Sansui 350A strange "clipping" problem

[jeromach]

I bought this receiver from a guy that recapped it, but could not get it to work.

Well, I need to say it's not an easy one indeed.

The issue is that when fed a plain 1kHz sine wave the lower part of the sine is fine, but the upper part is rounded off, not exactly like clipping, but in that direction.

I managed to track the issue to the F-1248 board, but I really can't find out what's wrong with it.

I disconnected the board from the main amplifier board. The sine wave up until the monitoring switch is looking fine. Form there on it is deformed.

The lytics have been replaced, they alle are mounted correctly (checked the board AND the schematics)
The transistors present were 2SC1845, just to be sure I changed them to Fairchild KSC1845
The rail voltage is more or less ok. The manual says 24 Volt, I measured 22,something.
All solder joints checked, it all seems ok.

I just do not understand what could cause this, especially since it is on both channels.

If someone has a clue, it's more than welcome!

 

[smurfer77]

Is the problem on both channels? If not, you may be able to see where the voltages are different between L and R. Do the voltages look okay around those transistors on that board? Any thump when flipping the monitor switch?

 

[DougBrewster]

is it distorted at all power levels or only at high power?

 

[jeromach]

The problem is equal on both channels.

I did some voltage measurements on the transistors and it differs from what is mentioned in the manual (although the manual isn't particularly clear);

TR701
E = 2,0
C = 4,83
B = 1,46

TR702
E = 1,41
C = 4,78
B = 2,0

TR703
E = 1,98
C = 10,09
B = 2,63

TR704
E = 2,01
C = 10,43
B = 2,6

To me it seems the B of 701 and 702 should be 0. The manual states for C = 9,2 V and for E = 1,3 V (or 13, it is very unclear).

Then for 703 and 704 C = 13V and no other voltages mentioned.

This is what comes out of it;

 

[jeromach]

Meanwhile I made a more or less readable copy of the schematic of this part of the problematic board (F-1248);

 

[sregor]

Can't figure out the scales and where exactly you are measuring output (for the scope trace) The tone amp voltages look ok to me. Are you measuring the output of the amp or the tone board. How large is the input signal. what does it look like at the collectors of tr701 and tr702.

 

[jeromach]

The trace looks like this at the collectors of both TR 701 and 702

The signal is just fine at the balance pot and where it enters the volume pot. From there on it gets distorted. Let's say at around half way open it's like this at the mid leg of the volume pot;

What surpises me is that this problem occurs on both channels likewise. I'd suspect transistors or electrolytics place wrongly, since it had been worked on before, but I checked it rather thoroughly and I can't find what causes it.

 

[sregor]

This is strange. Still curious as to voltages at scope (thinking - maybe problem is due to low gain later on, and tone amp is being over driven to get full signal) Also, just to throw some spaghetti on the wall, could the coupling caps between the volume and tr701 and 702 be in backwards (C705 and C706) Manual shows them correctly, assuming the board matches the manual. + side of cap goes to the transistors and should be on the outer side - (manuals cam be wrong but this looked ok) Good luck.

 

[sregor]

After some thought, I 'm thinking maybe problem with collector voltage being so low on 701 and 702. Those transistors are gain of 10 inverting amps, and could be reaching saturation too early and causing the distortion. I'd check the voltage at the 220 ohm resistor - looks like R733? Goes to the 24 volt signal. Also HFE affects the bias points - because of the way its biased, a transistor with higher HFE will have a lower base voltage and higher collector voltage. Maybe KSA1815? Or increase the 82K resistors across the B-E junction. Most people don't like changing values in a circuit, but as one person pointed out, you already are changing things when you change transistors. My 2 cents.

 

[jeromach]

Steve, I owe you one on this one!

I swapped the KSC1845's for KSC1815's and hey, presto, it plays as if its life depends on it. Very nice sounding little Sansui receiver we're having again.

I swapped so many 2SC458's with KSC1845 in Luxman gear without a glitch that it did not even occur to me it could be an issue with this Sansui. Also, the KSC1845 was advised here giving the impression it was tested and working.

Thanks for helping me out!

 

[jeromach]

Here it is btw, very clean and nice!

 

[sregor]

Glad to see you got it going. It's rare that you see a problem like that which is device dependent.