A 600w lamp isn't go to offer much protection in the scheme of things, normally a 60 or 100watt would be used.
So outputs and drivers are zapped. That's a common scenario for what happened. The drivers are old and obsolete and the big problem will be ensuring any replacements are genuine parts and not remarked 'fakes'.
They shouldn't read open... and probably don't when checked correctly. They should read like a diode between B and E and B and C although surrounding circuitry can give misleading results. They won't be open 🙂 Use a diode check range on your meter.
Q122 is configured as a vbe multiplier. They are not normally effected by such damage as has happened here. You should though set the bias to minimum when powering up after replacing parts. That means setting VR104 to minimum resistance.
https://en.wikipedia.org/wiki/Rubber_diode
You need to check the low value resistors around that area such as the 10 ohms, 47 ohms and 270 ohms around the drivers.
Don't be under any illusions about repairing this. While its an 'easy fix' in itself you may find that substituting transistors with modern parts causes such things as bias current being out of range and/or stability issues. Just be aware.
So outputs and drivers are zapped. That's a common scenario for what happened. The drivers are old and obsolete and the big problem will be ensuring any replacements are genuine parts and not remarked 'fakes'.
They shouldn't read open... and probably don't when checked correctly. They should read like a diode between B and E and B and C although surrounding circuitry can give misleading results. They won't be open 🙂 Use a diode check range on your meter.
Q122 is configured as a vbe multiplier. They are not normally effected by such damage as has happened here. You should though set the bias to minimum when powering up after replacing parts. That means setting VR104 to minimum resistance.
https://en.wikipedia.org/wiki/Rubber_diode
You need to check the low value resistors around that area such as the 10 ohms, 47 ohms and 270 ohms around the drivers.
Don't be under any illusions about repairing this. While its an 'easy fix' in itself you may find that substituting transistors with modern parts causes such things as bias current being out of range and/or stability issues. Just be aware.
I thought the higher wattage lamp would be more protection than a 60W because it will light up faster, be more easily noticed, and operate with lower resistance when lit, therefore remaining a more favorable path for current than the shorted device that follows it. Is that wrong?
I took this on because I'm interested in old hi-fi and thought it would be a good experience to try to get it working, rather than chucking it out or selling it for parts, and since it is not all that special a unit anyway, I can always walk away if it gets to be too consuming. Getting close now, but I think I can stick it out a bit longer. I liked this amp when it worked, though it seems NAD did not build these to last. I have a Pioneer from the early 70s I was given by my first wife's uncle. It has never been opened up as far as I know and we still use it daily in the house.
I'll definitely retest all those transistors. Honestly, it took hours to refresh my knowledge of PNP vs. NPN and identify the right terminals on everything, so testing came at the end of all that! The other amp I built was all tubes.
Thanks again!
in this NAD and most amps you need a minimum amount of current to the front end “on”, or you will chase your tail on issues that aren’t issues.
100w is plenty of protection unless you ignore the brightness and “cook” the amp for too long.
Most important part of lamp protection is direct short, so you don’t work hard fixing it only to blow up everything you replaced because of a resistor or cap you missed.
100w is plenty of protection unless you ignore the brightness and “cook” the amp for too long.
Most important part of lamp protection is direct short, so you don’t work hard fixing it only to blow up everything you replaced because of a resistor or cap you missed.
Yes.
Retesting transistors with a 2 v source meter has limited utility. It will catch a totally shorted one, but failing amps can damage transistors less than total failure. Sometimes they will withstand 2 v and not 25 or 50 or 100. An Iceo test at 24 v will catch the damaged ones too. However, with a light bulb in series with the AC line you can probe the DC voltages as the amp sits and probably spot damaged transistors & diodes too. Vbe <0.6, Vce zero or full power supply voltage with no current flowing are two bad signs.
As bullittstang notes, be sure the power supplies for the op amps are up to standard with the light bulb in series. I had to use a 1200 w room heater element in series with the AC line on a 1300 W amp with 10 output transistors per side.
The bulb tester relies on the low cold resistance of the filament which if the amp draws little current stays cold. If the current draw is high then the filament resistance rises, the bulb lights and current is limited. The lower the filament resistance and the more current is supplied to the load (the amp) which is what you don't want if there is a fault.
Try this for interest. Measure the cold resistance of your bulb. You will find it is really really low. Watts equals voltage squared divided by the resistance. So if you are on 115 VAC and you measure say 6 ohm (I've no idea what you will measure on yours) then the wattage drawn is (115*115)/6 which is well over 2kw. If the bulb is rated 600 watt then that means it has a hot resistance of 22 ohms.
A 100 watt bulb will have a much higher hot resistance and limit the current flow. The cold resistance is still low enough to allow a non faulty amp to power up OK.
I was just making an error, imagining the light bulb as if it were in parallel. Of course if a high current passes through the high-wattage bulb, that's whats supplied to the amp, whereas a lower wattage bulb, higher in resistance when lit, provides a lower current. I think I finally get it.
So, I think I'm hearing that it may be of limited value to test these output transistors with a DMM with no power supplied to the amp. The results I'm getting seem pretty ambiguous... the left channel output transistors seem obviously damaged, getting almost no voltage between B-C or B-E. On the right channel my meter is getting ~0.5 V between B-C on both output transistors, regardless of polarity, and a lot less, about 0.06 V, between B-E on both. What I think I'm hearing here, though, is that with these in-circuit I'll get a better idea where I'm at if I give them some voltage using the limiter and test them as diodes with that setup.
Can someone point me to any info on the numbers that get printed on these MT-200s below the part #? Both 2SC3264s on this unit, for example, have "12Y" printed on them, but the 2SA1295s have "13Y". I don't see the same numbers anywhere on the pics from the ebay sellers. Is this a situation where if I were to replace these I'd want to replace all 4 and match them? I'm an absolute noob about this, obviously.
So, I think I'm hearing that it may be of limited value to test these output transistors with a DMM with no power supplied to the amp. The results I'm getting seem pretty ambiguous... the left channel output transistors seem obviously damaged, getting almost no voltage between B-C or B-E. On the right channel my meter is getting ~0.5 V between B-C on both output transistors, regardless of polarity, and a lot less, about 0.06 V, between B-E on both. What I think I'm hearing here, though, is that with these in-circuit I'll get a better idea where I'm at if I give them some voltage using the limiter and test them as diodes with that setup.
Can someone point me to any info on the numbers that get printed on these MT-200s below the part #? Both 2SC3264s on this unit, for example, have "12Y" printed on them, but the 2SA1295s have "13Y". I don't see the same numbers anywhere on the pics from the ebay sellers. Is this a situation where if I were to replace these I'd want to replace all 4 and match them? I'm an absolute noob about this, obviously.
Couple things from your last post. First, if you have applied to power to the amp circuit (w/ light bulb in series) you will only test for voltages. Diode testing will only be likely to damage your DMM. So with voltage applied, you will be looking to compare voltages for the same part, good channel versus bad channel. Start with outputs, since they are big and likely to be higher off the board, gets more difficult as the transistor package gets smaller.
First if to get a good set of glasses and bright light and go over the boards very carefully. Looking for burned/charred resistors, cracked solder pads or damaged traces on the board. I have worked on 3-4 NAD boards and most had had a failed 1-2 watt low value resistor in the power supply to start, with a bad driver transistor and 1-2 NPN outputs shorted. Not always that evident and easy to find the fault though.
If that all looks good, then start by testing power supply voltages, making sure you have similar positive and negative voltages on both amp boards (remember voltages will be lower (5-10 volts) because of the current limiting bulb, but still should be similar channel to channel)
If your supply voltages look the same for both amp boards, then move on to other parts of the board, i.e. output transistors and check B-C-E and compare to the working channel. I usually set up an Excel or scratch pad with three columns and each channel right on top of each other for easy comparison. This way I don't have to remember the voltages very long. Important to notate the polarity positive or negative, as this can lead you to the issue.
Then you will move back one section on the board and test the drivers, then pre-drivers, etc. until you have voltages for each transistor.
On your inquiry about the transistor markings, the "Y" is usually the classification of the Hfe (Beta or gain) and you can find that information on the datasheet. Regarding eBay, generally speaking the parts will be fakes, not that they won't work at all, but generally speaking are of lower quality than what the original part was specified at. Best to find new parts at a reputable supplier like Mouser, DigiKey, Arrow, Newark or similar. Otherwise you will spend all this time reparing the amp and it will likely fail in a short time, rather than last another 10-15 years.
Good luck and come back as you have more questions.
First if to get a good set of glasses and bright light and go over the boards very carefully. Looking for burned/charred resistors, cracked solder pads or damaged traces on the board. I have worked on 3-4 NAD boards and most had had a failed 1-2 watt low value resistor in the power supply to start, with a bad driver transistor and 1-2 NPN outputs shorted. Not always that evident and easy to find the fault though.
If that all looks good, then start by testing power supply voltages, making sure you have similar positive and negative voltages on both amp boards (remember voltages will be lower (5-10 volts) because of the current limiting bulb, but still should be similar channel to channel)
If your supply voltages look the same for both amp boards, then move on to other parts of the board, i.e. output transistors and check B-C-E and compare to the working channel. I usually set up an Excel or scratch pad with three columns and each channel right on top of each other for easy comparison. This way I don't have to remember the voltages very long. Important to notate the polarity positive or negative, as this can lead you to the issue.
Then you will move back one section on the board and test the drivers, then pre-drivers, etc. until you have voltages for each transistor.
On your inquiry about the transistor markings, the "Y" is usually the classification of the Hfe (Beta or gain) and you can find that information on the datasheet. Regarding eBay, generally speaking the parts will be fakes, not that they won't work at all, but generally speaking are of lower quality than what the original part was specified at. Best to find new parts at a reputable supplier like Mouser, DigiKey, Arrow, Newark or similar. Otherwise you will spend all this time reparing the amp and it will likely fail in a short time, rather than last another 10-15 years.
Good luck and come back as you have more questions.
These are production codes and gain groups. Y is the top gain group according to the data sheet (70 to 140). I wouldn't worry to much over that, of more concern is buying from eBay as there are so many fake parts around.
Large transistors usually fail short circuit from C to E and that shows in circuit with a quick diode test.
There is no need to match these as they are just a single pair per channel. It is multiple parallel pairs where you match each set of N and P devices.
Not always. See post 24.
I would remove any obviously shorted transistors like the shorted outputs on one channel (by dvm diode test) before probing around for voltages with a lightbulb in series the AC line.
And I did said 'usually' and not that they always do 🙂
A short from C to E always shows itself when tested in circuit, you then have to carry the check through and make sure it is the device that is faulty and not something external across the transistor.
Amps with multiple parallel pairs need more care (the NAD C350 does not have multiple pairs) in that one shorted device can make it appear that the others have shorted as well.
Smaller devices such as drivers and small signal types certainly can and do often fail leaky while output devices (which have the full current delivery of the power supply behind them) do usually fail short circuit. It is very very rare to get such a device to fail in a leaky state in my experience.
A short from C to E always shows itself when tested in circuit, you then have to carry the check through and make sure it is the device that is faulty and not something external across the transistor.
Amps with multiple parallel pairs need more care (the NAD C350 does not have multiple pairs) in that one shorted device can make it appear that the others have shorted as well.
Smaller devices such as drivers and small signal types certainly can and do often fail leaky while output devices (which have the full current delivery of the power supply behind them) do usually fail short circuit. It is very very rare to get such a device to fail in a leaky state in my experience.
If an output device fails leaky (ie, an older process, too much age/stress) it will go short when you get full supply voltage across it. By the time it is identified as a failure, it’s shorted. A little leaky and not turned up to clipping the amp still works so you’re not in there messing with it. I did find a leaky 2N5630 in a Dyna 400 once - barely took 80 volts. It was still working when I tore it down for upgrades. If it hadn’t been a series connected output stage it would have poofed long before. Then it would have had four shorted transistors.
If one tranny shorts in a bank of several in parallel you’re supposed to replace them all anyway. I’ve been known to hang onto the rest of them, but they go in B-stock.
If one tranny shorts in a bank of several in parallel you’re supposed to replace them all anyway. I’ve been known to hang onto the rest of them, but they go in B-stock.
Today I received new 2SC3264 and 2SA1295 from bdent. I couldn't find any D669/B649 to replace the drivers, so bdent sent me C2690 and A1220 as substitutes (they went above and beyond; called me up and worked it all out over the phone). I've since read that NTE 373 and 374 are better replacements, but I'd rather not spend the massive shipping cost to get these from NTE, so I think I'll stick with the C2690 and A1220 for the drivers unless one of you experts recommends otherwise.
Meanwhile, I tested a ton of resistors, particularly those in the vicinity of that left channel's output section, and everything looks fine. After that I passed the time doing ESR tests on the remaining electrolytics (I already replaced everything on the power supply board), and similarly to what Trevor finds in this video, out of 44 caps I tested, almost all had either borderline or bad ESR values, so I figure I may as well get after those.
I'll probably replace the outputs and drivers and see where I'm at before I start on the capacitors.
Meanwhile, I tested a ton of resistors, particularly those in the vicinity of that left channel's output section, and everything looks fine. After that I passed the time doing ESR tests on the remaining electrolytics (I already replaced everything on the power supply board), and similarly to what Trevor finds in this video, out of 44 caps I tested, almost all had either borderline or bad ESR values, so I figure I may as well get after those.
I'll probably replace the outputs and drivers and see where I'm at before I start on the capacitors.
So here's the question: if I've found that one of my drivers (the B649) on the channel is okay, but the other (the D669) is bad, and I have different replacements (A1220 and C2690 because I couldn't find any B649 and D669), should I replace both the B649 and the D669, even though the B649 is okay, so that the two drivers on that channel have the same characteristics? I have no experience with this so no idea how important these differences are! Here are a few bits off the datasheets:
D669: hFE=[60, 320], fT=140 MHz, Pd=1/20 W, Ic=1.5 A
C2690: hFE=[60, 320], fT=155 MHz, Pd=1.2/20 W, Ic=1.2 A
D669: hFE=[60, 320], fT=140 MHz, Pd=1/20 W, Ic=1.5 A
C2690: hFE=[60, 320], fT=155 MHz, Pd=1.2/20 W, Ic=1.2 A
I wouldn’t fret over those differences. The reason to replace both is because if one was blown the other may have been stressed to the point where it might fail later.
That seems like good common sense. Now, things are coming along—recap is almost done—but I have another totally self inflicted problem. When I removed Q118 and Q124, which sit piggyback and share a heat sink, I did some harm to my PCB. It looks like the attached photo.
The shorter terminals across the top are ECB (left to right) of Q118, and the ones across the bottom are ECB (left to right) of Q124. The little hairs are from a q-tip. I've had no trouble removing other components, but somehow made a complete wreck of this set. Any pointers on getting these connections to work? I already attempted to solder Q118,B, and had no luck at all. Instead of trying the same thing over and over I'm soliciting advice.
The shorter terminals across the top are ECB (left to right) of Q118, and the ones across the bottom are ECB (left to right) of Q124. The little hairs are from a q-tip. I've had no trouble removing other components, but somehow made a complete wreck of this set. Any pointers on getting these connections to work? I already attempted to solder Q118,B, and had no luck at all. Instead of trying the same thing over and over I'm soliciting advice.
If the solder won't stick, first clean everything around there with 70% alcohol. You'll need to clean those bridged together pads upper right with solderwick (rosin dipped braid) or something to get the pads apart. The pads can't be shorted together, even if you have to scratch a crack between them. Then if rosin flux wire solder still won't stick, get a hotter iron. I had a ** of a time trying to use a western hemisphere 35 w iron. ****ese 40 w iron with variable temp from parts-express got hot enough to do the job. If you are in Europe or Asia I can't help you on what brand iron to buy.
Another trick on bad metal like steel is to use tin flux. Oatey, I bought it in a little tin can from the hardware store. Not acid flux, that causes later corrosion. You put it on before adding solder. The tin stuff does have to be washed off pretty thoroughly after use.
A real expensive solution is solder paste instead of solder wire with rosin core. Allows precise amount of solder to be applied cold, then heated quickly.
Another trick on bad metal like steel is to use tin flux. Oatey, I bought it in a little tin can from the hardware store. Not acid flux, that causes later corrosion. You put it on before adding solder. The tin stuff does have to be washed off pretty thoroughly after use.
A real expensive solution is solder paste instead of solder wire with rosin core. Allows precise amount of solder to be applied cold, then heated quickly.
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Easy fix. Scrape the lacquer off the good print the pins go to using something like a sharp tipped screwdriver and use a small piece of wire to wrap around the transistor pin and bridge the gap to the sound print. Don't try and bend the pins to reach, use a bit of wire such as a snipped off decent thickness resistor lead.
I use a Hakko FX880D which goes to 800F. I used that to build an entire amp a few years ago and it has done well for me. I have not tried solder paste, but tried different combinations of flux and solder and had no luck. In the end I went with short lengths of 24 gauge wire to make the nearest connections on the board. It's not pretty, and some melted insulation was inevitable, but all connections are testing okay and I can't find any oddities. Two of the terminals on Q124 did connect, and some adhesive to the heat sink tabs will give the whole thing a bit more structural hold.
I finally finished replacing most of the electrolytics and reassembling the outputs and drivers (with new outputs and drivers from bdent.com on the left channel). I removed a lot of transistors, ran diode tests on them, checked every connection after soldering, and checked for unintended connectivity as I went. According to the DMM everything looks okay.
When I turn on the amp with a 60W bulb in the limiter, the bulb flashes after a second, there's a simultaneous click, and that cycle repeats once a second. 1 second pause, light flash + click, one second pause, light flash + click, etc. On an analog meter, the +/-50V builds a bit between each flash, getting up to about 14V before seeming to level off. I haven't kept the power on long enough to see if it can go any higher.
I assume this is not good, and I don't dare turn the amp on without the limiter, since I don't trust the fuses alone.
One thing I'm wondering about are these 100 nF/63V capacitors on the +/-50V rails, if I'm naming those correctly. Here's a bit of schematic—I'm looking at C128 here.
I'm wondering if this surge of current when the amp is turned on has anything to do with those capacitors charging and is perhaps expected. They take a few minutes to discharge afterwards, and when I connectivity test across their terminals in-circuit with everything powered down, they test as shorted to ground while they charge (which takes about 15 seconds with a DMM), and that means everything coincident with them tests short to ground too. This question seems like pretty basic stuff and maybe I'm barking up the wrong tree with these, but they got my attention when I was probing for chassis connections with the power off. If the surge does have something to do with capacitors charging, I'd think it would not persist, and after a few of these flash/click cycles things would settle down, but they don't. I've kept the power on for a maximum of probably 15-30 seconds, and only powered up three or four times.
When I turn on the amp with a 60W bulb in the limiter, the bulb flashes after a second, there's a simultaneous click, and that cycle repeats once a second. 1 second pause, light flash + click, one second pause, light flash + click, etc. On an analog meter, the +/-50V builds a bit between each flash, getting up to about 14V before seeming to level off. I haven't kept the power on long enough to see if it can go any higher.
I assume this is not good, and I don't dare turn the amp on without the limiter, since I don't trust the fuses alone.
One thing I'm wondering about are these 100 nF/63V capacitors on the +/-50V rails, if I'm naming those correctly. Here's a bit of schematic—I'm looking at C128 here.
I'm wondering if this surge of current when the amp is turned on has anything to do with those capacitors charging and is perhaps expected. They take a few minutes to discharge afterwards, and when I connectivity test across their terminals in-circuit with everything powered down, they test as shorted to ground while they charge (which takes about 15 seconds with a DMM), and that means everything coincident with them tests short to ground too. This question seems like pretty basic stuff and maybe I'm barking up the wrong tree with these, but they got my attention when I was probing for chassis connections with the power off. If the surge does have something to do with capacitors charging, I'd think it would not persist, and after a few of these flash/click cycles things would settle down, but they don't. I've kept the power on for a maximum of probably 15-30 seconds, and only powered up three or four times.