    
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 3924
Registered: 7-2004
| | Posted on Saturday, September 08, 2007 - 11:25 pm: | 
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look for mods for these radios-they're all the same:COBRA 138 139 XLR (OLD) PRESIDENT GRANT, WASINGTON,MADISON , ROYBN 510 , 520D ,TRC-449 457 AND 458.
that said:(Mike circuit changes
1. UNSOLDER banded end of diode D21 and lift up.
2. UNSOLDER banded end of diode D22 and lift up.
3. SOLDER a 4.7 k resistor across where D21 and D22 was unsoldered.
4. Replace R78 (27k) with a 56k.
5. Replace R84 (4.7k) with a 2.2k.
(Transmit circuit changes
1. Replace C174 (56pF NPO) with a 82pF NPO ceramic capacitor.
2. Replace C173 (180pF NPO) with a 220pF NPO ceramic capacitor.
3. Re-tune coils L32,L30,L29,L28 and VR8 for maximum forward AM power.
The AM regulator 2SC1419 is WEAK. UNSOLDER it and toss it in the trash.
Replace it with a GE-66, ECG-152 or a NTE-152.
Relay buzzes on SSB, no am transmit, lights go dim, or the fuse pops sometimes?
Replace C179 ( a 2.2uF 25v tantalum cap located near finals) with a standard 2.2uF electrolytic capacitor.
PAY ATTENTION to the "+" and "-"!!! |
