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NAD Gone BAD (noob seeks help)

Well that was very clear thank you :thumbsup:.

The 12v is definitely present at pin 1 so it would seem that something is up with IC402 meaning that when the 12v shuts down the switching supply there's no more power supplied to the +5.6VD line and the amp shuts down again. Does that sound correct ?
 
Yes. Could be one of the diodes, the Vreg IC402, or one of the ecaps.

You say there is +12 volts on the wire that goes to pin 1. Try this: Lift one end of R404. This will prevent the switching regulator from shutting down when the +12 comes up. With one end of R404 lifted, and the wire reconnected to P402B pin 1, does the receiver still shut off or does it operate?
 
Hmm, I lifted r404, the amp stayed operational, than I put r404 back (with amp turned off) and turned it back on and now it works o_O

I also removed the LM7806 it tested okay but looks like a very generic brand. Could it be there was a bad contact internally and that heating the legs cured this ?
 
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BTW, there is no IC506. Remote still doesn't work, I checked the IR output with my phones camera and it works.
Also maybe this is important, instead of the 4.7v zener D403 I have a TL431 linear voltage regulator.
 
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Sounds like a couple of variations from the schematic I have. Is IC402 an LM7806 or a LM7805? Schematic shows a 7805 with a diode in the ground leg. A 7806 with the diode replaced with a jumper will give about the same output voltage, so would be fine. The '78xx' series of voltage regulators are/were made by just about everybody. A TL431 in place of the Zener at D403 is reasonable too.

Removing and replacing R404 and IC402 may have repair a bad solder joint. The LM78xx does generate some heat, and one of the solder joints could very well have failed from thermal cycling.
 
Troubleshooting the remote may be more difficult - a scope would be helpful. But there are a couple things you could check with a voltmeter.
On the display board, find IC504, D517 & D518. Confirm +5 volts on the cathode of D517 (and/or IC504 pin 3). Put the voltmeter on the junction of D517 (anode) and D518 (cathode). Aim the remote at the receiver and push a button on the remote. You should see some variation of the voltage at that diode junction.
 
Indeed, it's a variation from my schematic as well. I have the LM7806.
I didn't really suspect the solder joints because in my experience they rarely go bad on these double sided boards but it looks like this might have just been it. Just to be sure I think I will replace the electrolytics on the standby board and also the lm7806, the package really doesn't inspire much confidence and the legs are very flimsy. I saw some 50v 10µF, with green corrosion and a wet spot around some of the legs so these will go as well.

I did the test you suggested, I do have 5v on the junction of the 2 diodes and it takes a dip when pressing the remote so it seems to receive alright but not act on it.
 
Well, it goes from the IR receiver to a processor chip, IC503, pin 12. IR In and IR Out is also routed to the attached area on the PSU board. Think this is just a buffer to allow for a wired remote, but it might be loading the IR Out line. Could try lifting one end of D231.
Screenshot 2020-11-06 123440.jpg
 
I'll give this a try. I did notice there are a few 50V 10µF caps on here as well that, exactly as the one on the standby board, seem to have spilled some electrolyte. One of them is attached to this IC.
 
I spoke to soon, I replaced the components that seemed tired (all the electrolytics and the lm7806) and on the first try the old flash was back. Now I can only get it to work properly if I press down pretty hard on the heatsink of lm7806, sometimes it stays operating sometimes it shuts down when I release pressure. I can only see a bad connection somewhere causing this but I did a continuity check on all the components around the regulator and everything seems fine.
 
Looks like it was D405 (1N4007), when pressing directly on it with a plastic stick I got it to turn on but as soon as I released pressure it turned off immediately, I replaced it with a STTH2R06 (600v 2A fast recovery) and it now turns on and off like it should, meaning when I tell it to :).

That just leaves me with the remote issue. I tried resetting the remote and also changed batteries without change. I see the IR emitter flashing with my phone and there is a variation at the output of the ir receiver on the amp board but that's it. The manual states that the stand by button should blink when receiving signal from remote, this does not happen.
 
:thumbsup:

On the remote issue:
The manual states that the stand by button should blink when receiving signal from remote
Click to expand...
Assuming that is correct.... Not sure if this will be productive or not, but give it a try. Disconnect either P202B on the 'Main Board Preamp and Input Select' board, or P301A on the 'Tone Control' board. See if the unit will turn on with this disconnected, and if it does, see if the 'Standby' lamp responds to the remote in this configuration. (trying to isolate the "IR In" and "IR Out" lines.)
 
Yes the unit turns on with this disconnected but there's still no reaction from the remote. There are some more differences between schematic and reality, IC 503, actually printed IC 502 on the board is a surface mount 12c5a16s2 chip and some other components look different. I did trace the IR sensor out to one of the pins of IC503 with continuity check. Would the normal drop, when signal is detected, on the output of the IR sensor be about a diode drop because of D517 ? My measurement shows it switches from 5.26V to about 4.73V.
 
If you are looking at the same manual I am, check out the last few pages, in particular page 59. Shows a different variant that matches your description.
If you were using a scope on the output of the IR detector, IC504, you should see a rectangular digital waveform with a high level between +4.5 and +5.0 volts, and a low level of between 0 and +0.5 volts. With just a voltmeter, you should see a static level of either high or low, and when exposed to the remote transmitter, you should see a small shift towards the other extreme.
 
I didn't check the end of the manual, indeed it shows the circuit I'm looking at, for the standby circuit as well. It shows the output of IC 402 goes through a 1N4007 and drop to 5.2V which is what I'm getting at IC504. So far so good but what I don't like is that this would suggest there is a problem with the micro controller.
 
The diodes on the output of the IR receiver are clamps and are there to prevent the output from going above Vcc or below ground.
depaj said:
So far so good but what I don't like is that this would suggest there is a problem with the micro controller.
Click to expand...
If you can see the same voltage changes at the processor pin 8 as you do at the output of the IR receiver, then yes it does.
 
Hmm signal is present at pin 8. Do you know what exactly IC501 is doing ? Also I noticed that in each one of the 8x 10K resistor packages there is one resistor that measures high (14K and 12K) is this a problem ?
 
depaj said:
Do you know what exactly IC501 is doing
Click to expand...
IC50 is an RS-232 Driver/Receiver. I suspect it acts as a translator between the processor and the rear panel RS-232 connector. (but don't quote me on that :))

depaj said:
Also I noticed that in each one of the 8x 10K resistor packages there is one resistor that measures high (14K and 12K) is this a problem ?
Click to expand...
Most likely not.
 
Ylli,

I'm back, with a different problem on a different C375BEE. Wanted to say thanks again for all your input on helping us in the right direction on the last one.

So, I picked up another of these and this one won't even momentarily flash the blue front panel lights like the previous one did. When you press the front panel power button, it just goes from amber (standby) to red.

Since I have another working amp, I started comparing voltages. The standby board is working properly. I'd like to know how the 'auto-protect' circuit works, 'cause I'm leaning to it being the issue. The thermal switches on the amp boards all check the same.

Checking voltages... the bad amp is getting 73v to both of the amplifier boards (pin 1 of P209A & 210A) where the good amp is getting 55v. I think the problem is right after the two bridge rectifiers. On the working amp, BR201 output shows 127v. BR202 is 147v. (That doesn't make sense to me. Something else downstream pulling it down?) On the non working amp BR201 is 147v and BR202 is also 147v.

(Good amp)Voltages on TH201 are: pin 1=73v, pin 2=73v, pin 3=55v. TH202: pin 1 73v, pin 2 73v, pin 3 73v.
(Bad amp) Voltages on TH201 are: pin 1=73v, pin 2=73v, pin 3=73v. TH202: pin 1 73v, pin 2 73v, pin 3 73v.

The two large 2200mfd caps are in range.

I may be completely off track here but I figured maybe the auto-protect is activating due to over voltage?

Thanks for your time.
 
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