Hi everyone,
I recently obtained a NAD 3240PE for very little money. It had some dirty potentiometers and switches so I desoldered them and gave them all a good clean in the ultrasonic cleaner. However, I made a very (very) stupid mistake while soldering back the switches. Everything was fine, except that I appear to have shorted two pins of the output selector 🙁 😡
When I turned the amp on, it burnt a 3R3 resistor (R462) and blew a fuse. I replaced the resistor, fixed my soldering error, and now I measure -70,3V DC on the output of the right channel. Now, my electronics skills are not that great (I'm stil learning but it's a lot to take in), but if I understand correctly this means the Power Envelope circuitry is triggered to supply high voltage to the output stage, and the output stage may be shorted?
My question to you is; has anyone seen this problem before, do you think it can be fixed (given that I would be able to find replacement transistors) and what would be my best next step? I've printed out the circuit diagram and components list which I obtained from HifiEngine but I'm not (yet) good at reading schematics. All I see is a load of transistors... I am willing to learn!
Thank you in advance.
I recently obtained a NAD 3240PE for very little money. It had some dirty potentiometers and switches so I desoldered them and gave them all a good clean in the ultrasonic cleaner. However, I made a very (very) stupid mistake while soldering back the switches. Everything was fine, except that I appear to have shorted two pins of the output selector 🙁 😡
When I turned the amp on, it burnt a 3R3 resistor (R462) and blew a fuse. I replaced the resistor, fixed my soldering error, and now I measure -70,3V DC on the output of the right channel. Now, my electronics skills are not that great (I'm stil learning but it's a lot to take in), but if I understand correctly this means the Power Envelope circuitry is triggered to supply high voltage to the output stage, and the output stage may be shorted?
My question to you is; has anyone seen this problem before, do you think it can be fixed (given that I would be able to find replacement transistors) and what would be my best next step? I've printed out the circuit diagram and components list which I obtained from HifiEngine but I'm not (yet) good at reading schematics. All I see is a load of transistors... I am willing to learn!
Thank you in advance.
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You have probably caused the output and driver transistors to fail on that channel along with collateral damage including the 3.3 ohm resistors. The high voltage rail switching circuitry may or may not be damaged but this would not stop the amp working or cause other issues at this point.
Get the blown channel fixed first and then see if there is an issue with the rail switching once it all works.
Get the blown channel fixed first and then see if there is an issue with the rail switching once it all works.
Shorting the output of an amplifier almost always is cause of power and driver transistors failure. Get the schematic if posible and try to replace all components (diodes, resistors ans transistors) of the stage by as good pieces of the original as you can. Some caps if saw the polarity reversed because of the shorted output must be also repaced.
When replace of pieces is ready, get rid of the fuse at the primary source of the rig and replace it temporarily with a light bulb of the incandescent type. No fluorescent nor led. Pure filament lamp of the same power as the rig eats usually. If things goes wrong lamp will bright signaling you an abnormal condition. If it is all ok, measure DC voltages and "if and only if" measures are OK reconnect a load to the amp and give small signal to a test speaker or power resistor. Until you are sure all is ok don't use normal loudspeakers as load.
When replace of pieces is ready, get rid of the fuse at the primary source of the rig and replace it temporarily with a light bulb of the incandescent type. No fluorescent nor led. Pure filament lamp of the same power as the rig eats usually. If things goes wrong lamp will bright signaling you an abnormal condition. If it is all ok, measure DC voltages and "if and only if" measures are OK reconnect a load to the amp and give small signal to a test speaker or power resistor. Until you are sure all is ok don't use normal loudspeakers as load.
never found anyone on here who is like that mate they are all very helpfull, and like you i am learning, but compared to what i knew 6 months ago my knowledge now thanks to the guys on here, is realy quite good now, if not technician level yet(not sure ill ever get there lol) 🙂
heres a realy good copy of the schematic from the original service manaul and its great quality
Will try that first, thanks for the input! I'll first replace the channel components and check the rail switching later on. Makes sense to me 🙂
Good call on the capacitors, I hadn't thought of that. Good thing they're fairly cheap.
I like the idea of using a filament lamp instead of the fuse - that's clever, and saves me from blowing through fuses...
Yes, I have noticed! I'm sorry, I removed the statement on unhelpful comments. I have been a member on this forum for 6 years now but this is my first post. I've had some rough learning experiences as a software developer when I was younger on fora - other software developers tend to be condescending pricks when a beginner asks a question. So glad to see that's not the case here.
Thank you so much! I already had one printed out from Hifi Engine but it's barely readable in some places. This one is amazingly detailed... Thanks again.
My next steps will be replacing the components in the power stage of the right channel. Would someone mind guiding me a bit on the diagram? As in, which transistors do what exactly? I'm able to trace the right speaker output back to Q428 and Q430 (source and sink if I understand correctly) but I can't wrap my mind around what happens around those two. I can see the 70V rail (through PTC 1 & 2) and the 'normal' rail, the audio input signal and the output signal, but the rest doesn't immediately make sense to me.
If someone could give me some pointers (or guide me to good sources for learning about audio amplifiers / common transistor circuits), I'd be very thankful.
Everyone has their own methods for initially fault-finding on something like this.
As you have - (negative) 73 volts on the output I would begin by switching it OFF and then doing quick checks between Collector and Emitter of the two output transistors and the two drivers. If any read short circuit they are zapped. Check R456 and R460 as well as both 3.3 ohms.
The transistors can initially be checked in circuit for this.
(There is no quick way to explain how it all comes together to work as it does I'm afraid but the output and driver transistor combined make a 'super gain' transistor called a Darlington pair. Because they are so closely coupled failure of one often takes others with it. The lower NPN output and its PNP driver mean the amp is called a Quasi Complementary type where the NPN/PNP combination replaces the more conventional PNP/PNP pair... that doesn't help much does it. Books... read books)
As you have - (negative) 73 volts on the output I would begin by switching it OFF and then doing quick checks between Collector and Emitter of the two output transistors and the two drivers. If any read short circuit they are zapped. Check R456 and R460 as well as both 3.3 ohms.
The transistors can initially be checked in circuit for this.
(There is no quick way to explain how it all comes together to work as it does I'm afraid but the output and driver transistor combined make a 'super gain' transistor called a Darlington pair. Because they are so closely coupled failure of one often takes others with it. The lower NPN output and its PNP driver mean the amp is called a Quasi Complementary type where the NPN/PNP combination replaces the more conventional PNP/PNP pair... that doesn't help much does it. Books... read books)
Attachments
if you want a good book they dont come much better than 'The art of electronics' by paul horowitz 🙂
watch it with the parts in these ive had a few where people have gone realy cheap with the output stage and it doenst realy pay
Thanks for your clarification. First order of business: getting myself a new multimeter. Not even kidding, mine just cut out. No use putting time into fixing the damn thing as it was a cheap POS. Though I am getting tired of stuff breaking all the time 😕
Even though you don't think it helps much, it does, it really does. It gives me something to dive into (darlington pairs, push-pull, properties of transistors such as current gain, etc). I sincerely appreciate the fact that you took the effort to clarify the important section of the diagram for me.
Oh dear, a €70,00 investment... Not many second hand copies here in the Netherlands. I'll keep it in mind though! I managed to get some study books on Electrical Engineering for little money, as well as two books on transistors and transistor circuits from 1966 and 1972 respectively...
I know what I'll be reading in my spare time...Good point. I found a shop here in the Netherlands that sells all kinds of transistors from older electronics. I was thinking of giving them a call. They're not the cheapest, but they do appear to be original quality... Any suggestions for fitting replacements?
For my own future reference:
Code:
Q424 2SD669
Q428 2SD1047
Code:
Q426 2SB649
Q430 2SB817Thanks for the kind words 🙂 I'm pleased it helped a little.
When replacing transistors you really do need to be aware of fake parts. Many of these parts have been out of production for decades so you need to be sure any are NOS (new old stock) and not remarked generic parts.
If you have a found a reputable supplier then that is great.
Often parts can be substituted with more modern devices as long as care is taken in choosing suitable candidates.
You might find playing with a circuit simulator helps your understanding... click my signature line at the bottom of this post if you are curious.
When replacing transistors you really do need to be aware of fake parts. Many of these parts have been out of production for decades so you need to be sure any are NOS (new old stock) and not remarked generic parts.
If you have a found a reputable supplier then that is great.
Often parts can be substituted with more modern devices as long as care is taken in choosing suitable candidates.
You might find playing with a circuit simulator helps your understanding... click my signature line at the bottom of this post if you are curious.
especialy off ebay.There are 'original' 'M' parts on there for sale and they are empty cases so beware, only buy from the reputable people like digikey,mouser,RS etc or a realy good site i found for the hobiest
Bitsbox Electronic Components, Electronic Component Suppliers UK
some of the PNP stuff now is becoming pretty expensive as well, so try not to short stuff out as it gets expensive (like i found out 😀)
Bitsbox Electronic Components, Electronic Component Suppliers UK
some of the PNP stuff now is becoming pretty expensive as well, so try not to short stuff out as it gets expensive (like i found out 😀)
I have similar issues on the device of this model and want to modify this integrated amp in such kind, that the Class-G character is removed, because the front-end runs with +/-70VDC. When the parts of the front-end runs only with +/-40VDC, thermal stress is lowered a lot and reliability rises up - so I think.
By checking this forum I note, that there are a wide range of threads to this model - go to post #5 under
https://www.diyaudio.com/community/threads/nad-3240-pe-issues.333339/It seems that the previously known rule of not starting a new thread on a device model where a thread already exists no longer applies.
Is my estimate right and should I start a new thread again ?
The rules say not to start multiple threads on the same subject which is there to prevent individual members trying their luck by posting the same content in multiple sub forums.
If you start a new thread you are prompted with the message "Is this already being discussed?" and so if there is already a thread discussing modifying and disabling the Class G section then you could continue in that thread if you wish but you are not forced to do so. If nothing suitable exists then start a new thread.
I see - thank you very much for your quickly reply.
The rules say not to start multiple threads on the same subject which is there to prevent individual members trying their luck by posting the same content in multiple sub forums.
If you start a new thread you are prompted with the message "Is this already being discussed?" and so if there is already a thread discussing modifying and disabling the Class G section then you could continue in that thread if you wish but you are not forced to do so. If nothing suitable exists then start a new thread.
For those who would reside in France I have a cannibalised 3240PE, it has been used almost 20 years to power a dual coil sub. Parts are missing such as input selector and connectors but the main plate is there and fully working, just wanted to get out the transformer.