Nakamichi 620 Rebiasing Help Needed

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The right channel on a Nakamichi 620 amp I bought off EBay keeps burning out (it was advertised as such when I purchased it). My repair technician replaced the transistors once, it ran find for a few days, then burned out again. He's not an authorized Nakamichi serviceman, but he's been reliable over the years I've known him.

It's been suggested that the amp needs rebiasing when repairs are made to it. Hence, the help I need:

Can anyone provide me instructions for doing this? I need the bias settings, calibration method, where to measure, instruments needed, i.e., generally, how to go about getting this done: written instructions that I can pass on to my technican. I have a 620 service manual with schematics for all of the modules, including the output module, that I can provide if that helps. It does mention biasing in terms of measuring distortion levels but that's about it.

Second request is for replacement parts. Does anyone know of suppliers who carry replacement parts for the Nak 620? I've contacted Nakamichi directly and they have no parts suppliers or authorized dealers here in the US (at least, not for the 620).

Thanks, and

Regards.
 
If we are to assume that you would set the bias in a more conventional way, such as deciding on a reasonable idle current and measuring that current across emitter resistors, then any advice would hinge on the value of those emitter resistors.

A glance at the topology of the output stage would be very telling. Scan & post it.

Edit: Found a schematic online. Looks like 0.22 ohm resistors, with a 4.7 ohm in parallel (what for?)

Anyway, I have no idea what Nakamichi thought was a good bias current, but if you want to stop blowing it up, connect the positive lead of your meter to the output NPN emitter, and the negative lead to the emittter of the PNP output. Adjust the bias so the meter reads about 21mV, which will result in about 50mA of current flow.

I also noticed that the amp supposedly uses 2SD555's and 2SB600 output transistors. If your tech is buying transistors with these numbers, there's about a 99% chance that he's buying crap Chinese counterfeit parts. Buy some new On-Semi MJ21193 and MJ21194 transistors from a reliable source, such as Digikey or Mouser, and quit trying to source out-of-production Japanese parts. :smash:
 
IF YOUR TECH GUY

DOESNT UNDERSTAND .....

terms like bias and offset (without a schematic ) and also didnt check the 25 old unit if most of capacitors inside .... and simply replaced a couple of broken transistors ..... then he is bearly good to repair microwave ovens at the most .....

unit old like that to be made functional and operational again ( plus the potential to last another 25 years ) needs a complte check to almost everything inside ////

may be your tech guy needs to read this ( probably very carefully )

vintage amplifier repair upgrade manual

not much else to say
 
I design and build my own amps and the method I use is:

1/ Connect amp to a speaker.
2/ Turn the bias down as far as it will go.
3/ Apply 1 volt sine wave to input
4/ Monitor output on scope.
5/ Turn bias voltage up slowly until crossover distortion goes.

This method has the advantage that it adapts to different output transistors sets and sets the bias just enough to get rid of distortion without dumping heat into the heatsink unnecasarily.

Works for me every time......
 
EchoWars said:
On most amps with global feedback, this is not possible. The input stage will attempt to correct and nicely 'cover up' the crossover distortion. You'll never see it.


My amps all use global feedback and crossover distortion shows up very well on my scope with low bias.
Howeve,r you must make sure a speaker is connected for this to work.
 
Re: IF YOUR TECH GUY

sakis said:
DOESNT UNDERSTAND .....

terms like bias and offset (without a schematic ) and also didnt check the 25 old unit if most of capacitors inside .... and simply replaced a couple of broken transistors ..... then he is bearly good to repair microwave ovens at the most .....


vintage amplifier repair upgrade manual

not much else to say

I had to LOL at your "microwave" comment. He'll get a kick out of that one, too. He's actually a repair tech for musical equipment (PA, sound reinforcement, bass and guitar amps and the like); he only took this on as a favor to me after the repair tech that I used for my stereo equipment gave up trying to repair the 650 (he was totally baffled). Like I mentioned, Nak does not have any authorized repair shops in the USA for this amp according to them.

I've printed out your Vintage Amplifier Repair Upgrade Manual. I'll pass that on to him as well.

Thanks and

Regards.
 
Re: Re: IF YOUR TECH GUY

Dr. Mac said:


I had to LOL at your "microwave" comment. He'll get a kick out of that one, too. He's actually a repair tech for musical equipment (PA, sound reinforcement, bass and guitar amps and the like); he only took this on as a favor to me after the repair tech that I used for my stereo equipment gave up trying to repair the 650 (he was totally baffled). Like I mentioned, Nak does not have any authorized repair shops in the USA for this amp according to them.

I've printed out your Vintage Amplifier Repair Upgrade Manual. I'll pass that on to him as well.

Thanks and

Regards.


you got to be jocking of course .......

if he is a repairman for ----PA, sound reinforcement, bass and guitar amps and the like---- as you describe it .... a simple amp with 4outs should be a walk in the park .....

see you
 
This one, seems to me that has automatic control

Nothing to do with this amplifier... an almost perfect design..this one, made to listen to... and only.

Nothing to adjust i think.

Maybe i am wrong..but what i see is automatic.

This one is a hell creative design....bias is modulated by audio... send it to highly skilled people, alike Anatech.

Avoid even touching this wonderful machine ... This would be like to profane a temple.

regards,

Carlos

.......................................................................................................

I cannot send you the schematic..... if i could it would be already at your inbox, as i love to help...the one sent me this schematic said not to publish (entire....ahahahahah!)
 

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Hi Dr. Mac,
I just happened to see this thread, so my information may be too late.

You have a really nice amplifier, or at least it was at one time. There is no way to easily adjust the bias current. It is set by the Vbe drops in the output stage. This is something I call a "power diamond buffer", for want of a better explanation.

There are no special parts beyond the transistors used originally, so that means you will have to substitute wisely. If there are "NTE" or "ECG" transistors in there, it will not work.

The normal Nakamichi service network should be able to restore you amplifier to good running condition. As it sits now, you may cause further damage running it.

This design is not something I would suggest skimping money on. You will end up repairing it until the PCB is no longer reliable. Dead amp for a few bucks. :rolleyes:

If your technician doesn't understand this design, get it to someone who does.

-Chris
 
Anatech:

Do you know of a source for original transistors for the 620? The ones my tech got last time were listed as "original." Obviously, that was not the case. And you are right, more transistors blew out this time.

If you don't, then can you provide the name and source of "wise substitutes?"

My problem with your last and best suggestion is that there are not many, if any (none according to Nakamichi) Nak qualified service technicians here in the States.

Regards.
 
The number of good TO-3 transistors on the market now is pretty slim, and virtually all of them are made by On-Semi. My suggestion is in Post #2, and I'd be surprised if Chris has something different in mind (except maybe the MJ21195/96 or one of the old MJ1502x classics).

These transistors are commonly counterfeited too, which is why it is important to buy them from a vendor who sources them directly from On-Semi.
 
EchoWars said:
The number of good TO-3 transistors on the market now is pretty slim, and virtually all of them are made by On-Semi. My suggestion is in Post #2, and I'd be surprised if Chris has something different in mind (except maybe the MJ21195/96 or one of the old MJ1502x classics).

These transistors are commonly counterfeited too, which is why it is important to buy them from a vendor who sources them directly from On-Semi.


Dr. Mac said:
EchoWars:

I gave my tech your suggestion earlier this week. Actually, I printed out the entire discussion and gave it to him, but I highlighted yours and a couple of the others.

Thanks.

Regards.

belive me your approach is not correct .... you will not belive that very nice quality , originally made transistors , original brand and specs exist ...

but we dont know were to look for them .....

places to look is a strange story ....I ve done some searching in alternative places and i come up with amazing results ....

------- didtributors of semiconductors in small country like greece were forced to buy from toshiba for example the all line of 2sa and 2sb transitors .... so all common numbers are soled but all the rest is there waiting

-------then manufacturers may still have stock of original transistors

-------- and last but the best ....is JAPAN !!!!! you can belive how manu sub assempbly small factory were undertakers for keenwood sony sanyo pioneer .... and many good other brands that not only keep a stock of these transistors but very often complete pcb and other assemblies ...

through this way ....and so far i managed to import many original 2sB 554 and complementary ...alla brand new and all in the well known execelent japanese quality ....the only problem is that these small sub assembly village factory dont have a web page ...and beyond that they dont even speak english .....

i even managed to find pinch rollers for AKAI reels from the original manufacturer ( rpized almost nothing )

kawashima electronics


i had a trip sceduled to japan this spring ....but since i am a father now ...it will have to wait for a while ....
 
Originally posted by sakis
belive me your approach is not correct .... you will not belive that very nice quality , originally made transistors , original brand and specs exist ...
You are correct on that point, I do NOT believe it.

The orignal 2SD555's and 2SB600's were NEC and are long discontinued. If anyone else is making transistors with these numbers, they are counterfeit by definition and not to be trusted.

While there may be NOS transistors out there, it is difficult to tell these devices from a market which is flooded with Chinese counterfeits. I don't have the time or the financial resources to sort through them to find the good and bad ones, and neither do most other DIY techs. Your chances of buying a useless counterfeit device is 100x higher than actually finding a NOS original device. So why play that game? Buy a new device from a reliable source, and be done with it.
 
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Hi Glenn,
Hear hoofbeats: expect horses, not zebras
Nope. Around here, I expect cows. I'm normally right on that count! ;)

Hi Dr. Mac,
I would normally go for current Japanese, or On Semi. The 2SD424 and 2SB554 will work as substitutes I think. The Vbe is important in setting the bias current. Also, check the drivers.

If your technician can not find known good Japanese transistors, then On Semi is much better than some things you could grab. There is one other avenue available for you in extreme circumstances. Find an older unit that uses the same parts and reuse the ones from the good channel. The parts in a channel that has failed should be considered bad parts, even if they test good. Remember, this is a last resort. Do this only when you need those exact parts and they are no longer available.

I am very concerned that your tech does not understand this amplifier at all. Its not something you want to learn on for one. The simplicity belies the finesse of the circuitry. You have to be exactly correct in order to have it work properly.

if any (none according to Nakamichi) Nak qualified service technicians here in the States.
That is completely untrue, and it makes me unhappy to hear them say that. The old Nak service network is fully capable of servicing that if they were any good. An old Marantz tech could probably service it properly also. It's similar to a Marantz 300DC, but simplified. I have repaired a few of these and they are capable of truly great performance. They deserve respect and I know the one you have has not been shown any thus far.

-Chris
 
Hi
Just curious to hear what happened to the NAK 620 after all these good advice? I have a similar beaty and would like to use, but if I hand it over to someone who's not exactly a specialist in this model to be checked out, .... so what happened to the one mentioned in this thread. Mine is in Finland and it might be as difficult to find any new/old spearparts here.
 
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