[SOLVED] power supply help needed. lost 12v @ J4 pin3

[audioteknik]

I am building a TT kit. I know, and have read through the forums, as to the pitfalls of these kits and was careful to look over the boards very carefully to look for bad traces and obvious shorts and found none. In truth, the boards look great and the etching was very clean on all 4 boards (I have 2 kits). So I built the power supply board from the first kit and everything tested fine through that section. Had no hot spots and J4 measured fine with the 5.33v and 12v at the respective pins. I loaded the drivers on my laptop and was able to connect to it via USB and my laptop recognized it properly. Then completed the remainder of the IO board and worked through the VCO section of the first kit as well.

Heres where my issue is. I was going to test the VCO function with my O-scope and thought it best to recheck the J4 voltages before connecting the jumper from J4 to the main board. Now when I measure I get the 5.33v reference at J4 but my 12v is gone. Now IC23 gets warm. Warmer than it should. It takes a minute or so for it to heat up but it gets hot. I tend to keep the power on only as long as it takes me to take measurements and then shut it off before things get hot enough to damage any components. I tried a different IC in IC23 from the other kit and got no change at J4. That said I imagine that there's a short somewhere. I checked through all the previous stages and everything meters fine. The 5.33v is fine, the 5v rail is fine but no 12v.

[guest]

is the 12v present
with the mainboard disconnected

did you recheck j4 when you finished the ioboard

its possible the short is in the cable

[aminoacid]

see if theres a short to ground on theese:
r178 r179 c61

also check that they are the right parts.

[audioteknik]

The main board is disconnected. I tested it and had both 12v and 5.33v with just the power supply section completed but now that I've finished the remainder of the IO board I lost the 12v. I've looked for anything obvious and found nothing. My solder work is very clean and I couldn't identify any errand traces or overlapping solder joints.

Any thoughts on obvious points to look would be a great help. I have looked for but haven't found a reference diagram for the IO board operating voltages so I'm not sure which traces are 5v, 5.33v, and 12v. If someone has a diagram or a overall schematic of the XOX they can post I'm sure it would be a help to the community.

With the IC out of the socket at the IC23 location I have voltages as follows:

Pin 1 0v
Pin 2 0v
Pin 3 .10v
Pin 4 0v
Pin 5 5.33v
Pin 6 .10v
Pin 7 .10v
Pin 8 31v

Wall wart voltage measures 12VAC (it is a 9VAC supply).

I'm waiting on new 78L06 and LM336 components for my second supply which with any luck will be here tomorrow and I'll have in the board and (fingers crossed) am hoping that one will work. I'll try to post some pics of the boards later.

[audioteknik]

I am showing a short to ground through C61. I checked my other board and do not show a short through C61, although as mentioned above the 5v and 6v power regulators aren't present in the circuit so that may be affecting things. I'm going to pull and replace C61 on the completed supply and re test.

[audioteknik]

well with C61 pulled I still get ground from both points so obviously there is a short to ground on that trace somewhere . FORGET IT!!! DAMNIT! I removed the 3 pin cable connector at J4 and metered between pins 2 & 3 and noticed tie tiniest flake of of something between the two joints. It figures!! As it was under the connector I couldn't see the damn short!! Picked it off and got my voltages back.

Just goes to show... even when you're careful and doing a (...reasonably) good job of reviewing your work something as trivial as adding that damn header managed to derail me for couple hours. From now on I wont be saying things to myself like "nah... that wouldn't be it.

Thanks for the help anyway, at least it got me looking at that trace!

[aminoacid]

cool!!!