THAT1512 / Transformer 'blind test' mic preamp

[jamesk]

@DaveDC I reversed all the diodes as they were all using the same symbol. Or are you saying that D6 is wrong on the schematic?

 

[DaveDC]

No, D6 is okay as long as it is really the correct polarity, with cathode connected to the +48V output

 

[jamesk]

@DaveDC Just double checked and yes, cathode is connected to the +48V output. The 0.17V between the output and adjust pins is very strange, and I don't understand where R6 is sourcing all it current from. It should only have 2mA through it, not 30.

 

[DaveDC]

Doesn't the anode of D4 want to be GND?

 

[jamesk]

I just desoldered D5, D6, R5 and R6 and i still have 85.8V on the output and 85.6V on the adjust pin.

 

[jamesk]

@DaveDC It's connected to the unregulated +ve rail so up the available voltage for the +48V regulator, which it seems to be doing. The regulator doesn't seem to be regulating at all, though.

 

[DaveDC]

Did you destroy the regulator?

 

[jamesk]

@DaveDC I did wonder that. I'm trying to think how having those diodes all in back to front could have fried that regulator.

 

[DaveDC]

Backwards power diodes can do a lot of damage.

BTW, what is your transformer turns ratio?

 

[jamesk]

@DaveDC I removed the regulator entirely, along with R5, D6, D5 and R6, so that the only thing connected to the +48V output was C12 and still measured +55V on the output, which had me completely baffled. I then removed C12 and measured 0V. I then reconnected the other components, including the regulator, but with C12 removed, and still measured 85V on the output.

Perhaps the regulator is fried and is passing the full voltage to both the output and adjust pins. That 85V could then have destroyed C12, which is only rated at 63V. I still don't understand how I'm measuring 55V with the regulator disconnected, though.

The transformer has dual 18V secondaries.

 

[jamesk]

@DaveDC Okay, I've removed the regulator again and it turns out there's a dead short between the input and adjust pins, so the full unregulated voltage is ending up at the adjust pin. It looks like the regulator is fried, I think! Still not sure about that 55V reading with it removed, though.