| Author |
Message |
    
Cobra2000
Junior Member
Username: Cobra2000
Post Number: 11
Registered: 6-2007
| | Posted on Tuesday, August 07, 2007 - 12:22 pm: | 
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My Grant XL AM modulation crapped out recently...receive audio is fine, AM carrier is normal, SSB power/audio is fine. Even tried a diff. mic, although I knew that wasn't going to help. I did a quick check of TR22 and TR23, they are ok...C18 has been upgraded to a 1000uf 25V, and it tests OK. C109 is OK. C174 is OK. Radio has been working great, and just went out all of a sudden. Any ideas? |
Captian_radio
Intermediate Member
Username: Captian_radio
Post Number: 246
Registered: 9-2004
| | Posted on Tuesday, August 07, 2007 - 7:27 pm: |
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Check and see if the radio works in pa mode, if not could be the audio ic, this exact thing happened with an old realistic trc 451 and I found that the audio chip was bad.
CEF451/VE1CZ
Robert L. Spicer The days of radio are just beginning!
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Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2604
Registered: 9-2004
| | Posted on Tuesday, August 07, 2007 - 9:25 pm: |
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try replacing TR41.
its mounted to the chassis right near C174.
replace it with an NTE152, or a 2N6487.
dont forget to use a new insulator and heatsink compound.
that should do it.
matt
anyone wanting a "clean signal", just look to the left and build one of these!!!
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Cobra2000
Junior Member
Username: Cobra2000
Post Number: 12
Registered: 6-2007
| | Posted on Tuesday, August 07, 2007 - 10:34 pm: |
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I've already replaced TR41 with a 2N6488. I have good carrier, just no modulation. The PA audio is perfect. The receive audio is perfect. The SSB audio/power is perfect. Just no AM modulation. |
Cobra2000
Junior Member
Username: Cobra2000
Post Number: 14
Registered: 6-2007
| | Posted on Wednesday, August 08, 2007 - 5:08 pm: |
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I found the problem. R114 after TR21, the AM mod. sw. transistor, had a bad solder connection. I wasn't getting AM mod. into the audio IC.
The PA transistor, TR27, was earlier in the chain and didn't have to route through TR21 to get to the audio IC. That's why the PA still worked.
I had to break out the sams, o'scope and the big magnifying glass. I was ready to break out the hammer at one point. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2607
Registered: 9-2004
| | Posted on Wednesday, August 08, 2007 - 10:00 pm: |
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i only suggest that because it is a very common cause for the "no AM modulation, SSB is fine" symptom.
after posting that i thought i remembered that you had already done some audio mods to the radio that involved replacing this part.
2N6488 eh?
guess i hadnt thought of that, but im sure it works just great!
congrats on the repair, sounds like a head scratcher.
did you go with 10K on R114?
ive always wanted to try a 4.7K, or even a 2.2K in that spot, but ive never had the right setup to be able to tell the difference.
hmmmm....
wish i had a spec. analyzer.LOL
later,
matt
anyone wanting a "clean signal", just look to the left and build one of these!!!
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Cobra2000
Junior Member
Username: Cobra2000
Post Number: 15
Registered: 6-2007
| | Posted on Thursday, August 09, 2007 - 12:25 am: |
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Matt:
The 2N6488 was used because my parts guy didn't have the 2N6487. It's no better in this modification from a current standpoint, just higher breakdown voltage rating, that it will never come close to seeing.
I already have stepped down R124/126 before TR23, the first mic amp. I need to put them back to normal (10K) and change R114 to 10K. JustinD says it overdrives the mic amp. I'd have to agree with his logic, although it doesn't look too bad to me on the scope or through audio reports. I usually end up backing down the mic gain anyway, and using an Astatic 636L hand mic.
Have you ever swapped out C109, the 22uf for a 4.7uf? Not sure what that's supposed to do???
I need to leave this poor radio alone and get a RCI-2950DX or an Icom IC-706MKIIG or similar. |
