No sound output

[markmazoo]

After I got my two 990c's from John Hardy, I finished up the final bits of work on my J99....and thought I was home free. Just so you don't think I'm a complete klutz, I recently built an A12 that works fine. I've built a Hampton JFET micpre also, as my first DIY electronics project and I struggled some..but eventually got it to work correctly.

On the J99, I tested U1 and U2 and got +14V and -14V for each of them. I checked the voltages at U3, with pin 4 at -12V and pin 8 at +12V; so I have U6 and U7 in the correct spots.

I adjusted the 990c's to +/- 21V for U1 and U2. I inserted U1 and U2 into their sockets and applied power. Measuring between U2, pin 4 and U2, pin 6...the DC voltage ""didn't"" fall below 5mV and stay there. It measured 14V. Obviously, I've got some kind of problem here. I went on to adjust the input bias. I shorted CONN1 pin 2 and pin 3 by connecting them with the jumper. I put J6 and J8 into the adjust positions. My Fluke 8060A DMM was set at the 20 DC reading (sorry, I'm far from an expert on electronic terminology and theory). The 2 choices below the 20 are 2 and 200mV, neither of which would give a reading. I could get U1 to have a difference of .003 ( 17.55 CCW minus 17.547 CW ). U2 was harder to adjust and the smallest difference I could get was .103 (14.523 CCW minus 14.42 CW). I took J6 and J8 out of the adjust positions.

I mounted the J99 in the chassis (it has the latest PS03), and connected a mic to my M-Audio Delta 1010 and I could NOT get a sound out of it. I switched the same connectors to the A12 and it sounded great.

Does anybody have any idea where I may have messed up, and how to correct my mistake(s)? I reflowed all my solder joints, hopefully to catch any bad connections. Please let me know if I need to provide any more information.

Thanks..........Mark

[markmazoo]

Hope someone will come to my rescue.

[tpryan]

Measuring between U2, pin 4 and U2, pin 6...the DC voltage ""didn't"" fall below 5mV and stay there. It measured 14V.

Does this op-amp behave in a similar way if you move it to U1?

[markmazoo]

Hey Tim, Thanks for getting to my problem.

I guess the answer is Yes.
Lets call this one op-amp "A". When moved to U1 it measured about 17.59.


Don't know if this will help (or confuse the issue) but I made measurements for op-amp "B" also.
When in U2, measuring @ the 200mv setting, the reading fluctuated between -0.05 to +0.26.
When in U1, measuring @ the 200mv setting, the reading started out at about -70.0 and slowly (took a few minutes) dropped to about -0.05.

What do you think?

[tpryan]

It sounds like you have a bad op-amp.

[markmazoo]

I'm sending op-amp "A" back to Hardy for analysis.
Thanks for your help Tim.

[markmazoo]

I hate it when these posts come to no resolution, and you're left wondering what happened. Not so with this one.

Tim, you were absolutely correct. The op-amp was bad. Got the replacement a couple days ago, and finally got a chance to install it today. The J99 fired right up and sounds great. Thanks for your assistance Tim!