Unbalanced volume at low levels

[8bitmime]

Hello everyone,

I built my crack a while ago and have been using it but have noticed some issues. 

The biggest one I've noticed so far is that the volume is unbalanced at low levels.  If you imagine the volume control to go from say 7 o'clock through 12 o'clock to 5 o'clock, then between 830 and 5 it's totally fine - but between 7 and 830 the right channel comes through nicely (and quietly) but the left channel is nearly silent.  Once you pass about the 830 point the left channel comes to life at the same volume as the right channel (i.e. it jumps from very quiet to the same level as the right channel).

Has anyone experienced this before?

I attached some photos just in case they help.  Please let me know if you see anything else that needs attention - I've noticed some squeaky Styrofoam-on-Styrofoam like noises intermittently but haven't tried to debug that yet either. 

Thanks

8bitmime

[Paul Birkeland]

See Crack FAQ 3/4:
https://forum.bottlehead.com/index.php?topic=4295.0

[8bitmime]

How interesting - why does the problem only happen at low levels though? 

I just bought some resistors for another project, I think there should be appropriate values in that bunch. 

Thank you for your help, I'll report back when I get to try that. 

That just leave the Styrofoam noises. 

Best

8bitmime

[Doc B.]

Try cleaning the tube pins. The squeaky styrofoam noise (great description BTW) is oscillation that is often caused by a bit of oxidation on the tube pins. You can use contact cleaner, or a scotchbrite pad, or very fine (like 400 grit) sandpaper, or metal polish.

[8bitmime]

Hello Everyone,

I finally got around to adding the dropping resistors to the potentiometer. 

After i made the change, i decided to try the resistance measurements again just to be safe and they all checked out

then i went and redid the voltage measurements and thats where all heck broke loose

terminals 1-5 were all wrong with the exception of 3 which actually read 0. 

1 : -1.5 v
2: 1.5 v
3: 0 v (woo hoo i got one!)
4: 1.5 v
5: -1.5 v

before you ask -  i did check that my leads werent swapped on the terminals or on my multimeter - it was really reading -1.5v

i have no idea what happened - before i made the modification the amp worked perfectly - i was using it daily and had no issues other than the volume imbalance at low levels. 

now the voltages are all over the place.

any ideas what may have happened?

cheers

8bitmime

[Paul Birkeland]

Those voltages have nothing to do with the pot wiring.  Is the fuse blown? Are you measuring in AC volts on your meter by accident?

[8bitmime]

Sorry I was unclear - I know they have nothing to do with the volume pot - I'm saying "I must have touched something accidentally" cuz it was working fine and now it's most certainly not. 

Is the fuse blown - no I did check that first.

Was I using the ac mode on my multimeter - I do not believe so - I could have accidentally done that but I am pretty sure I set it to dc before taking measurements.   I can do it again this evening to be sure. 

The only other thing I did was fix a mediocre solder joint in the power supply but that was an in and out job, the component didn't come out of the hole in the terminal strip - i just snaked the iron and solder to it.  I guess that could have been it but the whole thing was working fine the other day. 

Could pulling the tubes in and out have caused issues?  I did that a few times. 

I'm stumped. 

Best

8bitmime

[8bitmime]

I also think I should _probably_ start a new thread, I vaguely remember a "one topic per thread" rule

[Paul Birkeland]

What DC voltage do you see at terminal 21, 13, and 15?

[8bitmime]

Hello Everyone,

I tested the voltages on the pins you asked for

21 - 247.5 v dc
13 - 247.3 v dc
15 - 247.2 v dc

and i retested all the other pins while i was at it too

1 - -1.2 v dc
2 - -1.2 v dc
3 - 0
4 - -1.2 v dc
5 - -1.2 v dc

6 - 0
7 - 0
8 - 0
9 - 0
10 - 0

i must have missread my dmm yesterday since today all of the first 5 pins have the same (wrong and incorrect) voltage on them. 

Today i am sure that i was using dc mode on the dmm though. 

Thank you for your help

8bitmime

[Paul Birkeland]

Terminal 13 connects to B5.  B5 connects to B2.  B2 connects to terminal 2.  Terminal 2 connects to terminal 4.

Your terminal 13 isn't making it to terminal 2.

[Paul Birkeland]

Looking at your build photo, when the instructions say to "attach" something, that means making a mechanical connection by bending the wire or component lead up and around the terminal.  In your build, things are cut and slid through holes and terminals, then just soldered.  By doing this, you will allow for the wires to slide out of the solder and come loose, and that is probably what has happened.  You can go back through and just bend each little end up 90 degrees against the terminal it's connected to, then add some more heat and solder and that will help a lot.

[8bitmime]

Gosh darn you are good - you were right on the money - 13 -> b5 came loose.  Funny enough it's the only cable in that chain that I did bend around the terminal - I must have just dropped the ball. 

I'll reattach it tonight and bend over the other connections and report back. 

Thank you

8bitmime

[8bitmime]

Well Paul you were right on the money, I stripped the wire back a bit and gave it a bit more wrap and soldered it.  I then checked all the other questionable no wrap bindings, found one more bad one and stripped it back and soldered it too. 

The voltages checked out and my amp is back to life. 

I'll take some time this weekend and redo more of the "no bend" connections.   Old habits die hard or something. 

The dropping resistors on the volume pot helped a bit, it's still very loud even at the bottom of the potentiometer but it's more balanced which helps greatly.  Tomorrow I'll scrub the pins on the tubes and see about decreasing my Styrofoam problems.

Thank you all very much for your help

Best

8bitmime