Nakamichi SR-2A Repair Problems

[Paul Strefling]

I just received a SR-2A; when powered on for the first time the left channel was silent. I opened it up and found a bit of corrosion on a couple of transistors, D1406Y and B1015Y, the ones near the center/rear of the board. I replaced these transistors with NTE377 and NTE378 respectfully. From what I can tell I no longer have any amplification; I cannot hear any music unless to volume is at its maximum. Both channels are working at this high volume setting though. I have testing the obvious things; the problem is not specific to the input, cd, aux, etc, the radio has the same problem and it is independent of the A/B speaker selection.

Some oddities I have noticed that may help is that when the "Tone" button is pushed there is a significantly loud static/cracking noise that is generated.

There seems to be an addition of a single wire and a few diodes on the bottom of the PCB, they look to be an afterthought, possibly from Nakamichi themselves, or perhaps from a repair shop. I can supply pictures if this might help the diagnosis.

I have replaced both the 377 and 378 transistors with the original ones and I still have no amplification. After pulling the 377 and the 378 I tested them, the 377 is acting like a NPN transistor, B to C=6.2 MOhms, B to E=13.67 MOhms, but the 378(PNP) is an open loop no matter which way I test it. Is this thing eating transistors?

I am hoping someone has had a similar problem...
Thank you

[Paul Strefling]

Noticing that my multimeter has a diode test feature I was able to test the transistors and all of them checked out.

My multimeter supples ~1.52v here are the results, in volts, maybe they will help someone someday.

B1015Y
BC .535
BE .538

NTE378
BC .545
BE .546

D1406Y
BC .568
BE .567

NTE377

BC .487
BE .493


I manage to locate the schematics and service manual for the for the SR-2A; I am slowing making my way through the board.

[Paul Strefling]

Fixed.

Transistor Q401 was bad. I hope you all enjoyed the conversation I have had with myself on here.

[Paul Strefling]

For completeness I will write a short summary on how I diagnosed and solved this problem. Knowing that the tone switch made a great deal of static when pushed I started my analysis there. I quickly learned that the tone control PCB was not receiving the correct +/-18 volts. It was something like -11 and -8. I traced the connector back to the main PCB and located the +/- 18v circuit. It was not trivial to find which one was bad; I actually ended up pulling all of them. Q401 was the culprit, I replaced the original A733 transistor with an off the shelf PNP from radio shack, 2N3906. This is a temporary solution though; I have a 512-2SA733P in the mail to replace it.