BP2 voltages slightly off (resolved)

[grosen]

Hi everyone,

This build has been smooth so far but I'm getting anomalous voltages on one channel at the check on p. 55 of the manual:

T6    94.5V  (100)
T16  69.2V  (100)

T25    9.6V (10)
T30  12.2V (10)

Any thoughts about what to check first?  Thanks,

Gideon

 

[Paul Birkeland]

Could you post some build photos?

I suspect on the side where you are seeing 69.2V that you have one of the resistors out of position around where the blue axial capacitor is.  The 94.5V isn't a problem.

 

[grosen]

Here's the resistor bundle on the bad channel:

 

[Paul Birkeland]

30U looks like it might need a more solid crimp and some solder reflow. 

What DC voltage do you get between pins 1 and 4 on that 4 pin socket?

What DC voltage do you get at pin 3 on that 4 pin socket?

 

[grosen]

Recrimped/flowed 30U.  Voltage between pins 1-4 = 4.9V.  The DC voltage at pin 3 (ref to ground) is maybe 10 mV, but it's a bit wobbly.  Not sure I'm measuring the right thing there.

 

[Paul Birkeland]

If you swap 300Bs, do the problematic voltages move to the other socket?

 

[grosen]

Nope.  That was the first thing I checked. Hmmm....

 

[Paul Birkeland]

Can you post some more build photos?

 

[grosen]

A random selection.  Happy to send more.  Thanks for bearing with me.

— Gideon

 

[Paul Birkeland]

You can use more solder on the joints on the terminal strips.  The terminal strips are phenolic and metal, and the teflon coating on the wires won't melt from your soldering iron, so don't be afraid to leave your iron on each joint for 10+ seconds, as there isn't really anything that's going to be damaged by doing that.  I suspect if you add more solder and reflow your joints that those problematic voltages are going to move around a bit. 

 

[grosen]

No luck.  I went back over everything upstream from the B socket with more heat and solder and the voltages are exactly where they were.  The resistance check before I installed the axial capacitor was perfect.  Does that localize the problem to the capacitor terminals (26U and 30U)? 

 

[Paul Birkeland]

Does that localize the problem to the capacitor terminals (26U and 30U)?

You are certainly welcome to remove the capacitor from those terminals to see if that does anything.  Looking at your photo, the lower joint below where the banded end of that capacitor connects has not had adequate heat applied to it to flow out properly.

 

[grosen]

Thanks, PB.  I removed the cap, checked resistances, reinstalled cap and reflowed everything I could find.  Resistances are spot on. Voltages are still as before.  I did a bit of prodding with a chopstick and the voltages are rock solid.  I'll give it a day or two and come back with fresh eyes unless someone has a better idea.  Thanks again for all the help .

 

[Paul Birkeland]

Could you please measure the DC resistance between 12 and ground?  How about 14?

 

[grosen]

Ah, great.  I think this is the problem.  12 to ground is open.  12 to Terminal D on the selector switch is 24k ohm; but D to ground is open, and I'm guessing that's wrong.

14 to ground is 22 ohms.


— Gideon

 

[Paul Birkeland]

If 14 to the chassis is 22 ohms, that's OK. 

12 to ground being open means you have a loose connection.  This could be on either side of the black/white twisted pair of wires leaving the balance pot, on the balance pot itself, the selector switch, etc.

 

[grosen]

OK.  I hate to be so needy, but I'm stumped.  What is the path to chassis ground from 12? I've got rock solid continuity from 12 to terminal D on the selector switch, and rock solid continuity from terminal C to ground.  The problem is zero continuity from D to C (if I understand correctly).  I've got continuity on the other side, from A to B on the selector switch (70k ohm).  So that's the obvious point where the channels differ.  So where's the path from C to D supposed to be?  Any chance I damaged the selector switch?  Or am I confused about how the path to ground from 12 is meant to go? 

Thanks one more time for all the help,

Gideon

 

[Paul Birkeland]

Center the balance pot, then measure the resistance of the upper pair of terminals without the 33K resistors across them, then measure the lower pair of terminals without the resistors across them.  Compare those resistances and let me know what they are.

 

[grosen]

Problem solved.  I was getting no continuity between the two terminals on the top rung of the balance pot.  I cleaned them up, got continuity of around 50K on the top and bottom, and now  the resistances and voltages are exactly right.  Man, my hunches about where the problem was were completely wrong. Glad I asked. 

I owe you a beer ...

Gideon

 

[Paul Birkeland]

  Man, my hunches about where the problem was were completely wrong. Glad I asked. 

I'm glad you were open to listening, let us know if you hit any other snags along the way.

-PB