Livewire Circuit Diagnosis

[redart]

Hi,
Looking for any help in diagnosing what might be wrong with the following circuit. It's the mains input stage for a turntable power supply, which I think is meant to feed the rest of the board (complex) with the indicated 13.5vdc coming from the collector of Q2. However when I measure this I get -120vdc, or 240vac depending on what meter range I use. The supply rail measures +120vdc. I can only get the indicated 115vac by lifting the -ve leads of C1 and C2, otherwise it's around 90vac. As there is mains voltage on the rest of the board I suspected the rectifier, however changing it hasn't helped. Any thoughts would be much aprreciated.

 

[JimB]

Looking at your circuit, I assume that a 240v mains supply is connected to "live240v" and "Neutral", and that a 115v mains supply is connected to "Live115v" and "Neutral", but not at the same time!

You show and incoming earth connection connected to chassis(?), also the 0v line connected to chassis.
With a circuit like this that is very wrong.
Depending on what voltage is on the neutral, the diode between neutral and 0v will get fried.

When making the (confusing) measurements, what are you using for the reference point, Earth, or the 0v line?
I suspect that you were using Earth.
To faultfind this circuit correctly, the -ve probe of you meter should be connected to the 0v line (ie the -ve side of C5), and not Earth. They are not (or at least should not) be the same point.

Try measuring again, and beware that the circuit is not isolated from the mains.

JimB

 

[redart]

Jim,
Thanks for the pointers. No there is no 115v mains supply connected to the board. The 115 is coming from the board, presumably via the voltage divider of C1/C2, or R2 to R5. The connector bringing mains onto the board just has the usual 3 wires, active, neutral and earth, but there is a 4th screw terminal marked "115", which has no wire but is soldered to the pcb.
The voltage across C5 is 1.2 volts. I had assumed that earth and 0v were going to be the same, however the zero volt line is actually at -120v! (-ve probe on -ve side of C5, +ve probe on turntable motor housing / mains earth terminal). Between the -ve of C5 and the +ve of C1 is 250vdc. Does this make any sense?.