Nakamichi Turning Off Randomly? Check your p/s solder joints!

[Johnny_Law]

Just finished getting this TA-2A back in action for my brother. When it was on, it would work great! But sometimes randomly it would shut off! Then other times it would halfway power on, lights would come on but nobody home. Sometimes I would push the power button, and it would remain totally dead, but would come to life if I nudged it.

Totally weird, right? So I looked at the power supply - did a couple things, but long story short, found a bad solder joint! Re-flowed it, and we're 100% back in business! (Kept looking and found a bunch more, re-flowed all of em!)

So what does a cold solder joint look like? Well here's the one that was causing this TA-2A to turn off, Q405 on the power supply. Look at the little circles on the solder joint. That's where the solder had broken and no longer making solid contact. A little bit of fresh solder and it's good as new!

-Ryan

 

[Fredrik 1977]

Nice!
you probably find a heatsink on that Q405 regulator. its probably pretty small too? usually a small piece of alu in a U shape if any at all, often there isnt even a heatsink. try putting a bigger heatsink on it if possible.

 

[sKiZo]

Good tip for most any intermittent issues like you describe. Brings to mind the infamous dolby board on the Sansui x001 quad receiver that would drop channels at random when you cycled the push buttons. The solder dries and shrinks over time, and whether the connection works or not is depends on temperature, luck, and maybe the phase of the moon,

Early SMD tv builds were also plagued with that sort of thing.

 

[Johnny_Law]

@Fredrik 1977, you know your Nakamichi! Yeah, Q405 does have that little U-shaped heat sink - will keep an eye on it. Thanks!

@sKiZo, it sure did seem related to the phase of the moon at first Maybe why I was so happy to discover the bad solder joints ... that actually made sense. I assumed it would require some kind of witchcraft to figure out.

 

[Freddymac2]

Both the Nak SR and TA series can have these problems, and can be a real pain to find, but a good magnifier can help. One of the other areas of failure is a 47K 1/2 watt resistor in the amplifier on both the left and right channels. Check it from the bottom of the main board. They are more prone to failure if you have 4 ohm speakers and the unit is pushed hard. These units also improve sonicly when they are biased properly.

 

[Johnny_Law]

Both the Nak SR and TA series can have these problems, and can be a real pain to find, but a good magnifier can help. One of the other areas of failure is a 47K 1/2 watt resistor in the amplifier on both the left and right channels. Check it from the bottom of the main board. They are more prone to failure if you have 4 ohm speakers and the unit is pushed hard. These units also improve sonicly when they are biased properly.

Yeah those resistors were my first shot at fixing it. I thought they did the trick until I moved it from the garage into the music room, lo and behold, it had got them ghosts back in it again.

 

[Freddymac2]

There are two of them in all of the Naks, 1 1/2 inches off of the heat sink front and back. Do check the bias it does make a difference. I do not like the way the variable loudness control works, so I always keep it turned full clockwise.

 

[northpaw]

Congrats to the OP for quickly finding the problem and the fix. That PS board is a pita to work on, being on a section of the chassis without a removable bottom cover and so much of the wiring to the board being soldered-on connectors. For me, that created a problem when manipulating the PS partially out of the chassis to work on, because I ended up pushing against that unusually tall heat spreader on U402 at times, and after several round trips, I created a problem with torn eyelets on the PCB, which I then had to fix.

The PS board on the TA-2, to me, is not as clean or as well thought out as the corresponding board on the earlier SR-2. Perhaps moving the large filter caps from the PS board (as in the SR-2) to the main board (as in the TA-2) has some benefits, I don't know, but other than that, the TA-2 PS board seems to be a bit of step down in quality, such as extruded Al heat sinks built into the board rather than folded sheet metal ones tacked on.

On the half-watt resistors on the main board that would benefit from a wattage upgrade, they are 309R and 309L, and should be 22KΩ, right? There is also the known issue on Q525 on the logic board (see service note that is appended onto the service manual available on hifiengine).

 

[Freddymac2]

You are right on northpaw, 22K and at least 1 watt. I confused it with the TA 4a that I have. Thank you for the correction