Sansui AU-101 Phono channel (right) issue

[tvlieg]

I possess a Sansui AU-101 amplifier, which I like quite much. Starting a few years ago, the right channel of the phono input began to make incidentally an annoying crackling kind of noise. It wasn't constant, but still very present and disturbing. Some wiggling with the RCA-connector sometimes seemed to temporarily decrease the noise.

Now, since a few months, the right phono channel has stopped working at all. Wiggling the connector doesn't help anymore.

Some things I tried/observed:

- Switching right and left RCA-connectors left the issue at the right channel. The problem is thus not in the turntable.
- I measured the connections with a multimeter. There is no break in the (beginning of the) circuit. I measured from the RCA-cable up unto the soldering on the PCB
- There are no problems with the other inputs. Aux, tuner, tape al work fine on both channels. So the problem must be somewhere in the beginning of the right phono circuit.

I hope you have any ideas/pointers as to where the problem may lie. Thanks in advance!

 

[Athalwolf]

Here is an old thread that may be helpful to you and welcome to AK.
http://audiokarma.org/forums/index.php?threads/diagnosing-crackle-on-au-101.430980/

 

[tvlieg]

Thanks for the replies! I'm going to try new transistors in the phono preamp, and maybe also new capacitors. I'll keep you posted!

 

[slimecity]

So the issue is isolated to one non-working channel in the phono section only? Yes I would look at replacing the small-signal transistors in the phono section in the first instance.

 

[tvlieg]

Would it be better to also replace the transistors of the other phono-channel with the same replacement transistors, so both channels use the same transistors?

As for the new transistors: I found in other threads that KSC1845FTA was a good replacement. I found however this threat that mentioned this transistor wasn't a good fit after all. Is there new knowledge regarding fitting replacements for 2SC781, that you know of?

 

[sregor]

A couple of people have found KSC1815 works better in some preamps than the 1845. I believe the limitations on the 1845 are when low voltages on the collectors, the gain goes down, so depends on the circuit. The 1815 worked well in this thread, where the 1845 didn't. If the collector voltage is a lot higher than the emitter or base voltage, then 1845 works very well.

 

[tvlieg]

Update: I just replaced the two 2SC781 transistors with KSC1815, and three capacitors in the phono preamp (just for the right channel), and that solved the problem! I now have sound on both channels again!

The reason I didn't also replace the transistors/capacitors of the left channel is so that I could compare both setups. I indeed notice that the new transistors sounded noticeably different than the old ones, especially in the bass sounds. Eventually, I might also try other transistors instead of the 1815's. If I do so, I'll let you know!