help in identifying component

[eduardo_gj1]

ok, I'll do it in a moment, but could you help me identify this component, it has written something like 450L

 

[rjenkinsgb]

From the position and connections, it's most likely the JFET input stage of the mic, that takes the signal from the condenser capsule.

I can't find any reference to the exact part so far.

The others appear to be:
2GW = BC850C (NPN)
4GW = BC860C (PNP)

 

[eduardo_gj1]

yes its look like 450l but i dont find any information on internet about that

 

[rjenkinsgb]

It's almost certainly either an equivalent, or very similar to, a J201 or J305 etc., which are common JFETs in that function.

 

[eduardo_gj1]

ok thanks, i will order all this and change for new one, its only 2€ with the ic,

 

[rjenkinsgb]

Are there an connections to that which could cause a direct short between the power track and ground? There do not look to be, so I don't see how that could cause a short even if faulty.

Have you tried scrubbing the board with isopropyl alcohol - a metallic fault seems more likely given the extremely low residual voltages.

 

[eduardo_gj1]

I did it, but there was no difference, there is no contact between positive and negative, and clean the pcb with isopropanol

 

[rjenkinsgb]

Ok..

Time for either a low-ohm meter or brute force - a single fully charged AA NiMH cell & some short thick wires with negative to the ground plane and positive to the track I marked in yellow back on post #36..

Touch the wire very briefly, as quickly as possible, and see if anything smokes or gets warm!
If nothing happens, try slightly longer - but don't let the PCB track itself get hot. It's so small it will not take much time to heat and burn..

1.5V is too low to cause any harm to the electronics byt can often locate - or clear- a PCB short.
It needs to be a NiMH as it may take several amps when you connect the wires & normal dry cells usually cannot provide that.

 

[eduardo_gj1]

and with a laboratory power supply? its 35v and 2.5a

 

[rjenkinsgb]

Yes, ideal if it will turn down to around 1V, or even lower.

Or with that, set it to 1V and use a 10 ohm resistor to limit the current, then use that and a voltmeter to find the lowest resistance point on that marked PCB trace? With a fixed current the fault should be fairly easy to find.

Everything seems to point to the fault being somewhere along that?