Marantz Amplifier Repairs
This worked okay but intermittently popped through the speakers with the current increasing at random. This uses a bunch of 2SC2705\2SA1145 transistors which had discoloured the board with the heat. I replaced one of the C2705's in the constant current circuit for the dual FET as there was distortion. I ended up replacing the other two in the current mirror circuit further up the line, worked okay afterwards. Unusual in that it uses a relay to switch in higher voltage tapping for 200W output. Normally runs +/-45V to the output transistors. The front end has a fixed +/-80V supply which is why it cooks the front end transistors. There is a small electrolytic capacitor in amongst the heat or perhaps two of them which could do with replacing.
No sound through the preamp, power amp worked okay, the tone circuit board held the problematical transistors 2SA702's, they had morphed into diodes according to my meter! Worked fine after replacing them with new ones.
Marantz 2330B Stereophonic Receiver
One channel wasn't working, if there was a major problem then the protection should have removed both channels! Turned out to be the volume control had chronic bad contacts, needed soaking in switch cleaner, made wondrous sounds after. (180W\Ch receiver)
This would cut out. I found the LH channel quiescent at 2x what it should be, readjusted, then dry joints found on the TO220 transistors and several on the power amp PCB, then back to the real fault cutting out at over ¼ volume. The protection circuit was dropping the relay out, turned out to be the capacitor across the over current i\p on the protect IC TA7317, worked okay after all that.
This required a modification to the amplifier which involved connecting a link to the chassis from a resistor. It worked fine after fitting it.
This needed a new STK module, the front end of the power amp on a chip, there was no o\p to one channels bias.
One channel had horrid distortion, the small board in the middle of chassis. The biasing was all wrong on the 2SC458's.