Sansui AU-5500 (Pre-Amp) Very Low Volume Output

[Zeus12]

Greetings fellow audiophiles!

Decided to start a new thread since the title is no longer relevant:
https://audiokarma.org/forums/index.php?threads/sansui-au-5500-driver-and-psu-board-recap.987740

Quick recap: All boards have been FULLY RECAPPED, Tone Board (F-2414) has had all 6 transistors replaced as well and the usual DEOXIT and cleaning of pots and switches.

Main Amp is WORKING GREAT!
Pre Amp is NOT working properly. The volume output is barely audible when Volume turned to MAX

Could the problem be in the PSU Board (F-2091)? Fuse F01 (1A) has blown twice already and I also noticed that Resistor R27 looks a little beat up.


So could the problem be in the PSU side of Board F-2091? Something is preventing it from giving the 30V to pin 24? Could it be D01 or D03 Diodes? Perhaps TR01?


The thing I don´t understand is how come I´m getting some output with Fuse F01 blown? Where else is this board getting its "volts" from?

Cheers!!!

 

[Zeus12]

Is the lack of sound from the pre-out from all inputs? If the unit was working before the tone board work, check the wiring to that board.

Yes, @Leestereo the lack of sound (barely audible) is from ALL INPUTS. Unit (pre-out) was NOT working before the tone board work, which is why I decided to change all caps and six transistors on it. Tone Board has been triple checked--all caps and transistors are in correctly, no lifted traces and the wiring to the board is correct. I marked each pin with it´s corresponding number before I took it out. I´m pretty sure the Tone Board is OK.

When I mean barely audible, it is BARELY AUDIBLE. I have to put volume at MAX and glue my ear to the speaker to hear a faint sound, which sounds like it´s "bleeding" through. Same thing through headphones. Balance knob, switches ,etc. all seem to work fine.

Cheers!!!

 

[PE9ZZ]

Are the DC conditions on F-2414 board OK? If pins of transistors are swapped they won't be. "Pretty sure" is not good enough...

 

[Zeus12]

Hi @PE9ZZ Thanks for helping out.

You are ABSOLUTELY RIGHT when you say "pretty sure" is not good enough...

Have a look for yourself:


Underside of Tone Board


Cheers!!

 

[PE9ZZ]

Hm. I cannot do measurements on an image... What are the collector voltages of T01-6 on the F-2414 board? And the voltage on pin#24?

 

[Zeus12]

Hi @PE9ZZ HEEL ERG BEDANKT!!

I haven´t taken any voltage measurements since I ran out of Fuses. Don´t know if it´s OK to have it on with Fuse F01 out but since it keeps blowing fuse F01--most likely, I think, there is a problem on board F-2091.

Looking at the Troubleshooting Chart I saw this:


The transformer can´t be, it´s working great in "Main In", so that leaves us with either a Diode or a Transistor--apart from the obvious F01 or F03 open.

TR01 is a 2SC1364 and the replacement would be either a KSC945C/protection board - KSC2690/demanding use. Which one should I use? I have both of these on hand.

The Diode is a 10D-1 (D01,D02). From what I have seen a good replacement would be the N5392 and N5393. Again, which one would you recommend?

Cheers!!!

 

[PE9ZZ]

Graag gedaan!

If you keep blowing fuses you have a major problem. First find and fix that. Most likely solder bridge somewhere or a trapped wire. Use your multimeter to hunt for shorts. No use testing board F-2414 if your system is eating fuses in its power supply. F01 and -3 supply the same circuit (TR01) so if one blows the other one goes as well. Use an external power supply and connect across C17. Slowly increase voltage until it starts drawing excess current. Now you can find the fault. Of course with the amp unplugged from the mains. Test TR01 first. It may not be faulty. KSC2690 is fine for replacement if it is. TR01 is a ripple filter that doesn't have to work very hard. The fuses are 1AT and that is very ambitious for the power needed in the preamp section. D01,2 can be replaced with 1N4001. If they are faulty. Check with your multimeter on diode test.

 

[Zeus12]

Hi there again,

Yes, it was the TR01 that was at fault. Looks like it was "dead" since it was reading 8pF.


Replaced it with a huge KSC2690:


And yessssssssssssssssssssssss!!! It´s singing!!! Love that feeling!! Like a blind man that sees the LIGHT AGAIN!!! So that was the problem. Also, found 2 1A fuses from an old mouser order!! Not blowing any fuses, BIAS is ROCK STABLE and DC is also within range!

Still pending to fix the pilot light, but in the meantime, I´M A HAPPY CAMPER!!

Obviously I couldn´t have done it without the help of all the audiophiles on this forum! Learned so much, and all the problems paid off in the end.

"Happy is the man who finds wisdom, And the man who gains understanding;"

Cheers!!!

 

[PE9ZZ]

What this doesn't explain is why it was eating fuses. An open transistor will not cause that. But good to hear you got it going!