Re: 4S-Tuner SWR Red LED


I had a similar problem with mine. I solved by connecting a 5k trim pot across the red led. With the tuner feeding a 50 ohm dummy load and adjusted for maximum dimming of the red led, I adjusted the pot until the led just extinguished. This does not really effect the sensitivity of the SWR indicator but rather compensates for the required base current into Q1 when Q2 is cut off. I left the pot installed as this is a balance situation so it would be very difficult to match the exact value required using standard value fixed resistors. It would also not be able to accurately calculate the value because it is dependent on the small signal current gain of Q1 and the threshold current of the LED. This worked for me hope it helps.   W3BNO

On Mar 3, 2021, at 9:49 AM, Don, W9EBK <donw9ebk@...> wrote:

I'm at work so I don't have time to look at the schematic but from the description it sounds like the current limiting resistance on the Red L.E.D, or the cutoff current of that L.E.D, is too low.  L.E.D.s are usually 20mA, 40mA, or 60mA for maximum brightness,  but the cutoff varies depending on the L.E.D.  If someone wants to do a bit of experimenting, the fix might be as simple as swapping out the Red L.E.D for one of a higher maximum current rating so it cuts off at the signal level of a low SWR.  If there is a resistor in series with that L.E.D then a larger resistor might do the job.

If I get some time tonight I'll take a look at the schematic. 

Don, W9EBK 

On Wed, Mar 3, 2021, 9:29 AM Adam Allen <adamallen@...> wrote:
I have a similar issue.  I can get the green LED to full brightness, but at best (even at almost 1:1 SWR according to other gauges) the red only gets dimmer.  It doesn’t turn off or even “fully” dim. I’ve learned to live with it but if there’s a fix floating around I’m interested. 

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