My QDX went almost deaf (something's wrong with op-amp bias?)


Adam
 

Hi,

Like in subject -- one day my QDX worked, the other day it went almost
deaf. I can receive only strongest signals, and the signal level is very
low even with high gain settings.

Audio filter sweep (done with the dummy load) shows USB at -46 dB.
Previously (when it worked) it was -14 dB.

https://ibb.co/qNLGrsB

I measured DC voltages on IC7 and IC9 amplifiers. All inputs are around
0.895 V and all outputs are around 0.815 V. Precisely:

- IC7A in+ (pin 3): 0.895 V
- IC7A in- (pin 2): 0.897 V
- IC7A out (pin 1): 0.815 V

- IC7B in+ (pin 5): 0.894 V
- IC7B in- (pin 6): 0.896 V
- IC7B out (pin 7): 0.815 V

- IC9A in+ (pin 3): 0.893 V
- IC9A in- (pin 2): 0.896 V
- IC9A out (pin 1): 0.815 V

- IC9B in+ (pin 5): 0.892 V
- IC9B in- (pin 6): 0.896 V
- IC9B out (pin 7): 0.815 V

I guess they should be more around 2.5 V...

Voltage supply is correct:

- IC7 vcc (pin 8): 4.994 V
- IC7 gnd (pin 4): 0 V

- IC9 vcc (pin 8): 4.994 V
- IC9 gnd (pin 4): 0 V

IC10 (PCM1804) gives 2.509 V on both VCOML and VCOMR outputs. This voltage
drops on R39 (and R36) -- after R39 it's 2.009 V.

Capacitors C71 to C74 are not shorted. Their inputs (from IC4 side):

- C71: 0.353 V
- C72: 0.356 V
- C73: 0.355 V
- C74: 0.352 V

I suspected that C71...C74 might be shorted only partially, but they don't
show anything on the meter (it has 60 Mohm range).

I also suspected that op amp inputs might be partially shorted to ground,
but there's around 5.8 Mohm between their inputs and ground.

Does anyone have any other tips on what could have failed?


Adam
 

Adam via groups.io <qrp-labs@...> wrote:

Like in subject -- one day my QDX worked, the other day it went almost
deaf. I can receive only strongest signals, and the signal level is very
low even with high gain settings.
Update.

I've read the topic called "QDX IC7 Voltages".

It's not a lock-up in my case. Slowly rising power (starting with 1.5 V)
didn't fix it, nor did shorting C71...74 or doing the factory reset. And
I'm running firmware 1_05 by the way.

Now the most interesting part.

During audio sweeps, the problem disappears on 40m and 60m bands. The
sweeps still look bad (on 40m USB is at -31 dB, on 60m at -33 dB), but the
voltages appear.

It's correlated with the BPF number. Proper voltages appear for bands with
the BPF number 1 (when I switched 20m to BPF number 1, voltages appeared).

Voltages at the input of C71...74 also differ during these tests. With BPF
1 selected, C71 receives 3.69 V from IC4. With other BPFs, it receives
0.349 V. It shouldn't have anything to do with IC7, unless there's a small
AC voltage there, but I can't measure any (after the sweeps complete), at
least with my equipment.

I measured BPF capacitors to ground. C30 and C28 have around 3.15 Mohm,
C31 and C29 have infinity (too large resistance for me to measure).

I measured the continuity of the BPF inductor -- it's OK for all taps.

The more I dig into it, the less I understand what's going on...


Al Holt
 

Adam,
I believe the bias is set by R17 & R18 show in the detail from the schematic below:


T2 is the trifilar wound inductor. I think one interesting test would be to see what happens to the voltage at the R17/R18 junction when the problem is there and when it's not. If it is then my guess is something up stream is getting shorted to ground. Maybe reflowing T2's connections?

I think you might want to consider the if T/R switch Q6 is working properly.

I'm familiar with Rev 3, 3a and 4 boards. If you have an earlier version some of this might not apply.

Good luck with your troubleshooting!

--Al
WD4AH


Adam
 

Al Holt <grovekid2@...> wrote:

I believe the bias is set by R17 & R18 show in the detail from the
schematic below:

T2 is the trifilar wound inductor. I think one interesting test would be
to see what happens to the voltage at the R17/R18 junction when the
problem is there and when it's not.
You're right. On 40m there's 3.694 V there, on 20m (BPF 1) 0.348 V.

If it is then my guess is something up stream is getting shorted to
ground. Maybe reflowing T2's connections?
Would T2 be affected with the band selection? It shouldn't...

I think you might want to consider the if T/R switch Q6 is working properly.
I checked, it seems fine. There's 8 mV on the drain during receive (on all
bands) and 12 V during transmit.

I'm familiar with Rev 3, 3a and 4 boards. If you have an earlier version
some of this might not apply.
It's 3a.

Good luck with your troubleshooting!
Thank you!

I might have found it. Maybe. Probably.

It seems that I have two unrelated problems (but if there was some spike
from the antenna, they might have the same source).

I cut the track between T2 and IC3 (pin 7) and measured both ends.

T2 didn't vary much, but IC3 pin 7:

- 20m (BPF 3): 46 mV
- 30m (BPF 2): 42 mV
- 40m (BPF 1): 3.841 V
- 60m (BPF 1): 3.841 V
- 80m (BPF 0): 1.569 V

I desoldered IC3, shorted pins 3 with 7 and 9 with 13, making a mono-band
receiver... and it works. At least on the audio sweep. I can't test it
with the antenna right now, because the antenna is in my other QTH.

But!

Even if audio sweep for 20m looks okay now, there are still strange
voltages on IC7 and IC9.

- IC7A in+ (pin 3): 0.887 V
- IC7A in- (pin 2): 0.890 V
- IC7A out (pin 1): 0.808 V

- IC7B in+ (pin 5): 0.886 V
- IC7B in- (pin 6): 0.888 V
- IC7B out (pin 7): 0.807 V

- IC9A in+ (pin 3): 2.376 V
- IC9A in- (pin 2): 2.497 V
- IC9A out (pin 1): 2.394 V

- IC9B in+ (pin 5): 2.396 V
- IC9B in- (pin 6): 2.516 V
- IC9B out (pin 7): 2.616 V

I suspected that IC7 might be bad, so I desoldered it and bias voltages
returned to around 2.5 V. I swapped it with IC9 just to be sure and...
now the new IC7 (old IC9) doesn't work (has 0.8 V), and new IC9 (old IC7)
sometimes works, and sometimes doesn't.

My suspicion is that a spike on the antenna killed IC3 and made IC7 and
IC9 unstable. I'll order new ones and see.

The other thing is that I want to avoid similar situation in the future...
I'm wondering what should I do. Maybe I should buy some protective device,
a surge arrester or something... or just a transil... or two transistors,
like T1 and T2 here: http://www.vlf.it/romero2/explorer-e202.html

I have two antennas right now, one is the open half-wave dipole (20m) and
another is a grounded end fed (it was just a random wire tied to a tree,
but works fine on 30m after adding a transformer), if that's relevant.


Al Holt
 

On Thu, Dec 1, 2022 at 12:10 PM, Adam wrote:

- IC7A in- (pin 2): 0.890 V
Adam,

Good work! Do you still have the caps shorted, C71-C74?

IMO, that would have to be a pretty good spike to travel that far through the chain to not messed something else up.

Surge suppression is always a good idea, as well as good grounding!!

You did a lot of SMD work; it sounds like you're on the right track!

--Al


Adam
 

Al Holt <grovekid2@...> wrote:

- IC7A in- (pin 2): 0.890 V
Adam,

Good work! Do you still have the caps shorted, C71-C74?
No, and the voltage at their inputs is around 3 volts.

IMO, that would have to be a pretty good spike to travel that far
through the chain to not messed something else up.
I hope it didn't kill anything more :) But I ordered three FST3253 (and
four LM4562) to have spares, so if it still doesn't work, I'll see if IC4
is fine and will replace it too.

I only hope that the ADC is good as it's not available locally. And of
course the MCU...

On the other hand, I have a strange problem with the PA (the 50 W PA from
QRP Labs) -- it probably sometimes spontaneously starts to self-oscillate
(I didn't try to diagnose it yet; in random moments it starts drawing
around 4 amps, R2, R3 and R4 get hot even with QDX disconnected, and L10
starts to smoke), so maybe one instance of it killed the QDX. It started
happening after I experimented with various (unmatched) antennas.

I'll get back to the PA as soon as I fix the QDX.

Surge suppression is always a good idea, as well as good grounding!!
Maybe this grounding is a problem... the 30m antenna is grounded, but the
transceiver (and the PC) isn't -- it was before, but I had ground loops
when using this 30m wire for VLF, so I disconnected it.

Maybe it would be good idea to reconnect it... or at least ground the 20m
dipole (on the unbalanced side).

You did a lot of SMD work; it sounds like you're on the right track!
Let's hope so :)


Adam
 

Adam via groups.io <qrp-labs@...> wrote:

I hope it didn't kill anything more :) But I ordered three FST3253 (and
four LM4562) to have spares, so if it still doesn't work, I'll see if IC4
is fine and will replace it too.
An update, if someone's interested.

Amplifiers arrived (switches still didn't -- I ordered them from a
different seller, maybe they'll arrive on Monday).

I replaced the amplifiers and now there's a good bias voltage on them
(around 2,5 volts), audio filter sweep looks very nice too.

https://ibb.co/m6yxB3M

RF filter sweep shows that the BPF is out of tune, but I'll check it again
when I install the switch IC (I think it can affect that).

So the op amps definitely were damaged.


Kelly Jack
 

My QDX is dead. No transmit and no receive.  I had a problem with my coax connector at the antenna and it was seeing high SWR (about SWR 5) which is high but not that high. Built for 12v operation I was running it at 11v to be on the safe side. But it still got fried.

I would prefer that there is at least some simple protection circuit, maybe a diode? as the failure conditions didn't seem that strenuous.

I won't be repairing this one not because the unit didn't perform well when it was operating but because 5w on FT8 from VK is not enough power for reliable DX contacts on 40m, which is where I like to operate.


73

Simon
VK3ELH


Al Holt
 

On Sun, Dec 4, 2022 at 08:05 PM, Kelly Jack wrote:
I would prefer that there is at least some simple protection circuit, maybe a diode? as the failure conditions didn't seem that strenuous.

 Protection-wise, I think the best bet is having a current limiting power supply set up for something like 1.2A. Right now I'm using a buck converter to knock a beefy 12V supply down to 9V. But, I haven't really put it to the TRUE test, yet.

Another protection method is having one of those LED SWR indicators built into an antenna tuner.  

Yes, 5W ain't much when it comes to DX, just another way to approach the hobby. 

I'd say give the QDX another go, the repair in and of itself is a rewarding experience.

All the best!

--Al
WD4AH


Adam
 

Kelly Jack <kellyjack1968@...> wrote:

My QDX is dead. No transmit and no receive.  I had a problem with my
coax connector at the antenna and it was seeing high SWR (about SWR 5)
which is high but not that high. Built for 12v operation I was running
it at 11v to be on the safe side. But it still got fried.
Most probably you fried the finals (and maybe the driver chip). It's cheap
to repair, you could give it a try.

I won't be repairing this one not because the unit didn't perform well
when it was operating but because 5w on FT8 from VK is not enough power
for reliable DX contacts on 40m, which is where I like to operate.
Have you tried the QCX PA?

https://www.qrp-labs.com/50wpa.html

Here's how it performs in my QTH -- multiple contacts across Europe and
a few DX-es -- not bad given that I work it only occassionally (1 hour
a week or so):

https://ibb.co/jMTtkLq


Donald S Brant Jr
 

On Mon, Dec 5, 2022 at 06:15 AM, Al Holt wrote:
I think the best bet is having a current limiting power supply set up for something like 1.2A
The problem with trying to protect semiconductors is that they generally blow up quicker than the protection (especially a fuse) can react.  Also, if you protect against excessive current and they are being blown by excessive voltage you are accomplishing nothing.
I use an SWR bridge, whose loss limits mismatch to ~2:1, between rig and tuner while the tuner finds a match, then switch it out.  
73, Don N2VGU


Kelly Jack
 

I have fixed a similar problem previously - replaced the si5351, driver chip and finals - I just don't see the point if it's going to blow up again.  I haven't had this problem with other QRPLabs products - I'm just finding the QDX to be less robust to my sometimes less than ideal operating practices. To be clear, I fully understand that I exposed the unit to conditions that caused it to blow up.

I suppose I could use it as a dedicated receive only monitor which would eliminate the possibility of a poor mismatch on tx taking everything out again.

73


Simon
VK3ELH


Kirk Maskalenko
 

My Rev 4 QDX went deaf and mute as well this weekend.  It's wired for 9V using a 9V 1A wall wort. It has been working fine for the last couple days with about 5 Watts output on all bands.

I was testing on 30M feeding a 50 watt QRP Labs amp tuning into a tuner/meter/dummy load. It was working fine until I accidently double clicked the tune to unclick the tune. I had been transmitting for 10 seconds or so. The second click showed only 100mw or so vs the 23 watts on the tuner/meter. Clicked again almost immediately. No drama, smoke or indication of issues. At first thought it was the Amp. Connected the QDX directly to the tuner/meter/dummy and still getting about 100mw of output. I connected the QDX directly to the antenna and only very strong signals were coming through but definitely is decoding.

I wired a jumper from the band pass filter input caps to a connection to the antenna and the noise level came up some but was still seeing only strong signals. All bands show similar results. X5 and X6 look good on band change. I replaced all 4 BS170s and the output is still about 100mw. I soldered a jumper at the Ant side of C33 to test power out and no change. I gave up playing for the evening  At this point, I suspect Q6 has failed as well as IC5 and think next steps are to remove both. I will need to place an order to get replacements for both and another set of BS170s.

Thoughts of other things to try?

73,

Kirk (wb8c)


Kirk Maskalenko
 

Pulled Q6 and put a .1 cap across the source and drain and the ears are back!

Ordering ic5, q6 and some bs170s to bring the TX side alive.

Kirk