Cheap pcb and HiZ ( microphone issues )

[Marc HENRY]

Hi all
i am very familiar making expensive point to point wired tube microphones
This time, i put a foot in reasonably priced transistor microphones

i have issues with two batches of pcb i ordered successively to cheap Chinese provider (jlcpcb)

Those circuits are supposed to be improved replacement circuit for Oktava MK012.
As i had crack and noise, i measured the pcb insulation (naked pcb, without any component)
i measured as low as 100Mohm, 40Mohm between tracks that are supposed to be perfectly insulated, at 2 or 3mm distance from one to another on the pcb !!
The worst one was made of...PTFE...!
the second one is very classical (green FR4)

Pcb's looks great, very clean...i try do dry them 12 hours at about 50°C (120-130°F), Z get higher, but one week later, with component in place, noise is back !!
i tried to air-wire HiZ (capsule bias and transistor gate) but there is still noise : little wind, and big wood cracking.

of course my components are those i use for usual microphone repair, i know them very well. to be sure i tested several FET and bipolar batches...

Sometimes (depends of the circuit and mod's i have tested) there is no noise. then I put the phantom power OFF for two minutes, and when i put it ON again noise is here again....!

Does anybody have experience like this ? with brand new pcb ? know what to do ?
Marc

 

[warpie]

Have you contacted them?

 

[Khron]

Pcb's looks great, very clean...i try do dry them 12 hours at about 50°C (120-130°F), Z get higher, but one week later, with component in place, noise is back !!

Photos of the boards?

 

[mihi_fuchs]

photos would be good. Maybe check your setup if you can measure insulation resistance that high properly. Some have troubles: https://www.giangrandi.org/electronics/gigaohm/gigaohm.shtml
Depends a bit what your meter says and how it reacts then to high resistance, normally it should just say too high, some may give you still values, which are incorrect.

I‘m home next week, at the moment i‘m enjoying awfully expensive finnland … i can measure boards then if that‘s of any help for you. (apologies to khron for the remark on finnland. but that one i had to bring somewhere - and here no one reads and gets it. Pls don‘t get it the wrong way and take it with a smile ;-) … i almost cried when i saw the bill for the 3 beers yesterday ).

if you made planes for grounding they can interfere with the hiz. If you have two planes on each side they can interfere with each other … that can cause noise in the hiz area too. Normally i keep my hiz areas separated from the rest of the board (separate hiz board with only one gnd connection). And i do not add planes there. But i guess the mk012 space is a bit limiting …

Ptfe should be actually better than fr4 - it has a higher cti value (breakdown voltage value) to my knowledge.

Maybe post some pics as khron suggested … back and front, if possible.

BR

 

[Khron]

at the moment i‘m enjoying awfully expensive finnland … (apologies to khron for the remark on finnland. but that one i had to bring somewhere - and here no one reads and gets it. Pls don‘t get it the wrong way and take it with a smile ;-) … i almost cried when i saw the bill for the 3 beers yesterday )

Why apologies? I'm a complete victim of that too And what ever possessed you to head this way in... dare i call it "summer"? You love mosquitos that much? At least in winter it's arguably pretty and bug-less

[End of off-topic-ness..?]