Wrong voltage on speedball big board [resolved]

[illusineer]

OA - 110V
OB - 102V
G - 13V
B+ - 206V

One of my voltages ain't right  :-\ it also reads 13v on the small board test for B-A/B. I plugged in my headphones and only got sound from the right. My amp was working fine before I put the pcbs on it. Also when I was soldering wires on the terminals for the big board sparks popped near the capacitors while I was affixing the wire ends with my pliers (this was after cutting out the 10k 3ws and before putting the big board on). The amp was unplugged but didn't blow the fuse, idk if that has anything to with it or I shorted out the capacitors somewhere while they still had charge.

 

[Paul Birkeland]

Please don't plug headphones into an amp that isn't working, as you may end up with headphones that also don't work.

13V on G means you have 13V on terminal 3 as well.  One of the black wires in your amp isn't well connected and is causing this problem.  This is not a Speedball issue.  I would reflow every solder joint in the amp where a black wire connects, then recheck G on the big board.

On the other hand, if your meter says 13mV, that's 0.013V and is plenty close to 0V to be ignored.

 

[illusineer]

I tested again, and what I noticed is that my multimeter reads 13-17v just with the black lead on 12u, not touched to the big board's G ??

 

[Paul Birkeland]

Can you post the model number or a photo of your meter?

 

[illusineer]

It's a klein mm400, read out all the voltages for when I built the regular crack just fine, got it only a couple days ago brand new

 

[Paul Birkeland]

The meter may need you to hit the Range button until it goes to auto or down to a low range where you can resolve something close to 0V.

 

[illusineer]

That is what I've been doing, I set the meter to volts then press range 4 times until it says 0V. Then I set it to MAX to get a reading that doesn't fluctuate all over the place when I touch the leads. Got my readings this exact way when I built without speedball. Without MAX it just reads 0-1V with 12u and G.

 

[illusineer]

12U didn't read 13-17V on my meters max mode on the regular crack voltage tests or any of the terminals that need voltage at 0 beforehand either if my memory serves me right.

 

[Paul Birkeland]

That is what I've been doing, I set the meter to volts then press range 4 times until it says 0V. Then I set it to MAX to get a reading that doesn't fluctuate all over the place when I touch the leads. Got my readings this exact way when I built without speedball. Without MAX it just reads 0-1V with 12u and G.

Is there a range that says "auto"? You should be able to measure 1.5V on a AA, AAA, C, or D battery as a test.

 

[illusineer]

Yes, I tried a brand new double a battery on auto and it reads 0.000V, I'm starting to suspect my meter is a $50 piece of brand new junk.

 

[Paul Birkeland]

If there's a Harbor Freight nearby, the cheap $7 meter they have there is more or less adequate for this kind of work, and the $23 meter is reasonably nice.

 

[Deke609]

Was the meter set to DC volts - parallel flat solid and flat dashed lines?  I have a bunch of cheap meters that I use for doing lots of simultaneous measurements and they require a lot of button pushes to configure properly. 

Don't use the "MAX" setting - it looks for and displays the maximum value measured.  Great for detecting one-off voltage spikes, but not so good for basic voltage tests (and don;t use "MIN" either, for a similar reason). You mentioned that if not set to "MAX" the meter reads 0 to 1V at 12 - and 0 is what you're look for.

And make sure the meter has fresh batteries. I've gotten some crazy readings when using batteries that were only marginally low.

cheers, Derek

 

[illusineer]

OK idk if you believe me but I figured out the correct voltage mode before you posted lol. I also redid solder joints on some of the black leads. Here are my readings

OA - 110
OB - 112
G - 0
B+ - 204-210

Everything seemed OK, plugged in my phones and worked for about 30 seconds, then started getting distortion in left channel even with nothing playing.  Not sure if it's related to my problem, but I did see voltage spikes on G a couple times while measuring it with my meter.

 

[Paul Birkeland]

Distortion is usually a loose wire.

 

[illusineer]

Left channel now doesn't have any sound :/

 

[Paul Birkeland]

No sound could be a loose wire/bad connection, or something else.  Have any voltages changed?

 

[illusineer]

Voltages haven't changed, any connections in particular I should be looking at? Maybe one of the terminals is changing voltage only when I'm not checking lol

 

[Paul Birkeland]

No sound would have me looking for debris across the RCA jacks shorting them out, loose or broken wires going to or from the volume pot, or loose/broken wires going to and from the headphone jack.  Many of the other problems that would cause a channel to drop would also make very noticeable voltage issues.

 

[illusineer]

I resoldered some more joints, checked voltages again. Now OB is getting 196V and B+ is getting 230, others didn't change and these two LEDS stopped lighting up (or maybe they were never lighting up?). Also looked at the RCAs and there doesn't seem to be any continuity between their underside soldered tips and the chassis. The pot seems ok and so does the headphone jack, I can take more pictures if it helps.

 

[Paul Birkeland]

One black wire on the heapdhone jack looks like it isn't captured by solder.

Is that OB on the big board or little board?  I would guess the big board by your photos.  I would suggest reflowing all the joints on the big board, giving a lot of extra heat to the center leg of the TIP50C.  I would suggest leaving your soldering iron on that middle lug for at least ten seconds.