I'm trying to suss out the source of fluctuating elevated DC offset in the left channel of a Sansui 771 receiver. The DC varies from nearly zero to as high as 100mV+, sometimes becoming audible through speakers.
I traced the instability to the negative side of C23 on the tone board. Replacing this 2.2uF 50V electrolytic with a Kemet film didn't make any difference. While taking measurements on the board, I also noticed a voltage difference between the two channels starting at the negative side of C07 and C08, though I'm not sure if it's related to the DC offset issue.
One other thing worth noting is that there was a factory mod made to a trace connected to C23 that isn't reflected on the schematic.
At this point, I'm going to check if C17 is leaky. Any other suggestions regarding the DC offset or the voltage difference at C07 and C08 are welcome!
I traced the instability to the negative side of C23 on the tone board. Replacing this 2.2uF 50V electrolytic with a Kemet film didn't make any difference. While taking measurements on the board, I also noticed a voltage difference between the two channels starting at the negative side of C07 and C08, though I'm not sure if it's related to the DC offset issue.
One other thing worth noting is that there was a factory mod made to a trace connected to C23 that isn't reflected on the schematic.
At this point, I'm going to check if C17 is leaky. Any other suggestions regarding the DC offset or the voltage difference at C07 and C08 are welcome!
Attachments
I tried pulling C17 to see if that calmed things down, but no change.
Measurements for the two BJTs and JETs.
TR01
E: 17.5V
C: 32.0
B: 17.8
TR02
E: 17.7
C: 32.1
B: 18.0
FET T01
S: 5.3
G: 5.5
D: 7.5 - 8.0 (fluctuating between these values)
FET T02
S: 4.55
G: 4.41
D: 12.32
Measurements for the two BJTs and JETs.
TR01
E: 17.5V
C: 32.0
B: 17.8
TR02
E: 17.7
C: 32.1
B: 18.0
FET T01
S: 5.3
G: 5.5
D: 7.5 - 8.0 (fluctuating between these values)
FET T02
S: 4.55
G: 4.41
D: 12.32
The FET's bias is completely isolated by C's. If the C's are not leaking, and short of blown resistors, it is the FET. The BJT's bias is also isolated, and both match pretty closely.
Ok, thanks. I checked the listed resistors in circuit, and none are open/all match up with the corresponding resistors in the other channel. C21 seemed to test fine, but I pulled and replaced it anyway to be sure. No change. As for the solder joints, they look ok, if not great, though I don't see any cold joints. I could reflow all of them.
I don't have a great way of testing the FET. It checked out ok in circuit using my DMM diode setting between the gate and drain and gate and source, and then reversing polarity.
Assuming it is the problem, I could use suggestions for a 2SK24 sub.
I don't have a great way of testing the FET. It checked out ok in circuit using my DMM diode setting between the gate and drain and gate and source, and then reversing polarity.
Assuming it is the problem, I could use suggestions for a 2SK24 sub.
Swapped the FETs between channels, and the DC offset issue moved too. So that confirms it.
The SM says this is a 2SK24, but what's installed are 2SK34 25C.
The SM says this is a 2SK24, but what's installed are 2SK34 25C.