Hobby Components 8x8x8 Cube Kit (HCKITS0050)

[andrew]

Yeah sorry again about that. It's because in the new revision of the PCB we've reversed the silkscreen print so where before it was black outlines on a white background, it's now white on black. So the negative side, the side with the stripe, of both capacitors needs to go the the middle (the round pad) and the positive sides to the outside (square pad). I'll get the instructions updated to reflect this.

[IBI]

I have my cube mostly assembled but as I was testing the fifth grid the blue LEDs stopped lighting up and the large white ones only light up inconsistently- exactly the same problem as users Cube1 and geekazoide had last year on page 5 and 6 of the thread. Unfortunately the resolution to either case was not posted publicly.

Problem description:

White LEDs come on dimly or not at all when the kit is plugged in. Touching pin 4 of U2 or the closest leg of Q1 to U2 with a finger or metal implement (but not a plastic implement) will change their brightness. Left alone at full brightness then they'll fade out over maybe 30seconds. It doesn't appear to exhibit any other behaviour.

Troubleshooting steps:

• Nothing appears to be getting hot rapidly (i.e. after having plugged it in for a couple of minutes).

• I'm powering it via USB cable. I've tried a couple of different USB power supplies. USB voltage at connector is 5.06V.

• Removing chip U3 doesn't change the behaviour

• Chip U3 doesn't have the faulty batch code on it although the kit was bought between those dates

• I've pulled U2 and U3 and none of the pins are bent

• R1, C2, C4, C13, C14, & X1 have no missed connections and I can't see any obvious faulty joints or solder smeared between two joints. Although my soldering skills are beginner level so it's possible

• Requested voltages from U2
  • pin 8 to pin 7: 4.99V
  • pin 20 to pin 22 5.06v
  • pin 8 to pin 9 0.73v
  • pin 8 to pin 10 0.51v

(I'm assuming the pins go anti-clockwise with pin 1 being the dot on one side of the top of the IC).

[andrew]

I have my cube mostly assembled but as I was testing the fifth grid the blue LEDs stopped lighting up and the large white ones only light up inconsistently

White LEDs come on dimly or not at all when the kit is plugged in. Touching pin 4 of U2 or the closest leg of Q1 to U2 with a finger or metal implement (but not a plastic implement) will change their brightness. Left alone at full brightness then they'll fade out over maybe 30seconds. It doesn't appear to exhibit any other behaviour.

If you can change the brightness of the white LEDs by touching certain points then it's likely the microcontroller (U2) isn't running. As you have a multimeter could you measure the voltage on pins 9 and 10 (the crystal pins) of IC U2 and let me know the result?

(I'm assuming the pins go anti-clockwise with pin 1 being the dot on one side of the top of the IC).

Yes that's correct.

[IBI]

Negative of pin 9, positive on pin 10 of U2 results in -0.13v

[andrew]

Ah sorry! I didn't make that clear. Can you measure the voltages on both pins with respect to GND (0V). I.e....

1) +Ve probe on pin 9 and -ve probe on pin 8 (GND),

2) +Ve probe on pin 10 and -ve probe on pin 8 (GND).

You should expect to see a small DC voltage on both pins.

[IBI]

0.74v between ground and 9 and 0.53v between ground and 10.

[andrew]

Thanks - that's exactly the voltages I would expect to see. If we assume for now that the microcontroller isn't running, those voltages at least tell me it is trying to start up and so is not completely dead.

However it could be that there's a problem with the crystal circuit. I know you said you've already checked this but could you give the solder joints for capacitors C13 & C14, and the crystal X1 another check. If you don't see anything obvious just try reheating the solder on these pads to be sure.

If that doesn't fix the issue can you try to lift the crystal X1 a little off the PCB just in case some flux or solder has gotten in between the underside of the can and the PCB.

[IBI]

Well I had a couple of goes at trying to re-solder stuff but no change so I decided to take the components off and start again. I started with the crystal and when that came off it had a collar on one leg:

Collar.jpg

Initially I thought it was from inside the crystal but having inspected the PCB I'm wondering if that's damaged:

Top.jpg

Bottom.jpg

If I have damaged the PCB and that's likely to be the cause of the fault, is it something I could repair? If not, if it possible to solder wires between that leg of the crystal and the pins of the other components it connects to, or another solution short of a new PCB?

[andrew]

That collar will either be the trough-hole plating for that pad or the pad itself. It does look like the entire pad on one side of the crystal is damaged so that will probably be what it is.

Fortunately the only things that pad connects to is a leg on the 22pf capacitor and a pin on the IC which are both close to it. So you could just do as you suggested and solder a wire to connect them. AS the legs of the crystal don't seem to be trimmed you can just use part of the leg. I've attached a crude drawing of what you need to connect together.

Fix.jpg

If you have no luck, one other option would be to send the PCB back to us and I'll fix it for you.

[IBI]

I've done the repair but there's been no change in behaviour:

repair.jpg

I flipped the board over and checked the resistance between the pin on the chip and the leg of the crystal, and the leg of the crystal and the leg of the capacitor and both values for both sides were nil so I think the connection is correct.

There were a couple of component legs I left long as I wasn't confident on the soldering job I'd done on them and thought I might need to re-do them later, the crystal was one of them. I have re-checked the others.

I haven't yet pulled the C13/C14 capacitors off entirely but I've had a couple of attempts at re-soldiering them. Is there any other area I should be checking specifically?