Subchannel
Active Member
I have just recapped a Nakamichi 410 preamp, but got into some strange problems.
I know all caps (Panasonic FC/ Elna Silmic) are mounted right, I have checked this many many times.
When pressing the TONE button it distorts/ cut sound on Left channel.
PSU measures correct, both connected and disconnected from main PCB.
But, on pin 6 on Tone Board (CN-7), there´s -5.8 to -10.2VDC (should be 0.2) depending on if CN-7 is connected to main PCB or not.
There´s poor quality on the PCB traces, but most joints are resoldered.
I have signal traced with scope from input to output, and without Tone control engaged the sound is as it should.
The distortion changes when turning Bass and Treble up/ down, so all logic sais that it´s the CN-7 that´s the problem, BUT, when disconnecting CN-7 there´s still negative DC on C410 to toneboard.
I have lifted/ cut PCB traces for both channels to switcboard so there´s only connection to Headphone circuit, but there´s still way too much negative DC from Headphone board to tone board (left channel only). Both channels measure -9.4VDC on Q403/ Q404
Pin 4 and 5 from Line amp measures good, so do the +/- supply to the Tone Board.
I´ve measured for many many hours now, but can´t wrap my head around why there´s DC there. I also replaced the new caps with other new caps in case of defects, but no difference.
The only thing that is not original is I replaced the light bulb with a LED and resistor, but no difference when in or out of circuit.
Service manual PDF too big to attach, but can be found here:
Anyone have any suggestions?
Thank you in advance.
I know all caps (Panasonic FC/ Elna Silmic) are mounted right, I have checked this many many times.
When pressing the TONE button it distorts/ cut sound on Left channel.
PSU measures correct, both connected and disconnected from main PCB.
But, on pin 6 on Tone Board (CN-7), there´s -5.8 to -10.2VDC (should be 0.2) depending on if CN-7 is connected to main PCB or not.
There´s poor quality on the PCB traces, but most joints are resoldered.
I have signal traced with scope from input to output, and without Tone control engaged the sound is as it should.
The distortion changes when turning Bass and Treble up/ down, so all logic sais that it´s the CN-7 that´s the problem, BUT, when disconnecting CN-7 there´s still negative DC on C410 to toneboard.
I have lifted/ cut PCB traces for both channels to switcboard so there´s only connection to Headphone circuit, but there´s still way too much negative DC from Headphone board to tone board (left channel only). Both channels measure -9.4VDC on Q403/ Q404
Pin 4 and 5 from Line amp measures good, so do the +/- supply to the Tone Board.
I´ve measured for many many hours now, but can´t wrap my head around why there´s DC there. I also replaced the new caps with other new caps in case of defects, but no difference.
The only thing that is not original is I replaced the light bulb with a LED and resistor, but no difference when in or out of circuit.
Service manual PDF too big to attach, but can be found here:
Anyone have any suggestions?
Thank you in advance.
