The board is "pre calibrated" so if you really want to check SWR, you'll need to set the radio up to How the Factory Originally Set The Auto SWR Meter.
(It can't do this itself - the FWD and REF settings are pre-determined thru a carrier set power level that sends the power thru the Sense wires - See the CAL "terminal"? That is where they "look" at the SWR meter and make sure it will work - they put in a 100 2W Carbon Comp at that ANT (J503 on Schematic) terminal and ground and look to make sure it Reads SWR above 1:1.5 - then they remove the jig and send along the board to final processing and shipping....VR1 "is set" to show that imbalance and if above the 1.5 - LIGHTS up the ANT light to WARN you.)
When you are in USB and the WARN light comes on, but the other modes - it doesn't - usually means a low-frequency IF leakage is happening - a TX Strip alignment is needed - stuff that isn't 27MHz or thereabout - like 10.7MHz if or 16MHz IF - isn't canceling out and working it's way down the strip showing up in other places - like the SWR - ANT WARN light or even goofy sounding Audio from even a stock mic when keyed up in that mode.
AM or FM mode - RF Power has to arrive at the meter to even make the FWD/REF comparison work.
So if you've set up the radio for a 1/2W swing and wonder why the light "blinks" on voice peaks, well, it needs to be recalibrated thru VR1 - or run and external SWR meter to verify the internal one is even sensing the RF power. Then you can use the EXTERNAL SWR meter to help calibrate VR1 to show internal SWR correctly.
Or...
If the SWR seems to go "flat" - then simply put on a 100 ohm 2W carbon Comp (100 Ohm Dummy Load) and adjust the RF Power Front Panel knob - can you set SWR to climb on the meter?
The problem - just may not be enough RF Power present to make the SWR meter even work. The dummy load is used to help you determine if the SWR board is bad (cracked or open lines - like No RF Power Out to the Antenna) or the RF Power setting needs to be reset thru VR1.
When you adjust VR1 - you have to make a choice.
One being; the SWR to even give you a moving needle when you are only throwing 1/2 watt or less RF power as Dead carrier - you adjust VR1 to show SWR, HOWEVER this may create an OVERSENSITIVE SWR condition and even Damage the Meter when you show SWR but have a wildly swinging carrier envelope
- Too sensitive - possible damage to the meter
- Too much attenuation - takes away the ANT WARN light from Blinking
- HOWEVER
- REMEMBER this goofs up your TRUE calibration of SWR condition
- - VR1 can make SWR (REF setting) appear too low, you have to adjust VR1 BACK to OEM with an RF Power output of at least 4 watts to accomplish this re-calibration
- - you should reset - to make SWR track true antenna SWR..
