Hi ,
I have been trying to help JIMK04 with hi DCG3 that was misbehaving. The good news being that I think that problem is sorted, no strange noises now. However, I think I have an issue with the DC servo.
Essentially without the opamp, I left the preamp to warm up for about 30 mins and then set trimmed the DC offset. It responds well to this and I can get it down to about +/- 1mV. So if then switch off and let the whole thing cool down and monitor the DC offset from cold it starts at about 15mV and slowly drops to 1mV over the next half hour. So pretty much what you would expect.
The issue is when I add the opamp. When that is done and I switch on, one channel goes almost immediately to about a 1mV and stays there so all good. The other channel goes to about 15-20mV and then slowly drifts down as the amp warms up. To me it looks like the servo is not working on that channel. I had a spare AD823, so I changed that and I get exactly the same result.
I am not very familiar with DC servos, so would be very grateful for any pointers on what to measure, check or change.
Cheers
Ian
I have been trying to help JIMK04 with hi DCG3 that was misbehaving. The good news being that I think that problem is sorted, no strange noises now. However, I think I have an issue with the DC servo.
Essentially without the opamp, I left the preamp to warm up for about 30 mins and then set trimmed the DC offset. It responds well to this and I can get it down to about +/- 1mV. So if then switch off and let the whole thing cool down and monitor the DC offset from cold it starts at about 15mV and slowly drops to 1mV over the next half hour. So pretty much what you would expect.
The issue is when I add the opamp. When that is done and I switch on, one channel goes almost immediately to about a 1mV and stays there so all good. The other channel goes to about 15-20mV and then slowly drifts down as the amp warms up. To me it looks like the servo is not working on that channel. I had a spare AD823, so I changed that and I get exactly the same result.
I am not very familiar with DC servos, so would be very grateful for any pointers on what to measure, check or change.
Cheers
Ian
Further to the above I would just like to state that Ian is being an incredible help and gentleman in assisting me with this.
The initial problem that he soon sorted out was my error in a misplaced R.
I wanted to add this because I didn't want someone reading in on this thread seeing me struggle and not knowing why...
It was my fault and a simple mistake that should have been avoided and in no way was a problem with the kit.
Apologies to Salas et al when asking if all components are as per BOM. I missed this one out when checking..
The initial problem that he soon sorted out was my error in a misplaced R.
I wanted to add this because I didn't want someone reading in on this thread seeing me struggle and not knowing why...
It was my fault and a simple mistake that should have been avoided and in no way was a problem with the kit.
Apologies to Salas et al when asking if all components are as per BOM. I missed this one out when checking..
A servo is an integrator. In this circuit there is an RC filter going from the preamp's output to the op-amp and a feedback capacitor across the op-amp. There's also an output resistor to the preamp's control node. It's a dual channel servo circuit. So it has to be checked for same resistance to ground connections and voltage readings on both sides socket pins as well as for same values good working passive components and no cold joints.
P.S. Happy to see diyers helping each other. Well done.
P.S. Happy to see diyers helping each other. Well done.
The picture is not complete, so the relay is wiring like below now?
-signal into com terminal, and out of no terminal, the same as previous version;
-then change is now the terminal 5, 6(which was not used before) shorted together and go to the ground on the right-hand side.
Please confirm.
Thanks