One day my NAD C352 made a loud pop and went into protection mode. After opening it up i found all the fuses blown, one of the Left Channel large transistors exploded. After probing around i found that both left channel large transistors had gone (2SA1295 and 2SC3264). I also found one of the smaller left channel transistors (2SA1930) was gone. I replaced these components.
I also took the liberty of replacing all of the electrolytics, being from 2004 i figured it was time.
Now, the amplifier powers up, it comes out of protection mode and everything seems fine except one thing. The repair manual calls for an alignment procedure: Connect the lead of a DC multimeter to the L speaker positive terminal and the negative lead to pin 1 of IC107. Oddly i get 5.79V! (should be less than 100mV). Also the main big heat sink starts getting hot REAL fast. Not wanting any trouble i turn the amp off.
I can probe IC108 and it gives me the correct reading so the issue seems to be limited to the LEFT channel. I have checked IC107 with diode mode on my multimeter and it seems to be ok.
I've checked all the diodes, looked for burnt resistors but cant seem to find any faults.
I am not that experienced with electronics repair. I have a multimeter and can solder. I have the repair manual but i cannot understand the electrical schematics enough to diagnose the issue.
Would be really grateful if someone could help me diagnose, i really like its sound and would hate to see it go.
I also took the liberty of replacing all of the electrolytics, being from 2004 i figured it was time.
Now, the amplifier powers up, it comes out of protection mode and everything seems fine except one thing. The repair manual calls for an alignment procedure: Connect the lead of a DC multimeter to the L speaker positive terminal and the negative lead to pin 1 of IC107. Oddly i get 5.79V! (should be less than 100mV). Also the main big heat sink starts getting hot REAL fast. Not wanting any trouble i turn the amp off.
I can probe IC108 and it gives me the correct reading so the issue seems to be limited to the LEFT channel. I have checked IC107 with diode mode on my multimeter and it seems to be ok.
I've checked all the diodes, looked for burnt resistors but cant seem to find any faults.
I am not that experienced with electronics repair. I have a multimeter and can solder. I have the repair manual but i cannot understand the electrical schematics enough to diagnose the issue.
Would be really grateful if someone could help me diagnose, i really like its sound and would hate to see it go.
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With all due respect, take it to a repair shop as the fault must be repaired first, then the failed components need replacing and setting up.
Never replace capacitors unless they are failing or have failed, you will cause faults!
You cannot check an IC using diode mode on your meter.
Professional repair will likely cost more than a working used C352; and many repair shops do not accept devices with extensive parts replacement. Ability to understand the schematic will greatly increase the repair chances and avoid making things worse, but some further tests may be done. At first I would try to swap the power transistors on the still working channel with the new ones from the failed channel, this will ensure that they are indeed the correct replacements. If you do this test, check the new transistors to be sure that they have not been damaged by the previous test and do not mount on the faulty channel the removed original transistors.
Good idea, I will try swapping the transistors.
Im not sure i understand the previous comment about never replacing capacitors unless they are failing or have failed- causing faults. I do not own an ESR meter so i cannot verify they are faulty. However a repair tech told it would be best to just replace them all due to their age (20 years). I was careful in selecting replacements and verified each polarity when replacing.
Im not sure i understand the previous comment about never replacing capacitors unless they are failing or have failed- causing faults. I do not own an ESR meter so i cannot verify they are faulty. However a repair tech told it would be best to just replace them all due to their age (20 years). I was careful in selecting replacements and verified each polarity when replacing.
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During that process you are likely to make mistakes - we all do sometimes.
So unless it is definitely a problem you should postpone this until the amplifier is in basic working condition. Then you can replace the caps if you feel like it - with e.g. 40 years old capacitors I would not think twice.
I would leave the working channel untouched - at least there would be a clean reference to compare the faulty one with.
The next step would be to power the amplifier up after DBT and start measuring voltages to find anomalies.
When using DBT one should make sure that the rectifier produces above +/-20V - there is a 18V zener in voltage regulator. Also you need to check if regulated voltages are at their expected levels - one would assume them to be symmetrical but zener is used only on one rail and this may cause anomalies when operating at reduced supply voltages. Just out of curiocity it would be interesting to simulate this solution in LTspice - how the significantly reduced input voltage affects the output..
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I'm just quickly skimming through what you said...
If it comes out of protection that suggests the offset is OK which would be a good sign. It obviously needs checking but any significant offset would keep it in protection.
This is the biggest concern and may simply be the bias setting is to high. Different (replacement) transistors will vary enough to make readjustment necessary.
You must turn the bias to minimum initially and that means turning the appropriate preset for that channel to minimum resistance which means the preset will measure 'short circuit' connecting the 390 ohm resistor directly to the 4.7 ohm above. The way to turn the preset could be in either direction so you must confirm by measurement first before turning on.
That made a difference, thank you! The amp no longer gets very hot on startup.
There is an issue i noticed after aligning VR101/VR102/VR103/VR104, not sure if this is normal given my replacement parts or sign of another problem.
To align VR101 i connect a negative multimeter lead to the LEFT Speaker Terminal negative and the positive multimeter probe to IC107 Pin 1. Multimeter is in mV range.
Normal or a sign of something else?
There is an issue i noticed after aligning VR101/VR102/VR103/VR104, not sure if this is normal given my replacement parts or sign of another problem.
To align VR101 i connect a negative multimeter lead to the LEFT Speaker Terminal negative and the positive multimeter probe to IC107 Pin 1. Multimeter is in mV range.
- I cant get the left channel to the correct setting. With VR101 turned all the way down (counter clockwise) the reading fluctuates between 0 and -20mv. It rises and falls between that range every 30s or so. If i turn VR101 up (clockwise) the reading increases further in negative value. This seems strange.
- When i do the same procedure to the RIGHT channel, it gives me a positive value and i can set it to less than 100mV with minimal adjustment, however this too fluctuates around 15-20mV every 30s or so.
- I can set Idling current just fine with VR103/VR104. It is set to 5.7mV for each channel without fluctuation.
Normal or a sign of something else?
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My bad, NAD instructions are not clear. Its POSITIVE multimeter probe to POSITIVE speaker channel under test, NEGATIVE multimeter probe to PIN 1 of IC107/108.
Everything seems to be working fine now.
The output offset voltage readings from IC107/108 PIN1 do fluctuate a bit, is this normal?
Worth it to check for transistor leakage?
Everything seems to be working fine now.
The output offset voltage readings from IC107/108 PIN1 do fluctuate a bit, is this normal?
Worth it to check for transistor leakage?
The offset instructions don't read correctly to me. Connecting a meter across the speaker terminals measures the DC offset but the offset on this amp should always be zero because it uses a DC servo to force zero offset.
Step 2 would need a second meter but all you are doing here is setting the conditions such that the servo output is around zero. You need to understand how servos work but basically the adjustment is just setting things so the servo can correct errors over the maximum possible range.
So you keep your single meters negative lead on the negative speaker terminal and measure the voltage on pin 1 of the opamp and adjust the appropriate preset to get approx 0 volts on the opamp output.
(Provided the opamp output voltage is between the opamp supply voltage the main amp DC offset will always be zero, this adjustment just puts it in the middle so it can swing equally above and below ground within the limits of the opamp supply)
Check again the DC offset across the terminals is zero.
Step 2 would need a second meter but all you are doing here is setting the conditions such that the servo output is around zero. You need to understand how servos work but basically the adjustment is just setting things so the servo can correct errors over the maximum possible range.
So you keep your single meters negative lead on the negative speaker terminal and measure the voltage on pin 1 of the opamp and adjust the appropriate preset to get approx 0 volts on the opamp output.
(Provided the opamp output voltage is between the opamp supply voltage the main amp DC offset will always be zero, this adjustment just puts it in the middle so it can swing equally above and below ground within the limits of the opamp supply)
Check again the DC offset across the terminals is zero.
Slow drift either way is normal as the servo corrects for thermal drift and maintains the offset at zero at the speaker terminals. It should not fluctuate rapidly and should only move over a relatively small voltage range.