This week started off with a very nice Marantz 2275 in for repair and restoration. The receiver was in stuck in protection and it was easy to see why when DC offset was checked. Since the speaker protection relay would not engage I tested at the relay itself. The left amp channel was putting out 44v, yes volts, of offset. Of course offset should be close to 0mV as possible and usually +/- 10mV is acceptable.
Upon checking the board a poorly substituted driver was used which was also causing the bias to be only adjustable down to 50mV, 10mV is ideal. I replaced the drivers with Fairchild semiconductors which brought the bias under control. Then replaced the differential pair with a well gain matched pair. Offset was still all over thr place and a pair of ceramic diodes were screaming to be replaced. After changing them out the speaker relay immediately clicked and offset was easily adjusted.
With repair complete it was on to refurbishing other sections. First up was the power supply. Panasonic FC and FR capacitors along with a new Omron relay. Adjusted the power supply for 35v easily.
Next was the preamp / tone amp which is removable and easy to work on. I did not take a before but here’s the after. Small value caps were replaced with Panasonic ECQ.
Next, on to the phono preamp. Caps were replaced along with all six transistors.
The receiver looked so sad with half its lamps out
Cleaned up and all meter and dial lamps replaced. Sounds sweet burning in. Will run for 30-40 hours before rechecking the offset, bias and ready to go home.