audiogymp

Active Member
Hi AK

I hope some of you might have some pointers for a recent Kenny purchase.

I bought a KA-7300 with a defective phono stage (as advertised). The amp otherwise works as expected but produced no sound at all through the phono inputs. The Tuner/Aux inputs that also pass through the board with the phono preamp work fine.

I opened the amp and (after some work extracting the preamp board) I realised that the power lines to the pre-amp board had been cut. I reconnected these and initially the phono inputs worked as expected. Then suddenly they stopped and instead of the sound signal there was just crackle and pop.

The preamp uses FETs and I understand they are difficult to source. The only silicon on the preamp board are the FETs (2SK68A (M) and (L)) and a few 2SA763WLs.

I suspect that the power lines to the preamp board were cut because of the problem. The way it worked and stopped makes me think I may have a transistor problem. The thing that is curious is that it affected both channels simultaneously with total loss of signal. Does this point to a possible cause?

There are 9 electrolytics in the board. At this stage I could swap them for cheap local substitutes to see if it is a cap problem. I'd appreciate any other suggestions about how to identify the problem.

Thanks, Jon
 
FET's are pretty reliable. I'd replace the electrolytics and the four bipolar transistors and cross my fingers that the FET's are OK.
 
Thanks, EW.

On
http://audiokarma.org/forums/index.php?threads/ka-5500-questions-for-echowars.704648/
you suggested KSA992FBTA as a substitute for 2SA763WL 4 or 5 (not in relation to a preamp board). The SM for KA-7300 lists the bipolar transistors as 2SA763WL 5 or 6. Do you think I can I use the same substitution here?

It looks like I will need to send off for the transistors, so I might as well get decent caps. Are Nichicon a reasonable choice?

Thanks again, Jon
 
Kenny phono stage is rocking on again with some new components (pic).

This is a late addition to the thread but I thought I'd add some comments in case anyone follows it with a similar issue.

I replaced the 2SA763WLs and this fixed the problem. I tested the old ones on a component tester as I removed them and they all tested good. This is surprising as the voltages are generally pretty low on this board. The legs had the black-furry corrosion I have read about elsewhere. Perhaps cleaning this off would have addressed the problem (which I have also read). But the board is a PITA to work on, so replacement is probably easier. I replaced the electrolytics for good measure, but the old ones tested pretty good - I don't think they have to work hard.

Beware of the detail in the circuit layout diagram in the SM and the markings circuit board. The transistors have a circle with a dash pointing towards the base. This was reliable for the 2SA763WLs but identify the E and C leg positions before removing the old transistors - SM was wrong at least once. The electrolytics have a + on the circuit board and the SM diagram but this can be ambiguous or incorrect. I became sufficiently mistrustful that I tested poles of the caps with a DMM to check correct polarity.
 

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