Diode next to power input faulty?

[wimmer]

I have a mixing desk that does not power on at all, the 5V jack input/connections has been tested fine, but right next to it is a diode marked D308, I tested Voltage with the power connected and 5V were going in (forward)? (cathode side) but nothing out. A diode test in circuit read 4.72 and reversed OL. Ohm check in circuit read 2.27k and reversed the readings were all over the place.  Should I assume this diode is faulty? I really don't know, so thanks in advance.

[DonKu]

The diode may function as a reverse polarity protector for the power jack. If the circuit expects a positive power tip, for instance, and you accidentally connect a negative power tip, the diode blocks the power connection.  If this is the case, you ought to expect about 4.3 VDC on the diode's cathode when it's connected to a 5 VDC adapter. You need to disconnect one leg of the diode to make any meaningful measurements. Otherwise, the remainder of the circuit interferes.

[wasedadoc]

I have a mixing desk that does not power on at all, the 5V jack input/connections has been tested fine, but right next to it is a diode marked D308, I tested Voltage with the power connected and 5V were going in (forward)? (cathode side) but nothing out. A diode test in circuit read 4.72 and reversed OL. Ohm check in circuit read 2.27k and reversed the readings were all over the place.  Should I assume this diode is faulty? I really don't know, so thanks in advance.

Are you sure the input voltage should be going through the diode?  With the external power unit plugged in and powered up, measure the voltage across the diode.  If the cathode (end with the band) is 5 Volts positive with respect to its other end, then the diode is wired in parallel with the input such that it only conducts if the input supply is reversed polarity.  If you do get that 5 Volts then the diode is not the cause of the problem with the mixer.

[donlisms]

Wouldn't that mean the diode would short the power supply and possibly blow it up just because someone used the wrong one?  Wouldn't it be better to use the diode in the forward direction and just remain harmlessly "open" if someone used the wrong supply?  I mean, I suppose it could be done either way, but it seems like a designer who understands the issues could take the more polite approach.

[Gyro]

Then you would suffer the additional forward voltage drop of the series diode, which may or may not be an issue depending on the amount of voltage headroom - is it a 5V regulated PSU? If so, the diode would drop the available voltage down to around 4.2V. Most PSUs have overcurrent protection.

Can you post the diode part number? It may be a Zener or TVS, designed to provide basic transient and overvoltage protection too.


Edit: Sorry, I didn't notice that you aren't the OP.

[wasedadoc]

Looking at the photo in the opening post it does appear that the anode of the diode is connected to the negative side of the 22uF electrolytic (C531).  That would be in accord with the diode being in inverse parallel with the power input.

[Terry Bites]

I'd put money on it being a reverse polarity protection too.

Wrong psu polarity; diode will get very, very hot. Your diode hasn't failed short, cap charging from your DMM giving rise all over the place readings.
It could have failed open. That would be of no consequence with the right psu plugged in. Your mixer would still work.

Unless the diode gets smoking hot, I doubt it is the problem.
There's no sign of scortching heat on the pcb where the diode sits.

You won't see anything "out" because the anode is at ground. No current flows though this diode unless ther is a reverse polarity incident.

Unsolder or snip off the K end and find out.

Does that revive your mixer. I think won't make any difference. The fault is elsewhere.

Follow the trace from the + side of the 22u cap. Whats along there. Maybe a regulator or noise filter or a MOSFET switch?

So. If you have a power supply with the correct voltage and plug polarity. What

[DonKu]

A closer look at the JPEG clears things up. As already noted, both D308’s anode and C531’s negative leg are connected to ground. The positive leg of C531 connects to the power pin’s positive island. A bottom side via almost  certainty connects D308’s cathode to the positive island.

If nothing else, D308 pulls ground down to one diode drop above a misapplied negative voltage. D308 is probably a Zener in parallel with C531 to provide a modicum of voltage regulation.

A two timing, half baked, cheap solution, so to speak.

[JustMeHere]

That diode looks like it's anode (the + side) is connected to ground.  This is a cheap way to protect the down stream circuit from reverse polarity, and avoid the voltage drop associated with the diode.  It causes and over current if power is connected backward.  This would blow a fuse or trigger some type of over current protection in the power supply.  Dave mentioned something about this in a video.  Can't remember what one. 

https://learnchannel-tv.com/en/electronics/rectifier-diode-how-it-works/reverse-polarity-protection-with-diodes/#:~:text=the%20subsequent%20circuit.-,Diode%20in%20parallel%20with%20the%20power%20supply,V%20or%20Si%200.7%20V).