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Sansui AU-717 static noise on left channel Phono 1/2, replaced transistors and got worse (noise and pops)

S

spunko

New Member
Hello, I have a Sansui AU-717 that have static noise on the left channel at Phono 1 and 2. Right channel is good. AUX, tuner, tape are good.
If I take the shielding cage, the noise becomes horrible.

I decided to replace all the transitors from the F2723 board with modern equivalents because I don't want any more failures in the future.

Google pointed at these two parts to replace all of them:

- 2SA992 replace = 2SA906 (TR 01, 02, 05, 06) / 2SA762 (TR 03, 04) / 2SA750 (TR 09, 10, 15, 16) - total 10
- 2SC1845 replace = 2SC1400 (TR 07, 08, 11, 12, 13, 14) - total 6

Now both channels at phono have louder static noise and even pops, so it got worse. Even AUX now has a little static noise, you can barely hear it but is there.
Maybe the new transistors aren't good as they are cheap chinese parts bought locally. I live in Ecuador, South America, and sadly you can't find any good spare parts, only cheap or fakes.

I'm going to buy from mouser and ship overseas, could you please lead me to the correct parts available?
I have all the original transistors so I can swap them again if needed.

Thank you very much.
 
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Replacements look fine to me (I guess you meant 2sa726).
Never a good practice to start swapping components before the fault is determined. There could be dirty switches, bad caps, cold solder joints etc.
 
Sorry, yes I meant 2sa726.
Boards have been recaped, switches are clean, the amp has been working great, except for that static noise on the left channel at phono (with the original TR) and now all the noise with the new TR
 
Yes, modern replacements install backwards because of pin layout. The amp works, and sounds good when music is playing.

I'm tempted to swap back to the originals and start all over again. Sadly I didn't identify each one with its corresponding number on the PCB (tr01, tr02, etc), but if its a faulty transistor, the noise should go to either side left or right.

Most likely the fault is somewhere else as you said. I just cleaned the switches, but I didn't inspect them out of the pcb. I've seen some old switches get conductive at that type of resin / glue that holds the terminals / legs.

Sorry for the bad english.
 
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Have you checked voltages on the phono board? Perhaps it is not supplied properly, which would point to a power supply issue. Or you could find a faulty tr that way.
 
spunko said:
Yes, modern replacements install backwards because of pin layout. The amp works, and sounds good when music is playing.

I'm tempted to swap back to the originals and start all over again. Sadly I didn't identify each one with its corresponding number on the PCB (tr01, tr02, etc), but if its a faulty transistor, the noise should go to either side left or right.

Most likely the fault is somewhere else as you said. I just cleaned the switches, but I didn't inspect them out of the pcb. I've seen some old switches get conductive at that type of resin / glue that holds the terminals / legs.

Sorry for the bad english.
Click to expand...

Did you just assume this pinouts are all backwards or did you download the datasheets? Did you use a transistor checking device to confirm? Very easy to make an easy mistake. If anything, the sound would NOT change if you swapped these transistors, certainly not get worse. It will not pay to reinstall the old ones. The problem could lie elsewhere and the new transistors could just have more gain and accentuate the noise.
 
If possible, can you snap several photos of the board? Make sure the transistor labels are also visible. I accidentally reversed one transistor on a driver board once and i only knew before-after checking photos of it.
 
There are four 82 ohm fuse resistors on the F2723 board that should be checked. R47 and R49 are for the left channel.
There are four VD1212 dual (blob) diodes that are potential trouble makers. D1 and D3 for the left channel.
 
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