Lab Series Preamp Debug

[strat68]

Finished it and after finding out I had both voltage regulators (7815, 7915)reversed I need to see why it does not work.
Doc: https://www.dropbox.com/s/dnizjzbetec84gu/aion-lab-series-preamp-documentation.pdf?dl=1

1. Since I had the VRs reversed should I expect damage to them or any downstream components.  They got hot when reversed and still get hot when inserted properly.

2. The AC power source measures about 12 volts and not 9.

3. When measuring voltages,  since I'm not a circuits expert, should the meter be set on AC or DC voltage?  The board has  +/- 15v testpoints.  AC or DC?   

Thanks!

[m-Kresol]

1) If you have the regulators correct now, do have effect? does it work as advertised? If yes, then your answer is No. If it doesn't work, I would start tracing the signal and see where it stops/acts weird. Could well be you fried an IC, but I'm not quite sure.

2) AC having 12V is not a big deal. did you measure it without anything attached, ie. no load? If yes, than that's normal anyways. Voltage will drop when you add a load. It's important to have at least 9V with the load.

3) The diodes in the beginning of the power section are a bridge rectifier. After that you will only have DC voltage.

good luck

[strat68]

Thanks.  I suspected there was a rectifier.  And yes, I measured voltage right out of the power plug. 

As an aside, I wonder why they just didn't go with DC power the whole way?  Those regulators get pretty hot.

[m-Kresol]

you have + and -15 V, which gives you more headroom for your opamp ICs. If you run it on 15V DC you have either + or - 15V DC, thus less headroom. You could achieve the +/-15V with a charge pump, but those can introduce whining sometimes.

I have to admit I don't understand how it's possible to get 15V DC out of the 9V AC. Rectifying to DC will give you 9V*√(2)=12.7 V, but you'd also loose the forward voltage of the rectifying diodes, so about 11.5V or so. Anyone care to explain how to get up to more than 15V so the voltage regulators work appropriately?

[aion]

you have + and -15 V, which gives you more headroom for your opamp ICs. If you run it on 15V DC you have either + or - 15V DC, thus less headroom. You could achieve the +/-15V with a charge pump, but those can introduce whining sometimes.

I have to admit I don't understand how it's possible to get 15V DC out of the 9V AC. Rectifying to DC will give you 9V*√(2)=12.7 V, but you'd also loose the forward voltage of the rectifying diodes, so about 11.5V or so. Anyone care to explain how to get up to more than 15V so the voltage regulators work appropriately?

Voltage doubler The rectifier also doubles the AC voltage, giving you +/-25VDC, which is then regulated down to 15 on both rails. Here's the best explanation I've found.

[m-Kresol]

oh thanks. I've actually learned about that about two years ago and build one that went to +80V from 12V. totally forgot about that. 

[strat68]

I finally got back to looking at this.  After replacing the 2 voltage regulators I'm getting sound at various points in the circuit but still none at the output.  I am measuring 21.5vdc volts at the test points and not 15vdc.  I'm getting 21.5vdc at IC1.  The output of IC1 is fine.   I'll re-check the power section to make sure I have the correct parts.  The 21.5v makes me want to make sure of that.

I've got to study the schematic for the various trace points and go from there. 

[aion]

I'd try replacing the two 3080s first. Those are the components in the circuit that are most susceptible to burnout if anything goes awry.

[strat68]

Thanks.  I'm off work this week so I will start looking at it in depth.  Considering that many of the traceable points have a signal, I'm optimistic.

[strat68]

Had some spare time.  A question about the schematic (see link above).  To the right of IC1 are 4 symbols labeled 4,5,6,8.  What do they represent?  I always have a good signal out to C4.  Nothing after it except I had good signals all over the board for about 2 minutes yesterday.  Not sure what happened to make it work/mot work.  Sounds like something might be loose.

I have only been working with channel 1.

Thanks for any responses.  I know I;ll have a couple more questions.

[somnif]

The 4, 5, 6, and 8 are connector pads for wiring one of the switch boards. The other switch board uses Letters instead.

[strat68]

"The 4, 5, 6, and 8 are connector pads for wiring one of the switch boards. The other switch board uses Letters instead. "

That's what I thought.  So I am assuming C4 connects directly to lug 4 of the bypass footswitch, I assumed it's that switch.  I'll have to trace the channel switch to.

Thanks Somnif!