[Elecraft] KAT2 Problem

[Thomas Beaudry]

> Sorry your having trouble.  Sounds like your still stuck
> trying to get the forward power measurement circuit working.

Yes.

> I figure that R6 is not installed.  So, when you go into TUNE,
> the K2 is still using the circuits on the RF board to measure
> power out level.

Correct.

> I figure that the K2 is making 5 watts - that DC current is
> up over 1 amp.

Correct.

> I figure that you have a good dummy load on ANTENNA 1
> and you have antenna 1 selected.

Correct.

> I figure that you did the null adjustment, C55, and had
> a little trouble getting a reading of zero - somewhere
> around 10 is the best you got.  (That is normal - is that
> what happened on yours?)

0.45 volts is the best I could do.  I also went out and bought a good 
analog meter to make the null adjustment easier as others have 
suggested.  BTW, with the power adjusted down to 1 watt so the K2 
display shows other than HI, the null value fluctuates between 20 and 
100 and once in a while displays 000.

> As to the expected value at the top of R1: That should also be
> 3 to 4 volts with 5 watts into the ATU.

With C55 nulled, I get 0.50 volts at the top of R1 with 5 watts.

> I don't know if you have checked the top of R2.  That should
> indicate reflected power.  You should not see much voltage
> there.  I'd guess less than a volt.

0.45 volts with 5 watts.

> Find some way to get a little reflected power - perhaps a 500
> ohm dummy load.  Make sure that the voltage at R2 goes up.
> Figure that you should see 3 to 4 volts at R2 with no load
> connected on antenna 1 with 5 watts in.  Of course, the K2
> won't like that very much.

At 5 watts and no load, it stays at 0.45 volts.

> If the reflected power, REFL, pin 4 on U1, shows reasonable
> changes when the dummy load is changed - say .8 volts with
> 50 ohms dummy load at 5 watts with R1 and R2 fully clockwise,
> and say 3 to 4 volts with a high VSWR dummy load, then the
> reflected power part of the SWR bridge is working.

U1-4 at 5 watts is 0.45 volts with a 50 ohm dummy load, increases to 
0.55 with no load.

> In that case, if FWD is still down around 0.8 volts, figure that
> D1 may be bad.  (Sometimes the reverse breakdown voltage
> on the 1N5711 diodes is ruined due to electrostatic discharge
> or something.)

D1 and D2 both show 1.7k in one direction and 105k in the opposite.

> On the other hand, if REFL doesn't do much of anything, then
> perhaps R3 is shorted out some how, or the RF path through
> the ATU to antenna 1 is blocked.  That should have caused
> odd readings when you did the null adjustment.

RF path shows good, no opens or shorts.  I pulled one end of R3 and it 
shows 196 ohms.

> 73 and GL, de Michael, AB9GV

Thanks.  With all the problems I'm having getting this simple circuit 
to work, you wouldn't think I used to repair prototypes in an 
engineering lab.  <frown>

-- 
Thomas M. Beaudry
k8la / ys1ztm
K2 # 3422