SX-939 AM alignement

[cq.ab0nf]

Going through the final two steps of aligning the AM section on this SX-939, and am not sure what I am supposed to be doing.

It calls for adjustment of VR4 and VR5 which go to pins 15 and 16 of this HA1138 chip.

The start of the procedure calls for 400hz, 30% modulation, so that's where I have my SG set to (I have a Sencore SG80). Step 7 and 8 say to tune to 1400khz, 36db. We have a station there, so I move to the nearest quiet spot @1450. The instructions say to adjust VR4 to 70mV at that power. Increase to power to 80db, adjust VR5 until output is 310mv.

I'm not getting anything close to this. 36dB the signal is too weak to really get a good measurement. 80 db the output is higher than that. I've got to be missing something. Any advice?

 

[rcs16]

You are confusing dBf with dBuV.
Pioneer calls for so many dBuV above a reference of 1uV into 75-ohms.
Take any high school or college math?Logarithms with base of 10.
dBuV = 20 log V/1uV or 10^(36/20)
So 36dBuV is 63uV,
80dBuV is 10^(80/20) = 10,000uV or 10mV.
Pioneer should have done the math for you and everyone else.
the SG80 is calibrated in dBf which is power and in uV which is voltage into a 75-ohm load Z(impedance)
dBf, is dB above 1 femtowatt (10^-15 W) relative to the load Z of 75 ohm
dBf=10log W/10^-15

Use uV attenuator scale, as it is calibrated into 75-ohm.

Relating dBµ and dBf

k6sti.neocities.org

 

[cq.ab0nf]

Thank you. That gave me much better results.

 

[larryderouin]

Oh Jeez Rick, I just got a Migraine and my Blood pressure went sky high reading your post. Where's that Icepack?!

 

[rcs16]

My dementia test for the elderly members
I remember having a small red paperback book of log tables. These days the calculator in the smart phone does the trick.
I am of the age where in school they stopped teaching us slide rules and vacuum tubes.
Now if it was calculus, then I'd be going for the icepack.