Re: QCX 20M with very low power

Mike Easterbrook

Just to adding my experiences with QCX20/low power -  & some help still needed.

I had a spare QCX 30 kit with V4 board which I  have now built up for 20m using latest updates i.e. R41/42 150R and 1.2K, C31 at 2.2uF;  R21 at 10K, C25/26 390pF ex Vishay/Digikey.

RF out per SWR/Power Meter (has been side-by-side compared with known(?) to be accurate meter in 1.0 - 5.0 W range) and found "similar". Following measurements made at 13.8 V at the connector i.e. before D3:

L1/2/3 wound per manual   ~1.0W
L1/2/3 wound to manual inductance tables using recently purchased Peak LCR40  ~1.5W
L1/2/3 adjusted for max output by squeezing/distancing turns on toroid and subtracting turns if necessary to peak output  ~ 4W  (final turns no. L1=13, L2=15, L3=13)  Observation was that only L3 adjustment had significant effect on power out. Reducing L3 to 12 turns gave output~5W.  However at this level there some evidence of intermittent instability/oscillation accompanied by immediate increase in total current from ~540 to ~990ma rapid heating of the finals and maybe 50% less power out.  Sending a series of dits about 1 in 20 were of much reduced power. The problem disappeared on reducing supply volts to 12 or increasing L3 to 14 turns. (i.e. keeping power out to below 4W)

On the assumption that the instability was in the key shaping section, I looked at the output with the scope and found complete absence of shaping.  I compared this with my QCX40 (on a V5 board built according to manual but with L1/2/3 turns reduced to match quoted impedance (17/22/17). This is measuring 5W out at13.8V with apparent good stability and OK shaping (see attached pics)  Grateful for any ideas on where to investigate the key shaping - component values and soldering look good.

Maybe unrelated incident - the LCD developed a fault - screen contrast suddenly very low but can be adjusted via R47 but random gibberish characters displayed and no communication with IC2. If I transfer this LCD to another QCX the fault remains.  Fault disappears with same board and a working LCD.  Not aware that the LCD was subjected to any trauma other than multiple plug-in/plug-outs while doing the L1/2/3 adjustments - anyway need to replace.  Anyone got a part no. for this item so I can order locally?  Don't want to add to my stock of wrongly ordered displays from old (non-QCX) project (pin outs not lining up, wrong colour, wrong backlight etc etc) 

Mike 9M2LXM

On Fri, May 8, 2020 at 8:11 AM ajparent1/KB1GMX <kb1gmx@...> wrote:
Something else is very wrong and usually if the caps are an issue the
power is in the 1W out range or more.  

IF anything check the DC voltage at the emmitter and collector of Q6
it should very near 12V when transmitting.  

Shorts to ground in the TX section could be a problem.

Other possible issue is the fets Q1-3 are in back ward or rotated 120 degrees.

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