First Radio Telescope Test - Success!

[robin_astro]

4 hours ago, Coto said:

I’ve ordered a MOSFET transistor to replace the broken one. Hopefully that’ll do the trick.

If it is just a blown  MOSFET the radio should work without it as  you dont need the bias voltage. If it still does not work without the MOSFET, it is likely there are more problems elsewhere unfortunately

Robin

[Coto]

3 hours ago, robin_astro said:

If it is just a blown  MOSFET the radio should work without it as  you dont need the bias voltage. If it still does not work without the MOSFET, it is likely there are more problems elsewhere unfortunately

Robin

My uncle managed to fix the power problem by unsoldering something and attaching something else so it feeds on 3.3V. Plugging the HackRF in and trying to listen to a radio station acts as if no radio waves are around, as if the antenna is not working and the hackrf isn’t getting any signal, so I expect the MOSFET to be the issue.

[robin_astro]

The damage may have spread beyond the MOSFET though

The radio should work ok you just remove the MOSET. (It is only there to switch the voltage to the bias T). If it does not then there are problems elsewhere.  (The fact that there is no 3.3v supply suggests this is the case unfortunately)

Robin

[Coto]

What do you mean there is no 3.3V supply? I plug it in and it recognizes it as an SDR and all that, it’s just that there is no apparent signal available to tune to. Are you saying if I completely unsolder the MOSFET and plug the antenna in, it’ll work fine? I don’t see why a component would be added to the board if it’s unnecessary in the first place?

[robin_astro]

Earlier in the thread you said the unit was dead (no indicator light) you then said your uncle had managed to fix the power problem by attaching something ( a 3.3v power source ?) 

The bias T circuit circled is to supply 3.3V DC power onto the antenna feed which  can be used to power a pre-amp for example. It is switched on and off under software control.  The MOSFET is the switch.  You dont need this for your application as you are powering your LNB through a separate bias T so you could remove all this circuit and it would still work.  If however, the MOSFET failed short circuit when you accidentally applied 12V to it through the antenna feed, the 12v would have passed through the MOSFET to the rest of the circuit, probably causing further damage.

To test if it could be recovered I would have done what I suggested here,

disconnecting the surge protection zener diode and bias T from the antenna feed, neither of which are needed for your application.  If this did not bring it back to life, this would mean the damage must be more widespread and I would probably have put it in the bin.

 

[Coto]

@robin_astro Shouldn’t I wait for the transistor to arrive (since I’ve already ordered it) and try replacing it first? If not, I could try what you said. Using the external Bias Tee without touching the HackRF at all (without shipping the MOSFET or anything) won’t work I assume.

This is what he temporarily attached for power (see image).

[Carl Reade]

Sorry if I missed it but have you turned on the internal bias T via the software and checked if there is 3.3 v coming out of the antenna?

I suspect there is a lot more wrong if it needs to be powered by the method being used. Is there a chance there was too much voltage used at the power end?

[Coto]

How can I turn on the bias T via  software? The only thing I tried was to use SDR# and see if I can hear anything.

[Carl Reade]

3 hours ago, Coto said:

How can I turn on the bias T via  software? The only thing I tried was to use SDR# and see if I can hear anything.

You would need to Google it bound to be info on it.

[robin_astro]

10 hours ago, Carl Reade said:

I suspect there is a lot more wrong if it needs to be powered by the method being used. Is there a chance there was too much voltage used at the power end?

Yes this is my conclusion too. 12v was applied to the antenna connection when the external bias T was inadvertently connected the wrong way round.  I suspect the 12v then got onto the 3.3v rail (and possibly also the pin that controls the bias T) via the MOSFET and fried the 3.3v supply and probably a lot more besides.  How do MOSFETS fail?  If it has failed short circuit, there would now be 3.3v on the antenna socket regardless of the control signal status.

[Coto]

So we took the MOSFET out (or at least the SP4 BSS84 transistor) and before we replaced it with a new one I just got, we tested both transistors with the same machine and they appear identical and OK, so we didn’t bother replacing it. We left it out so I could try what Robin suggested. Other than that, D1 seems OK so we’re not sure what’s the problem since the 12V couldn’t have gone through C64, so everything seems OK in that area (?). I’ll now try using the external bias T (how many volts should I supply to the HackRF killer (aka the bias tee)?).

Also, that chip that is marked with red (pic I posted above)? As soon as I plug the HackRF in, within a few seconds, that chip gets pretty hot (like 40C). Is this normal?

[robin_astro]

It is possible that the  MOSFET was not  damaged when you applied 12v to it but it may still have passed the 12v to the rest of the circuit and caused damage elsewhere.

Don't apply any voltage to the radio antenna socket. The radio should work fine with the MOSFET removed. If it does not then there are problems elsewhere.  Test it as a normal (eg VHF FM broadcast ) radio, with a piece of wire as an antenna,  not connected to  your bias T  LNB etc. 

[Coto]

@robin_astro I have that transistor removed but it acts just like it used to with the transistor attached (I assume the tiny transistor is the entire mosfet and not just a component - I can’t find a layout/topographic to be sure where exactly the mosfet is..). I already plugged in one of my antennas and it acted as if there were no radio stations around (I’ll try at higher frequencies tomorrow). I’ve only tried receiving. I doubt it’d be able to transmit and not receive.

But, didn’t you say that since I will be using an external bias T, I should remove the MOSFET? But now I shouldn’t use the bias T at all?

[robin_astro]

I f you check my posts you will see I  suggested you  try disconnecting  the internal bias t circuit after you had already damaged your radio by applying 12v to the antenna socket, to see if you could get the radio working again.

You  will need your external bias T (connected the right way round) to power your LNB but you dont need that to check if your radio is working.  The internal bias T is not needed to make the radio work either so if the mosfet in the internal bias t circuit was the  only problem, removing it should have got the radio working again.  You say you have now done this and the radio is still not working so there must be more damage elsewhere caused when you applied 12v to the antenna socket.  I  have no idea what further damage you have done but my suspicion is that the 12v got into the rest of the circuit somehow, probably via the internal bias t circuit and did extensive damage including blowing the 3.3v supply, which is why you are now having to run it off a separate supply to get any life out of it at all.

[Coto]

The surge protecting zener diode does not have a problem because my uncle measured the connector which is parallel to the zener diode and no shorting or leak has been found, so it must be OK, so there is no point in snipping that.

We're not sure where the inductor (L13) is located in this board. We can find it in the schematic (https://github.com/mossmann/hackrf/blob/master/doc/hardware/hackrf-one-schematic.pdf), but I haven't found any topographic which indicates where the inductor is. Any idea?

[robin_astro]

There is no  point in taking the inductor out of the circuit now if you have already removed the MOSFET (That was just my suggestion as the easiest way to isolate the fault) If the zener is ok and the mosfet has been removed and it is still not working then there are faults elsewhere. 

[Coto]

@robin_astro Hey, so I just found the topographic of the board, and that made me realize that we took the wrong MOSFET out (the one we tested and desoldered was Q1 which is way beyond C64!), so now that we have a topographic and know where everything is, we're going to do the following (in order):

1) Check C64 to make sure it's OK

 

2) Desolder D1 and test with SDRsharp

 

3) Desolder L13 as well and test with SDRsharp (if the removing D1 alone failed to resolve the problem)

(I've also got 3x MOSFET transistor spare part replacements (turns out Q1 is the same MOSFET as Q4), so I can replace that instead of keeping the MOSFET off the HackRF.)