Re: Diagnostics fail with no BS170s installed
#qmx
Nice job replacing that tiny thing!
What a strange fault.? Very glad itnis working for you now.?
Stan KC7XE?
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Re: QMX+ No transmit or receive
On Sat, Dec 28, 2024 at 02:57 PM, Nick Garrod wrote:
someone else who has looked at the voltages in the modulator under normal operation
Here are some waveforms of Q507 from a working QMX+.? All are of the transmission of a single 30wpm 'dit'.
My supply voltage was about 11.7V, scales as labeled, with zero baseline one line up from bottom.
?
1) note that at idle receive, Q507-gate should be at about 6V (1/2 V+), Q507 is 'on' and Q507-drain is at 12V (V+).? DAC is high (3V).
2) at start of transmit, cpu drives DAC low, to start the transmit shaping by starting the power at 0.? DAC low turns on Q513, which ends up turning off Q514, which makes Q507-gate go high, turning it off, making Q507-drain go to 0V.
3) cpu ramps up DAC, which gradually pulls Q507-gate back down toward 6, increasing the Q507-drain voltage to near full value.
4) to end the transmission, cpu ramps down DAC again, causing the reverse shaping happen at the end of the 'dit'.
5) after transmission is complete, cpu sets DAC high again during receive.
6) note that the Q507-drain voltage modulates the amplitude of the drive signal from 0-100%, but that drive signal comes from the TX and CLK2 signal into the BS170 drive transistors, so when TX is not asserted, no transmit occurs regardless of the Q507-drain voltage.
?
Screenshots:
Yellow trace is Q507-gate in all plots.?
Blue trace is Q507-drain in #1, #2.? Blue trace is DAC in #3.? Blue trace is RF out in #4.
Trigger is near left of screen at start of 'dit' (rising edge of Q507-gate).
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Re: A question about L401 windings QMX v1 vs QMX v2
thank you for the link Stan...73
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S
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I hit the 510s with circuit chiller while it was acting up.? No luv. Hit the rest of the amp parts as well.? Nothing.?
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regards,
Bryan, N0LUF
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Re: QCX Mini 20m - help please ??
Thanks Ron. I will do that. :)
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QMX, one of op-amps self-oscillates at 25 MHz (or picks up the system clock?)
For some time (several months) I had what I thought was some intermittent
QRM, where the band was covered in spikes spaced exactly 100 Hz apart.
Here's how it presents itself in the WSJT-X (there are differences on the
waterfall, because I was switching bands, and between digital and CW
modes).
When it happens, I can still see the band activity below it.
It came and went and I didn't think much of it, but today, when testing a
new antenna for another band, I noticed the same QRM and started digging.
Here's what I found:
In a speaker it manifests as buzzing. See the video (and pardon the mess
on a desk):
It doesn't change when switching bands. It does change (but not go away)
when switching between digi and CW. The same change happens when I enter
diagnostics menu (maybe QMX internally switches to CW when entering this
menu).
It doesn't come from antenna (tested with a disconnected antenna).
It doesn't come from the power supply (tested on batteries).
It doesn't come from a nearby RF field (tested by walking with QMX around
the house without any change).
So it's internal to the QMX.
I started poking and probing, looking for a bad solder joint or something
else intermittent. Everything seemed fine. Then I turned on the
oscilloscope and started poking around the input circuity.
I found some weird ~25 MHz oscillations on outputs and inverting inputs of
two op-amps (two other were fine), both in the same chip (IC406, LM4562).
Look (and pardon the mess once again):
Mind you, it's not present on any noninverting input.
My plan for now is to desolder this IC and see if it goes away (maybe
PCM1804 is injecting something -- the signal is also present on its VINR+
and VINR-, not present on VINL+ and VINL-), maybe replace this IC, but
it's still strange and puzzling to me.
Why is it intermittent?
25 MHz is the system clock, the output QRM is quite stable, which might
suggest that this signal is in phase with the system clock -- is it
related?
Maybe it's some bad decoupling somewhere, or some other element failed and
the op amp is good?
What am I missing?
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Re: QMX+ No transmit or receive
Okay Nick. I appreciate the quick response. I need to put the radio away for the night anyhow but i think if i find the source of the transmit problem, i may find the source of the receive problem as well. One other piece of info, because i can hear audio (cw receive white noise), but no actual signals, would that also mean that IC 407, IC 401 and IC 404 may perhaps be good? I think the audio out section must be good if thats the case. Okay. Thanks for your help? and enjoy your weekend and have a happy new year!
Now take this with a pinch of salt, but looking at the component values in the QMX+ a DAC output of 1.92V into the base of Q513 should turn the TX hard on.? However it’s late in the evening here and I may have misunderstood something fundamental. Good luck. Nick G0OQK
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Re: QMX+ No transmit or receive
Now take this with a pinch of salt, but looking at the component values in the QMX+ a DAC output of 1.92V into the base of Q513 should turn the TX hard on. ?However it’s late in the evening here and I may have misunderstood something fundamental. Good luck. Nick G0OQK
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Re: QMX+ No transmit or receive
Hi Joel, with your measurements the VGS of Q507 is -1.3V, which is not enough to turn it on. The specs for the FET are -1.5 minimum and -2.5V typical.
The modulator circuit is to my understanding for power level control and to give raised cosine shaping to the TX signal on key down and up.
Hans used a similar circuit in the 5W PA and provided a detailed description of its operation in the assembly manual for that unit. The main difference is that the modulator transistor is a FET in the QMX and a bipolar arrangement in the 5W PA.
I’m away for a few days, so can’t take any measurements on my QMX+, but would expect, the DAC output to be an active low voltage on key down, with a raised cosine leading and trailing edge.
Hopefully someone else who has looked at the voltages in the modulator under normal operation can offer some insights.
As my unit worked, I didn’t really give it much thought until trying to answer your question.?
If you look at the PA circuit of the QDX transceiver, the modulator isn’t present and the PA choke is connected directly to the PA supply, which is why the QDX is not suitable for CW transmit, as it lacks waveform shaping.?
I hope this is all helpful to you. 73 Nick G0OQK
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Re: Release of version 1.2 of ABS solution for QMX+ (Automatic internal antenna tuner, internal battery and internal speaker with audio amp)
#ABS
#ATU
#PSU
#QMXplus
Would love to be part of a group buy.
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72/72 de OZ1ES Erik
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Re: QMX+ No transmit or receive
BTW, one more thing....What sort of signal and p.p. voltage should i see on either the base or emitter of those transistors? thanks.
VY2KW
Hi nick. I have tested and yes, R506 and R507 coincide with the source and gate voltages at Q507. The next step i guess, will be to check the gate of Q507 with my scope and see if i am seeing a signal. R507 drops my 12v down to approx. 10.7v and then R506 drops the 10.7 volts down to about 9.25v. Source voltage at Q507 is 12v, gate is 10.7v and drain is .33v. I noticed there is a DAC input to the transistors Q512 to Q515. Is this correct terminology? thanks.
On your very first screen, the readout confirms what you first suspected in that Q507 is not turning on, hence one reason for no transmit. Might I suggest measuring Q507 VGS, the voltage between the gate and source and see how that compares with the voltage at the junction of R506 and R507.? The voltage should be the same as on the gate, as they are connected together. Nick G0OQK
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Re: QMX+ No transmit or receive
Hi nick. I have tested and yes, R506 and R507 coincide with the source and gate voltages at Q507. The next step i guess, will be to check the gate of Q507 with my scope and see if i am seeing a signal. R507 drops my 12v down to approx. 10.7v and then R506 drops the 10.7 volts down to about 9.25v. Source voltage at Q507 is 12v, gate is 10.7v and drain is .33v. I noticed there is a DAC input to the transistors Q512 to Q515. Is this correct terminology? thanks.
On your very first screen, the readout confirms what you first suspected in that Q507 is not turning on, hence one reason for no transmit. Might I suggest measuring Q507 VGS, the voltage between the gate and source and see how that compares with the voltage at the junction of R506 and R507.? The voltage should be the same as on the gate, as they are connected together. Nick G0OQK
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I should add that when installing the new pads, test for shorts and reset the bias. Too much bias can cause excessive heat.
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Bryan,
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I suspect that the connection to the heatsink is the problem.? It should be getting warm.? Try resetting the IRF510s on the heatsink.? I also suggest switching to AlN (Aluminum Nitride) insulators instead of the silicon pads that QRP-Labs provides.? Here is an example:
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This helped with my WA2EBY amp.
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73
Evan
AC9TU
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Re: New build QMX - high noise on all bands
Thanks for confirming Gus, I've been waiting to hear back from you. I've now got a new PCM1804 chip and I'm plucking up the courage to have a go at replacing it.
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Re: New build QMX - high noise on all bands
Just following up to confirm that what I had previously expected was the actual culprit. A bad PCM1804 was causing the AGC to engage constantly resulting in almost no signal reception on all bands. Chip was replaced and now functioning perfectly!?
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Gus
W9SSN
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Gus
W9SSN
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Re: QCX Mini 20m - help please ??
Lidiette, try it with only a dummy load attached and see if the problem persists.?
Ron
Ok, so the BNC connector was a little bit loose. I re-soldered and it is firm.? I connected the antenna which it is the antenna that I currently use for my home base station. I connected the qrp antenna tuner.? The problem still persist. I have to unplug and plug the power for the function to start again (but it will eventually happen again).
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I'm glad I checked the BNC connector though.?
?
Thanks,
?
Lidiette
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Re: QCX Mini 20m - help please ??
Ok, so the BNC connector was a little bit loose. I re-soldered and it is firm.? I connected the antenna which it is the antenna that I currently use for my home base station. I connected the qrp antenna tuner.? The problem still persist. I have to unplug and plug the power for the function to start again (but it will eventually happen again).
?
I'm glad I checked the BNC connector though.?
?
Thanks,
?
Lidiette
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Re: A question about L401 windings QMX v1 vs QMX v2
Scott,
See the description at this link, which is on the QMX page regarding a fix to a 20m receive sensitivity issue:
To take proper advantage of the new windings, you should also make the described change to the band configuration.
Stan KC7XE
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Lidiette Lacalle
Gus (W9SSN)
Ronald Taylor