You will all laugh at my ignorance but here goes. I did STF. When the unit powers up, it puts out a major hum through both the headphone jack and/or the speakers, then clicks into some kind of protection mode. I heard replacing the big things that look like C batteries could fix it, so I matched the numbers on the two capacitors to $4 parts on Digikey and soldered in these replacements, but with the same result. I read about shorting the cap before working on it and that was pretty neat. What do I do next? My options are to actually get an EE degree, mooch knowledge off a distant friend who builds spy satellites, or just chuck this thing I bought for $40 knowing it was broken and that I was stupid to take on an insurmountable challenge and I should just get back to my real job. Or, I could embarass myself in front of you all.
I am a rock-ribbed DIYer. I fabbed and installed my own Corian countertops, I change my own oil, I make my own CAT5 and AV interconnects. I once salvaged important hard drive data by buying the same model drive and swapping the controller boards. Am I wrong to attempt this new hobby of audio equipment repair?
I am a rock-ribbed DIYer. I fabbed and installed my own Corian countertops, I change my own oil, I make my own CAT5 and AV interconnects. I once salvaged important hard drive data by buying the same model drive and swapping the controller boards. Am I wrong to attempt this new hobby of audio equipment repair?
I have also reheated a bunch of the solder joints starting at the power supply, but no difference. There are two similar transistors for what I think are the two channels, based on the layout and they are connected to a huge heatsink, and when I put the meter on each of three legs of the first transistor and power up, I get current on all three. On the other transistor, I power up and get nothing. Mind you, the current only lasts a split second before the protection kicks in, but it's enough to show the difference between the two channels. With the current off, I get 2 different resistance values between the three legs, and both transistors measure the same. I did all of this with the pre-power jumpers out, but it made no difference if I put them in.
I have checked the speaker terminals for loose wire threads, and put an RCA in each input to clear any gunk. I still get the loud hum at poweron and then it kicks into protection. After ten seconds it goes into standby and if I hit the power switch it does the same thing again. I am using some cheap computer speakers connected to the headphone jack to prevent harm to real speakers, but it makes no difference to the symptoms.
Thanks in advance.
I have checked the speaker terminals for loose wire threads, and put an RCA in each input to clear any gunk. I still get the loud hum at poweron and then it kicks into protection. After ten seconds it goes into standby and if I hit the power switch it does the same thing again. I am using some cheap computer speakers connected to the headphone jack to prevent harm to real speakers, but it makes no difference to the symptoms.
Thanks in advance.
HI,
Usually among the first things you try with this kind of problem is to disconnect the power supply rails from the power amplifier section and leave the pre-amp section connected.
If the unit is fine without power section connected the chances are it has blown its output devices.
Get the data sheet for the transistors mounted on the big heatsink and check they arent short circuit between;
C-E/E-C/E-B/C-B
(E=emitter, B=base,C=collector)
I.E. the only significant conduction should be from base to emitter (B-E).
If they are short circuit then they are probably dead.
See what you get and post back, good luck!
Usually among the first things you try with this kind of problem is to disconnect the power supply rails from the power amplifier section and leave the pre-amp section connected.
If the unit is fine without power section connected the chances are it has blown its output devices.
Get the data sheet for the transistors mounted on the big heatsink and check they arent short circuit between;
C-E/E-C/E-B/C-B
(E=emitter, B=base,C=collector)
I.E. the only significant conduction should be from base to emitter (B-E).
If they are short circuit then they are probably dead.
See what you get and post back, good luck!
Most likely, one or more power transistors are broken.
Can you take a few clear pictures? I presume you have no schematic.
/Hugo
Can you take a few clear pictures? I presume you have no schematic.
/Hugo
Many thanks for the help. Disregard my previous transistor test. I downloaded the datasheet.
On one pair there's resistance all around, but for the other pair, one is shorted completely (zero resistance, base to emitter) and in that pair the other transistor has resistance that changes as I hold the probes on (increases).
I have no schematic, and probably couldn't read one anyway. So I removed the four 4A fuses and left the small pair of 1A fuses and while the unit powers on and does not move into protection mode, the tuner's station search light comes on although there's no display.
Sooooo, if I order a new set of all 4 transistors and use new thermal grease and solder them in, what are my chances of success? Digikey has the 2N3055, but not the MJ2955 in stock. Heck even my local radio shack has the 2N3055. Is there a substitute or upgrade I should use? Should I avoid buying a set of five of both on eBay from a guy in Hong Kong for $20?
On one pair there's resistance all around, but for the other pair, one is shorted completely (zero resistance, base to emitter) and in that pair the other transistor has resistance that changes as I hold the probes on (increases).
I have no schematic, and probably couldn't read one anyway. So I removed the four 4A fuses and left the small pair of 1A fuses and while the unit powers on and does not move into protection mode, the tuner's station search light comes on although there's no display.
Sooooo, if I order a new set of all 4 transistors and use new thermal grease and solder them in, what are my chances of success? Digikey has the 2N3055, but not the MJ2955 in stock. Heck even my local radio shack has the 2N3055. Is there a substitute or upgrade I should use? Should I avoid buying a set of five of both on eBay from a guy in Hong Kong for $20?
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I would recommend MJ21193 or MJ21195. The 2N3055 could be replaced by MJ21194 or MJ21196. Buy from a good source.
You will also want to check the smaller transistors between the big TO3 cans.
/Hugo
You will also want to check the smaller transistors between the big TO3 cans.
/Hugo
I would just like to chime in about those big things that look like C batteries as you called them (actually look REALLY small to me 😀). I'll refer to them as the filter caps and I would advise not to short them in the future. I would recommend a bleeder resistor instead.
Failure. As suggested I replaced the four T03's with the MJ21195 and MJ21196, as well as the two smaller transistors between them. I measured the old T03's, now out of circuit, and got shorts between the pins on at least one of them. Now instead of hum and then protection mode, it will blow two of the four 4A fuses immediately upon plugging it in. I guess I will go back and first check my solder work, and then try to take that whole channel offline. I have not been able to find the schematic for this model. But I did get one for my NAD 2200 so that's nice to have. Is there a likelihood that the power supply is causing this? The power switch? The startup relay? I'll measure the voltage coming off the transformer at the point it connects to each channel and see if there's a difference. Thanks again for any and all help.
Tim
Tim
Progress: replaced driver transistors and now works
I found and purchased the service manual. I just replaced what some have called the driver transistors after finding at least one bad. Four were replaced. Amp powers up and works fine, and I have made the first of two initial adjustments per the manual but have stopped short of really warming her up because there are a couple other transistors that get quite hot only five seconds after powerup, and I thought I saw some tiny wisps of smoke in that area, and the board there looks a little scorched. According to the schematic they are in the power supply section, but not the one the runs the power amp portion that was blowing fuses. They have small thin heatsinks not much larger that the devices themselves and are way too hot to touch. I have put a couple alligator clips on to help cool them but am afraid to take them off. In the schematic below they are Q507 and Q508, a 2SD669 and a 2SB649. These happen to be the same part numbers as the ones in the power section I already replaced.
Any recommendations as to how to fix/cool these? Maybe I should mention that the tuner portion works, as do the function lights, but the display showing the frequency does not. I'm not trying to fix that unless that problem is causing this one. Also, what does the X on the schematic next to the transistors mean?
I found and purchased the service manual. I just replaced what some have called the driver transistors after finding at least one bad. Four were replaced. Amp powers up and works fine, and I have made the first of two initial adjustments per the manual but have stopped short of really warming her up because there are a couple other transistors that get quite hot only five seconds after powerup, and I thought I saw some tiny wisps of smoke in that area, and the board there looks a little scorched. According to the schematic they are in the power supply section, but not the one the runs the power amp portion that was blowing fuses. They have small thin heatsinks not much larger that the devices themselves and are way too hot to touch. I have put a couple alligator clips on to help cool them but am afraid to take them off. In the schematic below they are Q507 and Q508, a 2SD669 and a 2SB649. These happen to be the same part numbers as the ones in the power section I already replaced.
Any recommendations as to how to fix/cool these? Maybe I should mention that the tuner portion works, as do the function lights, but the display showing the frequency does not. I'm not trying to fix that unless that problem is causing this one. Also, what does the X on the schematic next to the transistors mean?
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Hi timbert,
At least you bought the manual. Good move!
This seems to indicate that these parts normally run hot, but are now too hot. You can install heat sinks on them after you get it fixed. I do suspect they run the voltage amp section, but we can't see the relationship between this and the rest of the circuit. There may be interactions between different areas running off the power supply. No display is a clue possibly.
Now, I don't have the manual for this to help you out, could you scan it at a higher resolution and sent it direct please? bhomester at gmail dot com. Hugo may be able to help you as well if he has a copy to refer to. His recommended substitutes are excellent suggestions. They are far better parts than the originals.
When you replaced the outputs and drivers, did you use fresh heat sink grease and insulators? You may be able to reuse the old insulators but certainly not the grease. Also, please don't over tighten the new parts.
-Chris
At least you bought the manual. Good move!
This seems to indicate that these parts normally run hot, but are now too hot. You can install heat sinks on them after you get it fixed. I do suspect they run the voltage amp section, but we can't see the relationship between this and the rest of the circuit. There may be interactions between different areas running off the power supply. No display is a clue possibly.
Now, I don't have the manual for this to help you out, could you scan it at a higher resolution and sent it direct please? bhomester at gmail dot com. Hugo may be able to help you as well if he has a copy to refer to. His recommended substitutes are excellent suggestions. They are far better parts than the originals.
When you replaced the outputs and drivers, did you use fresh heat sink grease and insulators? You may be able to reuse the old insulators but certainly not the grease. Also, please don't over tighten the new parts.
-Chris
Hi timbert,
The 40 volt regulated supplies feed the voltage amp secton of your amplifiers. A fault with an amplifier could easily draw too much current and cause it to run hot or fail. Therefore, I suspect a front end fault as well.
-Chris
The 40 volt regulated supplies feed the voltage amp secton of your amplifiers. A fault with an amplifier could easily draw too much current and cause it to run hot or fail. Therefore, I suspect a front end fault as well.
-Chris
I am now up and running with several hours of music and have done all the adjustments per the manual. Meanwhile, I have read in another thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=93931&highlight=nad+701
that I should install some other resistors here and there, but I am unsure which of the four transistors should get them on their emitters.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=93931&highlight=nad+701
that I should install some other resistors here and there, but I am unsure which of the four transistors should get them on their emitters.
Heard back from NAD and their Lenbrook company. I talked to a guy there last week who suggested I email him so I wrote:
"I have a 701 receiver that I have repaired by replacing the big capacitors and the power and
driver transistors in the power amp section, and the amp sounds fine. I followed the procedures outlined in the service manual. I heard, however, that there were some updates that could be made, and because I replaced the TO-3 transistors with more modern ones those updates may be important for reliability."
The reply today from Lenbrook was that he scanned the docs for 701 updates and there were none, and he confirmed this with the lead engineer at NAD.
As it is now the amp works, any reason not to put it back in the kitchen where it will drive some little speakers? I already have fire extinguishers handy there.
"I have a 701 receiver that I have repaired by replacing the big capacitors and the power and
driver transistors in the power amp section, and the amp sounds fine. I followed the procedures outlined in the service manual. I heard, however, that there were some updates that could be made, and because I replaced the TO-3 transistors with more modern ones those updates may be important for reliability."
The reply today from Lenbrook was that he scanned the docs for 701 updates and there were none, and he confirmed this with the lead engineer at NAD.
As it is now the amp works, any reason not to put it back in the kitchen where it will drive some little speakers? I already have fire extinguishers handy there.
Hi timbert,
Oddly enough, I just turned down & dissuaded an "improvement mod" job on an NAD this afternoon. As I explained to the gentleman, there are all kinds of things written about "upgrading" and all kinds of people hacking equipment. There needs to be a logical approach and the studies on the stock condition and where to go are extremely time consuming. There is no guarantee of a successful change.
Some of us can make some improvements to the sound quality, but those people are in the minority. The rest of them are "experts" from reading on the web.
Keep in mind that modifications from the manufacturer will revolve around replacing discontinued parts and fixing weaknesses (problems). Once a unit goes into production, all engineering stops. The engineers are now working on the next model.
-Chris
Oddly enough, I just turned down & dissuaded an "improvement mod" job on an NAD this afternoon. As I explained to the gentleman, there are all kinds of things written about "upgrading" and all kinds of people hacking equipment. There needs to be a logical approach and the studies on the stock condition and where to go are extremely time consuming. There is no guarantee of a successful change.
Some of us can make some improvements to the sound quality, but those people are in the minority. The rest of them are "experts" from reading on the web.
Keep in mind that modifications from the manufacturer will revolve around replacing discontinued parts and fixing weaknesses (problems). Once a unit goes into production, all engineering stops. The engineers are now working on the next model.
-Chris
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