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NAD C352 Protect mode
NAD C352 Protect mode
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Old 10th June 2021, 08:10 PM   #51
Rick PA Stadel is offline Rick PA Stadel  United States
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OK, then R245 has opened again, or Q155 has failed. Q155 might seem OK at a glance because its Base-Emitter junction will still be forward biased by the string of three 680r resistors. But the rail will lay down without the transistor's current gain.

If you don't have a bench supply you can use to substitute for the +18V rail, you will need to start disconnecting stuff from both of the 18V rails. Pick someplace easy at first -- a plug or other connector going off the board. Both of the regulators should function somewhat, with little or no load on them. Something on that +18V rail is soaking up too much current.

Regards
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Old 10th June 2021, 08:59 PM   #52
Rick PA Stadel is offline Rick PA Stadel  United States
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I just noticed () that the motor drive circuit uses the +18V -- start by disconnecting that.

There are enough other places that +18V gets used, but motor drives are especially failure-prone. And this one doesn't have flyback diodes. Unsolder the Emitters of Q303 and Q304; replace R245 and see if the 18 rails come up and regulate.

The 18V problem must be solved before the power amps and protection circuits will work right. (Note that the power amp input diamond buffers use 18V to bias their cascode transistors.)

Cheers

Last edited by Rick PA Stadel; 10th June 2021 at 09:17 PM.
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Old 10th June 2021, 09:24 PM   #53
Lynyrd is offline Lynyrd  France
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The motor volume works fine
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Old 10th June 2021, 09:32 PM   #54
Rick PA Stadel is offline Rick PA Stadel  United States
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Where does it get +18V?

Take your time -- I've got something to do that'll take a little over an hour.
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Old 10th June 2021, 11:14 PM   #55
Rick PA Stadel is offline Rick PA Stadel  United States
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OK, I'm back -- for at least a bit 'til I gotta get dinner.

Oops -- that was almost two hours.

Last edited by Rick PA Stadel; 10th June 2021 at 11:17 PM.
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Old 11th June 2021, 01:45 AM   #56
Lynyrd is offline Lynyrd  France
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I don't now....mystery.
The volume knob turns very well with remote control.
But any led and any button no works on front panel.
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Old 11th June 2021, 04:00 AM   #57
Rick PA Stadel is offline Rick PA Stadel  United States
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OK, if it takes more studying the schematic, I'm in.

Might be worth trying to locate the missing section of the schematic -- the 8 unlabeled traces heading off to the right, bottom of page 6, from the 18V and 12V supplies we were working on.

But so far it seems you have a bundle of contradictions -- and I'm still trying to figure out how in 2 hours and 13 minutes, you managed to remove, test, and reinstall every component in a very complex power amp! .. and not induce ANY new problems!

Regards

Last edited by Rick PA Stadel; 11th June 2021 at 04:02 AM.
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Old 11th June 2021, 04:31 AM   #58
Lynyrd is offline Lynyrd  France
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No 2 hours,4 days....
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Old 12th June 2021, 07:03 AM   #59
Lynyrd is offline Lynyrd  France
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No bad parts ..... strange
R245 blown....
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Old 12th June 2021, 07:08 AM   #60
Lynyrd is offline Lynyrd  France
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No bad resistors or shorted capacitors.
No bad transistors.....
All parts around R245 desoldered and tested
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