Trouble with another 990C?

[Archmart]

Hey Hey!

Just finished my 2nd J99 Rev3.3 (2.5 years after the first! and I've got some trouble.

Everything's good up to the point of installing the 990Cs. I've got the voltage set at +/-24V and everything checks out at U1,U2, and U3.

Upon powering up with the 990Cs in, the voltage between pins 4 & 6 settles down to 50mV on one and 75mV on the other. That being more than a factor of 10 over the 5mV the instructions shoot for, I'm concerned.

I power off and swap 990Cs (between U1 & U2) and get similar readings.

I power off and swap 990Cs between my working J99 and this newly finished one. (I now forget what I read there...)

I power off and swap back again.

I look for the biasing sticky and can't find it. I find some biasing instructions in the latest version's instructions and try to follow those... (short pins 2&3 on Conn 1 and move J6 & J8 jumpers to "ADJ")

On U2, I get a consistent 24mV difference no matter how I set Bias 2.

On U1, I get within .1mV no matter how I set Bias 1.

I try U2 again, and this time I get a 414mV difference no matter how I set Bias 2.

I put the jumpers back in place and do the final full power up measurement again, and this time, U1 is reading 20mV and U2 is reading 16.4V!

Help?

Thanks,
Archmart

[tpryan]

1. Adjust regulators for +/-24V.
2. Jumper J6 and J8 so the center pin is grounded.
3. Install one 990C at U1. Don't install anything at U2.
4. Short pins 2 and 3 of CONN1.
5. Connect a voltmeter to U1, pin 6.
6. Set SW2 for minimum gain.
7. Measure the DC voltage at U1, pin 6 and record it.
8. Set SW2 for maximum gain.
9. Adjust R45 so the new voltage matches the voltage from step 6.
10. Repeat steps 6 through 9 until the voltages are as close as possible to each other. It doesn't matter what the voltages are as long as they're less than a volt or two.

When you get that working, let us know.

[Archmart]

Following the biasing instructions for the current rev. blindly, I was measuring at the empty sockets for U2 and adjusting U2's bias while a 990C was in U1, and vice versa.

I'll do this tonight and report back.

And I presume I should also follow these instructions for U2, yes?

Thanks,
Archmart

[Archmart]

Hey Hey!

Ok, adjusted for +/- 24V, J6 & J8 in the "ADJ" position, pins 2 and 3 of CONN1 shorted, and with trim full up...

990C (A) in U1/Nothing in U2:
0.0 mV at pin 6 at minimum and maximum gain.

990C (A) in U2/Nothing in U1:
1.3 mV at pin 6 at minimum and maximum gain.

990C (B) in U2/Nothing in U1:
16.3 V (not millivolts, but straight up volts!) at pin 6 at minimum and maximum gain.

990C (B) = Bad. Yes?

Darn! That's 3 out of 4! And these two were replacements for the original 2 bad ones, and it's been almost 3 years, so he's going to think I've just burned it up in use in the meantime...

Let me know if I'm missing something here, before I contact John...

Thanks,
Archmart

[tpryan]

You definitely have a bad 990C.

[Archmart]

Hey Hey!

Thanks. I'll talk to John. While I'm at it, I'm going to try again to talk him into selling me a couple of his very nice knobs. He's a tough nut to crack.

Thanks,
Archmart

[Archmart]

Hey Hey!

Finally got the replacement 990C in...

BUT... Here's where I sit... The biasing process went ok, with U1 ending up at -.1mV and U2 at 1.3mV.

The final, jumpers back in the rightful places, pins 2 and 3 no longer shorted, both 990Cs in place and powered up test results, though, are a bit off: U1 settles to -45mV and U2 settles to -33mV. That seems a far cry from the "less than 5mV I believe you prescribed last time.

What to do?

Thanks,
Kerry

p.s. Let me say for anyone who happens to read this that, talking with John Hardy I'm now convinced that the odds are against me on 3 of these 990C's having been bad. I probably accidentally shorted at least one of them unknowingly at some point. John's a good guy and totally fair, and the amount of testing and QC he does really doesn't leave much room for this many 990Cs to be bad...

[tpryan]

The final, jumpers back in the rightful places

And that means?

pins 2 and 3 no longer shorted

Keep them shorted or terminated with a 150 ohm source.

U1 settles to -45mV and U2 settles to -33mV

Measuring with respect to U2, pin 4, what voltage do you get at U2, pin 6 after 2 minutes when set to minimum gain?

[Archmart]

Hey Hey!

Sorry to be unclear.

I mean to say that the biasing went alright (-.1mV and 1.3mV) and THEN, as per V3.3, step 38, with jumpers J6 and J8 NOT set to "ADJ" and pins 2 and 3 (of Conn 1) no longer shorted, U1's output settles to -45mV and U2's output settles to -33mV (after several minutes).

In biasing U2, it WAS a bit tricky in that at full gain the voltage would very slowly rise, about a 10th of a millivolt a second, so I had to keep adjusting and adjusting until it finally settled. I don't know if some component was just sort of burning in at first or what, but eventually it did finally stabilize.

Too high? I haven't tried listening to a mic through it yet, not being sure if it's a good idea.

Thanks for your help,
Archmart

[Archmart]

Hey Hey!

(Sorry to be impatient, but I've got a big session I really want to use this on this Friday night...)

U1's output settles to -45mV and U2's output settles to -33mV (after several minutes).

I'll appreciate any help you can offer.

Thanks,
Kerry

[tpryan]

Have you measured again as suggested above, with the input shorted, gain set to minimum, and with the black meter probe directly at U2, pin 4? I suspect there's not a real problem and it will pass signal fine.

[Archmart]

Hey Hey!

Sorry, no. I haven't, as I'm a bit confused.

You're saying to short pins 2 and 3 of the input XLR, but do I also set the jumpers to "ADJ" or do I leave them in normal operating mode? I thought shorting pins 2 and 3 of the input was only for setting the bias with the jumpers set to "ADJ", and that checking the final load balancing (is that right?) of the twin servos was done without shorting pins 2 and 3 or setting the jumpers to "ADJ".

And when I do this, what parameters do I need to hope for? At what voltage reading (at pin 4) would I have reason for concern and not want to actually plug a mic into it to judge with my ears?

Thanks,
Archmart

[tpryan]

do I also set the jumpers to "ADJ" or do I leave them in normal operating mode?

Normal operating mode. We want to measure how close to 0v the servos bring the offset voltage.

At what voltage reading (at pin 4) would I have reason for concern and not want to actually plug a mic into it to judge with my ears?

The servos should bring the offset to within a few millivolts of 0v, but I wouldn't worry about 10mV or 20mV either way. More than 100mV, there's definitely a problem. Small errors like you're seeing are often a result of dirty probes or similar measurement problems.

[Archmart]

Hey Hey!

Measuring with respect to U2, pin 4, what voltage do you get at U2, pin 6 after 2 minutes when set to minimum gain?[quote]

-41mV at U2's pin 6

(and -45mV at U1's pin 6)

(and that's with pins 2 & 3 of Conn 1 shorted)

Not 0mV, but not 100mV either. What to do?

Archmart

[Archmart]

Hey Hey!

I went ahead and plugged a mic into it. I didn't die.

Just testing my own voice in a U87Ai, it sounded pretty good. I've not tried it with a matched pair of mics yet, nor even just switched between the two channels, as I really hate testing with my own voice.

Serving as my goto clean stereo pair for such things is exactly what I hope for these, which is why I really want to get to the bottom of this discrepancy.

So, yeh...

Measuring with respect to U2, pin 4, what voltage do you get at U2, pin 6 after 2 minutes when set to minimum gain?

-41mV at U2's pin 6

(and -45mV at U1's pin 6)

and that's with pins 2 & 3 of Conn 1 shorted)

Not 0mV, but not 100mV either. What to do? (Gotta hate luke warm...)

Archmart