Sony Amplifier Repairs
This was distorting the right channel, one of the 2SD88 transistors was found to be short circuit, replaced it with a D180. The main speaker connectors were replaced with decent connectors instead of the stupid phono sockets.
Cracks & pops were heard most of the time. Turned out to be the differential i\p transistors to the power amp which are on the tone control board. 2SA733's were the culprits I replaced all four.
This high end preamp was losing the phono socket outer connections. They were working loose with plugging\unplugging. The top row of the sockets can be stopped by soldering a length of braiding along the line of the spigots at the back of the sockets. As this unit also suffered from spurious howling\oscillations the braid was continued to earth points on the chassis, the volume case was also earthed. The chassis is polymer resin which may account for these problems.
From 1973, this was cutting out on one channel. The small thermal cut-out Sony used in the early 70's was tripping. Checking the transistors initially showed they were okay (in power amp) re-soldering the PCB provided an initial cure, then after re-soldering the other channel it started making pops etc through the loudspeakers. I ended up removing the old solder from the transistor connections and the old solder flux. This cured both channel faults. The solder that Sony uses (even now) leaves something to be desired and I suspect the old flux has become conductive.
This is not a repair as such just some information on the Sony V-FET amp from 1976-1978. The heat produced changes the resistance of the fusible resistors (the green coloured ones mounted away from the PCB) and causes the amp to fail. The screws holding the MOSFET's work loose over time and will need tightening. The V-FETs are only rated at 60 W power dissipation, so don't have much reserve. Also in the picture are the 3 diodes per channel toward the top of the PCB (black blobs) that are reckoned to expire and destroy the output FETs.
An early 70's high power amp 110W\ch, this amp's service manual had ©1970 on it. The amp had several faults on one channel, the o\p voltage was varying by about 1V with no signal, this was cured by replacing the input transistor of the differential pair (the second transistor is a higher power type in this amp for some reason known probably only to Sony designers),the transistors in the protection cct had to be replaced as well, (the 2SC633\2SA678) along with the SPS885 an odd numbered tranny-looks like a Motorola mpsu** type this was the main voltage amp\driver, the amp powered up okay and worked fine after setting the quiescent and o\p 0V.
The amp designers must have had a field day with this amp as it uses a thyristor in a cct to reduce\limit the power o\p connected across the bases of the o\p Darlington-connected transistors. I think this was a one off cct as the technology was just starting in high power amp designs 110W RMS was a lot of Watts in 1970, it also has a switch to reduce the power o\p by 25% and 50% which worked in conjunction with the thyristor cct evidently to protect the low powered speakers that were available at this time.