Voltage problem with J99 setup

[mikelevitt]

Hey guys,

I'm having a baffling problem with the J99 I just completed.

Voltages all check fine before installing the op-amps. When the op amps are installed I get about -11v across the output of U2.

Also, the -30v lamp on the power supply does not light when I have the op-amps installed. It does light when I have the module plugged in with the op-amps NOT installed.

I've double checked all of my work, looked for solder bridges, shorts, reversed diodes, etc. Nothing...

I've also tried a different "known good" pair of 990s. Same deal.

When the op-amps are installed, the 24v and -24v voltages on them are still testing good...

I haven't tried to pass signal, as I am worried about frying the power supply with that -30 LED not lighting...

Anybody have any ideas of things to check????

Thanks!

Mike

[tpryan]

Some suggestions:

> Voltages all check fine before installing the op-amps. When the op
> amps are installed I get about -11v across the output of U2.

It sounds like you've got a problem with the main op-amps or the servo. Lift one end of L2 before you do any more testing.

> Also, the -30v lamp on the power supply does not light when I have
> the op-amps installed. It does light when I have the module plugged
> in with the op-amps NOT installed.

Probably because the bad module is sucking too much current from the negative supply rail. This will likely go back to normal once you lift one end of L2.

> I've double checked all of my work, looked for solder bridges,
> shorts, reversed diodes, etc. Nothing...

Check closely around U3. Make absolutely sure all the resistors are in the right locations.

> I've also tried a different "known good" pair of 990s. Same deal.

This suggests the servo, not the 990s.

> When the op-amps are installed, the 24v and -24v voltages on them are
> still testing good...

What about the voltages at U3? Are the power supply voltages correct?

> I haven't tried to pass signal, as I am worried about frying the
> power supply with that -30 LED not lighting...

Don't bother with signal until you get the DC problems straightened out.

> Anybody have any ideas of things to check????

Short the input at CONN1. Measure the DC voltages at pin 6 of U1 and U2 and pins 1 and 7 of U3. Let us know what you find.

[mikelevitt]

Some suggestions:

> Voltages all check fine before installing the op-amps. When the op
> amps are installed I get about -11v across the output of U2.

It sounds like you've got a problem with the main op-amps or the servo. Lift one end of L2 before you do any more testing.

> Also, the -30v lamp on the power supply does not light when I have
> the op-amps installed. It does light when I have the module plugged
> in with the op-amps NOT installed.

Probably because the bad module is sucking too much current from the negative supply rail. This will likely go back to normal once you lift one end of L2.

OK....

> I've double checked all of my work, looked for solder bridges,
> shorts, reversed diodes, etc. Nothing...

Check closely around U3. Make absolutely sure all the resistors are in the right locations.

Will do...

> I've also tried a different "known good" pair of 990s. Same deal.

This suggests the servo, not the 990s.

> When the op-amps are installed, the 24v and -24v voltages on them are
> still testing good...

What about the voltages at U3? Are the power supply voltages correct?

Yes, the voltages on U3 are correct as well.

> I haven't tried to pass signal, as I am worried about frying the
> power supply with that -30 LED not lighting...

Don't bother with signal until you get the DC problems straightened out.

Thought so...

> Anybody have any ideas of things to check????

Short the input at CONN1. Measure the DC voltages at pin 6 of U1 and U2 and pins 1 and 7 of U3. Let us know what you find.

Thanks Tim. I'm tearing into it right now, and will report back with the findings.

Mike

[mikelevitt]

U1 - pin 6 -23.15

U2 - pin 6 -23.24

u3 - pin 1 11.16, pin 7 11.28

This is with one end of L2 lifted and the input shorted (pin 2 to pin 3)

And with L2 lifted, the PS -30v light is lighting, even with the 990s installed.

Thoughts?

Mike

[tpryan]

What's the voltage at the junction of R20 and R21? With the gain at maximum, what is the voltage at pin 1 of U1? And at pin 2 of U1?

[mikelevitt]

What's the voltage at the junction of R20 and R21? With the gain at maximum, what is the voltage at pin 1 of U1? And at pin 2 of U1?

U1 - pin1, -5, pin2 -4

I don't see a junction of R20 and 21, but here are the voltages...

R20 - "U3 side" 11.17, other side .005

R21 - "U3 side" .005, other side .002...

PS, I think I found something. The connection between pin 1 of U1 and the switch was not happening. I soldered through the bottom, and it did not flow to the top where the connection is. Fixing this now.

Would that matter???

Mike

[mikelevitt]

Oh boy...

I just resoldered that connection, reconnected L2, put the 990s in, and it is running without killing the -30 supply now...

After running for a minute, output of U2 (pin 4 and 6) is -.03v.... That sounds like it's fixed.....

I'm going to try some audio...

Mike

What's the voltage at the junction of R20 and R21? With the gain at maximum, what is the voltage at pin 1 of U1? And at pin 2 of U1?

U1 - pin1, -5, pin2 -4

I don't see a junction of R20 and 21, but here are the voltages...

R20 - "U3 side" 11.17, other side .005

R21 - "U3 side" .005, other side .002...

PS, I think I found something. The connection between pin 1 of U1 and the switch was not happening. I soldered through the bottom, and it did not flow to the top where the connection is. Fixing this now.

Would that matter???

Mike

[mikelevitt]

Passing audio, and sounds great.

Thanks for all of your help on this Tim... I'll be tracking tomorrow with these.

And for everybody else (watching and laughing) there's a lesson in this - don't overlook the small stuff... I'm amazed that I could make a mistake this silly - but the connection looked fine from the board side. It wasn't until I pulled my good J99 out of the case, and started comparing resistance readings from point to point that I found it....

Anyway, I am totally psyched to have these 8 killer preams finished!!!!!

Mike

[tpryan]

You beat me to it, I was just about to ask you to measure the resistance between U1, pin 1 and the junction of R20 and R21, the effective output of the first servo. The giveaway that something was wrong there was the large voltage difference between U1, pin 1 (-5v) and R20/R21 (.005v). The only resistance between those two points should be the 1538XL secondary winding, which is only a few ohms at DC. A large voltage drop implies a large resistance, or in this case an open connection.

[mikelevitt]

You beat me to it, I was just about to ask you to measure the resistance between U1, pin 1 and the junction of R20 and R21, the effective output of the first servo. The giveaway that something was wrong there was the large voltage difference between U1, pin 1 (-5v) and R20/R21 (.005v). The only resistance between those two points should be the 1538XL secondary winding, which is only a few ohms at DC. A large voltage drop implies a large resistance, or in this case an open connection.

Much more scientific than my approach, which was just to check the resistance between various points on the good J99 and the bad one and compare the numbers...

Anyway, thanks again for the help. The box is up and running and ready to go..

Mike

[dkatz42]

The connection between pin 1 of U1 and the switch was not happening. I soldered through the bottom, and it did not flow to the top where the connection is.

Most likely you had a cold solder joint. The holes are plated through, so it really ought not to be necessary to have solder necessarily coming out the top, though a properly heated connection will tend to do so anyhow.