Accuphase Amplifier Repairs
Stereo power amplifier from 1974 needing a repair, these are beginning to suffer from poor quality Fuji transistors, they cause DC drift, pops and general faults. They are the 2SA809 and 2SC1451, they are on the power amp driver board, there are four used as differential pairs and two drivers, (some versions of the P300 use different drivers as in the picture of the driver board), the driver boards also will need the solder going over to remove dry joints all the connections on the PCB behind the heat sink should be re-soldered also. It's best to remove the output transistors and re-apply heat sink compound as with the temperature the heat sink reaches it will have dried out by now, the non polarised electrolytic's are best replaced with new modern types. The large capacitors can leak electrolyte and corrode the area under them.
This wouldn't energise the speaker relay, the capacitors that couple the AC to DC circuit on the PSU board were open circuit, also the 1 ohm resistors in the emitters of the o\p transistors were open circuit along with a couple of the 56 ohm ones, (on each power amp board) the 3k resistor and 1uf capacitor across the 16 V Zener were replaced as they showed signs of stress, one of the DC o\p voltage pots was replaced, + lots of dry joints all over, notice while setting the DC to 0 V that the output voltage took about 10-15 seconds to reach a low enough level (<100 mV) to safely connect the speakers at switch on, a 10 µF capacitor connected across the 2,2 µF capacitor increased the time accordingly, the regulated supplies which should be +\-40V were +40 & -20, the transistor below the one on the small heat sink was faulty, the Fuji 2SC1451 transistors were working fine despite ones in the P-300 (see above) needing changing, the ones in this amp aren't run at as high a supply voltage and hence run much cooler. The 6.2 k and 5.6 k resistors were replaced and the 33 k at top of the PCB as they were showing signs of heat damage although they measured okay. A standard IEC mains inlet was fitted as the original was difficult to find a plug that fitted, made wonderful sounds after setting up.
This needed a repair, the speaker relay wasn't being energised. There were dry joints on the PSU board, several resistors had begun to disintegrate on the PSU board and each power amp PCB. The four main capacitors were replaced as two had leaked and begun to bulge, I used 12,000 µF 63 V.