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Re: QCX - Damaged Finals? (with IR photo)

joe@...
 

Alan -

Thanks!  I will follow your advice - this community is great because of all the thoughtful responses.  I have spares on hand - I’ll try them out!

-Joe


Re: Circuit Board Drills for cleaning out through-holes: Construction Tip

w9ran
 

I sometimes use orifice drills to clear PCB holes.  They're just twist drills in a piece of brass hex that works as a handle, which could be improvised a number of ways.     If you try this method you want to use a drill size that is considerably smaller than the ID of the hole so it's less likely to damage the barrel.

The key is not to "drill" but to heat the bit and pad with your soldering iron and gently twist it back and forth with slight pressure to shove melted solder out the other side.   Since the bits are steel, solder won't stick to them and since they have little mass it's possible to heat them to the point where they will conduct enough heat to melt solder in the barrel of a plated-thru hole, even  a thermal ground via.   A component lead can be used as well but since the solder will stick it doesn't remove the solder nearly as well.   Failure to maintain a light touch will probably result in board damage, so be careful!

73, Bob W9RAN


Re: QCX - Damaged Finals? (with IR photo)

Alan G4ZFQ
 

Q6 was also a BS170 - it is actually an MPS751, so pins 1 and 2 are the emitter and the base - and 150 ohms would be a low value, unless that is normal due to something else in the circuit.  So - I'll pull it and see if the short goes away, and if so, replace it and see if the problem goes away!

Joe,

Often Q6 has been damaged by a short-circuit BS170.
If Q6 is just shorted then the PA will work if all the BS170s are good. Q6 just turns them off.
Your situation is far from unusual. Look back, many posts about this.
You should replace Q1, 2, 3, 6 then hope that is all. Better, go through troubleshooting after removal of these components.

73 Alan G4ZFQ


Re: QCX - Damaged Finals? (with IR photo)

joe@...
 

"If its pulling 850ma and only putting out 1/2 watt your doing something wrong."

Yes - as I said, it was working, but was running hot.  Now I have no RF output, and the parts I mentioned are dissipating a lot of heat.  Running with a supply voltage of 15 volts is within limits (the manual says 7 - 16 volts), if you have proper cooling.  Maybe i did not.  I assumed Q6 was also a BS170 - it is actually an MPS751, so pins 1 and 2 are the emitter and the base - and 150 ohms would be a low value, unless that is normal due to something else in the circuit.  So - I'll pull it and see if the short goes away, and if so, replace it and see if the problem goes away!


Re: QCX(-like) for 2m #alignment #vfo

Guido PE1NNZ
 



On Friday, September 6, 2019, Kārlis Goba <karlis.goba@...> wrote: 
So, experiments might be possible, but the prospect is not that bright I think.
 
Probably true, but worth a try. The  Reduced noise figure performance probably could be compensated with a tiny VHF pre-amp.

Regarding your click at 80 MHz, it seems that you might hit the t_on+t_off speed limit of FST3253, which could be around 12ns.

Yes, that is probably what it is, I have no idea how it performs at 70 Mhz but at 50 Mhz it is seems sensitive enough for actual use.
Actually, I forgot about it, but at higher frequencies (167 Mhz) this FST3253 QSD is just working again.. Absolutely no clue why this is possible (maybe t_on/off times starts overlapping partially).


Re: QCX(-like) for 2m #alignment #vfo

Kārlis Goba
 

The PO3B14A is indeed a nice find but I'm afraid it won't really make much of a difference. I'm basing my view on what Chris Trask reported in his Mixer musings (https://www.mikrocontroller.net/attachment/146369/Mixer_Musings.pdf). He measured the switching performance (albeit in a different configuration than a QSD) for FSA3157 (see Table 1), which theoretically has t_on=3.4ns and t_off=2.1ns, so kind of close to 5ns of the 'Potato' chip. However, the mixing/switching performance starts to drop once you reach about 100 MHz. He lists several other alternative switches (SPDT though), but they might all have very similar performance (hasn't been exhaustively tested though). PO3B14A also stands out with quite high 'on' resistance unfortunately. 

So, experiments might be possible, but the prospect is not that bright I think.

Regarding your click at 80 MHz, it seems that you might hit the t_on+t_off speed limit of FST3253, which could be around 12ns.

Now, mixing on the 3rd harmonic is a workable idea, though it would require a RX preamp to cover the additional losses.

--
Karlis YL3JG


Re: Circuit Board Drills for cleaning out through-holes: Construction Tip

Bill Cromwell
 

Thank you Allison,

Multi-layer would mean three or more layers as was in the post where I replied with my question. Double sided boards are not usually included in "multi-layer" even though technically they are. That post confused me since I have believed the QCX to be mere double-sided board. I am not prepared to repair boards with "middle" layers. I don't want any radios that I cannot repair myself. So the QRP Labs gear is still on my wish list.

73,

Bill KU8H

On 9/6/19 11:06 AM, ajparent1/KB1GMX wrote:
QCX is two layer, top and bottom.  The display may be different but
that's a manufactured part.
So the answer is two, the main board and the display.
Allison
--
bark less - wag more


Re: QCX(-like) for 2m #alignment #vfo

ajparent1/KB1GMX
 

None of the Bus switches used are even close to fast enough for 2M nor quiet enough.

I tried fast switches at 6M and compared to a DBM they were terrible
and the system as a whole still too noisy for effective 6M weak signal work.
My HB radio has a NF under 4.5dB and is optimized for weak signal work.
I could not get that low with bus switches and had to resort to balanced fet
switching mixer.

To be effective on 2M you need low noise mixer and a RF amp (LNA).
A J310 fet and a pair of DBM can do that. 

Then the tx side needs work as the 74HC00 BS170 will not do much
power at 2M.

Basically stuff that works at 1-30mhz does not work above that or does
so poorly.

The basic idea would work but then anyone thats on 2M knows there
is not a lot of CW anymore though during contests there is.

Allison


Re: QCX - Damaged Finals? (with IR photo)

ajparent1/KB1GMX
 

If its pulling 850ma and only putting out 1/2 watt your doing something wrong.

For that current at 13V you should see more than 8W! 
That is more than 11W input for a mere .5W out.  

If the finals were running class E as they should the current
should be lower and the power out far higher.

For 2.5W out I'd expect about 400ma give or take a few ma.

If I had to bet you have a short in the output or a bad antenna or cable.

Likely the BS170s and the TX driver switch Q6 are fried most likely from
overheating.  However running on 15V is not a good thing and before
you do that make sure the power out and currents are consistent.

Allison


Re: Circuit Board Drills for cleaning out through-holes: Construction Tip

ajparent1/KB1GMX
 

QCX is two layer, top and bottom.  The display may be different but
that's a manufactured part.

So the answer is two, the main board and the display.

Allison


Re: Circuit Board Drills for cleaning out through-holes: Construction Tip

Bill Cromwell
 

Hi Tom,

How many multi-layer boards are in the QCX? I didn't know there were any.

73,

Bill KU8H

On 9/6/19 10:40 AM, Thomas Cobb wrote:
I just thought that it would be good, to point out,  that drilling out the through-holes, on a 4 layer PCB board, might be a bad idea.  On a lot of 4 layer PCB boards the power and ground plains are the center 2 layers, you could cause a short or open to one of the center layers.
Tom
--
bark less - wag more


Re: QCX - Damaged Finals? (with IR photo)

Ralf Rosche
 

Hi Joe,
150 Ohm in both directions?
This looks like defect of Q6. The best thing is to measure the voltages when the QCX is running. Desolder and use a FET tester.. There is a simple test method using a multimeter in case you have no special FET test equipment. 

I think high power consumption with low output is the reason of the damaged Q6.
Ralf


Re: QCX - Damaged Finals? (with IR photo)

Bill Cromwell
 

Hi Joe,

My copy of the manual for QCX may be from the dark ages but it shows Q6 to be bipolar transistor and NOT and FET. There is no gate nor drain. Using your ohm meter to check between the electrodes of Q6 reverse the leads for each connection. You should expect the base to emitter and base to collector to be different (a large difference) depending on the connections to your ohm meter. measuring it in-circuit can and probably will influence the readings depending on what other parts are in the circuit.

73,

Bill KU8H

On 9/6/19 10:27 AM, joe@... wrote:
Take a look at my 20-meter QCX.  Here is a thermal image of it running WSPR (the display is removed):
The board has just begun transmitting WSPR (the display is removed).  The board is connected to a 50 ohm dummy load.  The supply voltage is 13 volts, the current draw is 850 mA, the RF power output is 1/2 watt.  Q1, Q2, Q3, Q5 and Q6 are on fire, as you can see.  The current draw is obviously too high.
Just yesterday, this board was working.  I was running it at a high input voltage (15 volts), and I may have damaged it, but it was working when I shut it down. I provided some external cooling while it was running.  A visual inspection of the board shows no defects (ie burns).  With the power off, resistance measurements between the drain, grate and source of Q1/Q2/Q3 are all in the megs, as is the case with Q5.  Q6 however, reads 150 ohms between the gate and the drain (pins 1 and 2).  Gate to source, and source to drain were in the Ks or megs.  I do not know if this is normal - 150 is low, but I don't know what it was before I might have overheated it.
I'd like to ask the group: what should I do next?  Should I start replacing the BS170s?  I have spares.  Is there another check I can make to diagnose the problem?
Thanks!
-Joe
--
bark less - wag more


Re: Circuit Board Drills for cleaning out through-holes: Construction Tip

Thomas Cobb
 

I just thought that it would be good, to point out, that drilling out the through-holes, on a 4 layer PCB board, might be a bad idea. On a lot of 4 layer PCB boards the power and ground plains are the center 2 layers, you could cause a short or open to one of the center layers.

Tom

On 9/3/2019 4:24 PM, joe@... wrote:
For anyone interested, here is something handy to have if you have to replace components on PC boards:

PCB Drill Bits:
www.amazon.com/dp/B07N393BVK/ref=cm_sw_em_r_mt_dp_U_4SSBDbSQ60HAS <http://www.amazon.com/dp/B07N393BVK/ref=cm_sw_em_r_mt_dp_U_4SSBDbSQ60HAS>

These can be used to clean out through-holes after removing parts. Really comes in handy when you remove a part, and one lead connects to a heavy ground or power plane that is hard to heat and thereby clean with solder wick. For this situation, solder wick will get the hole mostly clean, then you can drill out the rest. Make sure you use a bit that does not make the hole larger...


QCX - Damaged Finals? (with IR photo)

joe@...
 

Take a look at my 20-meter QCX.  Here is a thermal image of it running WSPR (the display is removed):



The board has just begun transmitting WSPR (the display is removed).  The board is connected to a 50 ohm dummy load.  The supply voltage is 13 volts, the current draw is 850 mA, the RF power output is 1/2 watt.  Q1, Q2, Q3, Q5 and Q6 are on fire, as you can see.  The current draw is obviously too high.

Just yesterday, this board was working.  I was running it at a high input voltage (15 volts), and I may have damaged it, but it was working when I shut it down. I provided some external cooling while it was running.  A visual inspection of the board shows no defects (ie burns).  With the power off, resistance measurements between the drain, grate and source of Q1/Q2/Q3 are all in the megs, as is the case with Q5.  Q6 however, reads 150 ohms between the gate and the drain (pins 1 and 2).  Gate to source, and source to drain were in the Ks or megs.  I do not know if this is normal - 150 is low, but I don't know what it was before I might have overheated it.

I'd like to ask the group: what should I do next?  Should I start replacing the BS170s?  I have spares.  Is there another check I can make to diagnose the problem?

Thanks!

-Joe


Re: QCX(-like) for 2m #alignment #vfo

Guido PE1NNZ
 

There are (more or less) pin compatible alternatives for FST3253 that are faster like the PO3B14A.

This Potato chip :) has a total switching timing of 5ns, which in theory should cover 200 MHz.
With the regular FST3253 in my QCX, I have 50Mhz working and hear in the past weeks quite a bit of FT8 activity on 6m; for this I have modified the firmware and injected antenna signal directly into the QSD (without using T1 and inserted a 1k resistor in the bias). My impression is that the QSD is working up to about 80 MHz, then somewhere when tuning further up, I hear a change sharp click, still hear noise but it no longer detects any sensible signal. To receive 2m with FST3253, undersampling at 144/3 Mhz could be an idea.

Just some thoughts and experiences.
Guido


Re: Circuit Board Drills for cleaning out through-holes: Construction Tip

joe@...
 

From the OP ... Wow - great discussion!  I like the pin method.  I didn't mention that when using a drill to remove solder, you would 1) use magnification so you can see what you are doing, and 2) you would not use a motorized drill, but a just turn the bit by the shank by hand, or use a pin vise as David Wilcox suggested.

Also - the through holes on the QCX board are pretty large - the drill can be helpful if you run into ones that are much too small for a toothpick.  You might also have vias you have to clean out (say that are rotted from corrosion).  In that last case, you will need drill out the via and run wires through it reconnect it internally, and to both sides of the board.  The drill is then the only way...


Re: QCX(-like) for 2m #alignment #vfo

Kārlis Goba
 

The problem can be split in two parts. My two cents:

TX: the existing QCX architecture uses Si5351A to generate square wave LO and directly pass it to a class-E amplifier. As mentioned, class E operation can be tricky to tune without good instruments at such high frequencies. Alternative is to use class C final stage as done in U3s. Some efficiency will be compromised at the benefit of much greater ease of construction/alignment. This can be accommodated more or less by modding an existing QCX PCB (and requires modified firmware).

RX: this is harder. To keep the simplicity of the existing QCX design the RX should rely on a QSD detector. As mentioned, the analog switch used in QCX will not work directly at 144 MHz even if the Si5351A provides clean quadrature output. There are no easy and cheap alternatives to that. One way is to use 2 identical 'standard' (e.g. diode ring) mixers instead of the QCD, but that adds to the cost and requires serious redesign of the QCX board. Another, perhaps better alternative, would be to add a downmix stage in the RX chain and keep the existing QSD. No need to focus on an IF that lies exactly in an amateur band - anything can be used, and better yet with a monolithic crystal filter which can be readily sourced for 10,7 MHz, 21 MHz or 45 MHz. That still requires modifications, but could be accommodated as a small separate PCB in addition to the base QCX.


--
Karlis YL3JG


Re: QCX(-like) for 2m #alignment #vfo

Hans Summers
 

Hi Hans
 
I though the Ultimate3S has similar design and this can operate on 2m.
I'm not an expert ...

The Ultimate3S http://qrp-labs.com/ultimate3/u3s can produce output on 2m and even on 222MHz - the Si5351A can manage this easily (it is rated to 200MHz but can typically be pushed further). 

But the QCX http://qrp-labs.com/qcx question is a slightly different one. The Si5351A in the QCX generates two outputs in quadrature (90-degree phase offset). The question is about whether the Si5351A can continue to generate quadrature outputs up to 2m. 

73 Hans G0UPL


Re: QCX(-like) for 2m #alignment #vfo

Hans Summers
 

Hi Shirley
 
I remember reading somewhere that the upper limit for quadrature was somewhere around 110 MHz.

That's interesting. I'm not sure it is correct... there is a lot of misinformation written about the Si5351A. Not with bad intentions. Just because it is a complex chip and the documentation is poor so there are a lot of opportunities to misinterpret the datasheet. There are also a lot of inferior libraries floating around which exacerbates the problems. 

As far as I know, nobody else has done quadrature on the Si5351A until the QCX... so I don't know who would write about 110MHz anyway. 

The way the quadrature works using the internal even integer mode MultiSynth dividers with configurable phase offset in number of VCO quarter-cycles, it logically should work all the way to the top end of the Si5351A's frequency range, and not be limited to 110MHz. 

Though I have spent an enormous amount of time with the Si5351A chip I still feel it has further secrets waiting to be discovered. 

73 Hans G0UPL