| Author |
Message |
    
Johnhenre
| | Posted on Monday, January 12, 2004 - 12:00 pm: | 
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CAN THE VARIABLE KEY BE INSTALLED ON THE RF GAIN CONTROLL POT? IF SO WHAT DO YOU DO WITH THE WIRES?SO THE RF GAIN IS ON FULL ALL THE TIME, NOW WERE DO I GO ON THE PCB TO HOOK UP THE VAR. KEY TO THIS POT? AND DO I GOT TO HAVE IT REPEAKED AFTER THIS MOD. THIS IS A VIRGIN 29 LTD. |
Tshelburne5002
Junior Member
Username: Tshelburne5002
Post Number: 10
Registered: 3-2004
| | Posted on Saturday, April 02, 2005 - 11:32 pm: |
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I have done this mod before.... you need to take out the jumper in jp36 or jp6 cant remember which one but it is right next to the final..then solder in a 470uf cap and jump it on the solder side with a resistor. different value resistors will give you different dead keys. Then unsolder the wires that run the rf gain from the board. You put a jumper wire in where the two wires were unsoldered. Then hook the wires up to the two sides of the capacitor that is soldered in jp36 |
Patzerozero
Advanced Member
Username: Patzerozero
Post Number: 662
Registered: 7-2004
| | Posted on Monday, April 04, 2005 - 3:12 pm: |
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i'd personally put the variable onto the squelch, & keep the RF gain functioning...i know, i did it on the rf gain & cursed myself every time somebody drove near me, til i changed it!!! |
Inspector
Intermediate Member
Username: Inspector
Post Number: 142
Registered: 12-2001
| | Posted on Sunday, May 01, 2005 - 4:02 am: |
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Eventually the potentiometer will fail because it is under-rated for the current that will go through it AND you don't want runs of wire carying RF from the final circuit- to the gain pot- and then back to the final circuit. Using a TIP-120 darlington transistor, with the J-36 circuit, isolates the RF from the gain control saving it from failure and keeping stray RF at a minimum. The circuit using the darlington transistor can be found on the www... |
Patzerozero
Advanced Member
Username: Patzerozero
Post Number: 730
Registered: 7-2004
| | Posted on Sunday, May 01, 2005 - 3:29 pm: |
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10 years on my grant xl, all of hard use & abuse & the pot is just fine. same on my brothers 20 year old cobra 29. have NEVER had anybody return with a burned out pot after doing these mods for 30 years. quite possibly every mod i've ever seen for variable power is done the same way... |
Inspector
Intermediate Member
Username: Inspector
Post Number: 143
Registered: 12-2001
| | Posted on Monday, May 02, 2005 - 12:24 am: |
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well...I have seen many where the pot becomes very intermitant making unstable settings. Also seen internal interference from RF radiating from the unshielded wires added in the mod. I guess everyone has their own experiences...73s |
2600
Advanced Member
Username: 2600
Post Number: 560
Registered: 7-2002
| | Posted on Wednesday, May 04, 2005 - 3:40 am: |
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Hey Inspector,
there's a quick, cheap cure for the "RF all over the place" side-effect.
The jumper wire that gets pulled has the bypass disc cap for the driver stage on the FRONT end of the jumper wire. Adding a .02 or so disc cap to ground on the solder side of the REAR hole where the jumper came out really calms down this problem, along with the 'unstable carrier setting' problems that this can cause.
NONE of the front panel pots will survive if it is wired DIRECTLY in place of the jumper wire. Using the front-panel control to feed the base of a transistor, preferably a darlington, will not over work the control's wattage rating. That's why the control in the Grant XL has lasted this long. It feeds into a darlington circuit, made from two separate transistors in that radio. The TIP120 is my choice for that job on the AM-only radios. It places almost no load onto the control, and seems to last forever.
73
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