Re: QCX 40 NO POWER OUTPUT #problem #40m #qcx40


James Daldry W4JED
 

Hi, John

The voltage chart was written using the on-board "voltmeter". Since it has a relatively low input impedance it will pull down any voltage that has a high impedance source. Trust me - the actual voltage on pin 3 of IC8, as with any op amp wired as a voltage follower, is within a few microvolts of the voltage on pin 2. If you were to put a second voltmeter on IC8 pin 1 or 2 (they are connected together) while checking pin 3 with the on-board voltmeter you would find that all 3 pins would be at the same voltage - within microvolts. The reason the on-board meter is used is that that is the meter Hans knows that every QCX builder has on hand.

These op amps used in these radios have a DC open-loop gain of 140 db. That's more than the path loss between the 50000 watt FM radio station you are listening to and your radio antenna. Any difference in DC voltage between pin 2 and pin 3 is magnified by 140 DB.

73

Jim W4JED

On 2/26/20 8:27 AM, John Brock wrote:
It seems to me the voltage measurement table in the instruction manual is off in a  few instances. I am  looking through the receiver function on my 20M QCX. It seems too quiet to me. Don't get enough volume. Hear a few faint signals. I will admit my Icom isn't hearing a lot on 20M either..

So What confuses me on the voltage measurement table is the expected readings on the differential amps in the RCV chain. Like IC6, IC7, IC8 pins 2 and 3 are inputs to the amp. I think they should be equal. Same with pins 5 and 6. The manual says IC8 pin 2=2.46 while pin 3=0.65 for example.

Can someone set me straight on this?

Thanks
John
WA8US

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