After 8 years of daily home use my NAD 325BEE recently failed with a little puff of smoke. Initial indications were that at least one of the large 15000uF electrolytic capacitors in the power supply had given up (lots of black goo around them coating other components). I replaced both of these plus two other electrolytics in the power supply (the glue at the base of those fooled me into thinking there was something wrong) and cleared up as much of the residue from the blown cap as I could. With all that done the amp remains in standby (orange LED) and so won't properly power up.
Have read a number of other similar posts in this forum and have checked as much as possible given my limited knowledge and so am now stuck and looking for suggestions. Here's what I've checked:
In the power supply board:
All the fuses are OK.
All of the diodes are OK.
All of the resistors are OK.
All of the bridge rectifiers are OK (internal diodes fine).
The Vac on the outputs of the toroidal transformer are OK.
The Vdc on the outputs of the power board are quite a bit away from what I'd expect as a normal tolerance, although not sure what the tolerances are. I'm seeing +40.8 and -40.6V rather than +46 and -46V for the main outputs (disconnected from main board so no load) and +28.8 and -40.3 instead of +37 and -37 from secondary supply.
The main board isn't shorting any of the inputs and indeed connecting doesn't impact output voltages from the power supply board.
* First question: is the output Vdc values an indication of a problem and where to look further here?
The standby signal switches from 3.7V to 0V on pushing the power switch as seen at the main board of the amp.
Given this, the NAD service manual points to the protect circuit being the problem. (assuming the power output is OK)
The protect line is sitting at -0.4V regardless of standby state.
Any ideas as to where I should look next?
Thanks for any suggestions!
Have read a number of other similar posts in this forum and have checked as much as possible given my limited knowledge and so am now stuck and looking for suggestions. Here's what I've checked:
In the power supply board:
All the fuses are OK.
All of the diodes are OK.
All of the resistors are OK.
All of the bridge rectifiers are OK (internal diodes fine).
The Vac on the outputs of the toroidal transformer are OK.
The Vdc on the outputs of the power board are quite a bit away from what I'd expect as a normal tolerance, although not sure what the tolerances are. I'm seeing +40.8 and -40.6V rather than +46 and -46V for the main outputs (disconnected from main board so no load) and +28.8 and -40.3 instead of +37 and -37 from secondary supply.
The main board isn't shorting any of the inputs and indeed connecting doesn't impact output voltages from the power supply board.
* First question: is the output Vdc values an indication of a problem and where to look further here?
The standby signal switches from 3.7V to 0V on pushing the power switch as seen at the main board of the amp.
Given this, the NAD service manual points to the protect circuit being the problem. (assuming the power output is OK)
The protect line is sitting at -0.4V regardless of standby state.
Any ideas as to where I should look next?
Thanks for any suggestions!
Except for the 28.8V reading the voltages look ok. If there is really no load on the power board then D56 and C523 are all that can affect the 28.8V output. Do you have a scope to look at waveforms?
Thanks guys. Unfortunately I don't have a 'scope, just a trusty old Fluke 73 digital multimeter bought back in the 80s when I was just getting into electronics.
C523 was one of the caps I replaced, along with C524, both of which I think were actually fine - these were the two I described with glue residue at the base that I had taken as indications of damage having not realised this was just glue. C517 & C518 were the genuinely bad ones (also replaced), one had blown producing all the sticky brown residue (and presumably the smoke) and the other was bulging on top.
I've ordered a replacement for D56 although a basic check of voltage drop across pins (using the Fluke) appears to indicate that the 4 internal diodes are OK. In any case, hopefully that's the cause - I'll report back once it's replaced.
Thanks again for the tips.
C523 was one of the caps I replaced, along with C524, both of which I think were actually fine - these were the two I described with glue residue at the base that I had taken as indications of damage having not realised this was just glue. C517 & C518 were the genuinely bad ones (also replaced), one had blown producing all the sticky brown residue (and presumably the smoke) and the other was bulging on top.
I've ordered a replacement for D56 although a basic check of voltage drop across pins (using the Fluke) appears to indicate that the 4 internal diodes are OK. In any case, hopefully that's the cause - I'll report back once it's replaced.
Thanks again for the tips.
I will also check the +12,+18, -18 and +5V lines although I've not yet figured out where these originate. These aren't directly from the power board are they? Will work through the schematics and do the checks.
Thanks again.
Thanks again.
I've replaced the rectifier D56 and the +37V and -37V are now more balanced at +40.9V and -40.5V
The +46V and -46V are at +41.0 and -40.9V, which seems way too close to the +37/-37V lines actual values. This applies whether or not the main board is connected.
All-in-all the above all do seem quite a way off the expected values, but again no idea what the tolerances are meant to be.
+5V line is at +4.97V so that's good. That's driven off a little 5V regulator so I guess that's expected to have a very good tolerance.
The +18V and -18V lines are not looking good at all with
-18V reading -1.36V and +18 reading -0.25V
The +12V line has nothing, well 0.05V which is near enough nothing.
That the amp won't come out of standby might be influencing the +18, -18 and +12 as I've not got a handle on just how those voltages come about.
Any more suggestions much appreciated!
The +46V and -46V are at +41.0 and -40.9V, which seems way too close to the +37/-37V lines actual values. This applies whether or not the main board is connected.
All-in-all the above all do seem quite a way off the expected values, but again no idea what the tolerances are meant to be.
+5V line is at +4.97V so that's good. That's driven off a little 5V regulator so I guess that's expected to have a very good tolerance.
The +18V and -18V lines are not looking good at all with
-18V reading -1.36V and +18 reading -0.25V
The +12V line has nothing, well 0.05V which is near enough nothing.
That the amp won't come out of standby might be influencing the +18, -18 and +12 as I've not got a handle on just how those voltages come about.
Any more suggestions much appreciated!
Connector CB54 pins 1 and 2 are labeled +46V and -46V. These voltages are only reached in operation when D531 and D532 are turned on as high power is demanded by the program material. Then D56 is connected in the power path and the voltage is raised. Otherwise D51 provides main power at a voltage of around 40V (no load) or 37V loaded. NAD calls this switching to a higher voltage on demand "PowerDrive". Below is their marketing copy on this feature.
PowerDrive technology adds huge reserves of Dynamic Headroom without adding cost by ingeniously matching the amplifier to the speaker load. This is fully automatic in operation and adjusts the power supply parameters of the amplifier to best cope with the actual musical signal and specific speaker loading characteristics.
Your voltages at connectors CB54 and CB53 are correct for no load operation. The voltages 18V, 12V, and 5V are provided by circuitry on page 17 of the service manual in the upper right corner of the page. Check all the semiconductors for faults as a start.
PowerDrive technology adds huge reserves of Dynamic Headroom without adding cost by ingeniously matching the amplifier to the speaker load. This is fully automatic in operation and adjusts the power supply parameters of the amplifier to best cope with the actual musical signal and specific speaker loading characteristics.
Your voltages at connectors CB54 and CB53 are correct for no load operation. The voltages 18V, 12V, and 5V are provided by circuitry on page 17 of the service manual in the upper right corner of the page. Check all the semiconductors for faults as a start.
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