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

[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