Hi Ludwig
Sorry for the delay but family matters over the weekend.? I've done many measurements and am reasonably certain either the 'scope or the probes are limited in bandwidth.? See my notes and findings below...
Could you check
- the region of the BS170 for unwanted things, (See pix attached.? All pads and soldering look fine and check ok)
- T501 and the area around,? (see pix attached.? I rewound this in case I had made a mistake but the original looked fine and the replacement performs the same)
- the PIN diode system of the LPF section,? (I tested each diode, checked the configuration and voltages at each point.? All are per the document.)
- the G, S and D voltage at Q508 for RX and TX,? (TX G=0, D=10.6, S=0; RX G=3.3, D=0, S=0)
- the frequency response of your oscilloscope with the actual probe (e. g. checking a fast clock signal from the QMX).? (I used the three CLK's 0,1&2 with both probes and got the same results across 1.8MHz to 50MHz.? All 3 signals produced the same waveforms on each band.? Good square waves on 1.8, 3.5 and 7MHz and the waveform got progressively more round until at 50 MHz the signal was a sinewave.? I think the probes may be part of the problem.? I have ordered a better quality (bandwidth) probe.?
For No 1 and 2 it would be fine to show detailed photos.
(Pix attached) For No 3 you will find information reading this
document from the QRP Labs Wiki (section PIN diode switching). It's for QMX but simple to expand for QMX+.
It would be also fine to remove all BS170 and check the signals from IC503 for different bands.? I removed all BS170's and made the following measurements:
| Voltage measurement p-p (with scope) |
? |
? |
? |
? |
| ? |
? |
1.8MHz |
14MHz |
28MHz |
50MHz |
? |
? |
| Q503,505 gates |
? |
5V? |
4.2 |
1.3V |
IC503 outputs |
| Q504,506 gates |
? |
5V? |
4.2 |
2.0V |
No FETs in circuit |
? |
| IC501 pin 4 |
3.8V |
3.5V |
3.0V |
2.4V |
? |
? |
| IC502 pin 4 |
3.8V |
3.5V |
3.0V |
2.3V |
? |
? |
| ? |
? |
? |
? |
? |
? |
? |
? |
The step next is a visual check of the capacitors from "+12V" to GND and from VCC to GND.?
I checked all caps on both rails visually and with a DMM where possible and all appear ok.To get a better impression of your system:
- which power supply do you use? See pic
- which dummy load do you use?? See pic
Apart from having to replace Q507, I'm unsure how to proceed.? Maybe a better probe will give more meaningful readings/measurements.
Regards, Greg.
