Green Voltage Mystery

[Sleeper]

Testing out my greens today.
3 out of 4 are looking good, voltages are mostly in line with whats happening on the checklist.

PSU voltage on the neg supply is -15.2 and thats what I'm reading on the good boards at the negative posts of my op amp sockets, but on one of the boards I've got -16.4 at these points:?

I've checked for solder blobs and haven't found any...

can a bad decoupling cap cause this voltage to go up--or I should say down ? I've never seen this before... spooky :evil:

anyhow, I won't be at these again until tommorrow evening. but if anyone can steer me in the right direction. otherwise, I'll have to bust out my whiffle ball bat.


Sleeper

 

[tmbg]

have the chips in their sockets?

I find that most negative regulators will read high when unloaded, even though positive ones generally don't. My green board reads 14.98V and -17.04V when unloaded, but 14.98/-15.01 when loaded.

Then again, my greens don't work yet, so feel free to disregard

 

[Gus]

I like to load 3 terms at > 20ma. As posted before unloaded they will give the wrong readings.

Remember a simple rule 1ma across 1K is 1 volt

15V across 1k =15ma

Current ^2 X R(.015^2 X 1000) or voltage x current (15 X .015)will give the min power rating for the resistor. (.225 watts)

 

[Sleeper]

I'll clarify...
The chips are not loaded.
I have 4 seperate boards/channels and am testing them off af the same supply

3 boards are reading -15.2
1 board is reading -16.4

This is the mystery.

 

[alk509]

[quote author="Sleeper"]testing them off af the same supply[/quote]

Simultaneously? Unplug all the boards and measure the PS board unloaded... What do you get?

Peace,
Al.

 

[Sleeper]

No, until I get them tested out I'm running them individually off of the supply.

I do plan on running 4 on this one supply, it's the SSL supplyBTW, and all posts indicate that I've got the correct trafo and heatsinked regs to do the job.

I'll have to admit to being a big fan of troubleshooting by the process of elimination. I have 100% eliminated the psu as a source of error in this test setup.

Logically, from looking at what's laid out in front of me, it almost has to be a bad decoupling cap, without the ics socketed, there arent many components
connected to the -supply. for example the transistors aren't connected to the -supply so A bad trans. shouldn't cause havoc.


oops, I see a rply coming in all post this now

 

[Sleeper]

So, is this what happens here?
since my minus supply isn't really coupling to ground, it's coupled to the 0v center tap of the transformer could this be pushing the voltage up like this.

but wait a minute, how is this driving the voltage up

I'll have to check, but it seems like this is pretty similar to the voltage at the unregulated side of the power supply. Is a leaky connection to ground bypassing the regulator somehow....

late for work gotta go.

Kelly

 

[Gus]

you are not clear.

First what is the supply voltages not connected to the boards at all?

Next the one board that has a different voltage are the transistors in?

what style green PCB old or new one?

Maybe the one you are questioning does not have as much - current as the others. The unloaded voltage test of the PS will help.

 

[Sleeper]

you are not clear.

First what is the supply voltages not connected to the boards at all?

PSU voltage on the neg supply is -15.2

this is my unloaded voltage test

Next the one board that has a different voltage are the transistors in?

yes, ALL boards have transistors in. ICs have not been socketed.
and ALL boards are at the same stage of assembly.

what style green PCB old or new one?

new one

Maybe the one you are questioning does not have as much - current as the others. The unloaded voltage test of the PS will help.

I'm testing them with the same PS... the green has a molex connector for power. Picture me plugging one in, reading -15.2v unplugging that one, same with #2, same with#3 and then plugging in #4 and it moves to -16.4.


going to start replacing caps now, but I do hope someone can shed some light on Why this happens, I think this would be good info for other builders.

Thanks All
Kelly

 

[alk509]

[quote author="Sleeper"]going to start replacing caps now,[/quote]

I doubt the caps have anything to do with it... Something that draws current in your other three boards isn't doing it in the fourth one. For all practical purposes, caps don't draw DC current.

Peace,
Al.

 

[Sleeper]

hmmm- so maybe there's a bum resistor on this thing.

what I'm still not understanding is this, shouldn't an increased load on the PS cause the voltage to go down, in other words nearer to zero whether or not the rail is positive or negative?

this is why I mentally ruled out bad resistors.

Kelly

 

[CJ]

Put a current meter in series with the supply and record current draw on each of the four units.

 

[Svart]

I second the notion that the caps are *probably* not causing you this trouble, unless of course you used a tantalum somewhere that shorted but that usually causes dead shorts. I'll stop short of saying that they are not because i keep seeing strange things happening all the time. If you go ahead and take the board apart, take a part off, measure, take a part off, measure..etc. and let us know what exactly was wrong.

other than that what I'm getting from this is that the boards are not completed but you are voltage testing them at the same stage of completion regardless? are you using 1% metal films resistors? I don't want to cause a fuss and I'm not insinuating anything here but no one has asked yet so i'll be the evil one and do so.. have you double checked your resistor values? sometimes the colors look different on the blue bodies of the resistors especially if you use different brands... Just a thought that might help others too.

a difference of 1.2v.. that could be one resistor in the right places..

good luck! :thumb:

 

[Sleeper]

1% metal films all around.

I replaced a few of the caps-(surplus caps with new ones) and checked all the resistors. I also resoldered a couple of cold looking joints and took the exact-o knife to some of the tight traces. neatness counts don't it? I also went ahead and replaced the 7915 just to be safe and so far so good.

hooked it up and plugged in a mic, sounds pretty good on the bench here, I'm well impressed with how quiet this thing is...

One down three to go. just have to do the calibration on the other three. hope to have some pics by the weekend.

Thanks for the input.

Kelly