Good morning,

I received my 40m QCX last week and took a few days to build it. What an excellent kit with an amazing manual.

After power on everything appears to be working, I was able to align successfully and I'm able to hear stations clearly. I have sidetone and I hooked up the RF output and on transmit with a 12v power supply it shows 5w output (this seems high?).

I noticed a slight hum and that increased when adjusting the audio gain. Waving my hand around IC9 increases the hum the most.
Touching resistors in that area make a pretty loud hum (R35. R34, R33, R28 etc)

I took some voltage readings on the ICs in this area and came up with the following:

IC8 Pins 1, 2, 3, 5, 6, 7 are all 2.64v (4 and 8 are as expected).

IC9
Pin 1 4.84
Pin 2 4.84
Pin 3 4.5
Pin 5 2.6
Pin 6 2.6
Pin 7 2.6
Pin 8 11.8

Looking through other posts here it seems R59, and IC8 could be a problem areas.

Looking at R59 - ohms were 3.3k. Voltage on both sides and it appears to be 2.5. I would expect one side to see a drop? Does that indicate a short somewhere?

I've checked for continuity between all pins on IC8 and IC9 and found none. Solder joints seem to be OK under a magnifying glass. I've reflowed a few questionable pads but this has had no effect.

All I have is a multimeter to troubleshoot and not much knowledge (although I've certainly learned a lot already!)

Any pointers in the right direction??

Thank you very much!

I noticed a slight hum and that increased when adjusting the audio gain. Waving my hand around IC9 increases the hum the most.
Touching resistors in that area make a pretty loud hum (R35. R34, R33,

Rick,

I would not worry about those "symptoms" probably normal, there is a lot of gain. Maybe the slight hum is pickup from a transformer or your PSU is not smoothed enough.

IC8 Pins 1, 2, 3, 5, 6, 7 are all 2.64v (4 and 8 are as expected).
IC9
Pin 1 4.84
Pin 2 4.84
Pin 3 4.5

Pin 8 11.8

These voltages will not make much difference to the basic functioning but how can they be what you say?
Check IC6 & 7 pins 1.
I guess the slider of R27 is also 2.5?

Pin 5 2.6

Pin 6 2.6

Pin 7 2.6

Check R39 & 40 are correct values and that C24 is correctly oriented.

Looking at R59 - ohms were 3.3k. Voltage on both sides and it appears to be 2.5. I would expect one side to see a drop? Does that indicate a short somewhere?

Oh, there should be no flow so that looks good.

Check the IC6, 7 voltages.

73 Alan G4ZFQ

Thanks for the great reply Alan.

Am I correct in thinking that as long as these amps are biased (> ~0.6v) things are ok with the radio?

As for the requested voltages:
IC6 - All pins 2.64v (except 4 and 8)
IC7 - 1, 2, 3, 5 - 2.69v?
IC7 - 6 and 7 - 2.7v
IC7 4 and 8 as expected

C24 is oriented correctly and shows 6v
R27 Shows 2.68 on the top, 2.67 middle and 2.67 lower
R39 measures at 8.75k in circuit (and gets there slowly) 0v on the bottom, 6v on the top?
R40 measured 10k in circuit 5v on the right, 11.92v on the left (viewed from the back of the board)

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Hi, Rick

My guess is that you are hearing power supply ripple. Put together 8 penlite - c cells - flashlight batteries and try the radio on that. One of the downsides of direct-conversion receivers is that they are very susceptible to power supply hum, being that all the signal gain is at audio. If you have a gel-cell I will permit you to use that. I'm pretty sure you will find that the hum is gone.

Another source of hum, again power supply related, is a ground loop between the ground on the power supply and the ground for the antenna. Since the power connection and the antenna connection are at different points on the board, if there's any potential difference between the 2 grounds it will show up as hum in the audio.

Jim W4JED

Linux user since Debian Bo

Am I correct in thinking that as long as these amps are biased (> ~0.6v) things are ok with the radio?

Rick,

They will work, but if not biased correctly will distort earlier on large signal inputs.

However:-

As for the requested voltages:

I must apologise, coming to my senses it seems all is correct. The only major difference is IC9 pin 3 because you used your own DVM, not the one incorporated in the QCX therefore getting the difference.

C24 is oriented correctly and shows 6v

Which also shows I got the IC numbers mixed..

Sorry, I got you worrying unnecessarily.

73 Alan G4ZFQ

IC6 - All pins 2.64v (except 4 and 8)
IC7 - 1, 2, 3, 5 - 2.69v
IC7 - 6 and 7 - 2.7v
IC7 4 and 8 as expected

R27 Shows 2.68 on the top, 2.67 middle and 2.67 lower
R39 measures at 8.75k in circuit (and gets there slowly) 0v on the bottom, 6v on the top
R40 measured 10k in circuit 5v on the right, 11.92v on the left (viewed from the back of the board)

Hi: This is KA7QVE I built my 40 meter QCX and it does exactly the same thing . However, when I got it all together and in the case I can handle the knob and case and no hum is herd unless you take the knob off. I am not sure? about my 20 meter rig I didn't notice? any audio noise? problem on it when I touched the bare shaft on the volume control. My 40m QCX is very sensitive on receive I have to have the audio mostly turned all the way down or it distorts the audio on strong signals. I have had problems with my 20m unit with low power and an iffy control for frequency adjustment slow to fast adjust.

Thanks all,

I did hook grab a battery as the power source and the noise is gone.

The incorrect voltages still persist but I've been able to make it out via WSPR so I'm guessing things are ok for now??

I'll continue to try and work my way through and update this thread if I find anything out or have any more questions.

The incorrect voltages still persist

Rick,

Going right back to the beginning when in my haste I made my mistakes

IC8 Pins 1, 2, 3, 5, 6, 7 are all 2.64v (4 and 8 are as expected).
IC9
Pin 3 4.5
Pin 5 2.6

These are ALL as expected IF you used your own DVM. The manual's chart shows voltages using the built-in meter. Try that if you wish. The built-in meter loads the circuit and does not give true readings. A high impedance DVM gives correct readings.
The manual says "Bear in mind that in some of these measurements, the act of measurement will itself change the measurement. The “input resistance” of this DVM is only 13.3K" (Several others have been confused here, including me last night.)

73 Alan G4ZFQ

Alan,?

Understood. I read that note in the manual but I didn't expect the values to be that far off. Still waiting on the first QSO but things look good on WSPR.?

Thanks for helping a confused newbie!