I have a little trouble with PA on LM4702. DC voltage on both channel outputs is -25mV, but uPC1237 keep relay closed and there is no signal on output. BIAS is 15mA and has no influence to DC voltage on output.
Any suggestions on where to go from here would be appreciated.

 

First check all power supply voltages against the specification as these are common to both channels.

 

First check all power supply voltages against the specification as these are common to both channels.

Vcc +55,0/-55,0V
V +60,0/-60,0V
Checked

 

C103 and C106 voltages?
What's the history? One day it was working then next day it had this fault?
Working correctly, then stored for a long while, then it has this fault?
Other?

 

This is new construction, C103 +55V, C106 have to check later.

 

-25 mV is OK, You start to worry at +-750mV. many times there is no way to null the offset.

 

I don't worry but uPC1237 protector chip does not switch relays on.

 

Make the protection less sensitive by changing R103 to be smaller.

 

Do you have power feeding the circuit of upc1237 pin 4? If not, relays stay off

 

Yes, I have power on pin 4.

 

Yes, I have power on pin 4.

U101 Pin 2 detects an excess offset. It has a negative threshold of -0.12 volts and a positive threshold of 0.54 volts. And there are dividers from each channel into that pin, so that -25 mV should not trigger a fault. What voltage do you measure directly on U101 pin 2?

Now pin 4.... What voltage do you measure at U101 pin 4? Pin 4 should be no less that +0.9 volts, and no more than +10 volts.

 

I think problem is with different grounds from two symetric power supplies. I found that uPC is connected to Vcc, but ground from (V). So voltage on pin 2 is -45V.

 

The uPC should be connected to system ground. It is powered by it's own diode and filter cap from the power transformer secondary. How have you got the power supply configured? I wasn't able to find anything on this online other than the schematic you posted - are you working from any kind of instructions?

 

There are two symetric ps, +60/0/-60VDC for LM and +55/0/-55VDC for darlingtons,