J99 Input bias current adjustment.

[mondstein]

Hi there, Happy New Year

I just finished my J99 and I have some questions:
No problem with adjustment U8(AD797). Trim pot R45.
For U9, the value of the voltage between pin 4 and pin 6 with the switch turned fully CCW is steady. This is not the case by turned fully clockwise (not stable). Any clue?
Possible hint: Voltage regulators U4 and U5 have been initially wrongly installed and amplifier has been powered. Error has been fixed and supply voltage +/- 24 V has been adjusted according to instruction. Could this have damaged the op-amps AD797?

Note: Both capacitors C8 and C9 have been removed. Correct?

Thanks

thom

[tpryan]

If you're using the AD797s, adjust the regulators for +/-18v. They won't last long with +/-24v rails. The output of U9 may jump around a bit, especially if the module is completely outside of the chassis while making the adjustment. Just get it as close as you can. C8 and C9 are not required when using the AD797s, but C21/C22 and R27/R39 are.

[mondstein]

Hi,

Thanks for your reply. Once adjusted the PS-voltage to +/- 18V, my J99 with AD797 is now working correctly.

Note: Twisting the leads to the DMM reduces strongly instability of the DMM readings while adjusting R45 and R 50.

thom

[tpryan]

Note: Twisting the leads to the DMM reduces strongly instability of the DMM readings while adjusting R45 and R 50.

Excellent tip.