friday, April 19, 2002 - 7:53 pm: |
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its in the back, its black and has 3 legs.
if the front is facing you it should be on the left of the radio |
Kevster
| | Posted on Saturday, April 20, 2002 - 11:05 am: |
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Hi TAZ, thanks for the post. I wasn't sure if I had the correct one or not. The one in the back/left is grounded to the heat sink, correct? Thanks! |
Taz
| | Posted on Saturday, April 20, 2002 - 2:28 pm: |
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Yeah, Its screwed to the back. Its black and has really small white writing on it.
Good LUCK! |
Kevster
| | Posted on Saturday, April 20, 2002 - 9:19 pm: |
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Hey TAZ, just an update... I took it out and tested it, it's bad! Is it OK to replace it with a 2SC1969 or 2SC2312? I believe they are both 18W rated anyway. Also, any secrets on getting the modulation up on one of these things?? Thanks for the help! Kev |
Taz
| | Posted on Sunday, April 21, 2002 - 2:32 am: |
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Hmm, I never replaced on before. But tech181 or 307 could tell you right away if it would work. I think it will work, but not positive. |
Taz
| | Posted on Sunday, April 21, 2002 - 2:46 am: |
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Cobra 29 ltd
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The PLL is not modifiable when used in an AM only radio, the only method that will allow more channels is to disconnect the 15.360 Mhz signal that is coming from a tripler circuit and replace it with another one depending on the required freq. range.
Here are the instructions:
Lift one end of C87 (the side closest to IC2), then connect a switch's common pin to the open pcb hole, connect the "modified" switch position to a crystal oscillator (you can find a one you can build inside my build it / projects pages or just click here), then connect the "normal" switch position to C87, all you need to do is get a crystal to use for the oscillator, the original frequency is 15.360, you add / subtract the number of channels to / from the frequency
Say you want 26.515 - 26.955 (the low band) just deduct .450 from the original freq (450 khz = 1 band of channels), so a crystal of 14.910 Mhz will give you the low band.
OR to get 27.415 - 27.855 (high band) you will need a 15.810 Mhz crystal.
You may actually use both but the radio may not cover the freq. range ( due to limitations in the tuning circuits for the broadband requirements)
The only hard part is finding a supplier for the crystal, the mod itself is easy !
Step 1. Adjust vr-4 for modulation.
Step 2. Tune for max L14, L13, and L12 for max swing on channel 20, AM
Step 3. If possible inject 600 hz tone into mic, or say auuuudio, while tuning for max.
Channel 9 switch to Channel 19: Cut D32. Hard, huh?
GOOD LUCK! HAVE FUN! |
Sonoma
| | Posted on Sunday, April 21, 2002 - 6:43 pm: |
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I have the 2sc1969 in my cobra 29.It works great ,even a little more swing and out put |
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