Hello all,
Amateur but determined amplifier repairer here. This is my first repair project that I've thrown myself into.
I acquired a previously blown NAD 7240PE from ebay. This unit, when received, had one good channel (L) and one bad channel (R). The initial obvious failures were the main transistors (Q432, Q436, Q426, Q430, Q418, Q428, Q424) and a few electrolytic capacitors.
So far I have replaced every single electrolytic capacitor and the aforementioned transistors. I have also replaced the main transistors on the other side of the heat sync.
My current testing setup from the wall is a Mophorn Variac > 60W light bulb in wired series > NAD 7240PE.
Previously, when I powered on the amp, I could see the bulb shine bright for a few seconds then dim down as the amp began to draw more current.
I had recently replaced R471 and R472 (see center voltage adjustment section, service manual) and replaced the previously removed solder bridges. This is due to R472 burning out from over-current because I accidentally caused a short with the multimeter probe. During my next attempt to perform the center voltage adjustment, I observed a steady 1.5+-0.5vDC of output on the right channel and 0+-30mVDC on the left channel. During this time, turning VR402 resulted in no change in output current.
After a few minutes and restarting the amp, the light bulb wired in series stopped shining when the amp powered on. During this time, I was starting the amp with the variac at 0 and slowly increasing. At one point, I raised the variac to ~20v and the previously good left channel completely blew up and shorted 4/6 of the main transistors.
I am clearly missing something and would greatly appreciate any thoughts and insights into testing processes you would recommend. I'd like to isolate problems before mass replacing components.
I'm also open to cutting my losses and binning this project, just let me know if you think the same.
Amateur but determined amplifier repairer here. This is my first repair project that I've thrown myself into.
I acquired a previously blown NAD 7240PE from ebay. This unit, when received, had one good channel (L) and one bad channel (R). The initial obvious failures were the main transistors (Q432, Q436, Q426, Q430, Q418, Q428, Q424) and a few electrolytic capacitors.
So far I have replaced every single electrolytic capacitor and the aforementioned transistors. I have also replaced the main transistors on the other side of the heat sync.
My current testing setup from the wall is a Mophorn Variac > 60W light bulb in wired series > NAD 7240PE.
Previously, when I powered on the amp, I could see the bulb shine bright for a few seconds then dim down as the amp began to draw more current.
I had recently replaced R471 and R472 (see center voltage adjustment section, service manual) and replaced the previously removed solder bridges. This is due to R472 burning out from over-current because I accidentally caused a short with the multimeter probe. During my next attempt to perform the center voltage adjustment, I observed a steady 1.5+-0.5vDC of output on the right channel and 0+-30mVDC on the left channel. During this time, turning VR402 resulted in no change in output current.
After a few minutes and restarting the amp, the light bulb wired in series stopped shining when the amp powered on. During this time, I was starting the amp with the variac at 0 and slowly increasing. At one point, I raised the variac to ~20v and the previously good left channel completely blew up and shorted 4/6 of the main transistors.
I am clearly missing something and would greatly appreciate any thoughts and insights into testing processes you would recommend. I'd like to isolate problems before mass replacing components.
I'm also open to cutting my losses and binning this project, just let me know if you think the same.
I would avoid the use of the variac. Is pretty wonderful the bulb in series, but perhaps the amp may oscillate at low voltages when some transistors are just biased and others don't.
Hmmm, you may be on to something. Wish I had an oscilloscope for things like this.
Hi lucasgg,
I normally just use a variac and watch the current. The series lamp throws all kinds of variables into the mix.
Okay, this one is very roughly similar to a Carver in that it switches the voltage on the output transistor collectors depending on signal level. You have one pair of output transistors, Q428 and Q430. You can pull Q 432 and Q434 for now as they are the commutators (just switch supply voltage). Q424 and Q426 are the driver transistors for the outputs and may be damaged. Check R456 (1K0) and R460 (330R). If these are open you may not be able to control bias current. Q418 is your bias control transistor, if it is open your bias will be greatly excessive.
Check those things, one step at a time.
-Chris
I normally just use a variac and watch the current. The series lamp throws all kinds of variables into the mix.
Okay, this one is very roughly similar to a Carver in that it switches the voltage on the output transistor collectors depending on signal level. You have one pair of output transistors, Q428 and Q430. You can pull Q 432 and Q434 for now as they are the commutators (just switch supply voltage). Q424 and Q426 are the driver transistors for the outputs and may be damaged. Check R456 (1K0) and R460 (330R). If these are open you may not be able to control bias current. Q418 is your bias control transistor, if it is open your bias will be greatly excessive.
Check those things, one step at a time.
-Chris
Osvaldo de Banfield, you are exactly wrong!
People use light bulbs because they are too cheap to buy a variac and learn how to use it. Except for a very few cases, the variac is the correct way to do this.
People use light bulbs because they are too cheap to buy a variac and learn how to use it. Except for a very few cases, the variac is the correct way to do this.
Hi lucasgg,
That instrument is a basic need for what you are doing, but right now will not help you. You need a good DVM! That and your mind.
Filamentary lamps has current limiting capability. Variac's don't.
They cause the applied energy to vary unpredictably. That's what the current meter is for.
I'm a professional with a ton of years experience and factory training. I am telling you how to do it the right way. If you can't / won't, stop. Certainly do not advise others to do things improperly.
I'm a professional with a ton of years experience and factory training. I am telling you how to do it the right way. If you can't / won't, stop. Certainly do not advise others to do things improperly.
I have a Klein Tools MM600, so I'd like to think I'm doing well on the DVM side 🙂
I am curious, when starting an amp in a post-repair condition, do you recommending slowly increasing voltage with the Variac or powering everything on with the Variac set to 120v?
Slowly increase voltage while watching the current draw. Some products may draw more current at lower voltages. There is one badly designed Pioneer amp that draws very high current at lower voltages.
Never hit a new repair or a suspected faulty amplifier with anywhere near full line voltage to start. A variac can help you avoid further damage and assist in troubleshooting because you can sometimes run things at lower voltages. Enough to troubleshoot anyway. I always start from zero and increase while watching current draw.
SMPS type units require an advanced form of slow start. So does a Carver with a Mag coil.
Never hit a new repair or a suspected faulty amplifier with anywhere near full line voltage to start. A variac can help you avoid further damage and assist in troubleshooting because you can sometimes run things at lower voltages. Enough to troubleshoot anyway. I always start from zero and increase while watching current draw.
SMPS type units require an advanced form of slow start. So does a Carver with a Mag coil.
How and using what tool do you measure current draw? Just making sure I exactly understand the startup procedure.
Thank you
When commissioning an amplifier I find the safest approach is to use a current limited supply (even if its just achieved by adding couple of series resistors added between the filter caps ad amplifier PCB), and monitor the current (by measuring the voltage across such resistors for instance).
The filter caps store a lot of energy and can source 100's of amps in a fault situation, any attempt to limit current via the mains input isn't going to protect the amp from this source of energy, whereas a couple of small power resistors can do the job and prevent blowing up the amp like this. Something in the 50 to 100 ohm range might be good in the first instance, depending on the size of the amp.
Of course you can't drive much load like this, but you can check basic stability and operation before risking the full power supply and the direct connection to the filter caps. A well designed amp has enough decoupling on the amp PCB for stability and shouldn't rely on the filter caps for this.
The filter caps store a lot of energy and can source 100's of amps in a fault situation, any attempt to limit current via the mains input isn't going to protect the amp from this source of energy, whereas a couple of small power resistors can do the job and prevent blowing up the amp like this. Something in the 50 to 100 ohm range might be good in the first instance, depending on the size of the amp.
Of course you can't drive much load like this, but you can check basic stability and operation before risking the full power supply and the direct connection to the filter caps. A well designed amp has enough decoupling on the amp PCB for stability and shouldn't rely on the filter caps for this.
Hi Mark,
That's why you increase the voltage slowly whilst monitoring the current draw. No need for those precautions if you do it right.
Do not add anything that will change the normal operation of the equipment. Unpredictable results can and do happen. There is no dependable lazy way out of this. You have to follow the book and pay attention.
That's why you increase the voltage slowly whilst monitoring the current draw. No need for those precautions if you do it right.
Do not add anything that will change the normal operation of the equipment. Unpredictable results can and do happen. There is no dependable lazy way out of this. You have to follow the book and pay attention.
Hi lucasgg,
You would place an AC ammeter in series with the AC feed ot the amplifier you are testing. The better variacs have them built in, but you can always put a separate ammeter in a box with a plug and socket. Just insert that between the variac and unit under test. The circuit usually has the ammeter in the hot feed and sometimes a voltmeter across the output socket.
When powering up a new repair you always need to know how much energy it is consuming, and have an idea of what is normal current draw. That way you can catch a fault condition before anything is damaged.
-Chris
You would place an AC ammeter in series with the AC feed ot the amplifier you are testing. The better variacs have them built in, but you can always put a separate ammeter in a box with a plug and socket. Just insert that between the variac and unit under test. The circuit usually has the ammeter in the hot feed and sometimes a voltmeter across the output socket.
When powering up a new repair you always need to know how much energy it is consuming, and have an idea of what is normal current draw. That way you can catch a fault condition before anything is damaged.
-Chris
I would begin this by deliberately shorting the vbe multiplier (Q418) to force zero bias current.
You seem to be saying you observed one of these voltages as 1.5 volts @ -/+ 0.5v. If so then that is far from steady. The two 15k's feeding the offset adjust are a known failure item.
Altering VR402 should result in no change in output current. That is correct. VR402 is for DC balance (offset adjust). Also the 47k linking the two Zeners in the current source/sinks looks a typical failure item as well.
Pretty much all faults on these NAD's come down to the above and also failed (often intermittent failure) of the hot running voltage amplifier stages Q414 and 416. Failures here can take out the pre drivers and drivers. If that has happened, you need to check all the low value resistors in the output stage including those series ones in the supply rails.
The high/low rail switching will have no effect on the basic operation of the amp whether the rail switching is faulty or not... and often is faulty... but get the basic amp working first. If the rail switching is faulty, you will see the high 70v rails present at all times on the output stage. Check both - and + rails. If that is happening, then just remove Q432 and Q434 until the amp itself is fixed.
You seem to be saying you observed one of these voltages as 1.5 volts @ -/+ 0.5v. If so then that is far from steady. The two 15k's feeding the offset adjust are a known failure item.
Altering VR402 should result in no change in output current. That is correct. VR402 is for DC balance (offset adjust). Also the 47k linking the two Zeners in the current source/sinks looks a typical failure item as well.
Pretty much all faults on these NAD's come down to the above and also failed (often intermittent failure) of the hot running voltage amplifier stages Q414 and 416. Failures here can take out the pre drivers and drivers. If that has happened, you need to check all the low value resistors in the output stage including those series ones in the supply rails.
The high/low rail switching will have no effect on the basic operation of the amp whether the rail switching is faulty or not... and often is faulty... but get the basic amp working first. If the rail switching is faulty, you will see the high 70v rails present at all times on the output stage. Check both - and + rails. If that is happening, then just remove Q432 and Q434 until the amp itself is fixed.
Hi Karl,
Yes, exactly!
I didn't know the 47K resistors were prone to failure. I have never seen that before but I'll start checking in particular. My guess is they go high in value and eventually open?
Yes, exactly!
I didn't know the 47K resistors were prone to failure. I have never seen that before but I'll start checking in particular. My guess is they go high in value and eventually open?
Spec’ed as 1/6W, but dissipating about 2X that. Of course it’s eventually going to go open. They probably start going high from Day One. Replace with a 2W MOX.
Well, that males sense. Since I had never had a failure that I knew of, I would never bother to figure out the dissipation. So much for trusting the designers did a good job!