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NAD c350 protection module problem
NAD c350 protection module problem
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Old 28th September 2009, 07:04 PM   #1
hostarn is offline hostarn  France
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Default NAD c350 protection module problem

Hello airbody!

Some peeps has already post similaire or same problem but I have yet to see a solution. The fused resistors (R401 and R402) has been changed so that is not the reason.

The amp wont go out of protection mode (red light instead of green as it should be).

I saw that the NAD 370 often has a similair problem that is resolved by changing the electic capacitors in the protection circuit.

Maybee somebody could help me find that circuit in the NAD c350, already that would be a huge help. Even the service manual for the C350 doesnt seem to specify where this module is. I've might found it but I'm not sure.

What else could be the reason? Theoreticly. What could I do to advance in this frustrating enterprise?

Thank you in advance for your thoughts on this,

Best regards,

Erik
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Old 28th September 2009, 08:48 PM   #2
Bill_P is offline Bill_P  United States
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The protection module is IC105, type UPC1237H. From the schematic it appears to be on the main board.

If you have DC offset at the amp output the protection circuit is working as it should. You might need a scope to watch the output to see if it shows a DC voltage at power on.
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Old 29th September 2009, 12:44 PM   #3
hostarn is offline hostarn  France
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hi, thank you. Circuit located. The problem is not the same as on the 370 though, there it was a hot running diode that dries out a couple of caps. This is then not the case on the c350 circuit. And yes, I have dc offset. Dont know what to do know, any ideas?
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Old 29th September 2009, 06:44 PM   #4
hostarn is offline hostarn  France
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Seems like we have a really difficult one on our hands.

I have yet to read about ONE single DIYer on the wwooooole internet who have resovled this one except Burbank who sais that sometimes it's the C143. And offcourse R401, R402 who on old models was not already replaced by NAD.

Man, I'm dying here. So typical that people all over with the C370 solves this one so easy but for the C350 its a hard nut to crack.

I'm thinking total recap now, despertly. By the way, how do one test caps to see if one should indeed replace the same?

Cheers
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Old 29th September 2009, 07:01 PM   #5
hostarn is offline hostarn  France
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Oh, it's Burbeck not Burbank. Sorry, and thanx to Burbeck for that idea. Why could not that have been my problem, dang. I did change the c143 in the protection circuit but it did not help me.
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Old 10th October 2009, 12:01 PM   #6
hostarn is offline hostarn  France
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Apparently there was another cap that had given some people trouble, it was the C510. I changed it and unfortunately it did nothing for me.

I have now also on suggestion by ''King audio'' to verify the output transistors. They check out fine however.
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Old 10th October 2009, 03:47 PM   #7
reddish75 is offline reddish75  United Kingdom
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i've had one of these or maybe a c340 with similar problems, try replacing all the caps on the left hand side of the amp if the front is facing you, i seem to remember that these have a problem with drying out. it should'nt cost that much either.
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Old 11th October 2009, 10:59 AM   #8
hostarn is offline hostarn  France
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Thanx Reddish, at one one point or another I'm bound to shotgun this amp.

I should also tell that DC offset at speaker terminals is 3.8. If I compare that to my working Nad c320BEE (0.8) it's quite high. Should I meusure elsewhere ? Where exactly?
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Old 11th October 2009, 04:31 PM   #9
Bill_P is offline Bill_P  United States
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The service manuals for the C350 and C320BEE both indicate that the DC offset voltage at the speaker terminals should be less than 0.03 VDC. There are separate adjustments for the left and right channels and the reading is taken with no load connected to the speaker terminals.

What are you using to measure the voltage and do you have a load connected? Do both the left and right channels read the same DC voltage at the speaker terminals?

For the C350, if the protection circuit is activated, a relay should open the connection to the speaker terminals and you should read zero volts.

I would locate the datasheet for the UPC1237H and study it to determine what the voltages at each pin should be. Then measure the voltages at the UPC1237H pins in the C350 to find out what is activating the protection circuit.
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Old 11th October 2009, 08:40 PM   #10
reddish75 is offline reddish75  United Kingdom
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you should'nt be reading voltage on the speaker terminals if the relay has'nt fired! try measuring the voltage on the input of the relay from the power amp.
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