Hello all, I have a curious issue I am wondering if anyone here has advice or a solution.
I am finishing the restoration of a Sansui AU-919, recap, removal of problematic black flag capacitors, etc. The amp is finished and working, powers up, comes out of protect, bias and offset on the amplifier working.
I have low volume distortion that I have narrowed down to the flat amp / tone control board that disapears when the volume is turned up. I have been chasing my tail trying to find the issue... there are DC Offset trimmers on this board. When set to 0V DC the distortion is present, when I tweak the offset to allow for 50-60 mv of DC the distortion goes away. I'm wondering if this might be a lack of power to some of the transistors that is keeping them from turning on...
Schematic of this board below. Any ideas greatly appreciated. Thank you!
I am finishing the restoration of a Sansui AU-919, recap, removal of problematic black flag capacitors, etc. The amp is finished and working, powers up, comes out of protect, bias and offset on the amplifier working.
I have low volume distortion that I have narrowed down to the flat amp / tone control board that disapears when the volume is turned up. I have been chasing my tail trying to find the issue... there are DC Offset trimmers on this board. When set to 0V DC the distortion is present, when I tweak the offset to allow for 50-60 mv of DC the distortion goes away. I'm wondering if this might be a lack of power to some of the transistors that is keeping them from turning on...
Schematic of this board below. Any ideas greatly appreciated. Thank you!
Unfortunately I don’t have it on a scope. I can try to hook it up this weekend if that would help though…
When I exclude this board from the amp with the jump switch the distortion goes away. Also when I turn the volume up it decreases to almost nothing. Since it is both channels and in the front of the unit I thought it might be the power supply. This unit has a separate power supply for the flat amp… Hard to say…
You need to check all diodes and their capacitors. Also, R51 and R52.
There are some voltages mentioned on the diagram, check and confirm the voltages are okay...
There are some voltages mentioned on the diagram, check and confirm the voltages are okay...
Please check carefully the operating mode of the 2 fet bulbs T01a, T01b and transistors Tr01, Tr03. Looking at your waveform, I think it is the operating mode of those 4 transistors that causes it.
I would check the +/- 0.4V at the emmiters of tr13/15 first. Then D07 and voltages around it, it looks like output pair lost bias or the rails are low. Isn't C09 inserted backwards?
Last edited:
D07 is a double diode so if you replace it you need 2 1N4148 or similar in serie. C09 is a good tip.
Much appreciation for all of the help, thank you to everyone who commented. 🙏
Below are the measurements I took this morning. It does look like TR 13,14,15,16 are not turning on. I also checked resistors R51 & R52, they both measure approx. 39k Ohm. The schematic lists R51 at 120k Ohm. Strangely these are not listed in the parts list in the service manual for this board.
Would this group advice changing those to 120 ohm. ?
Thanks again in advance for any advice on this.
Below are the measurements I took this morning. It does look like TR 13,14,15,16 are not turning on. I also checked resistors R51 & R52, they both measure approx. 39k Ohm. The schematic lists R51 at 120k Ohm. Strangely these are not listed in the parts list in the service manual for this board.
Would this group advice changing those to 120 ohm. ?
Thanks again in advance for any advice on this.
Based on the voltages there is no bias voltage because the string doesn't conduct at all, which it should, over the diodes. Are you sure the C09/10 are installed correctly? Caps may cause short when in opposite polarity and that could take that Tr11/12 down there.
Rule of thumb is never start changing parts widely. Is that resistor marked 39k?
Rule of thumb is never start changing parts widely. Is that resistor marked 39k?
C09, 10 will not short with 1.xV applied or not in short time.
There is current in the driver stage, as indicated by the dc test
voltages. D08 is the main suspect for a short.
There is current in the driver stage, as indicated by the dc test
voltages. D08 is the main suspect for a short.
Thank you as_audio for your post. That solved it. It was my error.
When doing the recap, I replaced C09 & C10 with wima film capacitors. This made C11 & C12 unnecessary. When removing C11 &C12 I mistakingly replaced them with a jumper creating the short you described. I just removed the jumpers and the transistors biased. The amp is sounding clear.
Again, much appreciation to those who contributed to this thread.🙏 I had been chasing this problem for days. 😱
I have a full build thread documenting all of my work on Audiokarma if anyone is interested in what was done.
Build Thread
Peace and happiness to all.
BTW, this board was difficult to get to, photo below of my test setup, might be helpful to someone in the future.
When doing the recap, I replaced C09 & C10 with wima film capacitors. This made C11 & C12 unnecessary. When removing C11 &C12 I mistakingly replaced them with a jumper creating the short you described. I just removed the jumpers and the transistors biased. The amp is sounding clear.
Again, much appreciation to those who contributed to this thread.🙏 I had been chasing this problem for days. 😱
I have a full build thread documenting all of my work on Audiokarma if anyone is interested in what was done.
Build Thread
Peace and happiness to all.
BTW, this board was difficult to get to, photo below of my test setup, might be helpful to someone in the future.