Nakamichi 620 Rebiasing Help Needed

[Dr. Mac]

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.

[EchoWars]

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.

[Dr. Mac]

Thanks for the input, EchoWars. I'll pass it on.

I've posted the service manual.

Regards.

[Jan Dupont]

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

Could it be some kind of copensation if the 0.22 5W resistors are wirewound ?

[sakis]

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

[nigelwright7557]

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]

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.

[nigelwright7557]

Originally posted by EchoWars
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.

[EchoWars]

Unreliable method. He's asking here because the amp has apparently blown up more than once. I'm of the understanding that he's tired of repairing it.

[Dr. Mac]

Originally posted by sakis
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.