This is a quick overview of things you should know about the Q900, before I get the time to post more details. Since the PMR-171 and the Q900 V4 are very similar, most thing will apply to the PMR-171 as well.
Hardware
Receiver:
Let me start with the strong part of the Q900: Reception on HF bands is really great. Also, the Noise reduction works very well.
Reception on VHF (FM) is still very good with the right RF Gain settings. I compared to my Yaesu FT-3D and they are about the same. Reducing the FM bandwidth on weak signals to less than 12 kHz can improve a little.
But on UHF the reception is not so good compared to the FT-3D.
I can’t say anything about higher frequencies up to 2GHz. As far as I know not so good.
But the Q900 is also subsceptible to QRM from power supply, connected USB devices and even Power cables close to it without beeing connected. And it is subsceptible to common mode current feedback.
So it is better for portable use than at home. But I still manage to use it at home.
Antenna:
– On the Q900 V4 the HF Bands up to 4m are on the SO-239 connector. VHF/UHF are on SMA (TNC on the PMR-171) including DMR.
Broadcast FM Radio is received on the SO-239 socket. Witch antenna to use for receiving all the other frequencies is not documented.
– On the Q900 V3 with DMR module isntalled, TX and RX for VHF/UHF are not on the same antenna socket.
TX for VHF/UHF is on SMA.
RX is on SO-239.
RX and TX for HF on SO-239.
– The Q900 V3 without DMR module should have RX and TX for HF/VHF/UHF on SO-239.
TX audio issues
The original mic is of really poor quality. And there is a bug in the software in NFM mode with the mic gain.
See the microphone page for Details.
There is also a discussion about SSB on VHF on groups.io. It could be that the gain setting does not work there as well.
With an amplified mic (with variable output level) it should be possible to get very good TX audio even with the known bugs at this point.
TX lock
in different situations it can happen that the Q900 appears to be locked in TX mode.
Press the PTT key or the „.“ key on the Q900 a few times. Then it should stop transmitting.
In my case it happened using different Software (Bluetooth and USB) to control the Q900:
– QRadioBLE while using the tuning knob
– GHTerminal when disconnecting
– trying to get cat control to work with SDR software.
The good thing: I don’t know of any case where the finals did not survive this. And SWR protection seems to work well on HF.
Reset button (only V4 but not all of them and not documented)
There is a reset button on the Q900 V4 (not the first ones as far as I know) hidden behind a tiny hole next to the LAN port. It is about 4mm deep and probably not well centered behind the hole.
It will restart the Q900 after a short press. If a long press is needed when the Q900 shows no reaction, I don’t know.
If there is no hole, then you don’t have the reset button. If you have one I would suggest to carefully try to find the right angle, when the Q900 is turned off, in case you need it later…
Output power and power supply
In the specs you will find that the Q900 supports a power supply up to 32V.
Guehetec recommended in the WhatsApp group not to go higher than 13.8V. They calibrated at 13.8V.
Use a good power supply or a LiFePO4 battery pack to avoid any HF interference.
Also the min. 6A are really needed on full power output!
Output power on VHF/UHF: There is no High and low power mode and the PA level setting has no effect at this time. The output will be max or nothing when set to 0%.
And the output power really depends on voltage. At 13.8V it should be about 10W on VHF. I got 8.4W at 12V and over 12W on 15V. On internal battery it was a between 4 and 5W. UHF has about half the output from VHF.
On HF the output power is not linear but does not depend that much on the power supply. The PA level settings do only work below 14MHz. For 14MHz and higer the output poewer will not go below 6-7W.
So, there is now way to use an external Amp like the PA50 or XPA125b on 20m to 10m bands.
This is with V4 and firmware 3.4.1. I don’t know about older versions.
ACC/PTT output
The ACC jack has an output of over 0.2 V dc on TX according to groups.io. I measured 0.5V on TX without anything plugged in. It will not work with the XPA125B. I don’t know about other amps. Using a micro relay to invert the signal will work. Discussion on groups.io
Audio output:
The Q900 has 2 volume settings: SVOL (Speaker) and HVOL (Headphone)
– SVOL will set the volume of the internal speaker and the rear AFO jack (mono). No way of disabling the internal speaker, when using the rear AFO jack.
– HVOL will set the volume of the front PHONE (TRS) and the MIC jack.
The original mic has a speaker. When connected the speaker will drain the signal and the PHONE jack is of no use. If you use the original mic, the best would be to cut the wire to the speaker inside the mic. Then there is great audio on the PHONE jack.
In any case I would recommend using an external aplified speaker or headphones.
If you want to use another mic than the ones from Guohetec, be aware that the pins in the manual are not correct. The MIC jack is TRRS. 
The PHONE Jack on the PMR-171 is the same as the MIC jack on the Q900.
Don’t connect a stereo headphone (TRS), because the device will probably start transmitting.
Broadcast FM App: when using the broadcast FM app, audio will only come out of the internal speaker. Too bad, because reception is very good there…
DMR: Audio is not coming out of the headphone jack. Guohetec confirmd it (“only horn” in their words).
If it will ever change they did not tell.
A PMR-171 owner reported the same for DMR there.
Modules
DMR: (also applies to the PMR-171)
I cannot recommend the DMR module at this point (firmware 3.4.1).
– It is only Tier1.
– In the US nobody managed to connect to a relay as far as I know.
I managed to connect to a German relay but stopped testing because of bad reception on UHF. Now I own a GD-88 instead, so further testing is not on my priority list. But I will add anything reported to me.
– Also, there is no information shown on the display when receiving. Not even the signal strength on my device. But I saw s-meter level and waterfall signal on videos from others.
– The functionality is not even close to the other DMR radios like the Radioddity GD-88, Anytone at-d878uv or any radio supporting the open GD77 software.
But the received audio quality was very good.
GPS and Compass
My knowledge about these modules is limited at this point.
External encoder
The Q900 does not have an encoder / tuning knob. There are alternatives.
LAN port
The LAN port does not have any function at this time.
Early PMR-171 do not seem to have the LAN port, but it is actually hidden behind a sticker. Later they did not put the sticker over the LAN port.
Guhetec announced that they plan to use the LAN in the future.
Software
QRadioBLE:
Some people have trouble connecting the app to the Q900.
The pairing happens in the app. When you connect the first time:
hold BLE about 2sec and release. After releasing a new screen appears where you have to select “new”. Try 2 or 3 times if it does not work the first time. Also there have been reports of older Android versions not working at all.
Other software
Guehetec has a Youtube channel where they recently posted some promo and instruction videos for the PMR-171. Every Instruction video for use with software I watched could be used for the Q900 as well.
Guohetec Youtube channel