#86 - NAD 3225PE stereo amplifier repaired and tested

hello welcome to the workshop in this episode I have nad stereo amplifier model 3225 PE repair I like an indie equipment I own a couple of other units right now I use a more modern integrated amplifier c350 here everyday and I also have a receiver model 701 and I owned a couple of other units before to my taste they offer quite a good balance between sound quality and price this model is quite old probably around 1990 someone tossed it out and apparently this is an unfinished repair attempt the cover is loose and also the bottom plate is loose and the screws are missing and there are some other signs of an unfinished repair attempt inside let's have a look this is an old-school design no integrated circuits at all transistors only and unfortunately in this old design does not have much protection against shorting the output there are fuses here in PTC's but they are not fast enough for such a case so shorting the output is almost guaranteed to kill the output stage transistors and probably will cause some other cascading failures like transistors in the previous stage and some other parts around them resistors and so on maybe this is what happened here maybe and there was some other problem but I see that someone tried to fix the left channel this part of the board is power amplifier this is the left channel and this is the right channel almost identical sections like mirror images of each other and these two transistors in the output stage of the right channel seem to be original date codes 89 42 and 89 39 so they must have been made in 1989 but these two devices in the left channel are much newer date code here is 0 944 and 1001 here and they are not even the same part this is a classic P R 2 n 30 55 and MJ 2955 and PN and PN P and these are different MJ 15 0 25 and 15 0 24 and I don't quite like this I would much prefer having the left and the right channels identical and these transistors probably have quite different characteristics from the originals and it's not even clear that this configuration worked because I see a missing transistor here in the left channel it is present in the right channel and also I check the fuses here are four fuses and two for each channel and these two for the left channel alone and also I see a mission jumper here like this one for the left channel this is between the preamplifier and the power amplifier and I also see that three resistors in the left channel are not original so it's not clear if this configuration worked maybe it worked for a while then it failed and someone tried fixing it and did not finish or maybe it never worked and looking at the other side I would guess that this repair never worked look at these legs of the replacement resistors in the left channel let's use this ruler this is how close the bottom plate would be so I can hardly imagine that someone would leave these legs that lon if the intention was to screw the bottom plate back in place let's do some checking they would mount here this device is NPN so we put positive on the base this is the base and this is the emitter we have diode here this way and collector we also have a diode looks fine so far then we should see nothing the other way well but we do see something perhaps that's because it is in circuit and collector something is charging this way and collector to emitter something is charging here as well so it's not quite clear because it's in circuit but works sensible this is the PNP device so we put a negative on the base meter looks like a diode and collector here is a short let's try the other way yeah same thing of course so we have a short base to collector here and what about collector to emitter something is charging here so base to collector is shorted on the PNP device and it might be because of the previous stage it's not clear perhaps it's time to start desoldering things I want to replace the power devices anyway because they don't match the right channel i found the service manual with schematic which certainly helps a lot and on page 10 we have this piece of the diagram for this model with power amplifiers let's zoom in and there you are this is duet channel and this is the right channel not to complicate it just 9 times if there's per channel this is the output stage and these boxes around the transistors mean that they are mounted on a heat sink and this transistor is also mounted on a heat sink and we've seen a short here and between the base and the collector of this PNP device and you see it can be easily because of this transistor in the previous stage and I see that this resistor is not original because it is larger than in the right channel and it has a burn mark on it but still measures fine and I also see that these two resistors are not original they are also larger than in the right channel and this transistor is missing altogether so I'm thinking that perhaps would be a good idea to replace all these transistors just in case I need to order this type anyway and this one is of the same type so I might as well replace this one just in case these two are BD 1 3 9 must be cheap and easy to get this one is probably dead anyway and even if not it might be damaged and this one might be damaged too you never know so I am tempted to replace all of them and of course I need to check diodes and resistors and electrolytic capacitors as well here they are a transistors from the left channel ix is missing and perhaps you can see that these the big transistors have insulating mica pads stuck to them so I need to carefully peel them off they are fragile and this transistor also has a mica pad between it and the heatsink but that pad got stuck to the heatsink all right I managed to carefully peel off the mica pads and I wiped off the thermal compound from the transistors let's test them these three are PNP devices the rest are NPN let's start from the output transistors now let's see if I can hold it like so this must be the base so we should see diodes here and here yes nothing the other way around yes and nothing collector to emitter this one looks okay how about this one PNP device so we should see diodes here and yes nothing the other way around and collector to emitter nothing so this looks fine as well now the previous stage this one is NPN and the base is here we should see die out here and we don't and die out here no and the other way around nothing and collector to emitter nothing so this one looks completely open this is P and P so we should see the use here nothing and here nothing and the other way around also nothing and collector to emitter all there you go this is our short between the base and collector of the output transistor we have seen now this guy and PN and bases here so we should see diode here yes and here and the other way around should be nothing but we see something and collector to emitter does not look healthy at all so this one is dead this is of the same type so we should see diodes here and here yes and the other way around nothing collector to emitter nothing so this looks fine this one base must be in the middle and PN so we should see dark here yes and diode here this does not look filthy the other way around nothing and nothing fine and collector to emitter nothing so this one looks bad and this one is PNP we should see the huge here and here yes the other way around nothing which is okay and collector to emitter nothing so this one works fine this is not a conclusive test but at least on the first glance these two work fine the output transistors look fine and these four are bad and let's look at the resistors this is a 180 ohms resistor that is different from the right channel and it looks a bit burnt and these two also are different from the right channel and it's not even clear to me on this 33 ohm resistor is this burn mark or something looks a bit strange and also I see that this 68 ohm resistor is different from the right channel and here the you are in the right channel this one is 180 ohm resistor these two are 33 Omen 220 and this one is 68 home resistor looks like a metal film precision one I found that some capacitors are not very good these two big ones look fine but I found these two thousand microfarads 10 volts capacitors I disordered one from the left channel and one from the right channel and they are not very good let's have a look one kilohertz and let's set ESR is the second parameter so this one is 194 micro farad's and ESR is about point 4 ohms and this one is slightly better 266 micro farad's and he s are slightly lower point to 8 ohms but for comparison here is a good one 940 micro farad's and the ESR is about 10 times lower this is how it should be I ended up laplacian quite a few of these capacitors all over this unit including preamplifier some of them are not too bad but I replaced them anyway even if I had just a slight suspicion and I also replaced these suspicious resistors now let's look at how these transistors should be mounted here should be those mica pads we have seen before and the smaller pad for a transistor like this is still in place so these mica pads should insulate the cases of the transistors from the heatsink however the case is the collector and there is no pin so the only connection to the PCB for the collector is through these 2 bolts so the bolt should not be insulated from the case but look at this Hardware this fifth bolt is for that smaller transistor with the plastic keys so no insulation needed there at all but look at this for first of all they are not consistent these two look shorter and this shake proof washer looks larger than the rest so it is probably not original and these two bolts perhaps are not original and most importantly look at these two insulating plastic washers or bushings I don't think they should be here at all I think one such boat was used for device so there was still a connection for the second bolt not ideal but maybe fine this bolt does not have any insulation at all and this one has this small plastic sleeve and I believe this is how it should be I don't want to disturb this mounting of the original transistors to check but I'm quite sure this is the right way this is just to insulate the bolt from the heatsink to prevent it from touching the heatsink the hole here is much larger than the bolt and even slightly larger than the hole in the PCB but they had displaced a little bit as there is still some chance of touching may be small but anyway this is the right way and how did this happen I wonder perhaps someone took this thing apart and then put the project aside for a while maybe for some months or years and the hardware was lost and the right way was forgotten or maybe something else happened go figure the parts have arrived some from digi-key some from LCSC and as usually I bought more than necessary for example these are bg1 39 - 10 transistors I bought 10 just in case they are cheap so it's time to rebuild the amplifier I'm going to use pieces of this heat shrink as machine sleeves this size is just right I wiped off the old thermal compound and let's use some fresh one the left channel is fully rebuilt let's give it a go here is my test setup I'm going to use my key sight scope with built-in signal generator I have this BNC to RCA adapter this we can connect to the input of the power amplifier there is still no link between preamp and the power amp so I am bypassing the volume control here so I need to control the signal level on the scope and let's see we can put something in and get something out of this no load yet because I believe we need to check the DC zero-point and perhaps adjusted before connecting any load everything is ready and the amplifier is off for now and the output of the signal generator is off I set the output level 200 milli volts peak-to-peak 1 kilohertz so let's turn the amplifier on and let's turn the generator on and there you go we have 1.8 volts peak-to-peak output so let's bring the input up now we are at 300 millivolts peak to peak and now about 500 millivolts peak to peak input and the output is seven point seven or eight volts peak to peak this looks great and now the same check on the right channel just to make sure the right channel is working and for comparison I removed the link same a 500 millivolts peak to peak input and about the same seven point nine or eight volts peak to peak output now let's check the DC zero point I turned the amplifier on a few minutes ago so the voltage stabilized the negative hundred millivolts on the left channel and I think it's too high let's check the right channel and I don't have to reconnect the negative I'd checked the schematic in this amplifier it is the same ground so here we have around 13 millivolts so we need to adjust the DC offset in the left channel here is the adjustment procedure in the service manual there are 4 adjustments here - in each Channel this one is 0.4 the left channel they call it Center voltage this one is idling current and this is the procedure speaker impedance which should be in a tone position which it is no load connected then measure across the output of the channel and adjust to be within plus minus 30 millivolts and for idling current we need to remove solder shorts across two resistors one in each channel they are one ohm resistors in output transistors collector circuit and we need to measure across these resistors and adjust to be within 26 to 30 millivolts and then we need to remember to short those resistors again I adjusted the zero point in both channels this is the left channel and I switch to millivolt so we are below a millivolt and it drifts up and down a little bit and it doesn't have to be this precise just within plus and minus 30 millivolts and I adjusted the right channel as well we had about 12 millivolts there now it's close to millivolt and a half let's say and now do the idling current here are the solder shorts we need to remove this one is in the right channel and this one is in the left channel here is the idling current in the left channel it is too high let's see which way I need to turn no the other way it should be within the range from 26 to 30 let's see around 28 something like that and this is the idling current in the right channel it is just a tiny bit above 30 millivolts perhaps I'll bring it down slightly to the middle of the range but I don't really have to and I didn't have to touch the zero point in the right channel as well it was just fine I found the right screws and put the amplifier back together now let's take care of the missing link for the left channel I have this broken cable I keep in a junk bin precisely for such cases when I need to build something custom so I'm thinking about taking these connectors apart and improvising something you then you link sticks out a little more but it shouldn't be a problem there was banana terminal stick out even more than that now let's give it a full test with some load and all the way through the preamplifier I have this power resistor for testing it is 50 Watts 8 ohms register I am afraid that the marking on it is too faint and probably will not show on camera and let's do a quick calculation how far we can push things I'm using Ohm's law here very simple if we multiply both sides by voltage we get power on this side in what here it is and the solving for voltage will get this formula now we plug in our numbers the amplifier is rated continuous power into 8 ohms is 25 watts and up to 40 watts clipping so let's use this continuous power that gives us 14 volts and that is DC or AC equivalent would be RMS and I could ask the oscilloscope to measure voltage RMS but we can easily convert to voltage peak to peak first of all we double it because it goes both ways positive and negative and this factor of square root of 2 gives us a voltage peak to peak and that is about 40 volts but let's not go all the way to the limit let's go to let's say 30 volts peak to peak that should be good enough for a test I don't want things to get too hot test setup is ready a piece of plywood here just in case I don't want to burn the bench I said 250 millivolts peak to peak output from the signal generator I'm using CG left channel the amplifier is on but the volume is at zero let's crank it up there you go let's go to approximately 30 volts peak-to-peak and here it is no problem and let's see the volume knob is almost exactly at 3 o'clock and now the same test on the right channel this is about 30 volts peak-to-peak and the volume knob is approximately at the same position great no problem at all the test would be incomplete if we don't play some music I have these good old small Sony speakers they sound great they have 6 ohms impedance and can handle up to 50 watts let's try some copyright free music from YouTube [Music] [Music] there you go nad 3225 PE repaired and tested thanks for watching bye

Channel: FeedbackLoop

Views: 22,649

Rating: 4.9297366 out of 5

Keywords: diy, electronics, repair

Id: 1I7wv7wJ7zs

Length: 31min 2sec (1862 seconds)

Published: Sun Mar 29 2020