NAD C370 ISC Adjustment

[ConradH]

Fixing up a C370 for a friend and got the easy bake oven protection circuit taken care of. That's all that seemed to be wrong. Offset and bias are straightforward, but I think the ISC section of the service manual is vague and the test points are backwards in terms of ground and signal according to the schematic and ohmmeter. They seem to want -50 to -100 mV between the two test points, but don't specify which way 'round the meter should go. The voltage can be adjusted to either side of zero, though it doesn't seem to be the most stable thing in the world- it's very sensitive to line voltage.

Not clear on the circuit operation, but it appears they use the opamps as comparators? (no local feedback) and with no signal the adjustment point should be a tad negative in relation to ground so the comparators aren't switched on (high).

Is there a way to test the operation of the ISC?

Does anybody have a definitive word on setting this up?

 

[Binkman]

Service manual: https://www.hifiengine.com/manual_library/nad/c370.shtml
See pdf page 12 circled in red are TP9 & TP10 for reference for readers.

I have no clue on definitive set up, I do see some references noted and 1% resistors in the circuitry but layout is advanced for me.

 

[Binkman]

Just to add; The target mv is 0v +/- 50mv? so does it really matter measuring + or - if close to zero is established? You have the unit so what do you read reversing the test points? Steps A-c for adjustments. Do your readings indicate that A and B affect C? or is ISC adjustment independent?

(must be 'vague schematic month'.. seen too many already)

 

[ConradH]

Maybe I just want too much certainty! The problem is the page showing the adjustment locations has the test points labeled backwards from the schematic, TP10 on the location outline is ground, as proven using an ohmmeter. I went with the logic (from another forum) that the voltage is in reference to ground, so set TP10 (per the schematic) to -75 mV in relation to ground (TP9, per the schematic). They do seem to be clear about the range as -50 to -100 is shown in the manuals for several different amps. FWIW, this voltage is bias dependent, line voltage dependent and warm-up dependent. The best thing to do is use 3 meters, one for the bias on each channel and one for the ISC voltage. At turn-on the bias will start out at 50 mV and the ISC about 1 V. After a minute things come down closer to where they should be and after 5-10 minutes it all stabilizes. At that point one can fine tune the bias to about 5.5 mV and the ISC to -75 mV. Correct? Hell if I know, but it does sound very good.

 

[Binkman]

Well if you glance at the sm and the main circuit board reads "NAD in reverse?

10 minutes to stable and all is well and fat lady sings? Conrad for all we know you probably did a factory set up. I mean I'd send you my stuff to calibrate.. if you say it's reads XXXxmv.. good enough for me.

I don't see anything in your diagnosis wrong. Intuition on your side. I'm sure you double checked all as you do. Close it up.
bink

 

[Binkman]

Saw your thread at DIYaudio, too. (http://www.diyaudio.com/forums/solid-state/303640-how-adjust-nad-c370-isc-circuit.html)

Is the ISC designation a 'current' sense circuit? I can understand with both ic's comparative so does this trigger the protect circuit? (pardon my lack of knowledge... I have seen op amps used in audio stages to compare both channels in pre amp stages for an auto adjustment or is the circuit protection related? (pardon my lack of knowledge) thanks

 

[ConradH]

"Done and dusted", as the assassins in one of the Jack Reacher books said. On to the next project!

ISC is NAD's proprietary impedance sensing circuit that adjusts the power supply based on the load being driven. I understand more or less what it does from a 60,000 foot altitude view, but not enough to know what the voltages are in normal operation. AFAIK, it's independent from the protection circuitry unless it isn't.