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Another Super High End Phono Stage! No expense spared...

Failure

Yesterday (Friday) I went to the family room to see if I could quantify how much lower the right channel is vs the right. I have been chasing this imbalance in my TT and cartridge combos for months. I have been hoping to add a bit of gain to the left channel of the CNC to compensate.

The sound from my Shure test record was a little garbled at first, so I turned the TT and CNC off, then back on.

Surprise! BOTH channels from the TT are playing through the left speaker and nothing through the right.

First thing I did was switch the 5000A to FM Stereo and everything sounds OK. Went back to AUX and swapped the output RCAs on the CNC and the combined channels came out of the right speaker. I switched to Phono and moved the TT RCAs from the CNC to the receiver and things are normal.

Then I reconfigured everything back to the 'original' configuration tried it again, but I still get both channels on the left. BTW: I turned the TT and CNC off each time I changed the configuration.

Seems to me that I have isolated the problem to the CNC but I cannot begin to imagine what failure mode could be causing this. The last thing I was doing on Thursday was probing points on the CNC with my TEK 468 scope to read the output levels for each channel. Don’t think I bridged any pins with the probe, but could this have zapped one of the opamps? It was playing ‘normally’ when I quit this line of testing on Thursday.

This morning (Saturday) I went back to take another look. Now everything seems to be like it was before yesterday. SHE! (Self Healing Electronics) I am bamboozled.

Now that the sun is shining I'll go cut up a downed tree, mow some grass, then come in & try it again.

Anybody got any ideas or constructive suggestions?

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I had a problem with another phono stage, turned out a single strand of wire was shorting the RCA socket when a RCA plug was pushed in, it can be something that minor.

Like Pio1980 said, go over everything, you never know.
 
I checked each set of joints with a 10X loupe and an ohm meter during the build, but it won't cost me much to check it all again.

Right now I am just waiting for that second GOTTCHA!

Thanks - - -

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I'm with Pio, with the probing it could have boogered a less than perfect solder joint. I'm all for rewetting.

The problem could be either power supply or signal path. One joint opened and you are out one channel. It should be easy to test the chip legs for the power supply voltage. With the 'scope you can verify how far the signal gets. Well, maybe not the input level, I don't think my 'scope will read that low in voltage. Be sure to turn off the 10:1 divider on the probe, go 1:1 on the signal.

Good luck.
 
I carefully examined all the solder joints and have verified good joints and no whiskers.

I was using the point probe of the scope, checking the voltage levels at pins 1 & 7 of each opamp to read the L&R output for each stage. My practice has always been to place the point where the lead goes into the socket, giving a positive connection and minimizing the probability of bridging 2 leads with the probe.

The opamps are the only thing common to the two channels other than the ground trace, so the problem must have been in one of the opamps. Seems wierd to me that the problem persisted through several on/off cycles of the preamp, then was OK the next day.

Guess that the only thing I can do now is to keep an eye on it and enjoy the sound of this fascinating preamp.

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Only other thing is to verify the chip leads are indeed properly inserted and seated and not somehow folded in intermittent contact, other than that I got nuttin' left since it's not a common-mode prob with both channels.
 
I also think the input and output connections are less reliable than your board traces. Is it possible one of those are intermittent?
 
Pio1980 said:
Only other thing is to verify the chip leads are indeed properly inserted and seated and not somehow folded in intermittent contact, other than that I got nuttin' left since it's not a common-mode prob with both channels.
Click to expand...

Good call, but I checked that last night. Carefully pulled each opamp,checked that the leads were OK, then reseated them.

Grainger49 said:
I also think the input and output connections are less reliable than your board traces. Is it possible one of those are intermittent?
Click to expand...

The real problem was that I was getting both channels on the left channel out of the preamp, and nothing out of the right. Wierd problem. and then it cleared up overnight.

I used solid wire from the PCB to the connectors, bent the wire thru the hole in the connector, crimped it down, then soldered it. Checked them all last night and the connections are solid.

Checked it again this afternoon and both channels are separate and playing out of the correct speaker. It almost seems like I had a bad dream (for an hour?) and took good notes in my sleep.

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Yeah, that makes no sense. As I think you said, nothing is common but the power supply. The dead channel's input couldn't physically get to the operating channel's input.
 
Take a close up pic and share .. maybe it's something glaring you in the eyes and need a 2nd pair to see it. Could be just about anything but I'd put my $ on a wire issue to/from the inputs or outputs and the ground.

Cheers,
Bob
 
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