A friend has asked me (!) to have a look at his NAD 7020i Receiver which had been working with no issues for around 15 years. Turned it on one day and nothing - no bang, flash or smoke, no burning smells, just nothing!
The NAD was DOA on arrival with no signs of life whatsoever, so I suspected a Power Supply problem? My main area of expertise is vintage valve radios and Dansettes with the occasional dalliance with more modern hi-fi equipment.
Got the covers off and carefully checked the circuit boards for any signs of over-heating or frazzled components but nothing obvious? Did some basic checks on the mains plug fuse, on/off switch, internal fuses F503-F504, diodes D505-D508 and the two big smoothing caps C508-C509 but no issues found?
Firstly I wanted to ensure that I had no shorts on the very large mains transformer and partly extricated that from the case - what a beast! The AC mains input to the transformer are the white and brown wires shown at the very top of this picture and I am measuring around 10 ohms across this - tends to imply that at least the Primary winding isn't shorted?
View attachment 984744
At this point things become a bit more challenging for me as I have the group of wires shown in this next picture which are RED, ORANGE, YELLOW and BLACK, but which is which? These also appear to by Primary inputs from the small board to the right of the transformer? I'm looking to measure their resistance but I've no idea about the colour coding?
View attachment 984745
There is an even larger group of wires (Secondaries?) on the opposite side of the transformer and again I have no idea where to start with those!
View attachment 984746
It may be that the mains transformer is ok but I just wanted to ensure there were no dead shorts before proceeding to turn it on and check the voltages in the Power Supply section.
Any help, assistance or clarification would be welcomed.
The NAD was DOA on arrival with no signs of life whatsoever, so I suspected a Power Supply problem? My main area of expertise is vintage valve radios and Dansettes with the occasional dalliance with more modern hi-fi equipment.
Got the covers off and carefully checked the circuit boards for any signs of over-heating or frazzled components but nothing obvious? Did some basic checks on the mains plug fuse, on/off switch, internal fuses F503-F504, diodes D505-D508 and the two big smoothing caps C508-C509 but no issues found?
Firstly I wanted to ensure that I had no shorts on the very large mains transformer and partly extricated that from the case - what a beast! The AC mains input to the transformer are the white and brown wires shown at the very top of this picture and I am measuring around 10 ohms across this - tends to imply that at least the Primary winding isn't shorted?
View attachment 984744
At this point things become a bit more challenging for me as I have the group of wires shown in this next picture which are RED, ORANGE, YELLOW and BLACK, but which is which? These also appear to by Primary inputs from the small board to the right of the transformer? I'm looking to measure their resistance but I've no idea about the colour coding?
View attachment 984745
There is an even larger group of wires (Secondaries?) on the opposite side of the transformer and again I have no idea where to start with those!
View attachment 984746
It may be that the mains transformer is ok but I just wanted to ensure there were no dead shorts before proceeding to turn it on and check the voltages in the Power Supply section.
Any help, assistance or clarification would be welcomed.
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Hi - you can find the service manual for free, on the hifiengine website (you need to register, but it's free and the site's 'clean'). Various transformer wire colours are shown, but I doubt they'll help much, and I'd be somewhat surprised if the transformer had suddenly given up the ghost with no indication.
NAD 7020 Stereo Receiver with AM/FM tuner Manual | HiFi Engine
Personally, I'd be inclined to just power the receiver up using a 'dim bulb tester' and start measuring. The 'DBT' is essentially just an incandescent bulb in series with the main power supply to the receiver (there's loads of detail on the net if needed), that'll provide protection in the case of a dead short, and should prevent further damage to the receiver. A 40W or 60W bulb should be fine for a 20W/Ch receiver.
NAD 7020 Stereo Receiver with AM/FM tuner Manual | HiFi Engine
Personally, I'd be inclined to just power the receiver up using a 'dim bulb tester' and start measuring. The 'DBT' is essentially just an incandescent bulb in series with the main power supply to the receiver (there's loads of detail on the net if needed), that'll provide protection in the case of a dead short, and should prevent further damage to the receiver. A 40W or 60W bulb should be fine for a 20W/Ch receiver.
NAD 7020i
Thanks Goldie99 for your quick and helpful response.
I've been tracing as many wires as I could from the transformer, both Primary and Secondary, and have to admit that there doesn't seem much wrong with the windings i.e. no dead shorts that I can detect. As you say, it seemed unlikely that the transformer would fail without some indication!
So I went ahead and powered up the unit using my DBT as you call it, in the vintage valve radio world we call it a Series Lamp Limiter. The receiver is completely dead although I'm measuring power coming in via the on/off switch through to the transformer? Through caution to the wind (!) and connected directly to the mains - still completely dead??
I'll try measuring some of the voltages associated with the Secondary wires running to the main board.
Thanks Goldie99 for your quick and helpful response.
I've been tracing as many wires as I could from the transformer, both Primary and Secondary, and have to admit that there doesn't seem much wrong with the windings i.e. no dead shorts that I can detect. As you say, it seemed unlikely that the transformer would fail without some indication!
So I went ahead and powered up the unit using my DBT as you call it, in the vintage valve radio world we call it a Series Lamp Limiter. The receiver is completely dead although I'm measuring power coming in via the on/off switch through to the transformer? Through caution to the wind (!) and connected directly to the mains - still completely dead??
I'll try measuring some of the voltages associated with the Secondary wires running to the main board.
I'd leave it on the limiter as long as you're fault finding - the worst it'll do is reduce all the voltages somewhat, the best it'll do is prevent further damage...
If there's no dead short, I'd expect the bulb to come on, and dim almost immediately, probably to a barely glowing filament depending upon the bulb wattage. A dead short, it'll come on, and stay on.
With the receiver powered on the DBT, you'll be able to get a good idea if the various power rails are all there (they'll be a couple of volts low probably, from the dbt). You can also 'pull' the 'Pre-Out / Main In' links at the back, and try the pre-amp & power amp stages separately (assuming the rail voltages are there).
One other area I'd be suspicious of is the start-up muting FET's (Q309 & Q310) - if they're not being turned ON, the pre-amp signals stay cut off from the main amps = silence.
If there's no dead short, I'd expect the bulb to come on, and dim almost immediately, probably to a barely glowing filament depending upon the bulb wattage. A dead short, it'll come on, and stay on.
With the receiver powered on the DBT, you'll be able to get a good idea if the various power rails are all there (they'll be a couple of volts low probably, from the dbt). You can also 'pull' the 'Pre-Out / Main In' links at the back, and try the pre-amp & power amp stages separately (assuming the rail voltages are there).
One other area I'd be suspicious of is the start-up muting FET's (Q309 & Q310) - if they're not being turned ON, the pre-amp signals stay cut off from the main amps = silence.
NAD 7020i
Thanks again Goldie99.
Forgot to mention that when I used my lamp limiter there was no evidence of a dead short? Indeed there was no sign of life at all from the incandescent bulb?
I have used my lamp limiter on countless occasions over the years so am well versed in what to look for. I made sure it was working properly by firing up one of my radios.
Not sure how to interpret the results from the lamp limiter so don't want to proceed any further until that is clarified?
Thanks again Goldie99.
Forgot to mention that when I used my lamp limiter there was no evidence of a dead short? Indeed there was no sign of life at all from the incandescent bulb?
I have used my lamp limiter on countless occasions over the years so am well versed in what to look for. I made sure it was working properly by firing up one of my radios.
Not sure how to interpret the results from the lamp limiter so don't want to proceed any further until that is clarified?
Plug in the AC cord and switch the power on.
Stop at the first place that has no AC voltage, in this order:
Check for AC line voltage before the power switch.
Check for AC line voltage after the power switch.
Then on the secondary wires, before the impedance switch.
Then just after the impedance switch.
Then just after both secondary fuses.
Stop at the first place that has no AC voltage, in this order:
Check for AC line voltage before the power switch.
Check for AC line voltage after the power switch.
Then on the secondary wires, before the impedance switch.
Then just after the impedance switch.
Then just after both secondary fuses.
Powered up the receiver again today but increasingly I have the feeling that either there is something obvious that I am missing or that there is something seriously wrong with it! I'm fully prepared for an outcome that may see the receiver end up in the electrical items skip at our local Recycling Centre.
Tried to take some measurements and that is where my doubts started to increase. Normally I would attach the black lead of my DMM to the chassis and use the red lead to take any readings. Not so here and I had to clip the black lead to the incoming neutral (blue wire) from the AC mains or it may be that I need to go back to the Electronics Remedial Class!
Anyway, with the DMM connected as per above I'd confirm again that the power switch is working and I have a healthy 242V AC mains supply coming into the receiver. Lots of strange connections and tracks on that small board to the right of the transformer but I do have the same voltage for the Primaries.
Following the various points in Ray's Post above I checked all the wires connected to the Impedance Switch and everything measures around 11.2V AC. Similar situation for the various Secondaries running to the main board as shown in this image where I'm getting about 11.7V AC:-
There are two fuses shown in the above image (F503 and F504) and I am measuring about 10.8V AC either side of these - so far so good? I think the next components to check are the diodes (D505-D508) which may be shorted but I'll need to remove them from the board to be sure?
Tried to take some measurements and that is where my doubts started to increase. Normally I would attach the black lead of my DMM to the chassis and use the red lead to take any readings. Not so here and I had to clip the black lead to the incoming neutral (blue wire) from the AC mains or it may be that I need to go back to the Electronics Remedial Class!
Anyway, with the DMM connected as per above I'd confirm again that the power switch is working and I have a healthy 242V AC mains supply coming into the receiver. Lots of strange connections and tracks on that small board to the right of the transformer but I do have the same voltage for the Primaries.
Following the various points in Ray's Post above I checked all the wires connected to the Impedance Switch and everything measures around 11.2V AC. Similar situation for the various Secondaries running to the main board as shown in this image where I'm getting about 11.7V AC:-
There are two fuses shown in the above image (F503 and F504) and I am measuring about 10.8V AC either side of these - so far so good? I think the next components to check are the diodes (D505-D508) which may be shorted but I'll need to remove them from the board to be sure?
When you're measuring the AC voltages on the impedance switch, or the fuses, where is the other DMM probe ?
I think you're voltages are ok, but you need to use the right 'reference' points.
For one secondary - put one probe on F503 and the other on F504 - that should give you the VAC on that secondary. Similarly, for the other secondary, one probe on F505 and the other on F506. I think they should be something like 22.5VAC, and 36.3VAC resp.
You can then just forget the diodes for now, and go straight to measuring the various DC supply voltages. I wouldn't remove any parts until it's absolutely essential.
For the DC voltages - the DMM neg. probe can now stay on the chassis, and you can check with the pos. probe at say;
the cathode of D505 or D508 (should be ca. +30V)
the anode of D506 or D507 (should be ca. -30V)
the junction of R511 / R513 (should be ca. +50V)
the junction of R512 / R514 (should be ca. -50V)
I think you're voltages are ok, but you need to use the right 'reference' points.
For one secondary - put one probe on F503 and the other on F504 - that should give you the VAC on that secondary. Similarly, for the other secondary, one probe on F505 and the other on F506. I think they should be something like 22.5VAC, and 36.3VAC resp.
You can then just forget the diodes for now, and go straight to measuring the various DC supply voltages. I wouldn't remove any parts until it's absolutely essential.
For the DC voltages - the DMM neg. probe can now stay on the chassis, and you can check with the pos. probe at say;
the cathode of D505 or D508 (should be ca. +30V)
the anode of D506 or D507 (should be ca. -30V)
the junction of R511 / R513 (should be ca. +50V)
the junction of R512 / R514 (should be ca. -50V)
Thanks Goldie99 - I've never really had a problem measuring voltages, until now! 
The Impedance Switch has three pairs of connections:-
LEFT - Yellow and White
MIDDLE - Orange and Brown
RIGHT - Orange and Blue
When measuring voltages I have the black DMM probe on the neutral AC mains input (blue wire) and the red DMM probe on one of the above connections. Maybe I should be measuring across these pairs of connections?
The Impedance Switch has three pairs of connections:-
LEFT - Yellow and White
MIDDLE - Orange and Brown
RIGHT - Orange and Blue
When measuring voltages I have the black DMM probe on the neutral AC mains input (blue wire) and the red DMM probe on one of the above connections. Maybe I should be measuring across these pairs of connections?
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No, you should indeed not be referencing everything to the mains AC neutral.
I can't tell from the manual what the wire colours on the impedance switch are, that's why I suggested to use the fuses - they're on the output side of the impedance switch, so there's no possible confusion. If the voltages there are good, then the impedance switch by deduction must be ok.
I can't tell from the manual what the wire colours on the impedance switch are, that's why I suggested to use the fuses - they're on the output side of the impedance switch, so there's no possible confusion. If the voltages there are good, then the impedance switch by deduction must be ok.
You're measuring AC voltage - there is no negative, so which probe goes to which makes no difference.
As I feared I'm getting no readings whatsoever when measuring AC across F503 to F504 or across F505 to F506!
You can safely assume that all my previous readings are suspect!
Looks like the transformer is 'goosed' or as we would say up here 'gubbed'. Can't think of any way to find a replacement or how it could be installed what with the multitude of wires! That small board next to the transformer with the AC mains input, on/off switch connections and Primary inputs still has me confused?
I've probably taken up enough of everyone's time but really appreciate the help and support. So let's just call it a day and consign this receiver to the skip?
You can safely assume that all my previous readings are suspect!
Looks like the transformer is 'goosed' or as we would say up here 'gubbed'. Can't think of any way to find a replacement or how it could be installed what with the multitude of wires! That small board next to the transformer with the AC mains input, on/off switch connections and Primary inputs still has me confused?
I've probably taken up enough of everyone's time but really appreciate the help and support. So let's just call it a day and consign this receiver to the skip?
Personally, I doubt very much if the transformer is dead - but, if you're determined to get shot of it, I'd suggest to just put it on the famous auction site. Someone will take it on for 'spares / repair' and you'd at least get a few notes back.
I'd offer to look at it, but will shortly be out of the country for the next couple of months so won't have time..
I'd offer to look at it, but will shortly be out of the country for the next couple of months so won't have time..
Intrigued to know what makes you think that it’s not the transformer?
I suppose I’m still perplexed by my inconsistent readings on the Primaries:-
1. DMM on the Ohms range shows around 10 Ohms.
2. DMM on the Diode/Continuity setting indicates a dead short.
While at the same time my Lamp Limiter showed no activity at all - no dead short or even the faintest of glows from the incandescent bulb.
I suppose I’m still perplexed by my inconsistent readings on the Primaries:-
1. DMM on the Ohms range shows around 10 Ohms.
2. DMM on the Diode/Continuity setting indicates a dead short.
While at the same time my Lamp Limiter showed no activity at all - no dead short or even the faintest of glows from the incandescent bulb.
Transformers don't go bad often. When they do, they usually stink. Exception, ones with built in fuse.
Solder joints & traces do go bad often. Possible in this case. Push in, crimped, or soldered connectors lose contact often.
AC side of transformer, one side of meter should go to AC in lowest side. Make sure to use AC volts of meter.
DC side of transformer (out), one side of meter should go to analog ground. Before rectifiers use AC volts of meter. After, DC volts of meter.
Do NOT measure any voltage over 25 with 2 hands. (as AC input) Voltage >25 through the heart can stop it. Use an alligator clip lead for black probe with same hand that you measure with. No metal on hands wrists or neck. High current through metal can burn your flesh to charcoal.
Test post 12 said fuses 503 504 are on the DC side of transformer. That is way too late to trace AC problem . You need to find a bit of metal that connects to the AC side of the input of the transformer. To find where the AC is getting lost. AC Goes into transformer, doesn't come out on F503/4 the transformer or the wiring or connections are bad.
If this has IEC socket for the AC cord, continuity can be lost in that socket. It is a complex RF filter.
You may have a power switch problem. Or wiring to the power switch.
There should be a fuse or circuit breaker between the AC input , the switch, and the transformer. My one page schematic doesn't show it.
DMM ohms of modern meters uses "safe" voltage for ohms, and somewhat higher voltage for diode scale. They lie a lot, I'm finding. All the hall effect sensor wires are open on all my bike hubmotors, my new klein DMM says. Probably they are 2 diode drops. I'm going to have to build an ohmmeter out of an analog meter to find the results I used to get with my 1985 Craftsman DMM that used 2 v source for both scales. That was stolen 9/20.
Damaged transformers stink because a turn has shorted through the paint to another turn. Shorted turns take all the energy, nothing comes out. Transformers are wound with painted wire. You can't detect the short with an ohmmeter. If I want to check a amp transformer, I don't remove it, I remove the output wires and connect to a big resistor. One equal to using up the rated load current, if I have one that size. Usually logs I use to test for power out rating on speaker are big enough.
The impedance switch S601-2 is shown on my schematic. It changes value of one of the DC voltages out of the Power supply to lower it for 4 ohms speakers. One of the complexities of this amp. Does not appear to be anywhere near the input side of the transformer.
Solder joints & traces do go bad often. Possible in this case. Push in, crimped, or soldered connectors lose contact often.
AC side of transformer, one side of meter should go to AC in lowest side. Make sure to use AC volts of meter.
DC side of transformer (out), one side of meter should go to analog ground. Before rectifiers use AC volts of meter. After, DC volts of meter.
Do NOT measure any voltage over 25 with 2 hands. (as AC input) Voltage >25 through the heart can stop it. Use an alligator clip lead for black probe with same hand that you measure with. No metal on hands wrists or neck. High current through metal can burn your flesh to charcoal.
Test post 12 said fuses 503 504 are on the DC side of transformer. That is way too late to trace AC problem . You need to find a bit of metal that connects to the AC side of the input of the transformer. To find where the AC is getting lost. AC Goes into transformer, doesn't come out on F503/4 the transformer or the wiring or connections are bad.
If this has IEC socket for the AC cord, continuity can be lost in that socket. It is a complex RF filter.
You may have a power switch problem. Or wiring to the power switch.
There should be a fuse or circuit breaker between the AC input , the switch, and the transformer. My one page schematic doesn't show it.
DMM ohms of modern meters uses "safe" voltage for ohms, and somewhat higher voltage for diode scale. They lie a lot, I'm finding. All the hall effect sensor wires are open on all my bike hubmotors, my new klein DMM says. Probably they are 2 diode drops. I'm going to have to build an ohmmeter out of an analog meter to find the results I used to get with my 1985 Craftsman DMM that used 2 v source for both scales. That was stolen 9/20.
Damaged transformers stink because a turn has shorted through the paint to another turn. Shorted turns take all the energy, nothing comes out. Transformers are wound with painted wire. You can't detect the short with an ohmmeter. If I want to check a amp transformer, I don't remove it, I remove the output wires and connect to a big resistor. One equal to using up the rated load current, if I have one that size. Usually logs I use to test for power out rating on speaker are big enough.
The impedance switch S601-2 is shown on my schematic. It changes value of one of the DC voltages out of the Power supply to lower it for 4 ohms speakers. One of the complexities of this amp. Does not appear to be anywhere near the input side of the transformer.
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