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The_horseman
New member
Username: The_horseman
Post Number: 1
Registered: 11-2007
| | Posted on Monday, November 19, 2007 - 8:04 pm: | 
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Hello all, a quick introduction! Have been using radio's for a long period, recently tested and passed for my General Class Amatuer license, and since have this fascination with electronics. Totally new, for the most part.
Here is this issue:
Cobra 148GTL Malaysia (yea I know, not exactly the best board) 08/1996 - uses the RCI MB8719 and EPTo14811Z board.
This radio has only been peaked and tuned, second or third owner here, but I cannot find any obvious tweaks or hacks to mod the radio. I used it in my Kenworth for about 6 months, between my 29LTD. This radio was a good talker with range and clarity, both on AM and SSB.
It has this quirk I call it now, it will power up, but no recieve or transmit. No movement of the meter when key up, will modulate through the PA circuit, but not AM or SSB. Occasionally, if I leave it sitting on the desk powered up, it will come to life, and work normally until it is turned off and allowed to sit. I have tried 3 different mikes, same result with all of them.
Cannot find any obvious broken/bad solder connections on the switches or board. I know some of your responses will be to trash the radio, since it is a Malaysian made, but I am wanting to learn.
Some hints please as to were to look for the solution of this intermittent problem. Eventually, if I can solve this issue, I plan to change the chip and use it for 10 meter SSB experimentation. (As noted earlier, I am a licensed ham)
Also, I have not been able to locate the correct schematics for this board, any help there would be apprectiated.
Thank you in advance! |
Tech858
Moderator
Username: Tech858
Post Number: 60
Registered: 10-2007
| | Posted on Monday, November 19, 2007 - 8:51 pm: |
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Could be a number of things but I would first check the PLL chip solder joints for dry joint or it could be the chip is faulty , second check the mixer IC joints or chip for faults , third check the oscillator transistor for possible fault , fourth check the channel change control and board for dry joints/faulty control , check crystal solder joints , the list is quite long but these are the ones that I would check first , best to get yourself a Sams or a decent copy of the service manual so you can check all the right points on the board and IC's for the correct voltages/frequencies as it could be almost impossible to pin-point the problem without them. Good Luck and hope you get it fixed .
Tech858/Jim
MM0TXO
CEF#858
HAM#230
CVC#116
Wavin' a hand at ya' fae Scotland 
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The_horseman
New member
Username: The_horseman
Post Number: 2
Registered: 11-2007
| | Posted on Monday, November 19, 2007 - 8:34 pm: |
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OH, I should have noted, that on power up, there is static heard through the external speaker, the same static is also heard when switched to the PA side. |
The_horseman
New member
Username: The_horseman
Post Number: 3
Registered: 11-2007
| | Posted on Monday, November 19, 2007 - 10:12 pm: |
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OK, interesting find, in trying to find a dry/cold joint. On the solder side of the board, a trace that leads to the 11.325 crystal, their appears to be a diode (possibly D35, still trying to cypher from a Philipines schematic),
If I briefly touch a jumper to each side of this little black box (there are actually two of these on the solder side, the reaction is the same on both), the unit comes to life, transmit and recieve. turn the unit off, nothing, jump across this diode looking black chip, and the radio comes back to life.
The second one attaches to traces for J27 and ??.
Oh well, need to go move the old fridge, so the new one can be put in its place in the morning.
Any ideas as to why jumping/shorting across this would wake this old critter up?
thanks, |
Teham
Member
Username: Teham
Post Number: 77
Registered: 1-2004
| | Posted on Monday, June 07, 2010 - 2:22 pm: |
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try replacing the diode. but before u do heat solder connections. maybe when u jumper it it connects again. tony cef 259 |
Funtimebob
Intermediate Member
Username: Funtimebob
Post Number: 259
Registered: 5-2002
| | Posted on Wednesday, June 09, 2010 - 3:27 am: |
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I Have a TRC-490 (same board as as the original 148)
I had this happen to me before and it turned out to be a dirty PA switch
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