IC Power regulator weirdness

[Potato Cakes]

Hello, Everyone,

I had just finished another GSSL build and I have encountered a strange issue. When I initially power on the unit, all the power seems fine except for the +12V that should be going to the Ratio board on the + connection of the header connection. When I switch it off, right when the power caps drain the power LED (connected to ratio PCB) flickers and when turn it back on it's fine. This happens every time I power the unit on. I traced the problem back to the 78L12 regulator on the board as the Vin pin is correct and the Vout will give me -0.9V and +12V respective of the powering on issue above. I'm ordering a new part, but I'm just curious if it is possible for an IC regulator to intermittently output the incorrect voltage on its own.

Thanks!

Paul

 

[Brian Roth]

bad regulator?  possible!  screw up in the gizmo?  LIKELY!

<g>

Bri

been there and have MANY t-shirts

 

[JohnRoberts]

May be a turn-on latch up issue. First time it has to charge up capacitors and can't handle it, second time is easier.

Look at regulator data sheet to see if they have anti-latch up advice.

JR

 

[Potato Cakes]

bad regulator?  possible!  screw up in the gizmo?  LIKELY!

<g>

Bri

been there and have MANY t-shirts

That's what I initially thought. I try to be very careful anymore posting this kinds of things until I feel confident it's not the typical solder error. I will keep digging around until the part arrives.

Thanks!

Paul

 

[Potato Cakes]

May be a turn-on latch up issue. First time it has to charge up capacitors and can't handle it, second time is easier.

Look at regulator data sheet to see if they have anti-latch up advice.

JR

I'll dig into the datasheet and see what I can find. The other GSSLs I have all have push buttons and they seem to be fine though.

I'm still hoping it is a stupid easy error, but there aren't many things to check in that particular voltage path. Same results with all the peripheral boards are disconnected and it's just the main board connected to power.

Updates to follow soon...

Thanks,

Paul

 

[JohnRoberts]

IIRC the 78"L"12 is a pretty low current regulator. Excessive capacitance on the output could current limit at start up (just another guess).

JR

 

[Potato Cakes]

I replaced the 78L12 with the same results. I looked over the data sheet and there was no info regarding latching. I thought I found a 100nF where a 100pF capacitor was supposed to go, but that was just my eyes playing tricks. The kit included all 22uF 50V electrolytics, which my be a bit high regarding voltage, but I have built a number of these same kits with no problems like this. Worse case, I can remove the 78L12 and jumper the 12V relay power that isn't being used as the power off the 7812 which is working properly, but I'd rather find the problem. I'll swap out some caps and even the board from a working unit and see if anything changes. Still hoping for a bad solder joint or misplaced component, but so far it seems that I didn't mess that part up this time. Shocking!

Thanks!

Paul

 

[Potato Cakes]

It tried the 12V relay power to bypass the 78L12 and it does work for that end of the circuit board, but it kills the +15 rail on the other end, making it -1 to -3V, so it won't pass audio as the op amps aren't getting the right amount of power. I think it's getting to the point where I need to post this question over on the GSSL help page. I'm just not fully understanding why if I'm getting the correct voltage on the Vin of the 78L12, then why would I not get the correct power all the time at Vout? I was hoping it would be just a faulty IC regulator, but something else is going on. I'm afraid it's going to replacing a bunch of parts and hopefully it'll start working.

Thanks!

Paul

 

[JohnRoberts]

To troubleshoot you can stick a few hundred ohm resistor in place of the VR and it will tell you how much current the circuit is drawing...

JR

 

[Potato Cakes]

So...

For one last double check on something before I started pulling stuff, I disconnected everything down stream of the 78L12 and double checked voltages on either side. Before, I thought the rail feeding it was always correct. This time, it measured incorrectly on both sides, leading me to the 7815, which was -0.9V instead of +15. But when there was a brief power off then back on as stated before, it was fine. I remembered with another project there as a power up issue as I had to replace the caps with smaller ones to accommodate for the increased channel count. Since I'm using a CRC with this build, I pulled the four 1000uF that were on the main PCB PSU section and now everything thing seems to be good. I think that the Expat guys were using the older GSSL boards for testing their circuits which have only two of the 1000uF caps.

So it wasn't the typical obvious issue, as I had hoped, but I think I learned something in the process.

Thanks!

Paul

 

[Majestic12]

This sounds like the well known latch-up issue.
Some regulator brands seem to be having this problem (especially "ST" brand sold by Reichelt electronic in Germany).
Replacing the regulator with ones from Fairchild did solve the problem on every GSSL that I have troubleshooted so far (and which are a lot).

 

[Potato Cakes]

Good to know about this problem related to brands. This happened to be the ST branded version regulator that was having the issue. I'll keep it in mind in the future.

Thanks!

Paul