I bought my C-70 on Ebay for cheap. When I got it, the shipper didn't remove the shorting plugs in the normal outputs, so the jacks got crushed into the back panel in the box. I managed to repair the jacks pretty easily, and I have been using the preamp for the last several months. It sounds great on all inputs, including the MM phono. I decided to at least rebuild the power supply, which I did today with all new Nichcon PW caps and new thermal compound on the regulators. Easy enough.
However, when I went to perform the MM/MC amp adjustments in the service manual, I can't get the right channel readings at the Rec Out jacks to change when I move VR102 or VR104. The left channel adjusts fine to +- 0.1mv for both MM and MC adjustments and responds to VR101 and VR103. But the right channel just fluctuates between around -1mv and +2mv and randomly jumps around in that range no matter where I turn the VR's.
Upon closer inspection, I can tell that someone has worked on this preamp before. There is charring around C251/C250 on the Equalizer board, and those caps have been replaced with Panasonic FM's. The voltages for TR211 and TR212 look fine compared to the schematic, and it looks like the diodes and resistors in this area were also replaced. Lots of freshly done solder joints on the transistors in this area.
C103, C104, C105, and C106 have also been replaced by Nichicon UKL's on the EQ board. There are some lifted traces that the previous repairman bridged using wire, and those occur on the left channel side that works fine. The right side in that area has also been worked on, as I can see fresh solder joints on many of the transistors, but no lifted or bridged traces.
I reflowed all of the pins that connect the MM/MC amp boards to the EQ board, and still no luck. I thought IC102 on the right channel MM/MC board might be bad, so I took the voltages from all 8 pins. Here they are, along with the voltages from the corresponding pins on IC101. None look very far off.
........IC102.....................IC101
Pin 1...+64mv..........Pin 7...+71mv
Pin 2...+5.6mv.........Pin 6...+2.7mv
Pin 3...-2.0mv..........Pin 5...-0.8mv
Pin 4...-15.53V........Pin 4...-15.65V
Pin 5...+3.1mv.........Pin 3...+6.5mv
Pin 6...+5.6mv.........Pin 2...+7.6mv
Pin 7...+37.5mv.......Pin 1...+32.2mv
Pin 8...+15.48V........Pin 8...+15.59V
I cleaned up any of the solder work that looked iffy on the EQ board, but I still can't adjust the MM/MC for the right channel. Any advice is appreciated.
However, when I went to perform the MM/MC amp adjustments in the service manual, I can't get the right channel readings at the Rec Out jacks to change when I move VR102 or VR104. The left channel adjusts fine to +- 0.1mv for both MM and MC adjustments and responds to VR101 and VR103. But the right channel just fluctuates between around -1mv and +2mv and randomly jumps around in that range no matter where I turn the VR's.
Upon closer inspection, I can tell that someone has worked on this preamp before. There is charring around C251/C250 on the Equalizer board, and those caps have been replaced with Panasonic FM's. The voltages for TR211 and TR212 look fine compared to the schematic, and it looks like the diodes and resistors in this area were also replaced. Lots of freshly done solder joints on the transistors in this area.
C103, C104, C105, and C106 have also been replaced by Nichicon UKL's on the EQ board. There are some lifted traces that the previous repairman bridged using wire, and those occur on the left channel side that works fine. The right side in that area has also been worked on, as I can see fresh solder joints on many of the transistors, but no lifted or bridged traces.
I reflowed all of the pins that connect the MM/MC amp boards to the EQ board, and still no luck. I thought IC102 on the right channel MM/MC board might be bad, so I took the voltages from all 8 pins. Here they are, along with the voltages from the corresponding pins on IC101. None look very far off.
........IC102.....................IC101
Pin 1...+64mv..........Pin 7...+71mv
Pin 2...+5.6mv.........Pin 6...+2.7mv
Pin 3...-2.0mv..........Pin 5...-0.8mv
Pin 4...-15.53V........Pin 4...-15.65V
Pin 5...+3.1mv.........Pin 3...+6.5mv
Pin 6...+5.6mv.........Pin 2...+7.6mv
Pin 7...+37.5mv.......Pin 1...+32.2mv
Pin 8...+15.48V........Pin 8...+15.59V
I cleaned up any of the solder work that looked iffy on the EQ board, but I still can't adjust the MM/MC for the right channel. Any advice is appreciated.