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DreadPirate 28th November 2013 04:21 PM

Power Issues Nakamichi SR-4A
1 Attachment(s)
Unit is exhibiting fm tuning issues due to power supply not providing right voltages. In auto, it just scans and in manual it appears that only one channel tunes in, no matter the dialed in value.

I checked voltages on the "Logic Board" and found that CN12 values were 10.5, 12.7, and 30.3 instead of the expected 5.6, 10, and 30. CN28 also is off, giving 12.7 instead of 10. CN11 is giving 10.3 instead of 5.6. CN20 is ok.

Looking at the power supply board, I'm reading bridge D409, 12VAC between the outer legs (~) and 12.7 VDC for the innter (+,-). At fuse F405, 6VAC to Ground. Seems to me the voltage is coming off too high from the bridge (should be 10V?) and something is preventing down-voltage to the required 5.6V. By inspection, is it possible to determine what is causing this?

Mannegizen 10th December 2013 06:57 PM

If you haven't checked it already and the problem is just for FM, is the center voltage adjusted according to the service manual?

Step 3 under 3.2.1 FM tuner section on page 5.

DreadPirate 12th December 2013 12:35 PM

That adjustment cannot be made, it doesn't change but slightly, I suspect it is because the voltages are not correct coming off the power supply.

Mannegizen 12th December 2013 05:42 PM

You are probably right, I would suspect one or more of the following parts:


Mooly 12th December 2013 06:24 PM

As above.

Just check the voltage across the zener diode. Can't make it out on the diagram but it will be a 6.8 volt zener. If you have higher than that its duff.

DreadPirate 13th December 2013 06:26 PM

Those would be the likely suspects for failure in this circuit, but do they serve to reduce the voltage? What are those transistors there for? Is this functioning as a voltage regulator such as in:

Why would one do this in lieu of a voltage regulator such as the lm type?

Mooly 13th December 2013 06:41 PM

The transistors make a very simple regulator and ripple filter. The diode produces a stable voltage but the current available is tiny and limited by the two resistors feeding it. To get useful current we apply that voltage to the base of a transistor. That allows the emitter to deliver useful current to our circuit. This particular circuit uses two transistors, one feeding the other for even more current gain. The control input allows the voltage to be shut off (you can't do that with a normal 3 pin reg) when the unit is in standby etc.

So yes, it is a voltage regulator. Three measurements will tell you where the fault is.

1. Voltage across the zener.
2. The voltage across the base and emitter of Q402.
3. The voltage across the base and emitter of Q403.

If either 2 or 3 are above 0.7 ish then the transistor is faulty.

DreadPirate 13th December 2013 08:02 PM

Great explanation! Exactly what I was looking for!

By coincidence I'm also working on an SX-580 with FM power issues and has same type of voltage regulation, with 2SD712 and 2SC1384 being used (but not cascaded like on the Nak). The SD712, in particular, gets hot and is prone to solder cracking issues.

The TA-4A is showing the same arrangement as for the SR-4A, but with an 2SC1815 instead of the 2SD1406. So commonly used for FM power.

DreadPirate 14th December 2013 12:55 AM

3 Attachment(s)
Interesting findings. First pic shows how NAK made it tricky to access the PS board, it cannot be tackled from beneath. Second shows some solder bead issues on C402, the 5.6V power connector, J3, C408, and general area. There was a piece of black foam attached to this area upon dis-assembly. Final pic is a foam pad underneath the PS board. I believe this is the cause of the fault, it has resulted in some conduction between the previously mentioned solder joints. This foam pad has to go. Wonder why it was there to begin with. There is some decent clearance between the pcb and the case.

Voltage across ZD403: 7.03V
Voltage b-e 2SC945 (driver): 4.25V
Voltage b-e 2SD1406: .65V

Since I have the board in the tilted position, I have had no issues on power up. Before, the unit took awhile to come out of protection mode on startup (power light stayed red, now comes green immediately). Either some solder joints are being affected or the foam is also to blame for this problem.

DreadPirate 14th December 2013 02:34 AM

I have some 2SC1345 F to use for the 2SC945, but gain is much higher on these (600-800 vs. 150-450), is that ok (pin out to be confirmed)?

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