开云体育

Last night while playing with my qmx the receiver went deaf. ?This happened after txing on ssb with IQ mode on and panadapter open. ?I was using a mic connected to the radio. ?The S meter with antenna dropped to nearly nothing and I can only hear white noise now.?

?

The radio will still put out 5w. I ran my little signal generator into qmx and on receive I hear nothing at -33dbm. ?At 0dbm (I know quite strong) the signal will get through. ?Any thoughts? ?

Thank you

Ryan?

KK6DZB

Check all the enamelled wire soldering. I had the same symptoms a month after a?

successful build. After re heating a suspicious joint, problem solved.

I don’t remember which toroid, maybe L401?

?

Dave

ve3zzy

?

Awesome. ?Thanks Dave. ?Will do this tonight. ?I hope to not be swapping IC’s. ?It does get a signal if you throw a strong enough one into the antenna port.

?I forgot to mention that it did this (went deaf) and I turned of and turned on qmx and it had receive again for another few minutes then quit again. ?

?

Hopefully it’s just a cold joint on my part. ?

Ryan

KK6DZB

Well I checked and re soldered all of the? joints for the toroids and still no fix.? Tried a factory reset hoping it would be that simple but nothing.? Now it seems to be even more deaf than before as absolutely nothing will register on the S meter unless I feed the 0DBM signal into the antenna port.? I checked for continuity through the toroid board connections and that seemed fine.? With the signal source connected and checking with a scope the signal seems to get lost at one side T401.? But all of the connections test to be solid and I re-flowed all of the joints.? I am also unsure whether I am tracing this out right with the scope.? I am going through the schematics.? TX still works fine.? Hopefully I don't have to replace IC407 which I have read failed on older versions I have V4.? Any help is appreciated I would like to get this working again.??

?

Thank you

Ryan?

KK6DZB

Does the signal get lost at the input side of T401? (pin 9 of IC402)

Do you have the qmx on with the same frequency tuned as your input signal? (IC402 is the rx band selector).? Do you see your signal on the appropriate Yx input of IC402 for the band you are on?

Very common issues that silence rx on all bands are 1) the T401 connection to IC402 and its pair; and 2) the input connection to the BPF toroid(s), rx_in.

Stan,

?

Thank you for the response. ?Yes signal is at 7.020mhz using an elecraft signal generator. ?At -33dbm I can see on the Oscope the signal comes into T401 but not out. The connections have continuity on the board and I reflowed the joints. The receiver is deaf with -33dbm coming in and listening on 7.020mhz. ?If I increase the signal to 0dbm the tone will break through and I can hear it no problem on 7.020mhz. ?So I am assuming the ic’s are ok. ?I am at a loss as the radio was working fine then just lost all sensitivity going from tx back to rx. ?Which makes me think that whatever is controlling rx/tx is not returning to rx and a strong enough signal is breaking through. ?But this is where I am stuck.

?

Ryan

KK6DZB

I went through and checked again tonight.? Reflowed all of the joints in the rx section. Ran the signal generator.? I see the signal on pin 9 of IC402 and pin 9 of IC403 and the reciever is still deaf.? I am at a loss as to what to try next.? ?I am baffled by the fact I still have tx and it failed without warning.? Really don't want to shelve this thing.??

?

Ryan

KK6DZB

I'm glad you can see the signal on pin 9 of both IC402 and IC403.? You should also see it on pin 7 of IC403 (the third winding of the trifilar toroid),?

?

Next, look at the outputs of the op-amps that feed the PCM1804.? This is after the mixer, so the frequency will be very near 12kHz rather than 7MHz, and will be 10x the voltage swing (if all is well).? Here is the description of this test from Jeff as recorded in the wiki:

?

?

And if you see the signals as described, either you have a PCM1804 failure, or you have a failure in your audio output path.

?

To determine which of these, consider the following observations: do you get receive audio via the USB computer interface path?? e.g. receive ft8 data in wsjt-x, or see signals with IQ mode enabled in an sdr program like NaP3 or HDSDR?? If you get audio via this path, the PCM1804 is fine, and the fault lies in the audio-out circuits.

?

Note that complex ICs do have a non-zero failure rate (even the good ones), so while it is much less likely on an R4 board to have a failure in the PCM1804 or one of the audio-out chips, it can happen.

?

Good luck in finding the culprit,? and let us know what you find next.

Hi Ryan. Did you ever check to see if Q508 was working properly? This is the switch that turns off the receive signal input while transmitting.

Ron

I will check all of this hopefully tonight. Thanks for the responses.

?

Ryan

KK6DZB

Stan,

?

I have been using quisk while trouble shooting with IQ active.? ?I only use linux here so no NaP3 or HDSDR. I get a stream coming into the sdr software but the noise floor is very very low just as the swr meter indicates on QMX.? If I but a very strong signal into the antenna connector I do see the entire noise floor rise in quisk.? ?I do not see anything at IC405 and IC406.? I am guessing the PCM1804 is good otherwise I would have nothing coming into quisk when driving into the radio with a heavy signal.? I guess ic405 and ic406 could be bad but I would like to know for sure before ordering parts and digging into the board.??

?

Ryan?

KK6DZB

Are you looking at 12kHz (rather than 7MHz) when looking at IC405/406 outputs??

I have never heard of IC405/406 failing before, so I would rather suspect the IC403 - check the four inputs of IC405/406, where you should see a 12kHz sine wave at about the same amplitude as you saw 7MHz at the two IC403 inputs; if not, they are not coming out of IC403.? Also make sure there are clock signals at the A/B inputs of IC403; without those, IC403 won't act as a mixer.

Stan,

?

Yes.? With a very strong signal coming into the antenna I get a 7mhz nice clean sign wave up until I reach the input of IC405 and IC406 which is still a clean signwave but at? much lower frequency (I havent measured actually frequency but it is significantly lower)? the output from IC405 and IC406 are not any stronger P2P then the input but the signwave is distorted on the output.??

?

Ryan

Hmmm.? This is strange.? And as Ronald noted, it would be good to check the state of Q508, which blocks the receiver during transmit.? Perhaps it is stuck in transmit mode (always off), and requires a super-strong signal to make it through the off-state leakage.? This could also happen with a shorted D516; or with the correct LPF not enabled; both easy to check.

?

It would be very unusual for both IC405 and IC406 to fail.? I would inspect them and IC407 carefully under strong magnification to see if there is any questionable soldering.? Then I would a) measure that frequency at the input of IC405/406, and make sure it is very near 12kHz - because something significantly different would also cause a deaf receiver, Next, if possible, use a 2nd channel on your scope to verify that the signals on pins 3 and 5 are 180 degrees out of phase with each other - if they are somehow in phase, it would goof up the output of the differential amp.? And the signals on IC406 should be 90 degrees out of phase with those on IC405.?

?

And exactly how strong is the input signal at input of IC405/406?? If it is above about 0.4Vpp, then it will distort on the output due to the 10x gain, and the 5V Vcc power to those chips.? I imagine if you are using the 0dBm output of the elecraft sig gen, that you would have about 0.6Vpp - 10x this would be 6Vpp, far beyond the range of the Vcc power rail of this amplifier; so I don't know what it would look like.? Unless, of course, that original 0dBm was significantly attenuated before this point.? An ideal test signal is something like -10dBm; the -33dBm is hard to see on the scope, being only 14mVpp.? Do you have an attenuator you could put in line with the 0dBm??

?

I guess my thought is that your latest measurements don't correspond to any common-sense failure, so I'm wondering what could be wrong with the measurements or our approach.

?

Stan KC7XE

?

?

Stan,

?

?

I do have a signal in and out of q508. ?The reason I suspect that ic405 and ic406 are bad is the fact there is no difference in gain between the input and output. It was noted earlier thar there should be 10x the input on the output but there was no change. ?Yes I am on 0dbm. ?It is very difficult to make out the -33dbm signal on the scope. ?I could attenuate the input a few db but I would have to make one up. ?If there is no gain from the amps I am going to assume they are bad. ?I don’t have much else to go off of. ?Also the part number on ic405 and ic406 do not match the schematic or the build manual. ?I found on here that they were changed to a different part. ?

Ryan

KK6DZB

On Thu, May 15, 2025 at 07:28 AM, Ryan Wesolowski wrote:

Like Stan wrote before, it looks like IC405 and IC406 are in overdrive state but not broken. It would be the first time reading about broken ones or find this when repairing a QMX(+).

?

Please check these voltages using your DMM:

For Q508 in RX mode the voltages are at S 0 V, at G around 3 V and at D near 0 V.

IC402 pin 9 should be around 2.5 V and depending on the active BPF one of the pins 10, 11, 12 and 13 should be the same.

IC403 pin 7 and 9 should be around 2.5 V. Pin 2 and 14 should be around 1.6 V. Using your oscilloscope you should see there (2 and 14) the LO signal with low at 0 V and high at 3.3 V (or so).

To repair QMX(+) I had to replace Q508, IC402 and IC403 already.

?

Could you show please screen shots from Terminal - Hardware tests - RF filter sweep and Test ADC I/Q for a lower band and a higher band?

?

73 Ludwig

Ok. ?I will check that this evening. ?But even when the signal level was at -33dbm there is no gain from the input to the output of ic405 or ic406, ?in fact I believe there was nothing at all on the output of ic405 or ic406 from what I remember with the signal level at -33dbm. ?

-33 dBm is around 14 mV pp, enough to trace using your oscilloscope.

?

Also without amplification by IC405 and IC406 (amplification = 1) there should be enough input voltage for the PCM1804 for good receiving.

It was written before but you may read this document (second part of section Diagnostics) from QRP Labs Wiki for the procedure to test until the input of PCM1804.

?

From your post 2 days before

Strange, there is a strong magnetic coupling between input and output. Voltage at input but not at output is strange.

I'm courious about the voltages at IC402 and IC403.

?

73 Ludwig

?

?

Ludwig,

?

Here are my results so far

?

IC402

Pin 9? ?394 mv

Pin 10 394 mv? 40m active

?

IC403

Pin 9 393 mv

Pin 7 393 mv

Pin 2 1.64 V

Pin 14 1.64 V

?

Q508

S 0

G 3.3V

D 0

?

All measured in VDC certainly not meeting the 2.5v on pin 9 and 10.??

And this is a QMX mini not a plus though I doubt it matters in this section of the circuit.

?

Thank you for the help thus far.? Hopefully can get this repaired and not have it fail again!

?

Ryan

KK6DZB