As of now, nothing still has happened to it. It's sitting in my closet, unused.
My tech couldn't get any of the transistors suggested, so I took it back from him. I decided not to chance it further. A repairer from another city, who claims experience with Nakamichi, said he could fix it. I would have to box it up and ship it out to someone I don't know.
I found another 620 to drive my second set of Wharfedale E 70's. It works fine. So I won't need the other one, or get into any trouble, unless I find a third set of E'70s. I'll probably have to get a divorce if I do.
Regards.
My tech couldn't get any of the transistors suggested, so I took it back from him. I decided not to chance it further. A repairer from another city, who claims experience with Nakamichi, said he could fix it. I would have to box it up and ship it out to someone I don't know.
I found another 620 to drive my second set of Wharfedale E 70's. It works fine. So I won't need the other one, or get into any trouble, unless I find a third set of E'70s. I'll probably have to get a divorce if I do.
Regards.
Hi Dr. Mac,
Well, what you have posted is both very sad and completely ridiculous! I can't believe you can't find a technician capable of repairing your amplifier! It is similar to Marantz 240, 250, 170, 300DC and related models. Therefore, the design is solid, and repairable!
I was one of the first warranty depots opened back when Nakamichi went direct in Canada. My early experiences were fixing many products that were "bouncers" from other shops. The same theme applied with most brands (Marantz, Carver, Teac and many others, even Yamaha!), with the exception of Revox and McIntosh. Those companies demanded higher levels of competence from their service technicians in the warranty network. Over many years, I had taken note of what sectors of service companies were the best, and what were the worst for audio service. Very simply put, the only service people I would trust are those who have a focus on audio service that included distributor training sessions. That service has gone by the wayside and there are almost no places where service people can obtain advanced training today. That means you are stuck with us old folks. I'm 52 now, and one of the younger people who has had extensive training in audio.
So now you are in a situation where an amplifier is unusable simply because your technician hasn't had enough training. Repairing this amp is pretty simple if you understand how it works. Also, a good understanding of parts is required. Sticking with this one technician is a pretty half-hearted effort. At least you have found another amp, but what to do with your closet case? It's too nice an amp to have someone learn on. I don't know how your current amp sounds, but it should be extremely clear and clean. If not, it needs service.
I don't know where you are located, but there must be a good technician who knows what he / she is doing close to you. There must be someone! Remember, an audio technician. Your technician is mostly pro-audio, which we did a fair amount of also. There is a major difference (mind set) between high quality audio and pro musical service is done. They get things back on the road running as quickly and inexpensively as possible. The high end audio technician returns the unit to a level of performance beyond the specs, if the design allows it. They will match parts and replace everything that needs replacement - even if the part looks like it survived. So a Pro audio tech may not be the direction you want to go in. Remember that really good audio techs may also do pro audio - but that pays his rent, audio will be his passion. Avoid the younger guys who design their own superior amps BTW. They have a skewed viewpoint and too much pride often enough. All this preceding info comes from years of observation. So, either get that amp into the hands of someone who knows what they are doing, or into the hands of another owner who will.
-Chris
Well, what you have posted is both very sad and completely ridiculous! I can't believe you can't find a technician capable of repairing your amplifier! It is similar to Marantz 240, 250, 170, 300DC and related models. Therefore, the design is solid, and repairable!
I was one of the first warranty depots opened back when Nakamichi went direct in Canada. My early experiences were fixing many products that were "bouncers" from other shops. The same theme applied with most brands (Marantz, Carver, Teac and many others, even Yamaha!), with the exception of Revox and McIntosh. Those companies demanded higher levels of competence from their service technicians in the warranty network. Over many years, I had taken note of what sectors of service companies were the best, and what were the worst for audio service. Very simply put, the only service people I would trust are those who have a focus on audio service that included distributor training sessions. That service has gone by the wayside and there are almost no places where service people can obtain advanced training today. That means you are stuck with us old folks. I'm 52 now, and one of the younger people who has had extensive training in audio.
So now you are in a situation where an amplifier is unusable simply because your technician hasn't had enough training. Repairing this amp is pretty simple if you understand how it works. Also, a good understanding of parts is required. Sticking with this one technician is a pretty half-hearted effort. At least you have found another amp, but what to do with your closet case? It's too nice an amp to have someone learn on. I don't know how your current amp sounds, but it should be extremely clear and clean. If not, it needs service.
I don't know where you are located, but there must be a good technician who knows what he / she is doing close to you. There must be someone! Remember, an audio technician. Your technician is mostly pro-audio, which we did a fair amount of also. There is a major difference (mind set) between high quality audio and pro musical service is done. They get things back on the road running as quickly and inexpensively as possible. The high end audio technician returns the unit to a level of performance beyond the specs, if the design allows it. They will match parts and replace everything that needs replacement - even if the part looks like it survived. So a Pro audio tech may not be the direction you want to go in. Remember that really good audio techs may also do pro audio - but that pays his rent, audio will be his passion. Avoid the younger guys who design their own superior amps BTW. They have a skewed viewpoint and too much pride often enough. All this preceding info comes from years of observation. So, either get that amp into the hands of someone who knows what they are doing, or into the hands of another owner who will.
-Chris
Hi Dr. Mac,
Just another note. I missed an earlier post by nigelwright7557 in post #6.
This works in most situations. I'll provide more information on this for you.
You need a THD meter, first and foremost. Every single good audio tech has one on his bench, so the method goes without saying. You do as nigelwright7557 says, but you monitor the output with with a dummy load and a distortion analyzer. Take the output of the distortion analyzer and monitor that with an oscilloscope. The crossover notches are plain as day, very clear to see. It goes without saying that the source of the sine wave is much cleaner than the equipment you are testing. You will find it easier at frequencies between 10KHz and 20 KHz . A 4 ohm load makes this easier as well, but check it at 8 ohms to make sure.
There is no adjustment controls for this design, so you are performing a check only. As nigelwright mentioned, this adjustment procedure will result in an amplifier running at lower bias than the service manuals will indicate. The only times it will not work is when there is no minimum to find (high bias increases THD), or the design is so poor that distortion is a feature of the design - you're stuck with poor performance without a redesign. An amplifier with a fault will also show excessive distortion many times.
nigelwright is dead right on this.
-Chris
Just another note. I missed an earlier post by nigelwright7557 in post #6.
This works in most situations. I'll provide more information on this for you.
You need a THD meter, first and foremost. Every single good audio tech has one on his bench, so the method goes without saying. You do as nigelwright7557 says, but you monitor the output with with a dummy load and a distortion analyzer. Take the output of the distortion analyzer and monitor that with an oscilloscope. The crossover notches are plain as day, very clear to see. It goes without saying that the source of the sine wave is much cleaner than the equipment you are testing. You will find it easier at frequencies between 10KHz and 20 KHz . A 4 ohm load makes this easier as well, but check it at 8 ohms to make sure.
There is no adjustment controls for this design, so you are performing a check only. As nigelwright mentioned, this adjustment procedure will result in an amplifier running at lower bias than the service manuals will indicate. The only times it will not work is when there is no minimum to find (high bias increases THD), or the design is so poor that distortion is a feature of the design - you're stuck with poor performance without a redesign. An amplifier with a fault will also show excessive distortion many times.
nigelwright is dead right on this.
-Chris
Sad, yes, but true.
I live in the midwest (USA). When I moved, here, from the south, some 33 years, ago, there were 3, maybe 4, high end audio stores in this city. Only one carried Nakamichi equipment. I bought my original equipment in the town I moved from, one much smaller in population, yet one that had quite a few more high end stores.
The high end stores went the way of the dinosaur, beginning in the mid-80's due to the influx of the big-box chain department stores. Given the brands they were carrying back then, and now, the interest in visual equipment, computers, and compact (re: ipods, etc.) types of media, technicians that are capable of repairing the unit are few and far between. I did find another one in the area who wanted to charge more than the asking price for the unit when Nakamichi was selling them. And as I stated, the others I found were all out of town and required mail-ordering.
It's a sign of the times. Technicians of today are trained on what's selling. A lot of those units are so inexpensive that its cheaper to buy a new one than get it repaired. The result is that those in the business learned about repair of the systems that will keep food on the table and older quality units get pushed aside because of a lack of skills in repairing them.
It is what it is.
BTW: you are correct - the tech I was referring to is an pro audio tech. But he is the second tech that's work on the unit. The first tech was audio electronics. He had worked on other components of my systems, and done so successfully. The first tech put in the bad transistors that made things worse; he's why I tried someone else. To the credit of my pro tech, at least he tried to research the problem, looked for good parts, and didn't rush hastily into a repair to would have made things worse; that's also why I came on this site looking for help. The unit set on his shelf for a long, long time while he did that. I finally told him that I would seek another solution.
Regards.
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Hi Dr. Mac,
Man, I wish you lived up here. I could have that unit running just fine. I am not the only tech capable of that. I get frustrated when I see stories like yours.
Your first tech is buying from the wrong places, and is not aware of how to tell a fake from the real thing. When in doubt, I tend to reject the parts. If he is to survive in the service world (no matter what he fixes), he's going to have to buy from good suppliers. Their prices are naturally higher, but who cares? They are less expensive in the long run (plus the medium run - and even in the short run!). Your second tech has my respect. He's thinking. I like that. He may simply be cautious and unfamiliar with this circuit configuration. There will be some experimentation with transistor types, but it is repairable. He should have several transistor types on hand already though,so it's not an order in type deal with him.
You may very well be further ahead by allowing him to work on your amp when he is slow. Curiosity may get the better of him as this is a chance to learn. As I mentioned, this design is related to some Marantz products and probably many others. Looking over those schematics may give him some ideas. So, how's that for confidence? I'm suggesting you allow your pro tech to play with it. He must use a variac or variable bipolar power supply for testing. This allows the current to be limited to reduce / eliminate further damage. No one likes to clean up after another tech either, so I can understand his reluctance too. This tech should have an AC line variac on his bench.
-Chris
Man, I wish you lived up here. I could have that unit running just fine. I am not the only tech capable of that. I get frustrated when I see stories like yours.
Your first tech is buying from the wrong places, and is not aware of how to tell a fake from the real thing. When in doubt, I tend to reject the parts. If he is to survive in the service world (no matter what he fixes), he's going to have to buy from good suppliers. Their prices are naturally higher, but who cares? They are less expensive in the long run (plus the medium run - and even in the short run!). Your second tech has my respect. He's thinking. I like that. He may simply be cautious and unfamiliar with this circuit configuration. There will be some experimentation with transistor types, but it is repairable. He should have several transistor types on hand already though,so it's not an order in type deal with him.
You may very well be further ahead by allowing him to work on your amp when he is slow. Curiosity may get the better of him as this is a chance to learn. As I mentioned, this design is related to some Marantz products and probably many others. Looking over those schematics may give him some ideas. So, how's that for confidence? I'm suggesting you allow your pro tech to play with it. He must use a variac or variable bipolar power supply for testing. This allows the current to be limited to reduce / eliminate further damage. No one likes to clean up after another tech either, so I can understand his reluctance too. This tech should have an AC line variac on his bench.
-Chris
with a bit of here and a bit of there still we manage properly Chris .... thank God and diyaudio for that !!
poor nak ... if i had it over here it will be just a walk in the park ....
hope you and family are doing well
Best regards
sakis
420 and 620 similar?
I once fixed a problem in a Nakamichi 420 power amplifier.
I don't know if the 620 and 420 are similar in this respect, but:
the 420 uses anodized aluminium isolators between the power transistors and the cooling fins. When one of these develops a short circuit to the cooling fin, the power transistors of this channel will blow.
It was not always possible to measure a short with an Ohm meter. Problem happened after some time with amplifier working, not immediately.
I replaced the original isolators with mica ones, and no more problems!
I once fixed a problem in a Nakamichi 420 power amplifier.
I don't know if the 620 and 420 are similar in this respect, but:
the 420 uses anodized aluminium isolators between the power transistors and the cooling fins. When one of these develops a short circuit to the cooling fin, the power transistors of this channel will blow.
It was not always possible to measure a short with an Ohm meter. Problem happened after some time with amplifier working, not immediately.
I replaced the original isolators with mica ones, and no more problems!
Hi 968driver,
I haven't seen the schematic for the 420, so I can't say. However, those insulators sound familiar for other products. They could be anodized Al or similar. Replacing with mica is the route I would have taken as well. This must have been fun to troubleshoot. I've had similar when manufacturers have used only the anodizing on the heat sink for insulation. I use mica whenever I see that these days.
Were they mounted dry by chance?
The 620 used mica insulators, so the fault wouldn't have the same cause.
-Chris
I haven't seen the schematic for the 420, so I can't say. However, those insulators sound familiar for other products. They could be anodized Al or similar. Replacing with mica is the route I would have taken as well. This must have been fun to troubleshoot. I've had similar when manufacturers have used only the anodizing on the heat sink for insulation. I use mica whenever I see that these days.
Were they mounted dry by chance?
The 620 used mica insulators, so the fault wouldn't have the same cause.
-Chris
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