Yamaha Amplifier Repairs
Right channel low when switched on. Switching the tone circuit on would cause the other channel to disappear. Fault found in preamp, it needed the relays replacing, not available now - the Mitsubishi ones with odd pin outs. I made PCB's to hold\interface g5v ones, replaced the 4 for the output mute and tone section.
Yamaha CA-1000 MkII
Worked okay for 20 minutes then cut out, then would be okay again. I found the o\p DC slowly increasing from 100mv to 1v over a time in the LHS channel before the protection kicked in. Removed the PCB and found a film of something over the PCB, after cleaning it off and replacing it, I removed the glue around the capacitor. Now had 10 V on the output, tried changing the transistors, turned out to be the 8k2 r was o\c feeding the 6V Zener from the o V rail, the glue had attacked the resistor and damaged it, the regulator caps were u\s replaced them 220 µF 100V and 100 µF 63V.
Yamaha CR-1020 Receiver
This buzzed through speakers with a varying frequency depending on the tuning and if switches were engaged, turned out replacing the 13 V transistor in the regulator 2SD400 stopped it but the stereo decoder started to buzz in stereo. Guess it was goosed.
From 1974, one channel was going off with distortion, fault was in the preamp. Replacing the transistors on the filter board behind the switches cured it.
This classic tuner amp was not giving any output and needed repair. Remove the chassis from the wooden sleeve, make sure the tuning curser is toward the LH side of the scale or a bracket will rip the wires from the pointer bulb. Remove the screws on LH and RH sides at the front and two screws just behind the tuner PCB, the whole of the front will pivot upward revealing the PSU PCB. The front RH 2 transistors are the +/-50 V regulators for the power amp driver stages. The RH one +50 V reg expires due to inadequate heat sink.
Another Yamaha CR-1000 needing repair, it had low output with distortion, on the back of the PSU board there is a mute signal (MU) that swings from about 0 V to -15 V at switch on, 2 wires are connected one goes to a PCB that has swung up with the chassis to mute the pre amp output, the other one goes to the filter board, this second wire was low resistance to chassis, leaving it disconnected cured the fault and didn't cause any problems. FET's are used for the mute and suspect they had expired, and they are now unavailable.
The 47 ohm fusible resistors in series with the +50 V reg was open circuit and bulbs in the meters needed replacing 9 V.
This worked but there was a plop through the LH speaker when the speakers were switched on-off. The fault was on the tone board, one of the tantalum caps was slightly leaky putting about 90mv on the output. There were several dry\bad joints on the board, as well on the resistor in series with the supply to it.