Sansui 2000x quiet channel

[Layton Hayes]

Hello! First post. I've been using AudioKarma as a guide for a bit, but am now stumped.

I have a Sansui 2000x that has a quiet left channel that becomes distorted at higher volumes. Trying to fix it, I replaced the 2sc458 transistors with ksc1845 on both the Tone Board and Power Amp Board. I also replaced all transistors on the Power Amp Board with new ones. Still the same issue.

If I flip the Power Amp Board 180 degrees in the socket, the right channel becomes the quiet one, leading me to believe the Power Amp Board is definitely the issue. The quiet channel happens on both headphones and speakers.

Any ideas/thoughts on what it could be? Could this issue be caused by a bad capacitor or resistor?

Thanks in advance!

 

[larryderouin]

Check the fuses in the back (2 marked L & R) for continuity WITH A METER 1st. Visual inspection canbe and usually is misleading if the filament separates on the end.) It could be a cap, but since you replaced the transistors, I'd look there 1st at the Transistor pinouts. IIRC the KSC1845 is 180* out from the 2sc458 (ebc vs cbe for example.) Download the Datasheets for the transistors and match up the pinouts with the schematic then transfer to the boards. It could also be an output cap.

 

[Layton Hayes]

Thanks larryderouin. Checked the fuses with a meter, they're good. I was having this issue before I replaced the transistors (plus some noise). Replacing them made the noise go away, but still quiet on the left channel. The pinouts of the replaced transistors match what's on the board.

Is the output cap on the Power Amp board? Because flipping the Power Amp board 180 degrees reverses the issue (right channel becomes quiet), I still think it's something on that board. I could be mistaken though.

 

[mbz]

Have you checked/set the bias on the problematic channel, The point of the exercise is to confirm that the
transistors respond correctly to a bias adjustment. Failure to bias would point to faulty transistors.

 

[larryderouin]

I'm just wondering if your power supply voltages to the amplifier board are nominal. You should have 62VDC (+/-10%) @ pins 8C on the AMP board (the connector pins are 9-10 or K-L). Also you should have about 65VDC at pins 8D or 8E (connector pins are 8-11 or J-M). Also check the grounds @ pins 2-3(B-C) and 16-17(T-U) on the connector.

Another thing is to CHECK the PREOUT - MAIN IN JUMPERS. Quite often they don't make good contact. Pull, spray with DeOxit, and scrub with a Green Nylon Scotchbrite pad. Then Rinse with DeOxit. Push them in the holes a 1/2 dozen times to spread the deoxit on the contact points. DON'T BOTTOM them out when you install them and leave them. I noticed on FiSHERS that they can short across on the outer ground of the plug. Insert to bottom and back off 1/4" or so.

This is also an opportunity to actually check to see if the problem actually lies in one of the preceeding boards.

Get 2 RCA CABLES
1.) Place the 1st cable in the LEFT PREOUT, and RIGHT MAIN IN jacks (this takes the signal from the left preamp to the Right Amp)
2.) Place the 2nd cable in the RIGHT PREOUT, and LEFT MAIN IN Jacks (Signal is now from RIGHT PREAMP to LEFT AMP).

Turn on Unit. If NO CHANGE in quiet side, the amp needs work. If the QUIET SIDE reverses the problem is further back. Start with the F1222-1 Tone Control board and work back toward the inputs. If you have another amp, take a junk RCA Cable, and wrap a .1uf 600v cap (one side) around the center lead and solder it on. Then use the other end of the cap as a probe with the RCA cable inserted into the AUX plug on the other amp. Keep the volume down!!!! Follow the signal path back (comparing identical points, side to side) until you get an even level btwn sides. Go back one point and verify either equal or imbalance. If imbalanced, there's your problem child. Toss it out with the Bath water, replace and verify operation and bias/offset (No offset as it's cap coupled.).

BTW if you haven't yet, and you should have it, get the MANUAL and the Schematic. Free registration @ hifiengine. https://www.hifiengine.com/manual_library/sansui/2000x.shtml

3 LARGE CAN CAPS on the 2000X. the one off by itself is the Main Filter cap. The 2 that are paired up are the output couplers.

 

[Layton Hayes]

Thanks for the great tips. I checked the supply voltages - 63.6v for pins 8C and 66.8v for pins 8D and 8E. Grounds look good too.

I gave the jumpers a good cleaning with Deoxit and made sure not to bottom them out. I tested the left preout to right main in and right preout to left main in - the left channel is still lower so it points to the Driver Amp being bad.

At this point I'm not sure how to diagnose the problem with the left channel on the Driver Amp. Any ideas on how to start troubleshooting if it's a bad component or bad connection? Looking at the solder joints, nothing stands out. I also swapped all transistors from right to left side of the board. Same issue, so not the transistors. Really stumped on this one...

 

[larryderouin]

Replace ALL of the Electrolytics on the board. Nichicon UKL's are Low Noise and ideal for this. Use them on the Head amp(phono) and the tone control amp. UPW/UHE's on the P.S. and regulator boards. It's old enough to have wholesale replacements done and shouldn't cost over $40.00 for the whole thing including the Main and output couplers.