I can't adjust the bias correctly

[elbartolo]

I have a J 99 Rev 3.3 with AD 797 OP Amps and I can't adjust the bias correctly.

This is the way I do it, let me know if I do something wrong:

Step 34. I short CONN1 pin 2 and 3. I turn R28 and R32 counter-clockwise until I hear a soft click and when I measure U1 and U2 between pin4 and pin3 it is ok I find 14V, when I measure between pin4 and pin5 the voltage is -14V.

After that, (the jumpers J6 and J8 are at ADJ position) I measure U1 and U2 at pin 6 and 4, with SW2 clockwise and after counter-clockwise and it's impossible to make the 2 measurements close. I tried to adjust R45 and R50 in every way and the measurements are very far from each other.

When I plug a microphone, the sound is very loud and with distorsion, and when if I turn the GAIN a big noise appears.
Thank you for your help.

[tpryan]

Step 34. I short CONN1 pin 2 and 3. I turn R28 and R32 counter-clockwise until I hear a soft click and when I measure U1 and U2 between pin4 and pin3 it is ok I find 14V, when I measure between pin4 and pin5 the voltage is -14V.

Can you adjust R28 and R32 for +/-18V in step 36?

After that, (the jumpers J6 and J8 are at ADJ position) I measure U1 and U2 at pin 6 and 4, with SW2 clockwise and after counter-clockwise and it's impossible to make the 2 measurements close. I tried to adjust R45 and R50 in every way and the measurements are very far from each other.

Are you following steps 37 through 48 exactly? The object is not to get the voltage at U1 to be the same as at U2, it's to make the voltage at U1, pin 6 the same at minimum and maximum gain, then to repeat the procedure for U2. Even without this adjustment, the servos should work and the module should pass a signal.

When I plug a microphone, the sound is very loud and with distorsion, and when if I turn the GAIN a big noise appears.

What is the entire signal chain? What gain setting causes the "big noise"?

[elbartolo]

I don't understand how to measure in step 36.
The assembly instructions says:

" check the voltages at U3. U3 pin 4 should be
around -12V; U3 pin 8 should be around +12V."

Can you please, tell me where do I have to put the meter probes of my DMM?
U3 has 8 pins but no numbers on it.

[tpryan]

Can you please, tell me where do I have to put the meter probes of my DMM? U3 has 8 pins but no numbers on it.

http://www.linear.com/designtools/packaging/pdip/N8.pdf

Put the black probe on any convenient PGND point like J8, pin 2.

[elbartolo]

OK, now I know how to measure in step 36, but is it 12 or 16 V, the assembly instructions says 12V and you tell me 16V ?

[elbartolo]

Sorry, 12 or 18 V ?

[tpryan]

In step 35, you're looking for +/-12V at U3. In step 36, you're adjusting R33 and R37 to get +/-18V at U1 and U2.

[elbartolo]

OK,the voltage at U1 and U2, pin 6 is the same at minimum and maximum gain, , I have 11,70 V at U3 pin4 and 8 .
What is the next step?

My J99 is a REV 3.3 version. My assembly intructions ends at step 38:

http://www.joselarraceleta.com/j99_asmbly.pdf

[tpryan]

OK, can you adjust the voltages as described in step 37? If your board looks like that pictured in the rev 3.3 instructions, you're done. If it looks like the board in the current instructions, use those for the adjustment procedure.

[elbartolo]

GREAT !

The J99 works perfectly, thanks a lot for your help.