Sansui AU-9900 Left Channel Low Volume/Distortion With Tone Control On

[Thorbr]

Removed the Tone boards F-2575/F-2576 and desoldered ecaps & transistors. Did some testing and the caps appear fine, but with some
very low ESR values. The 47 microF reads below 1 and the 3.3 microF reads between 2 and 3. As I understand low is good.
For the transistor 2SC1708, HFE values for 3 are under 500 (spec. min.) at between 460 and 470. There is only 1 over 500.
For the 2SA798, the ones from the F-2576 board HFE values read between 288 (low) to 366 (high). Spec. min. is 250.
For the F-2575 board HFE values read between 510 (low) to 553 (high). All reading were taken with a Peak Atlas DCA55 tester.
I realize these are far from perfect tests, as I am not at rated voltages, but typically the Atlas has served me well in the past.
Anyone seen similar readings or should there be more dramatic differences?

 

[Thorbr]

Changing out the ecaps and transistors did the trick. I now have both channels functional, with Tone/Filters enabled. Still have to button
everything up, along with remaining clean-up, but looks promising so far. Thanks all for the help given.

 

[Thorbr]

Thought I was done with this, but after about a week, the right channel output, with phono 1 or 2 input selected, is very faint. Along the lines of
what I went through with the Tone/Filter board upgrades. The phono input specific Equalizer circuit board, F-2570, had work done by the previous
owner, with ecaps and all transistors, except for the 1708s replaced. I have pulled all transistors, specific to the Right Channel, including the previously
replaced ones and all are testing within spec. Planning on replacing the 1708s, but wondering if there is still something else, that I may be overlooking.
There is not much else on the F-2570, beyond resistors, some film and ceramic caps. If anyone experienced something similar, and/or can offer any input,
it would be much appreciated.

 

[lgdavis]

Rather than shotgunning this, I suggest that you signal trace it...
Inject a tone, (through a small capacitor) into same points in both channels, while monitoring a known good point, such as the inputs to the power Amp.
(assuming here that the power amp is OK,, verify that first by feeding the direct inputs - from a volume controlled source , be careful doing this !! )
Starting at the known good point, work your way from there, towards the main inputs, comparing the channels at each point.
Hopefully this will pin down where your problem is. My $$ is still on a contact somewhere in the signal path, but - maybe not .

Good Luck!

 

[Thorbr]

Rather than shotgunning this, I suggest that you signal trace it...
Inject a tone, (through a small capacitor) into same points in both channels, while monitoring a known good point, such as the inputs to the power Amp.
(assuming here that the power amp is OK,, verify that first by feeding the direct inputs - from a volume controlled source , be careful doing this !! )
Starting at the known good point, work your way from there, towards the main inputs, comparing the channels at each point.
Hopefully this will pin down where your problem is. My $$ is still on a contact somewhere in the signal path, but - maybe not .

Good Luck!

Thanks for the reply. As it turned out, it was the 1708s on the F-2570 board. Working fine now. In regards to signal testing on the Phono input,
what do you recommend? I have a frequency generator, but the lowest output setting is 100 mV. Fine for other inputs, but too high for Phono.

 

[lgdavis]

Thanks for the reply. As it turned out, it was the 1708s on the F-2570 board. Working fine now. In regards to signal testing on the Phono input,
what do you recommend? I have a frequency generator, but the lowest output setting is 100 mV. Fine for other inputs, but too high for Phono.

Not sure why you want to signal trace the F-2570, since that board is the phono preamp - and you said it's working fine now.
But you could just use a pot on the output of the signal gen to lower it's output - or make a simple divider out of fixed resistors.

Hope this helps...

 

[Thorbr]

Not sure why you want to signal trace the F-2570, since that board is the phono preamp - and you said it's working fine now.
But you could just use a pot on the output of the signal gen to lower it's output - or make a simple divider out of fixed resistors.

Hope this helps...

Just asking for future reference. Not planning to touch the AU-9900. Don't fix what is not broken - lol.

 

[Thorbr]

Hello, Reopened this thread as I may still have remaining issues with the phono section on this amp. Recently when playing some vinyl, I found that the volume needs to be turned up considerably compared to another audio input. For reference, on the volume control I am turning it from -51 to -42 db. This is in a relatively small office type room where I do not need much volume to begin with. I noticed a slight discrepancy, when I first made the repairs, to the phono input section, as outlined previously in this thread, but it seems to be getting worse, or maybe it is my hearing - lol. The TT cartridge is an Ortofon Red and my Phono 1 settings on the amp are 2mV and 50 kohms. Specs for the cartridge are 5.5 mV and 47 kohm. If anything, I should adjust the mV setting on the amp to the next higher setting (4mV), but I think that will only make it worse. Probably need to revisit the phono input board and make further updates, but I wanted to run this by others for some feedback/advice. Thanks in advance