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On my mid-band QMX ....I wasn't going to do it. I'd read several horror stories but then, as an "experimental amateur radio station operator" I couldn't help myself. The SSB calibration routine was a disaster. Blew the 2A inline PSU fuse, and took out the BS170s.?

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I replaced the BS170s with new ones on hand and applied enough heat, visually checked for other damage (none found), then sparked it up. Receives fine and all functions appear to work (Mode, Volume, Step, Band etc). However, when I try to transmit on CW into a dummy load, I'm only seeing about 340mW on 40m and progressively less as I go up in frequency...

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Any ideas as to where to turn to next would be very much appreciated...

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73,

Robert VE7ZN

Robert, maybe this document from the QRP Labs Wiki is helpful.

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73 Ludwig

Revision to original post...

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I just checked again and I'm actually getting about 4W on 60m but then it drops to 350mW on 40, ~100mW on 20m, 75mW on 17m and 0mW on 15m.

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Help!

73,

Robert VE7ZN

Ludwig,

Thank you for your reply. Please note my amended post above. I ran Diagnostics in Putty and see the following RF Output:

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The SMPS appear normal and other conditions on the Diagnostic screen appear normal (unless I'm missing something). I've checked all 1N4007 diodes as well as several others on the board - all check out OK.

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73,

Robert VE7ZN

Thats strange that you get 4w+ at 60m??? Reset factory defaults to erase calibration??

Robert, did you measure using an external power meter? If not maybe something is wrong around T507. Please check this document from the QRP Labs Wiki.

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73 Ludwig

And did you check voltages at G of BS170 (see the link in my first post)?

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Yes, I am getting a genuine 4W+ output on 60m using my calibrated LJ-2K+ Power Meter.

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Hmmmmm, will give that a try - never thought of doing that. I will advise later.

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73,

Robert VE7ZN?

Richard,

Tried factory reset - no change. Same RF Output....

Hmm, I haven't tried this yet, the only dummy load I have is a bird termaline 5w, wondering if that's up to the job?

Ludwig,

Yes, they’re all ~3.0v

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On Tue, May 20, 2025 at 06:28 PM, Robert Dale wrote:

Yes, they’re all ~3.0v

Robert,

this would be wrong. There are rectangular pulses with 0 V for low and 5 V for high and duty cycle 50 %. Your DMM should read the mean value of 2.5 V. If realy 3 V IC503 could be wrong. This could result in a wrong switching of the BS170.

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73 Ludwig

Ludwig,

I see. Well I decided to send it off to Jeff (W1NC) to have it fixed. If it comes back working, I will NOT be doing the SSB calibration again, that’s for sure.?

Thank you for your input. Much appreciated.?

73,

Robert VE7ZN

Do not do it if your not comfortable.? Hans said calibration is not required and the unit works fine in SSB without it.

Would it help to feed the QMX under update with a lower voltage, so as to reduce the strain on the BS170's during calibration?? For example, if I feed my 12V QMX with 9V, the power drops by about half.? I assume some of the issues during calibration are due to high transmit duty cycles that occur during the calibration process.??

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I did successfully run calibration on my knock-around QMX.? Fed it to a 50 Ohm dummy load and didn't encounter any issues.? But when I update my daily-driver QMX, would reducing the supply voltage help to add margin during calibration???

No, Mike, you don't want to do that, unless you also want to run the QMX at that lower voltage.

This is because the calibration routines store the phase compensation values for voltages across the full modulation range - and you don't want the high voltage ones to be empty, since it may cause distortion when those voltages are used in operation.? So you want to run the calibration at the highest source voltage that you will run your QMX in operation.

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Note that the majority of QMX/QMX+ devices have no problems with the calibration.? I have 2 QMX and 2 QMX+, on which I have run the calibration many times without any issues.? [I ran them many times during the beta period, due to updates and working around early calibration problems.]? So the odds are in your favor.

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But for some as-of-yet-unknown reason, there is something about certain QMX/QMX+ builds/units that makes them sensitive to the calibration procedure, and the finals and sometimes the PA voltage modulation transistor Q507 burn out.? Hans already made several changes in the firmware to reduce the chance of it happening, but for some reason on a select few QMX* it still happens.? Some conjecture that it is because of already-weakened finals, an already-failed Q507 (but in the release code Hans put in a check for that one), voltage spikes from L502, a poor connection to the dummy load, or other build-related issues - but nobody really knows.

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So if you definitely do not want to take any chance of breaking your QMX*, don't do the calibration.? The SSB performance of QMX* will still be very good. ? If you have a greater risk tolerance, and are willing to change the very inexpensive PA transistors if they do blow, then certainly do the calibration and enjoy the 'better SSB performance' that is promised.

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Stan KC7XE

Thats what I said I have to reach for perfection in my build couldn't help myself ?

Two comments:

1.? If calibrated at a lower excitation voltage then operated at higher excitation there will be co compensations for the higher voltage.? This is the same as if a calibration does not complete.? Hans assures us this is fine if not perfect.

2.? At 80m and particularly 160m at full power the voltages caused by reflections from the LPFs back into the BS170 exceed the manufacturers rating, so if your not competent at replacing BS170s that may fail I ould not calibrate for those frequencies.

Unfortunately I have misplaced the voltage pictures.