Hello everybody!
I bought recently a NAD 7020i (was aiming for the 3020 but none was available) that worked just fine until this evening when I tried to compare head-to-head two CD players (same disc in both playing the same song at the same time, switching between sources from the amplifier). All of the sudden some relay inside the amplifier started switching on and off a couple of times and then the sound became horribly distorted and muted. The preamplifier part is working correctly there was no smoke :lol: and nothing seems fried inside. Could I expect something as mundane as old, dried out caps? At a glance, there are two "dried puddles" around the filter (?) caps...
Any suggestions as to where to start looking are highly appreciated.
Thanks!
I bought recently a NAD 7020i (was aiming for the 3020 but none was available) that worked just fine until this evening when I tried to compare head-to-head two CD players (same disc in both playing the same song at the same time, switching between sources from the amplifier). All of the sudden some relay inside the amplifier started switching on and off a couple of times and then the sound became horribly distorted and muted. The preamplifier part is working correctly there was no smoke :lol: and nothing seems fried inside. Could I expect something as mundane as old, dried out caps? At a glance, there are two "dried puddles" around the filter (?) caps...
Any suggestions as to where to start looking are highly appreciated.
Thanks!
Attachments
Of course you can wait, anything is possible and they really look bad, change them to the same value in capacity and voltage.
Don't have the circuit? You can download the older model (7020 without I) from here and check first if those capacitors are the original values, the PSU should be the same for the 20 watt power of both models.
But, I am struck by that bare wire (bridge) that is blacked out by overheating ...... it could be precisely the consequence of the failure of the electrolytic capacitors ...... You need some knowledge of electronics and a tester If not, he would take it to a trusted repairman.
NAD 7020 Stereo Receiver with AM/FM tuner Manual | HiFi Engine
Don't have the circuit? You can download the older model (7020 without I) from here and check first if those capacitors are the original values, the PSU should be the same for the 20 watt power of both models.
But, I am struck by that bare wire (bridge) that is blacked out by overheating ...... it could be precisely the consequence of the failure of the electrolytic capacitors ...... You need some knowledge of electronics and a tester If not, he would take it to a trusted repairman.
NAD 7020 Stereo Receiver with AM/FM tuner Manual | HiFi Engine
Was that "damn" glue still being used in those years? 1988?
I remember that they were a typical failure of Sansuí, but it was with the models before that date ..... I think, lately my memory is playing tricks on me. I called my wife by the name of my first great love !
I think that was just a optical illusion. the wire looks fine, the PCB looks fine as well. no signs of overheating in that area. I guess I'll attempt a recap and see where things lead. I'll probably post impressions and questions as I go along. My multimeter can only measure capacitance up to a couple hundreds uF...
I'm totally new to electronics but I want to learn more. I don't thing replacing some caps require more than patience and an organized and methodical approach.
For the time being I'm knees deep (for my skills) in another amp)
There's also a little story as a prologue
Kenwood KA-3700
I'm totally new to electronics but I want to learn more. I don't thing replacing some caps require more than patience and an organized and methodical approach.
For the time being I'm knees deep (for my skills) in another amp
)There's also a little story as a prologue
Kenwood KA-3700
Rather than guess about the photo image or the history of glues, your description of the relay switching on/off a few times at least tells us that there is an electronic fault, probably a serious one. The relay will respond specifically to excess output current at the output of the amplifier, so indications are that one channel of the amplifier likely has a fault which is more than something you can fix with just a new capacitor or two. I wouldn't jump to conclusions yet, but this often means expensive transistor failure.
First, with no input signal, speakers disconnected and volume at minimum setting, does the amplifier come on normally by just switching power on? If so, can you measure any (even as little as 100 mV) steady DC voltage across either pair (i.e. the red and black terminals) of each channel?
Note that if the relay is faulty, you cannot have any audio or DC voltage to measure at the speaker terminals. You would then need to inspect the relay to find the relevant channel power amplifier connection to the relay and test for any DC voltage there. The black (ground) connection is common to both circuits so you can leave that in place.
NB: There is no conventional relay in this amp, the "relay" is a thermal magnetic switch at the ouput of each channel power amplifier, which opens when current heats up a bimetal strip. If you hear a "click" there is a change I'm not aware of.
First, with no input signal, speakers disconnected and volume at minimum setting, does the amplifier come on normally by just switching power on? If so, can you measure any (even as little as 100 mV) steady DC voltage across either pair (i.e. the red and black terminals) of each channel?
Note that if the relay is faulty, you cannot have any audio or DC voltage to measure at the speaker terminals. You would then need to inspect the relay to find the relevant channel power amplifier connection to the relay and test for any DC voltage there. The black (ground) connection is common to both circuits so you can leave that in place.
NB: There is no conventional relay in this amp, the "relay" is a thermal magnetic switch at the ouput of each channel power amplifier, which opens when current heats up a bimetal strip. If you hear a "click" there is a change I'm not aware of.
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Look at this, and if you search YT you will find many videos.
Testing on the electrolytes of a PSU usually comes down to two situations, the capacitor is either shorted or "open" (not charging and discharging in both cases).
And if it's at a loss or exploited, you don't need to measure anything, just look at it. They look good.
how to test electrolytic capacitor | xuanx electrolytic capacitors
The problem is at the output stage, not the power supply or its large capacitors, according to the evidence. There are 2 amplifiers and both are powered from the same supply. One still works so the PSU is not where the problem lies. Further, the protection devices (they are not relays and there is no related protection circuit) are thermal-magnetic switches, AKA circuit breakers TB1, TB2 in the 3020i schematic. These only open or close slowly when the output current flowing is more than an amp or so, continuously.
3020 and 7020 power amplifiers are virtually the same and share the same schematic design. These particular little circuit breakers are very old components now. They seldom fail but I believe there are still some copies available from China, if needed. First though, check that there is continuity between the breaker's terminals (unpowered). No amount of guesswork or parts replacement in the amplifier will change anything if its contacts are burnt or it won't close when the output current is normal.
The problem with such a simple device is that the contacts are just plated metal strip - only good for a small number of operations at maximum power (1.5A), before the contacts burn, since they move slowly and are only useful for occasional accidents with shorted speaker leads etc. However, never assume that any fault in an audio amplifier is restricted to its protection device. The relay or rather, circuit breaker, failed or opened for a high current reason - likely a shorted speaker lead or an extended period of high output power.
I have seen this problem with a 3020i model too - many years ago. Be aware that these old NAD models all had power transistors of an early early silicon transistor type (hometaxial) that was crucial to 3020 type amplifier sound quality but now virtually unobtainium. Modifications to the circuit are needed to adapt it to work with modern versions of 2N3055/MJ2955 but rather than take this risk when it became necessary, NAD just ceased production.
3020 and 7020 power amplifiers are virtually the same and share the same schematic design. These particular little circuit breakers are very old components now. They seldom fail but I believe there are still some copies available from China, if needed. First though, check that there is continuity between the breaker's terminals (unpowered). No amount of guesswork or parts replacement in the amplifier will change anything if its contacts are burnt or it won't close when the output current is normal.
The problem with such a simple device is that the contacts are just plated metal strip - only good for a small number of operations at maximum power (1.5A), before the contacts burn, since they move slowly and are only useful for occasional accidents with shorted speaker leads etc. However, never assume that any fault in an audio amplifier is restricted to its protection device. The relay or rather, circuit breaker, failed or opened for a high current reason - likely a shorted speaker lead or an extended period of high output power.
I have seen this problem with a 3020i model too - many years ago. Be aware that these old NAD models all had power transistors of an early early silicon transistor type (hometaxial) that was crucial to 3020 type amplifier sound quality but now virtually unobtainium. Modifications to the circuit are needed to adapt it to work with modern versions of 2N3055/MJ2955 but rather than take this risk when it became necessary, NAD just ceased production.
well, after a few days with the amp gutted on my dining table ) I found the following:
regarding the dead transistors, what kind of modifications should be made if replacing them with new 2N3055 and MJ2955 (by On Semiconductors that I can find on Mouser)? and what does actually happen if I simply replace them?
since no other parts show signs of damage (resistors, caps, diodes etc), is it possible that these transistors simply failed of old age?
thank you!
) I found the following:- both breakers show continuity - in-circuit, unpowered (they are marked E-401 and E-402 in the schematic)
- both output transistors of the right channel (2N3055 and MJ2955) are shorted across all terminals
- all other transistors read ok in-circuit. I've pulled a few that I wasn't really sure about and all measured ok off-circuit as well.
- all diodes in the amplifier stage read ok
- the preamplifier stage and radio work fine when preout and main in jumpers are removed and connected to another amplifier
- the voltage of the power supply feeding the amplifier is around +-24V when the amplifier section is disconnected
regarding the dead transistors, what kind of modifications should be made if replacing them with new 2N3055 and MJ2955 (by On Semiconductors that I can find on Mouser)? and what does actually happen if I simply replace them?
since no other parts show signs of damage (resistors, caps, diodes etc), is it possible that these transistors simply failed of old age?
thank you!
Output transistors are usually blown by shorted speaker wiring, as a 1/4" phone plug pulled partially out. Or a bad speaker, say a high value crossover capacitor allowing current to flow to both woofer & tweeter at too many frequencies. Or a shorted turn in one driver.
Or the mica washer under the TO3 transistors could have shorted across. These are 30 year parts, and should be replaced with the transistors. Silicon rubber insulators require no heat sink compound, but cost more than mica.
2n3055 and MJ2955 are rather minimal these days. I'd upgrade to MJ15015 MJ15016, or MJ21194 MJ21193. Note new TO3 packages come with smaller holes, and require 3 mm or #4 screws instead of #6 or 4 mm. Try to get the screws in the same box for the same E9 freight.
Double check the emitter resistors on the output transistors. They frequently blow also.
BTW, the modification Ian Finch was talking about necessary for faster modern output transistors, is a 10 ohm 1 to 3 watt resistor between the driver transistors and the bases of the output transistors. You may need to cut the leads and drill tiny holes in the boards to install these resistors. The legs of the new parts can be bent down to cross the old leads, and soldered to them. I use a general brand pin vise to drill these holes, usually a #41 or 45 (US number drills) to drill the hole. These tiny drills will not fit in 3/8" chucks of electric drill motors. The inductance of these resistors if wirewound or metal film is usually enough to keep the output transistors from oscillating in the RF band.
Or the mica washer under the TO3 transistors could have shorted across. These are 30 year parts, and should be replaced with the transistors. Silicon rubber insulators require no heat sink compound, but cost more than mica.
2n3055 and MJ2955 are rather minimal these days. I'd upgrade to MJ15015 MJ15016, or MJ21194 MJ21193. Note new TO3 packages come with smaller holes, and require 3 mm or #4 screws instead of #6 or 4 mm. Try to get the screws in the same box for the same E9 freight.
Double check the emitter resistors on the output transistors. They frequently blow also.
BTW, the modification Ian Finch was talking about necessary for faster modern output transistors, is a 10 ohm 1 to 3 watt resistor between the driver transistors and the bases of the output transistors. You may need to cut the leads and drill tiny holes in the boards to install these resistors. The legs of the new parts can be bent down to cross the old leads, and soldered to them. I use a general brand pin vise to drill these holes, usually a #41 or 45 (US number drills) to drill the hole. These tiny drills will not fit in 3/8" chucks of electric drill motors. The inductance of these resistors if wirewound or metal film is usually enough to keep the output transistors from oscillating in the RF band.
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well I'd exclude a shorted speaker wiring as I didn't touch the cables and it worked fine with them for a month prior to the incident. the speakers are working fine as well.
I'm either missing something or there's no emitter resistor on the output transistors in the schematic of the 7020i. the emitters are linked together...
I'm either missing something or there's no emitter resistor on the output transistors in the schematic of the 7020i. the emitters are linked together...
As posted already, the replacement output transistors need to be hometaxial type to match the sound quality of the original amplifier, as you can hear from the working channel. Sure, you can modify both output stages to suit newer epitaxialtransistors of almost any complementary epitaxial TO3 transistor type but that does not quite restore the original NAD 3020 sound quality. It may be good enough though.
As I understand the OP, you bought the amplifier to hear the original NAD sound quality. It's seems you were misled by the seller if the second channel doesn't work now but you must have shorted the output somehow if its burnt now. Anyway, I don't think you are going to be satisfied with modern 2N3055/MJ2955 substitutes as I found from 2 customers, when I replaced them after similar damage to one channel.
There are several threads about the problem because so many amplifiers were produced. Here's a good example with an explanation of the problem. Look particularly at post #2 here: NAD 3020 (& related NAD output stages)
Edit: I guess you now understand why there are no emitter resistors.
As I understand the OP, you bought the amplifier to hear the original NAD sound quality. It's seems you were misled by the seller if the second channel doesn't work now but you must have shorted the output somehow if its burnt now. Anyway, I don't think you are going to be satisfied with modern 2N3055/MJ2955 substitutes as I found from 2 customers, when I replaced them after similar damage to one channel.
There are several threads about the problem because so many amplifiers were produced. Here's a good example with an explanation of the problem. Look particularly at post #2 here: NAD 3020 (& related NAD output stages)
Edit: I guess you now understand why there are no emitter resistors.
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As that thread indicates, RCA quit making homotaxial transistors in the late 70's. others a few years later. The RCA NJ factory is now a county park. I think Comitern in Eastern Europe made the last of them. You are not going to find any. Of course, you can buy any number you want on aliexpress or ebay. See "my transistors are they fake" at the head of parts forum. I am trying to repair a replacement swell engine board where the factory tech installed 27 "transistors" of the same number as the print required. Only they aren't transistors, they are little black pieces of plastic with 3 wire legs.
And add the emmiter resistors. I use .47 ohm, most use .33.
If you want a particular sound, buy a graphic equalizer or a DSP. Frankly, the Dynaco ST120 with homotaxial transistors 1966-1972 was panned by the critics, sounded much worse than tube amps of the time. The "TIP3055" mod of 72, which replaced the homotaxial output transistors with epitaxial TI ones, produced a lot less harmonic distortion of high frequencies. As proved by a sim run by a diyaudio poster. I confirm, with an additional mod to keep idle bias current up to 20 ma, my ST120 sounds much better on high frequency sources like piano and tinkly bells than my tube amp, a ST70.
And add the emmiter resistors. I use .47 ohm, most use .33.
If you want a particular sound, buy a graphic equalizer or a DSP. Frankly, the Dynaco ST120 with homotaxial transistors 1966-1972 was panned by the critics, sounded much worse than tube amps of the time. The "TIP3055" mod of 72, which replaced the homotaxial output transistors with epitaxial TI ones, produced a lot less harmonic distortion of high frequencies. As proved by a sim run by a diyaudio poster. I confirm, with an additional mod to keep idle bias current up to 20 ma, my ST120 sounds much better on high frequency sources like piano and tinkly bells than my tube amp, a ST70.
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There is no emitter resistor.
The supply rails for the power amplifier section should be +/-28 volts if you measure those on the cases of the 2N3055 and MJ2955 in the power amplifier channel that is still working.
I had a friend who had a similar experience to you when switching digital sources like CD players only he was less lucky as this caused a two channel power IC module to fail.
You could have been loading one CD player output momentarily with the second CD player output when switching between these where the inter-reaction has caused an oscillation that has migrated into the failed channel of your power amplifier.
The service manual for this amplifier shows the output transistors as plain 2N3055 and MJ2955 if these were Hometaxial types that would be denoted by the suffix H which would also be stamped onto the transistor cases. If otherwise they are Epitaxial types which is a later and cheaper manufacturing process with superior high frequency characteristics.
I have no problem with your buying On Semiconductor replacements as you propose.
The supply rails for the power amplifier section should be +/-28 volts if you measure those on the cases of the 2N3055 and MJ2955 in the power amplifier channel that is still working.
I had a friend who had a similar experience to you when switching digital sources like CD players only he was less lucky as this caused a two channel power IC module to fail.
You could have been loading one CD player output momentarily with the second CD player output when switching between these where the inter-reaction has caused an oscillation that has migrated into the failed channel of your power amplifier.
The service manual for this amplifier shows the output transistors as plain 2N3055 and MJ2955 if these were Hometaxial types that would be denoted by the suffix H which would also be stamped onto the transistor cases. If otherwise they are Epitaxial types which is a later and cheaper manufacturing process with superior high frequency characteristics.
I have no problem with your buying On Semiconductor replacements as you propose.
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indiana, might your "little black pieces of plastic with 3 legs" just be a different type of device, such as a "digital" or pre-biased transistor for switching purposes? These add a couple of resistors on the chip to ensure precise switching action without the need for additional components. The major semi brands all now churn out several grades and package types of these by the truckload.
While all components on one channel seem to read ok (most readings in-circuit), the sound is extremely distorted and muted. Hopefully it has something to do with the present imbalance of the two channels, otherwise there's something wrong there that I haven't found too.
But I just checked the service manual for the original 3020 and the parts list specifies MT2955 and 2N3055 for the output transistors (no H suffix). Is there any other way to tell on from the other?
While it's true I wanted (and still want - but now I see things with a different set of eyes) a 3020 to experience the 'legendary' sound, I do admit my ears are not the sharpest out there, and, except a true A/B test, I wouldn't probably tell one from the other.
Right now I'll just consider it a project from which I can also learn a thing or two (since I'm totally new to electronics). Of course, since my aim is to make it sing again, I might as well do it as good as I can (albeit in a 'monkey sees, monkey does' manner).
Thank you!
But I just checked the service manual for the original 3020 and the parts list specifies MT2955 and 2N3055 for the output transistors (no H suffix). Is there any other way to tell on from the other?
While it's true I wanted (and still want - but now I see things with a different set of eyes) a 3020 to experience the 'legendary' sound, I do admit my ears are not the sharpest out there, and, except a true A/B test, I wouldn't probably tell one from the other.
Right now I'll just consider it a project from which I can also learn a thing or two (since I'm totally new to electronics). Of course, since my aim is to make it sing again, I might as well do it as good as I can (albeit in a 'monkey sees, monkey does' manner).
Thank you!
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