Hey guys,
(Canada, 120VAC, 60Hz)
Got my hands on a Velodyne sub that shuts off after just a while of use. Trying to fix it.
The issue is that it shuts off after a while of use and afterwards you just hear a relay clicking away sort of it trying to reset itself, unsuccesfully.
I don't know this thing's condition so I opened it up and I'm testing everything I can think of. I see three PCBs, a signal/input handling/preamp board, a power board and what I suspect is an amplifier board enclosed inside a metal box. This thing passed the visual and smell tests so I decided before I start probing and testing things I'd see how well the grounding is overall.
A bit more info (and do correct me if I'm wrong). The IEC plug that plugs into this thing has no earth connection, just Line and Neutral, so this makes this a (Class 2?) PSU. This supported by the fact that I see suppression X and Y caps in the form of:
Namely referring here to the two Y caps going from Line to chassis to Neutral, forming some kind of "earth" there. This earth then goes out from the PCB with a cable bolted to the back plate if the sub.
So I got my DMM set to measure AC voltage, and I do this between the metal black plate and earth, real actual earth from the wall socket. I'm measuring this while the sub is plugged in and it's power switch (which interrupts the LINE cable, neutral goes in directly) turned on.
I'm measuring ~60VAC, I turn the subwoofer's switch off, measure again, ~120VAC! What?
So now I'm thinking: I've handled the sub's back plate back when it was completely assembled, turned on, off, plugged in, not plugged in. Didn't get a single shock, not even a tingle, I didn't die. So these voltages, while they are high, must be very low current.
I tested this. With the sub plugged in, switch OFF. I measure~120VAC open voltage, 0.41mA short circuit current.
Sub plugged in, switch ON. I measure~61VAC open voltage, 0.21mA short circuit current.
Is this normal?
Are these Y caps dying (Marked as 472M, 4.7nF)? They both measure ~8.5nF with my meter.
Where are these high voltages (120V!) coming from? (Especially when the switch is off so only Neutral is connected to the PCB, LINE is interrupted by the switch and the plug isn't inverted or anything, although I don't see how that would affect anything as the primary side looks pretty symmetrical)
Here's some pics of the PSU board and the AC connections:
(Excuse the shitty graphics, I've flipped/rotated the board pics so that they match)
PSU Top:
PSU Bottom:
AC connections: