| Author |
Message |
    
Rippedradio
| | Posted on Saturday, July 27, 2002 - 11:25 am: | 
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please help me i am not a radio tech but i do understand electronics and have been into radio's for a long time but this one has me stumped???? i have a cobra 2000 that is the most beautiful radio i have ever seen but it does not transmit much power! it keys about a watt and a half and swings to about three, i checked the modulation and it has not been clipped i turned it up and it is over 100% with a silver eagle! i changed the final and driver i even put in a 2sc2312 and now after retuning i barely get a three watt swing! i also tried the mod with tp8 hooked to 12 volts and it raised the carrier to like 2 1/2 watts but still only swings three?????? i have tried adjusting L38 and L37 wit this as the result! the only thing i can possibly think is the voltage regulator but i am not sure of anything anymore and the radio is making me mad! also the recive seems a little low my 142 was much more sensitive and i have allways heard the 2000 is the best! anyways any help would be greatly appreciated! thanks, rippedradio! |
Taz
| | Posted on Saturday, July 27, 2002 - 9:03 pm: |
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hmm, how about your meter? I would also guess voltage reg! |
Ss8541
| | Posted on Saturday, July 27, 2002 - 10:58 pm: |
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with what you have given(poor rx and tx), my 1st thought would be low vco 'injection' level. has the radio always been like this, or did it just start(the tx and rx problem)? if it has always been like this, then the last owner probably screwdrivered the pll/vco area giving low vco output to both the tx and rx sections. and if it just started(for both rx and tx) then there is most likely a bad component in the pll/vco area giving the same results. answering the questions above would help in narrowing this down more. |
2600
| | Posted on Monday, July 29, 2002 - 12:16 am: |
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Just a long shot, but the combination of weak receiver AND transmitter may be just what Ss8541 suggested. I have seen L20 fail in a number of models that use this circuit board. The Cobra 2000GTL was made for 13 or 14 years. The oldest ones (made in Japan) are old enough to buy a drink legally. The newest (Philippines) still get the cheap ticket at a movie theater.
Looking into the top of the radio with the front towards you, find the PLL chip at the front left (your left)of the main circuit board. It may say MB8734 or MB8719 if it's been changed. Look about an inch to the rear for a vertical black (or brown) plate about an inch wide and a half-inch tall. This is the VCO module, and will have the letters 'UHIC' on it. Butted up to its left end is L20. If L20 appears to peak the receiver best WITH THE SLUG DEAD EVEN TO THE TOP OF THE CAN, then it is probably bad. You aren't really seeing a proper peak if the slug is at this position. If the coil is good, it will peak in TWO PLACES. Once, with the slug at a position ABOVE the top of the can, and again AT LEAST ONE FULL TURN below the edge of the hole. The lower peak will usually be a little stronger. IF IT DOESN'T HAVE BOTH THESE PEAKS then it isn't REALLY a peak and the coil is bad. Getting another coil type LA-195 from Cobra might not be so easy, but with 20+ years of 148, 142, Washington, etc. radios around you can probably find a junk chassis to cannibalize. They don't go bad that often, it's just an educated guess, but at least that's how to tell if this is the cause.
73 |
Rippedradio
| | Posted on Tuesday, July 30, 2002 - 6:01 pm: |
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i have no clue all i can tell u is i bought the rig like this, the recive is not that bad but not good, the output is the most of the problem! all i can tell u is the rig is a 7a3 taiwan model and had never been touched peeked or anything as far as i can tell and u can usualy tell! i have seen lots of radio's and it looks to be untouched until i tried to peek it with no suscuess! thanks for the anwsers guys! any more help would be greatly appreicated! thanks, chris |
2600
| | Posted on Wednesday, July 31, 2002 - 1:19 am: |
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Well, since you DID try to peak it, just look closely at L20. If the body of the tuning slug is DEAD EVEN with the top of the can, it's a clue. If the slug peaks down inside the hole, L20 ain't the root of the problem. If ONLY the receiver was weak, or if ONLY the transmit was weak, L20 would be a waste of time to worry about. When BOTH are wimpy, it's worth eyeballing this one. If you tried to "peak" the radio and DIDN'T turn L20, you missed a step. L20 feeds to BOTH the transmitter and receiver mixers. If it is NOT tuned to a peak, BOTH will be weak. Guaranteed.
73 |
Rippedradio
| | Posted on Wednesday, July 31, 2002 - 5:46 pm: |
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2600, i did tune L20 and it is inside the pot it is not even close to even! i emailed 307 and he said it could be one of two things witch i do not understand completly, 1. local oscolator is to low in amplitude and 2. the output tank curcut is miss tuned (shorted capicator) thanks for the replies and keep em comming my rig is sick! also 2600 i was wondering if you have ever heard of a problem with the cobra 146gtl, it has a dead key but no audio and on ssb it hums and does 22 watts also no audio! i thought i heard about this in the past but cannot remember what it was!the audio chip and voltage regulator have both been replaced! thanks again, chris |
2600
| | Posted on Thursday, August 01, 2002 - 12:00 am: |
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Bummer about L20 checking okay. If it WAS bad, that would be simpler. Makes it sound like two problems, one in the receiver, and one in the transmitter. The local oscillator that 307 mentioned is the module that feeds INTO L20. They don't go bad too often, but nothing is fail-proof. Nothing. Without an oscilloscope with 50 MHz minimum vertical bandwidth, I don't know how to look much deeper. Sounds like it's time to drop back and check the main power supply. Do the meter lights blink severely when you modulate it? They should normally blink ONLY a little.
I do have one suggestion about the transmitter power. Check the idling current on the final and driver. If one of them won't set high enough (40 mA. for the driver, 60 mA. on the final), you may have one or more small electrolytic capacitors going bad. Any problem that keeps one or both readings low or on zero will also hurt your power on AM and SSB.
I hate to use such a nasty word, but the 146 sounds like sabotage. Maybe by accident, but a slug-twister who isn't careful can make one act that way. Looking for solder shorts and tracking down all the switching diodes and transmit-receive switching transistors that might be popped would try your patience even with a complete manual. The only one Cobra ever offered was stamped "preliminary" on the front. I don't think they ever spent the money to make it "final". There is NO parts placement or circuit-board trace picture. Just a schematic and adjustment locations with a parts list. This just adds to the labor needed to track down a weird problem. The schematic alone is fine, but that parts placement diagram still reduces the labor needed to track down weird stuff.
I just remembered one thing in your original post. The Cobra 2000 final stage will work better with the original 2SC1969 part in place. The 2312 needs a slightly different setup to deliver its full potential.
73 |
Ss8541
| | Posted on Friday, August 02, 2002 - 9:02 pm: |
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b4 u start 'repairing' any other areas, i would suggest you find someone who can do an alignment correctly. i would almost bet the last owner either personally screwed up the pll/vco area, or payed a 'tech' to do it. it may not be this, but the vast majority of problems like yours are due to this. |
307
| | Posted on Friday, August 02, 2002 - 9:19 pm: |
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Would you like me to do it for you? If so email me direct at tech307@copperelectronics.com
307 |
Ss8541
| | Posted on Friday, August 02, 2002 - 11:16 pm: |
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well there you go, drop him an email. |
Funtimebob
| | Posted on Saturday, August 03, 2002 - 1:36 am: |
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He Da Man! |
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