trouble adjusting J99's

[tpryan]

It's probably jumpy because the board is out of the chassis and there's no termination at the input. If you're having trouble getting a steady reading, try shorting pins 2 and 3 at the input connector and avoid touching the bottom of the board.

[dandeurloo]

Ok, I did both of those and ideas and it did help with one of the kits. The other kit still is jumping all over the place. I triple checked solder joints and parts to make sure everything was good. I also, tried grounding the kit to the chassis. No matter what i do I can't get these to get bias like the other kit? Any more ideas? What happens if it I can't get them close?

Also, it seems like R50 may not adjust far enough for me to get it close. That one clicks out everytime.

Just so you know what I mean about jumpy. The meter will bounce from say -.60v to +2.26v a few times and then whatever it decides to stick with it will slowly drop in value for a few mins.

I really seem to have hit a brick wall. I spent about another 2 hours tonight on it and just can't get it to work. Any ideas would be great. I would try and send you readings but they are so sporadic it would probably be just a waste of time.

Help. I want to use these babies.

[tazman]

I am also having problems adjusting BIAS. I can't get them close at all.

[tpryan]

Adjust each op-amp separately, starting with U1 and U2 removed from the board. It may take a lot of turns to make a noticeable change. Keep in mind that this is an iterative process and takes some time. You won't get the voltages close all in one go.

[tazman]

As I turn R45 and R50, both the value ccw and the value cw move in the same direction, thus never converging.

Also, I thought you were supposed to do the BIASing with the op-amps installed. I did U1 with only U1 installed and the same with U2, but I could never get them to converge.

U1 reads 4.6mv ccw and 23mV cw
U2 reads 9.8mV ccw, 49.6mV cw

[tazman]

Just so I make sure I understand:

When I turn R45, it only affects U1, right? As I turn it cw, the value read with SW2 fully ccw and fully cw, both move up.

The opposite occurs when I turn R45 ccw.

The same happen with U2 when I turn R50.

Is this correct? How will they ever converge? The pre seems to work fine, BTW.

Also, it doesn't take a lot of turns to make the values start moving.

Looking at U1 and U2. Could someone id the pins on the chip. I just want to make sure I am not picking the wrong pins:

U1 (and U2)

-------
__| |__
__| |__
__| |__
__| |__
------

Thanks,

[tazman]

well, I tried everything. I keep getting the same thing. U1 and U2 are installed, both ccw and cw move in the same direction. The pre is working fine as far as passing sound, so I at this point I am not sure what to do. The instructions on the sticky are somewhat vague, I think. I can't tell if I am doing something wrong or what

[tpryan]

As I turn it cw, the value read with SW2 fully ccw and fully cw, both move up.

Yes, and as you turn ccw they'll both move down. At the high end of the gain range, the voltage will move more. One way to attack the adjustment is to observe the voltage (call it V1) at minimum gain, switch to maximum gain, then adjust the trim pot to match V1. Switch back to minimum gain and measure a new value for V1. Repeat the process until the voltages are as close as you can manage. Keep in mind that, in the end, all this adjustment does is help the servo settle faster. It doesn't make the preamp sound better or worse.

Could someone id the pins on the chip.

http://www.analog.com/UploadedFiles/Dat ... /AD797.pdf

[tazman]

Rob

[tazman]

ignore

[tpryan]

Are you reading my posts? If you can't make sense of the information in this thread, I don't think a telephone conversation is going to help.

[tazman]

Ok, I think I was testing between the wrong two pins (as I figured). This is what I get now:

U1 (U2 removed)

18.00V (ccw) and 18.01V(cw). When I put J6 back to normal operation the voltage between U1 pin 6 and pin 1 J4 (PGND) goes down to 2.8mV.

U2 (U1 removed)

18.01V (ccw) and 18.06V(cw). When I put J8 back to normal operation the voltage between U1 pin 6 and pin 1 J4 (PGND) goes down to 4.4mV.

All this with pin 2 and pin 3 shorted at CONN1.

My second J99 has the following readings:

U1 (U2 removed)

18.00V (ccw) and 18.02V(cw). When I put J6 back to normal operation the voltage between U1 pin 6 and pin 1 J4 (PGND) goes down to 4.0mV.

U2 (U1 removed)

18.01V (ccw) and 18.04V(cw). When I put J8 back to normal operation the voltage between U1 pin 6 and pin 1 J4 (PGND) goes down to 3.8mV.


Are these numbers correct?

Thanks and sorry for the pain. I just want my 2 J99's to work correctly


Roberto

[tpryan]

You're still not measuring all of the voltages at the right locations, but the final offset voltages are fine. I'd say you're done.

[tazman]

I am going to stop.

Just out of curiosity, I was measuring at pin 4 and pin 6 (per the sticky) based on the picture in the link you posted.

http://www.analog.com/UploadedFiles/Dat ... /AD797.pdf

[tpryan]

The sticky refers to pin numbers on U1, which is a 2520-style discrete module. You're measuring the IC pins directly. As you can see on the schematic, the pinout is not the same. The directions in the sticky work for either style of op-amp, however, as long as you measure the pins at U1/U2 and not U8/U9.