intermittent white noise in one channel during warm up [resolved]

[maryc27182]

Yeah, I'm skeptical about my third measurement. I was waiting about an hour between tests, but as I mentioned, I can't be sure I got white noise. Ah well. I'm torn between taking another measurement and re-wetting those joints. Scientific curiosity versus engineering pragmatism. Thanks again for the help.

[maryc27182]

Well, since I had the preamp out, I took the Breg measurement. Pretty stable over 10 minutes of operation. 99.3 VDC about 15 seconds after powering on, and this drifted slowly down to 98.3 VDC at 10 minutes.

I am going to put this aside for now--probably put it back in the system this evening and see if the white noise is still there.

[maryc27182]

It turns out that having the preamp powered down for an hour, hour-and-a-half *isn't* long enough to get it back to whatever initial condition that causes the white noise. (I put the preamp back into my system and listened. When powered down for six hours, I got the white noise; when powered down for an hour, I did *not* get the white noise.)

So the OA and Breg measurements are probably OK (they were both done on a system that had been powered down for more than 12 hours.)

The OB and IA measurements were done on a system that had only been powered down for about an hour, so I'll redo those. I also got a new 12BH7, so having that option, along with the option of re-doing solder joints, should keep me busy, since I only like to make one change at a time hahaha.

[maryc27182]

Here's IA (VDC) versus time. The preamp had been powered down for 16 hours. Ambient temperature was probably 5 degrees F colder than when I took the previous measurements.

Summary: IA was ~226 VDC until about 9 minutes after powering on. Then it fell to 160-170 (fluctuated) for about a minute, then dropped below 150 (fluctuating) to a minimum of 128 at about 11 minutes after powering on. There was a cluster of points fluctuating around 140 from 11.5 to 12.5 minutes, then a cluster of 160-170ish VDC from 13-14 minutes, and a linear rise from 180ish to 226 from 14 minutes to 16 minutes (ish). Stable at 226 from 16 minutes to 20 minutes, at which time I powered down.

I'll re-attach the OA vs Time measurement from previously. Difference was that the OA voltage dropped at 6 minutes, but these data were measured previously.

My knowledge of voltage regulators is slim, but shouldn't it give 225 VDC no matter what happens 'downstream'? (Again, I don't know enough about how it works to know if the 'stuff' on the C4S board should be able to pull IA down.)

[Paul Birkeland]

Well, that's very helpful.  The IA voltage is indeed the output of the regulator back by the 12BH7.  There's a slim chance that this is a 12BH7 acting up (though very doubtful, a failing 12BH7 would actually cause the regulated voltage to go up usually), but more than that, there may be a solder joint acting up thermally back on the PCB above the 12BH7. 

I would now recommend measuring Kreg on the PCB over the 12BH7 on the offending side to see if it drops too, but at the same juncture I would reheat all the solder joints on that board as a precautionary step.

-PB

[maryc27182]

Thank you! I was just about to get the preamp out again, as it's been about six hours and should exhibit the behavior again. I will measure Kreg on the board above the 12BH7. Let me take a look to make sure I can figure out which side is the right channel....

I am assuming the "A" Kreg on that board is the right channel?

Would you like the measurement, or would you like me to remove the board and reheat everything first? Thanks again.

[maryc27182]

Re-reading your post, I think you want me to re-do all the solder joints first.

[Paul Birkeland]

I'd reheat everything on that board first, yes.

[maryc27182]

Thanks again. I will be slowed down a bit. While looking at that board, I saw that one of the 220 ohm grid stop resistors was under tension--I remember being concerned about that when I mounted the board, but I didn't know (then) that these resistors are mechanically fragile.

I tried to relieve the mechanical stress yesterday (I had a couple of mm of lead protruding through the hole) but I wonder if the damage is already done. Anyway--I will get some carbon comp 220 ohm 1/4W resistors before I continue.

[Paul Birkeland]

Any old 100-500 Ohm film resistor will do the job down there.
-PB

[maryc27182]

Yeah, all I had in my parts box were 5W metal oxide film resistors, and they are, um, hefty. I can't even remember what I used them for.

p.s. Woah.. so it's 50 posts where you stop being a "Jr. Member" or whatever I was. I don't feel so bad being so verbose now.

[maryc27182]

It wasn't the grid stop resistors being in tension (and perhaps damaged.) I think it was a bad solder joint on a 2N2907A transistor leg. I can't be totally sure, since I re-wet everything on the shunt regulator pcb (as well as the tube socket for the 12BH7.) However, it was the only joint I saw that looked suspect--I had trouble re-wetting it and spent more time on it than anything else.

Although my head knew it's silicon, I built a couple of guitar-related things with germanium transistors recently, and I'm sure my reluctance and anxiety was driven by feeling how warm the 2N2907A was getting as I tried to get a decent joint, as well as my past fear over solder wicking up and bridging to the metal body.

Anyway--I've done two listening trials now, and so far, no white noise (oh--voltage checks were all good.) Thanks for all your help with this.

[Paul Birkeland]

Good job tracking that down!

Yeah, it can be a little tough sometimes to know how much heat parts can take.  The little HLMP diodes don't take a lot (they also don't need a lot to be well soldered), then the MJE5731 takes tons and tons of heat to get properly soldered.  The PN2907 is somewhere in te middle.

If the issue comes back, we'll be here to help.

-PB