7915 pinout on schematic incorrect?

[BrianJSiegel]

I'm working on an EMT/Barco 981 and am not getting the -15V from the power supply. I found where the fault is but I'm having trouble understanding the pinout of this 7915 as it is shown on this schematic...pin 2 should be the input and pin 3 should be the output from what I understand looking at the 7915 datasheet but I'm still fairly new at this so I could be missing something or just simply confusing myself.

That being said, this is the exact spot where its losing the voltage.
I've replaced the regulator and it gives me the same incorrect readings.
What am I doing wrong or not understanding? Please any help would be very much appreciated.
Thanks fellas in advance for your help.
-Regards, Brian

 

[Khron]

Nevermind the schematic - what's on the board? What are pins 2 and 3 connected to, respectively?

Or what if the pinout wasn't mentioned on the schematic? It rarely is...

And, on the other hand, how certain are you that nothing is shorter downstream, between the -15v rail and ground? Is the 7915 getting warm/hot?

 

[gyraf]

pin numbers could refer to a different package?

 

[Walrus]

Is C812 a Tantalum capacitor and possibly short circuit?

 

[buckethead]

The pinout is OK like this, (in some datasheets) L79XXCV have weird pin numbers (see attached picture - taken from STM L7915CV). Maybe C812, R807 or the connector is shorting? Where are you trying to measure the -15V? Could be the PCB trace/pad on the output of the regulator is broken.

 

[BrianJSiegel]

Nevermind the schematic - what's on the board? What are pins 2 and 3 connected to, respectively?

Or what if the pinout wasn't mentioned on the schematic? It rarely is...

And, on the other hand, how certain are you that nothing is shorter downstream, between the -15v rail and ground? Is the 7915 getting warm/hot?

7915 is not getting hot. I get -23V at pin 3 and .04V at pin 2.

I even replaced the 4001 diode but no change. What is the diode doing circuit-wise? Sorry, still trying to understand.

 

[BrianJSiegel]

Is C812 a Tantalum capacitor and possibly short circuit?

It was an electrolytic and I replaced it hoping that was the case. Doesn't seem to be. Should I put a tantalum in there instead?

 

[Khron]

7915 is not getting hot. I get -23V at pin 3 and .04V at pin 2.

I even replaced the 4001 diode but no change. What is the diode doing circuit-wise? Sorry, still trying to understand.

Is there any way you can disconnect the circuitry downstream of C812? Perhaps by cutting the trace (if necessary)?

We still don't yet know whether the issue is the regulator itself (not very likely), or whatever it's powering (much more likely).

 

[buckethead]

7915 is not getting hot. I get -23V at pin 3 and .04V at pin 2.

I even replaced the 4001 diode but no change. What is the diode doing circuit-wise? Sorry, still trying to understand.

The diode is for protection of the 7915. Those regulators don't like it when their output voltage is higher than their input voltage (could happen under some circumstances at power-off). No need to put a tantalum in there.

Are you measuring the voltages directly at the pins of the regulator? Maybe check the resistance from output to GND (with the supply turned off). And could you check the resistance from the GND pin of the regulator to GND somewhere else on the board?

 

[BrianJSiegel]

Oh I had no idea

Is there any way you can disconnect the circuitry downstream of C812? Perhaps by cutting the trace (if necessary)?

We still don't yet know whether the issue is the regulator itself (not very likely), or whatever it's powering (much more likely).

I will check it out right now, and report back shortly. thank you for the help btw. much appreciated

 

[BrianJSiegel]

The diode is for protection of the 7915. Those regulators don't like it when their output voltage is higher than their input voltage (could happen under some circumstances at power-off). No need to put a tantalum in there.

Are you measuring the voltages directly at the pins of the regulator? Maybe check the resistance from output to GND (with the supply turned off). And could you check the resistance from the GND pin of the regulator to GND somewhere else on the board?

Yes, measuring on the pins of the 7915. I'll go check those resistances and report back shortly.

 

[BrianJSiegel]

The diode is for protection of the 7915. Those regulators don't like it when their output voltage is higher than their input voltage (could happen under some circumstances at power-off). No need to put a tantalum in there.

Are you measuring the voltages directly at the pins of the regulator? Maybe check the resistance from output to GND (with the supply turned off). And could you check the resistance from the GND pin of the regulator to GND somewhere else on the board?

I'm getting shorts from pin 1 to ground, pin 3 to ground, and pin 1 to pin 3. And also from GND to GND elsewhere on the board. And I'm getting 500ohm from pin 2 to ground.

 

[BrianJSiegel]

Is there any way you can disconnect the circuitry downstream of C812? Perhaps by cutting the trace (if necessary)?

We still don't yet know whether the issue is the regulator itself (not very likely), or whatever it's powering (much more likely).

oh that is smart. I can try that, where am I cutting the trace? Just before C812 or after?

 

[buckethead]

I'm getting shorts from pin 1 to ground, pin 3 to ground, and pin 1 to pin 3. And also from GND to GND elsewhere on the board. And I'm getting 500ohm from pin 2 to ground.

Interesting! Just to be sure, with pin 3 you mean pin 3 as in your schematic? In any case, you shouldn't get a short between any two pins. Do you have the connector to the mainboard still connected?

 

[BrianJSiegel]

Interesting! Just to be sure, with pin 3 you mean pin 3 as in your schematic? In any case, you shouldn't get a short between any two pins. Do you have the connector to the mainboard still connected?

I meant the two outside pins (as 1 and 3) and I do not have the main board connected.

 

[buckethead]

I meant the two outside pins (as 1 and 3) and I do not have the main board connected.

Ok great. So in my opinion there are the following options to consider (I know you changed some of those parts, but still):

- C812 is shorting (maybe a solder-blob between the two pads?)
- R807 is shorting

Those are the only parts connected to the output of the regulator (besides the protection diode and the connector J800).

So since the regulator itself is not getting warm/hot and the fuse right before the regulator is OK, there's the possibility of the new 7915 being faulty. Maybe desolder it and measure resistance between pin1 and pin3 again.

Could you also measure the resistance from the -15V fuse to the middle pin of the 7915?

 

[BrianJSiegel]

Ok great. So in my opinion there are the following options to consider (I know you changed some of those parts, but still):

- C812 is shorting (maybe a solder-blob between the two pads?)
- R807 is shorting

Those are the only parts connected to the output of the regulator (besides the protection diode and the connector J800).

So since the regulator itself is not getting warm/hot and the fuse right before the regulator is OK, there's the possibility of the new 7915 being faulty. Maybe desolder it and measure resistance between pin1 and pin3 again.

Could you also measure the resistance from the -15V fuse to the middle pin of the 7915?

Unsoldered the 7915 and the short between 1 & 3 goes away. Re-soldered it back in and it's measuring 1k between 1 & 3. Also getting a short from -15V fuse to middle pin.

 

[Whoops]

You should post the full schematic of the PSU.
You should also replace all the Electrolytic and tantalum caps in the PSU, quite possibly that solves the problem

 

[BrianJSiegel]

Replaced all electrolytics and will do the rest f the tantalums as well.

 

[buckethead]

Unsoldered the 7915 and the short between 1 & 3 goes away. Re-soldered it back in and it's measuring 1k between 1 & 3. Also getting a short from -15V fuse to middle pin.

Sounds good! Short from fuse to middle pin verifies the regulator pinout. There's a chance your -15V rails is working now - I'd give it a try and power it back on again.