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Hi everyone,
I've completed my #391 QCX build for 20m. The receiver works well, but I discovered a strange problem with the power going out, as on the video:
I'll try to trace the path of signal with my oscilloscope soon, but maybe someone here experienced such behavior or has any idea about the probable cause. The PA section isn't too complex so there's just a few elements to check.
Lovely rig:-)
Olgierd SQ3SWF
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Hi Olgierd,
I had some similar problems with my 20m QCX.? The issue of output power changing after key down was due to the two inner low pass filter capacitors, C25 and C26 I think.? I changed them for some 200V ATC capacitors and the power output then remained stable after key down.?? The next issue started after trying WSPR.? This is the same as your key down sometimes with high power and sometimes with low power.? After touching up all the solder joints and getting nowhere, I changed the PNP key shaping transistor Q6 (2N2907) and it was fixed.? This transistor is running close to its limits and has been replaced with an MPS751 in later kits I understand.
Cheers, Glenn VK3YY.
On 19 October 2017 at 18:20, <opilarczyk@...> wrote: Hi everyone,
I've completed my #391 QCX build for 20m. The receiver works well, but I discovered a strange problem with the power going out, as on the video:
I'll try to trace the path of signal with my oscilloscope soon, but maybe someone here experienced such behavior or has any idea about the probable cause. The PA section isn't too complex so there's just a few elements to check.
Lovely rig:-)
Olgierd SQ3SWF
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Hi Glenn, thank you for a response.
Another few trials of transmitting into a dummy load resulted in power dropping to less than 1Vpp.. and staying there. :-(
I suspect MPS2907 is dead, voltage on BS170's drains is close to 0V during key down. Drive is a correct 0-5V square so I'm pretty sure it's the transistor.
I know MPS751 is a good replacement, but I'm not sure if I'll be able to easily obtain it - do you know any other replacements from BC series with >=1A Ic ?
Regards,
Olgierd SQ3SWF
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Funny thing.
As I'm looking for a suitable replacement for Q6, I decided to remove the transistor and short emiter with collector, providing 12V directly to BS170 drains. Obviously there's no envelope shaping now, but I'm only transmitting into a dummy load.
So, I set keyer to Iambic 60WPM, pressed the key and here's what I observed:
Yellow channel is power output, measured at 50 Ohm resistor, blue channel is "RX" line - gate of Q5 and pin 14 of Atmega IC.
As you see, when the power output is correct (nearly 20Vpp = ~3 Watts), the gate is properly grounded. When the output power is low, Q5 oscillates (!) and lets the power into the RX transformer! I measured high amplitude signal (tens of volts) on the primary winding of the transformer. Surely that's not how it should work.
"good" (high power, no oscillations, no RF power on the RX part - behind Q5):
"bad" (low power at the output, Q5 letting the RF power into RX chain):
When Q5 start oscillating, finals get hot pretty quickly as they're no longer working in class-E circuit. Funny thing - the issue is quite random and I'm wondering if it will happen with proper envelope shaping in place.
Wondering if everything is OK with Q5 transistor - I'll try replacing it with another one. Possibly, adding a small choke to the gate may improve the situation and block the oscillations.
Regards,
Olgierd SQ3SWF
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Hi Olgierd
SQ3SWF
? Why is the "dah" only twice the duration of the "dit"?
? John, VK6JY needs to understand.?
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On Fri, 2017-10-20 at 00:00 -0700, opilarczyk@... wrote: Funny thing.
As you see, when the power output is correct (nearly 20Vpp = ~3
Watts), the gate is properly grounded. When the output power is low,
Q5 oscillates (!) and lets the power into the RX transformer! I
measured high amplitude signal (tens of volts) on the primary winding
of the transformer. Surely that's not how it should work. Is the lack of a DC reference on the drain of Q5 a possible cause ? Is the gate being properly driven when the problem occurs ? PeterO G0DZB
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On Fri, 2017-10-20 at 08:23 +0100, G0DZB wrote: Is the gate being properly driven when the problem occurs ? Ignore that question as I now see the yellow trace IS the gate voltage (I had wrongly assumed it was the source). PeterO G0DZB
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@Peter - yellow trace is the output power, 50 ohms on the BNC connector. Blue trace is the gate of Q5.
@John - these are all equal length "DAHs", some of them are randomly transmitted with higher power, some with lower. Soom look like they're longer, but it's just how my oscilloscope shows things. If I zoom in a little, a gap between there would be clearly visible :-)
Olgierd SQ3SWF
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On Fri, 2017-10-20 at 01:16 -0700, opilarczyk@... wrote: @Peter - yellow trace is the output power, 50 ohms on the BNC
connector. Blue trace is the gate of Q5. Gate-source confusion AND Yellow-Blue confusion ! I better not do anything important today as I seem to be getting everything mixed up LOL ! PeterO G0DZB
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Hi Olgierd,
I quickly tried to duplicate your setup, monitoring the RF out (top trace) and the control signal to Q5 at pin 14 of the processor.? This is my trace, it looks OK.? One thing that did not make sense for me after replacing Q6 to resolve the key up power issue similar to yours.? When my QCX was transmitting low power (~1W) the current draw was nearly an amp.? When the key up produced a high output (~4W) the current draw was about 500mA.? Maybe my Q5 was doing something similar and shunting RF to the front end????? Anyway, it did go away after replacing Q6.
On 20 October 2017 at 18:30, G0DZB <g0dzb@...> wrote: On Fri, 2017-10-20 at 08:23 +0100, G0DZB wrote:
> Is the gate being properly driven when the problem occurs ?
Ignore that question as I now see the yellow trace IS the gate voltage
(I had wrongly assumed it was the source).
PeterO
G0DZB
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On Fri, 2017-10-20 at 19:32 +1100, Glenn S wrote: Hi Olgierd,
I quickly tried to duplicate your setup, monitoring the RF out (top
trace) and the control signal to Q5 at pin 14 of the processor. This
is my trace, it looks OK. One thing that did not make sense for me
after replacing Q6 to resolve the key up power issue similar to yours.
When my QCX was transmitting low power (~1W) the current draw was
nearly an amp. When the key up produced a high output (~4W) the
current draw was about 500mA. Maybe my Q5 was doing something similar
and shunting RF to the front end???
Anyway, it did go away after replacing Q6. Early on in testing of my 17m QCX I found that if I slowly increased the PSU above about 13V the output would suddenly drop and the current increase (luckily I had a current limited PSU). This seems consistent with your observations. It eventually killed Q6. I don't think I tried increasing the PSU after I changed Q6 for something beefier. I'm wondering if the rf signal seen on the gate of Q5 is the cause of the problem or is it a symptom and a consequence of a problem elsewhere ? PeterO. G0DZB
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Hi Glenn,
thanks a lot. Yes, when transmitting with "low power" finals get hot quickly, I assume the current goes into RX front end. Well, not an assumption - it actually does - I've seen it on the scope ;-)
I found a few possible replacements (2A PNP) for dead Q6, unfortunately none of them is a drop-in replacement due to a different pin order... But I'll somehow squeeze it and I hope it will resolve the problem. Will let you know!
Olgierd SQ3SWF
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Hi Olgierd / Peter,
Just finished dinner and had a closer look.? I can simulate Olgierd's issue with low power out and high current on the first PTT after power up.? I have noticed this before and ignored it.? It is repeatable in my case. The second and subsequent PTTs after power up are fine. The scope captures some oscillation on the gate to Q5 on the first PTT, even though there is a bypass capacitor on first PTT.
Cheers, Glenn VK3YY.
On 20 October 2017 at 19:45, <opilarczyk@...> wrote: Hi Glenn,
thanks a lot. Yes, when transmitting with "low power" finals get hot quickly, I assume the current goes into RX front end. Well, not an assumption - it actually does - I've seen it on the scope ;-)
I found a few possible replacements (2A PNP) for dead Q6, unfortunately none of them is a drop-in replacement due to a different pin order... But I'll somehow squeeze it and I hope it will resolve the problem. Will let you know!
Olgierd SQ3SWF
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Hi all
There seems to be some oscillation sometimes in some of the kits. I am not 100% sure of the cause yet. I am interested to know if anyone has seen it, with the new MPS751 transistor at Q6??
Also - some people mentioned that instability is cured by reducing the value of the 1nF capacitor C33, to 150pF. Can I suggest you try this also? There is a 180pF capacitor already in the kit, that could be used for this purpose.?
73 Hans G0UPL
On Fri, Oct 20, 2017 at 12:15 PM, Glenn S <glenn.vk3yy@...> wrote: Hi Olgierd / Peter,
Just finished dinner and had a closer look.? I can simulate Olgierd's issue with low power out and high current on the first PTT after power up.? I have noticed this before and ignored it.? It is repeatable in my case. The second and subsequent PTTs after power up are fine. The scope captures some oscillation on the gate to Q5 on the first PTT, even though there is a bypass capacitor on first PTT.
Cheers, Glenn VK3YY.
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Hi Hans,
I tried adding 680pF SMD cap underneath C34 (something about capacitor inductance), but it didn't really change the behavior. I got two replacements for the dead Q6 (2SA-1315 and 2SB-892), will check with those.
1nF cap impedance is about 11 ohms @ 14 MHz, 180pF @ 14 MHz is 63 Ohms - wouldn't that reduce gain of the receiver?
73 Olgierd SQ3SWF
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Hi Olgierd
Yes, reducing C33 to 180pF will reduce the gain a bit. But there is plenty of gain anyway; and if it is the only way to stop the oscillation then it would be a good tradeoff...?
I am still not sure why a few people seem to have this problem but most do not.?
73 Hans G0UPL
On Fri, Oct 20, 2017 at 2:48 PM, <opilarczyk@...> wrote: Hi Hans,
I tried adding 680pF SMD cap underneath C34 (something about capacitor inductance), but it didn't really change the behavior. I got two replacements for the dead Q6 (2SA-1315 and 2SB-892), will check with those.
1nF cap impedance is about 11 ohms @ 14 MHz, 180pF @ 14 MHz is 63 Ohms - wouldn't that reduce gain of the receiver?
73 Olgierd SQ3SWF
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I have seen it on a 17m yota model that had the shorted Q6. Perhaps not everybody reporting this while running it with a broken transistor?
Could it be a undefined DC voltage on Q5 drain that somehow starts the drain-source diode conducting when a fast rising RF pulse comes along? If so, a solution could be a high value resistor from Q5 drain to +12V to keep it away from negative voltages.
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Hi all,
I put a 4k7 resistor from the drain of Q5 to the +12V rail, and for me, this has resolved the high current draw and low power output on first PTT.? I tried about 30 power cycles without noting the issue, where as previously, it was repeatable on every first PTT. Olgierd, it would be interesting to see if the resistor helps your problem.? I have put a photo of the resistor placement in my blog. Also I checked the MDS of the receiver and as expected, it did not change.? The MDS is roughly 0.2 uV in my case (I reckon I could copy CW at this level).
Cheers, Glenn VK3YY.
On 21 October 2017 at 00:52, Daniel Ekman (kng) <knegge@...> wrote: I have seen it on a 17m yota model that had the shorted Q6. Perhaps not everybody reporting this while running it with a broken transistor?
Could it be a undefined DC voltage on Q5 drain that somehow starts the drain-source diode conducting when a fast rising RF pulse comes along? If so, a solution could be a high value resistor from Q5 drain to +12V to keep it away from negative voltages.
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Hi Glenn,
I have tested the solution provided by Hans yesterday, to replace 1n C33 with 180pF - it works fine, no oscillation. I will try your solution as well as it seems a bit more elegant (retaining the original sensitivity), but...
I'm only getting 20 Vpp output on 50 Ohms which is?1 Watt!?- over 11V DC of supply on the drains, about 30V peaks while keying. Not sure what's wrong now. Drive on the gates looks correct, I removed one of the finals and it's still nearly the same output. Wondering if one of them may misbehave..?
I'll hapily use about 2-3 Watts output, looking at my previous screenshots 20 Vpp was the maximum I could pull out. Compared with FT-817 on the scope and yes, it's surely 1W output, I miscalculated the power using?wrong formula :(
But I'm closer and closer ;)
73 Olgierd SQ3SWF
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Hi Ologierd,
I also replaced C33 (1nF -> 150pF) some time ago because I had the same issue. I did not see any oscillations since that.
It seems that many people had the same problem. Most of the dead Q6 might be caused by these oscillations because current was over 1A with no power output.
I wonder if the oscillations are specific to the 20m version (as mine and yours) or if all bands suffer from that issue.
Unfortunately one of the controller pins died during one of these "events". It is one of the keying inputs so the QCX is keying now all the time. At least I found a workaround while waiting for the replacement controller. I simply set initial keying speed to 0 and can use the QCX as receiver
73
Karsten- DL3HRT
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Glenn S
Daniel Ekman SA2KNG