LED Troubleshooting in Speedball

[dpitch40]

Hey all, I have begun assembly on my Speedball kit. I just finished soldering the components onto the main PCB. While checking the various components and connections, I discovered that two of the installed LEDs didn't work. For two of them, namely the two closest to the edges of the PCB, I get an open circuit with the black (neutral/common) lead of my DMM on the red/non-banded side of the diode, and a voltage drop of about 0.673 V with the black lead on the silver/banded side, as expected. For the other two that are farther to the center, I get an open circuit in both directions. To my dismay, the other four LEDs that I haven't installed yet also all have this problem. I am a bit suspicious that six of my eight LEDs would be defective after I had no problems with the ones in my Crack, and that the working ones would happen to be the two that I installed in matching positions on the PCB. Is it possible that I am testing them wrong, is there some other reason only those two work, or am I just that unlucky?

[Paul Joppa]

Normal LED voltage drop (at least these "standard red" formulations) is about 1.57 volts. Some meters will light them up in the right direction, some won't.

[Paul Birkeland]

Of the 8 meters that I have, 2 will light the HLMP diodes, the other 6 will not.

Work through the upgrade, then look for issues (if any) during power up.

[dpitch40]

It worked fine for a few days...now I keep getting loud, persistent crackling and softer 60 Hz humming in the right channel after 5-10 minutes of being powered on (so after the tubes have already warmed up). Given that I recently installed the Speedball, could this be related?

[Paul Birkeland]

It sounds like an iffy solder joint, I'd get a magnifier and double check the installation.

[dpitch40]

I figured it was probably Speedball-related. To save time, which board corresponds to the right channel?

UPDATE: I think circuit board "A" corresponds to the right channel. On inspection, I can't see any obvious soldering problems. Also, it seems to be random whether the soft hum, loud popping/crackling, or both occur; sometimes it starts immediately, other times I don't hear anything for a while. The crackling is always intermittent, not continuous, and the hum is sometimes fixed by shifting the RCA cables in their sockets. Is there any way to narrow it down?

Also, interestingly, it now seems to be amplifying the vibrations of me running my finger over the chassis; tapping the chassis produces a loud pop in the right channel only.

[dpitch40]

Thinking it was a ground connection, I reflowed and reinforced all the ground-related (black-wired) solder joints I added for the Speedball. This seemed to work for a few hours, but when I put the chassis back in place  and powered it on again, the crackling returned within 10 minutes. I am pretty stumped.

[Paul Birkeland]

Do you have tubes you can roll?

If you can reproduce the crackling by tapping one of the RCA jacks, then I would focus there, though you should be able to mitigate this effect by turning the volume control all the way down (in other words, with the volume control all the way down, tapping in the RCA jack would only cause crackling if this was coming through elsewhere).

Were your voltages all looking pretty good?

Also, if the screws holding the TIP50C transistors to the heatsinks aren't somewhat tight, they will begin to overheat after a few minutes

[dpitch40]

No; I don't have any extra tubes I can try.

The crackling still happens even with the RCA jacks unplugged and the volume all the way down, starting after a few minutes. EDIT: It is very erratic; once again, I have been listening for about an hour with no crackling.

My voltages were all fairly good, within 15-20% of what they should have been, so i wasn't too concerned about that.

The screws holding the TIP50C transistors are definitely tight enough.

[dpitch40]

It just started crackling again after listening for about an hour--could it be heat-related or something?

[Paul Birkeland]

I would check the tightness of the screws that hold the TIP-50C's against the heatsinks.

A spare set of tubes will set you back $10-20, it might be good piece of mind to be able to rule that out.

[dpitch40]

They were already pretty tight before, and definitely are now. I didn't know the tubes were that cheap. Where could I order some? (Or other tubes that would fit into the sockets--this may be a good time to start tube rolling)

[Paul Birkeland]

www.tubesandmore.com and www.tubedepot.com are good places to start, eBay generally has tons and tons of options too.

Don't worry about paying for brand new tubes, used tubes are a good way to go.

[dpitch40]

How do I know if the tubes will fit the Crack? Is it just any octal or 9-pin tube?

[Paul Birkeland]

How do I know if the tubes will fit the Crack? Is it just any octal or 9-pin tube?

You need one 12AU7 and one 6080.