Electronics > Repair

NAD C325BEE Will not power.

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paul_g_787:
Hi Everyone. I have here a NAD C325BEE amplifier which will not power and I am a little stuck.

Service Manual: https://elektrotanya.com/nad_c325bee-sm.pdf/download.html

It was dropped off to me with the report being that it was in use, then started distorting and crackling, then it switched off and had an amber light and would not come back on.

So. I firstly found an issue where the main capacitors for the +46/-46 DC rail were bulging (C517 and C 518) and there was +39V and 0V respectively. As reported the amplifier had an amber power light but would not turn on.

I have replaced these now and I am getting +40V / -40V, which is still a bit low...  :-// Not sure here..... But progress...
I also replaced C523 and C524 as their ESR was pretty high. These are for the +37/-37V rails which measure just fine.

It is my belief these capacitors failed / were in the process of failing as they are exactly next to the main heat-sink... a quick search shows this is a common failure in this model.

So that being sorted (I think) I now have the issue where there is no power light at all. Not even amber. And the amplifier won't respond to the power button.

The service manual check sheet (Page 24) says to check the 5V supply circuit on the power board. However there is nothing labelled 5V at all on the power board schematic. I am assuming this would be the pin labelled "TRIG" ??  Confused here...  :-//

Power supply schematic on page 19 of the SM for those playing along.
I have attached a copy of this with my current readings written on it.

All the bits in green I am sure are good.  :-+

All the red bits are where I am confused.  :-//

The TRIG pin is at 0V
The AC DET pin is at 5V AC and -25V DC
The I-V pin is at 0V

What are these ping exactly. I am not sure.??  :-//

So my questions are, firstly what should TRIG, AC DET, and I-V be measuring?
And also why would the +46 and - 46V rails only measuring +40 and -40?

Can anyone make any suggestions here?

up8051:
5V is made by IC45 AMS1117 form 8V (CB53 pin 6,7)

IC45 schematic page 17 right-up corner.

paul_g_787:

--- Quote from: up8051 on December 23, 2021, 10:58:02 pm ---5V is made by IC45 AMS1117 form 8V (CB53 pin 6,7)

IC45 schematic page 17 right-up corner.

--- End quote ---

Aha well spotted sir!! Even though the SM says "on power board"... No wonder I didn't see that!

I will test that part of the circuit next.

Kim Christensen:

--- Quote ---All the red bits are where I am confused.  :-//

The TRIG pin is at 0V
The AC DET pin is at 5V AC and -25V DC
The I-V pin is at 0V

What are these ping exactly. I am not sure.??  :-//

So my questions are, firstly what should TRIG, AC DET, and I-V be measuring?
And also why would the +46 and - 46V rails only measuring +40 and -40?
--- End quote ---

It looks to me like the rails marked 46V are actually capable of being switched between 40V and 51V depending on the voltage level on TRIG. (Voltages approximate)
When TRIG is zero, then the 28.5VAC transformer windings are suppling the power via the D51 bridge rectifier.
But when TRIG goes "logic high" then both the SCRs (D531 and D532) are turned on which connects the D55 bridge rectifier outputs (which is fed from the 36.4VAC windings) to the filter caps (C517 & C518) and thus the voltage on "46V" rails (CB54) increases from 40V to 51V.
I would guess this is supposed to happen when the user turns up the volume and the amplifier needs to put out more power.

feedback.loop:

--- Quote from: Kim Christensen on December 23, 2021, 11:14:21 pm ---
--- Quote ---All the red bits are where I am confused.  :-//

The TRIG pin is at 0V
The AC DET pin is at 5V AC and -25V DC
The I-V pin is at 0V

What are these ping exactly. I am not sure.??  :-//

So my questions are, firstly what should TRIG, AC DET, and I-V be measuring?
And also why would the +46 and - 46V rails only measuring +40 and -40?
--- End quote ---

It looks to me like the rails marked 46V are actually capable of being switched between 40V and 51V depending on the voltage level on TRIG. (Voltages approximate)
When TRIG is zero, then the 28.5VAC transformer windings are suppling the power via the D51 bridge rectifier.
But when TRIG goes "logic high" then both the SCRs (D531 and D532) are turned on which connects the D55 bridge rectifier outputs (which is fed from the 36.4VAC windings) to the filter caps (C517 & C518) and thus the voltage on "46V" rails (CB54) increases from 40V to 51V.
I would guess this is supposed to happen when the user turns up the volume and the amplifier needs to put out more power.

--- End quote ---

I believe this is so-called NAD PowerDrive. It is supposed to switch from the higher voltage rails to the lower voltage rails when more current is needed.

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