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NIKKO STA-4030

This vintage Stereo receiver is the result of a dumpster dive. Overall in good condition but no sound or light when turned on.
So now the fun starts - what is not working ? All lamps blown out !  All fuses ok !

1. Plugged in a headphone and start to turn the knobs and flip the selectors. Vague noice now and then... good, not totally dead.
2. Connected a cd-player in the AUX - input. Yes, the music weakly present in both channels but when turning the balance from left to right, one channel seems to be ok but the other sounds weakly. The same fault regardless using the speakers or headphone.
3. Ideas 
    - Could it be the variable resistor pot ?
    - Any capacitor died of old age ?

4. Found the schematics on the net, added below.

5. After a study of the schematic, the main amplifier circuitboard seems to be a good part to start from. Did some measurement with an ESR-meter and find a bad cap (C701, 0.47uF). Compared the value with corresponding cap on the other channel and find it faulty.

6. Put a fresh cap in parallel with the old and YES - the weak channel sounded as good as the other.

7. I replaced the capacitors in both channels, C701 and C702.

8. Ordered some new pilot-lamps (8V, 250mA) to get some light in the front.

9. Replaced the lamps. The remaining issue is the STEREO indicator. The Lamp driver from the IC (NEC PC554C) does not give the correct voltage. A pulsating sound is present when in STEREO quality.

10 Problem solved !  The lamp for the stereo indicator was blown/shortened. Replacing it made stereo indication worked!

Lesson learned:











ą
Benny Saxen,
4 Jul 2014, 00:25
Ċ
Benny Saxen,
29 Jun 2014, 11:10
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