NanoVNA V2 / Forum

Note: this page is a mirror of
Click here to join and see most recent posts.

can I repair this V2 plus 4?

barnc4br 2021/06/21 11:49

I have a V2 Plus 4 that has a spagetti display above 140MHz. I understand, from what I have read, that this is caused by a bad MXD8641 - U1, U3, and/or U801.
1st - Is my understanding correct?
2nd - Which device(s) need to be replaced given one band appears to be OK?
3rd - Would it be advantageous to replace all of them with MXD8641H given that the H has a higher input tolerance of 36dBm vs 27dBm of the plain 8641?
4th - Would the 0.1dB higher insertion loss cause a problem, and if so is there a way to mitigate it?
5th - This is the BIG one - is there instructions on replacing these devices, that I can hardly see, without disturbing other nearby parts or the back side of the board? - I have only tried to remove single SMD devices from scrap boards without much success.
6th - Has anyone successfully added ESD devices like a ESD101-B1-02ELS to the input without an effect on measurements?

Siegfried Jackstien 2021/06/21 23:24

if you are not good in smd rework ... then let it be done from somebody
who has the right tools and is used to work on smd

are you SURE it is defective?!?

0.1 db more loss will never hurt (it is calibrated out)

the higher input tolerance is maybe useful if you measure an amplifier
... but with enough attenuation its not needed

maybe a bit safer?!? i do not know that

instructions for replace?? .. hmmm ... take a hot air gun and peel them
off the board ... use solder paste and place new one on the board ..
check for shorts :-) ... easy job if you did it a few times ... NOTHING

greetz sigi dg9bfc

Am 21.06.2021 um 20:49 schrieb

barnc4br 2021/06/21 15:38

No, I am not sure anything is defective, just that I have a spagetti display above 140MHz.
Given what I have read, there are 3 bands, The first one ends at 140MHz. There are also 3 MXD8641s. Therefore, logically 2 of them are used for bands 2 & 3.
I could assume they are U3 and U801 but I don't like assumptions, so I asked the question.
I could practice SMD work more as I have a few scrap SMD boards, but I still need to know which ones might be bad.

Hector Pascal 2021/06/21 23:20

Before rushing in and trying to take anything off the board, I personally would be inclined to get a can of electrical circuit freezer spray, and chill a few areas to see if the spaghetti suddenly behaves while they are cold.  It's the best way to detect any dry joints, that may only need re-soldering.  You can even sometimes detect and confirm bad ic's this way.  But keep the spray away from the screen.

I would REALLY advise against trying to change chips at this level of circuit construction without the right tools, good eyesight, a microscope and a considerable amount of smt soldering experience.

OwO 2021/06/22 00:16

Do the two blue LEDs (LED601 and LED401) light up?

barnc4br 2021/06/22 15:51

only LED401 is on for either range that I run it - 50MHz to 140MHz (OK) or 140MHz to 1000MHz spagetti

barnc4br 2021/06/22 15:59

hard tapping on the board produces no change in the spagetti display and I have carefully inspected it with a 30x glass, so I do not think it is a bad solder issue. Components are warm but none appear to be overheating and the problem exists from the moment it is powered on. So freeze isn't going to tell me anything.

OwO 2021/06/22 20:29

If LED601 is not lit then U601 has a bad solder joint. Try applying flux to U601 and drag soldering.

barnc4br 2021/06/23 09:42

same issue. I touched each lead on u601 and u401 with iron. anything under 140mhz is a sold s chart. anything over 140mhz is all over the lace.

OwO 2021/06/25 09:00

LED601 unlit means the PLL U601 is not locking. You can ask the seller for a replacement under warranty. If you want to fix it you need to apply lots of solder and flux when drag soldering.

barnc4br 2021/06/25 19:44

It is intermittent, so does that mean 601 is bad or can something else be causing it not to lock?

To reply to this topic, join

View this thread on