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Technics SA-303 Help

I agree, lets focus more closely.
Lets do dcv comparisons between Left and Right channels:
Feeds both channels, Q603:
E:
B:
C:

IC601 diff amp pins
1:
2:
3:
4:
5:
6:
7:
8:
9:
Q601:
E:
B:
C:
- on C609:

IC602 diff amp pins
1:
2:
3:
4:
5:
6:
7:
8:
9:
Q602:
E:
B:
C:
- on C610:
 
Let me get those.
let me ask, how does the positive 34 V show up all over the place with Q601 out of circuit?
 
Q601 is the constant current source input to the IC603 inputs and is supposed to be controlled by IC601 to keep IC603 pins 4 and 7 at 0vdc (controls dc offset). Q603 is a ripple killer that smooths the +40vdc main dc supply down for the +39vdc supply to the Diff Amps IC601 & IC602. Some of the +34 comes from pin 6 of IC601 to control Q601. If you noticed, with Q601 out the voltages on pins 0, 1, 4 and 7 of the STK (resistors) all dropped to -30vdc, close to the negative rail voltage. Q601 is doing part of its job pulling up those points, but not correctly. Somewhere in this circuit the dc negative feedback to pin 2 of IC601 is being disrupted or there would be 0vdc on pins 4 & 7 of the STK.
 
Here are the readings. Then I will read your explanation above.
Q602
E 33.0v
B 33.8v
C 34.2v

IC601
1 -80mv
2 -83mv
3 -85mv
4 not used climbs from 0v to 240mv
5 -6.3v
6 32v
7 31v
8 33.2v
9 33v

Q601
E 32.7v
B 32v
C 32.7

C609
-46mv

IC602
1 -71.4mv
2 -69.2mv
3 -350mv
4 0
5 -6.3v
6 32.2v
7 31.7v
8 33.2v
9 33v

Q602
E 33v
B 32.5
C 1V

C610
-33mv
 
I forgot to mention the only thing out of circuit now is R601 and the STK. And the ribbon cable from the output to the headphone and speaker.
 
It looks like there is a short from Q601 E to C because they are both the same voltage. Measure dcv directly at E to C, E to B and B to C to get a more accurate reading. A transistor in saturation would still have a bit of a voltage drop E to C. It does look hard on because the BE voltage is ,7vdc. Look at the traces in the area, follow them around looking. What transistor is in place for Q601?
 
powered down it does not read as shorted.
Also before you got involved I had to replace C627 because it popped. I do not remember the conditions at the time. I am sure that is why I lifted the ribbon cable.

Here is so more info I should have mentioned.
When I first got this thing. I am not sure its real history. But the guy just said left channel is out. So I hook up it up right channel first. playing music. As soon as the left channel is hook up, terrible noise. I am pretty sure that both speakers can be connected as long as speaker switch is off or if balance i all the way to the right. Speakers ohm out and are balanced.
 
Last edited:
Didn't you say both STKs you have worked in the right channel correctly? And neither one worked in the left? And both of the ICs work in the right but not the left? I usually advise people to have the circuit in good shape before installing a 'new STK'. We could make both channels match by putting resistors in the right channel also. I think there is still a problem on the board we have not located. Your call, I have been wrong before, and have not worked on this model.
 
Correct I have moved the components and the problem remains the same. I just want to let you know everything i can think of. The right channel always works. the left has never worked. I am ready to make any changes you can think of.
 
OldZofTorC said:
Correct I have moved the components and the problem remains the same. I just want to let you know everything i can think of. The right channel always works. the left has never worked. I am ready to make any changes you can think of.
Click to expand...
Yup, just doing a quick recovering of where we are at so I don't ask you to do something, and caution against something, to damage things further (including your wallet).
 
Since it was not working when you got it we need to look at a bigger picture and consider what mistakes someone else made.
I thought I saw a post where you said C627 popped and you replaced it, but now I cannot find it again? Was I imagining that?
Unsolder and lift one end of R626 and measure resistance, sb 100Ω.
 
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