You are right.
Emitter to base Q601 is also .75
Emitter to base Q601 is also .75
Technics SA-303 Help
- Thread starter OldZofTorC
- Start date
You are right. Q601 E-B is also .75v
merlynski
Curmudgeon Electronicist
Replace Q601 and Q603, if that does not bring IC603 pins 0 and 1 back into balance around 0vdc, then I believe IC601, AN7060 is blown.
Replacement for Q601 suggestion: 512-KSA1013YBU
Replacement for Q603 suggestion: 610-2SC1815
(Mouser numbers)
EDIT: One other possible test, unsolder and lift one end of R601 (2.2kΩ), power up and see if those pins balance.
Replacement for Q601 suggestion: 512-KSA1013YBU
Replacement for Q603 suggestion: 610-2SC1815
(Mouser numbers)
EDIT: One other possible test, unsolder and lift one end of R601 (2.2kΩ), power up and see if those pins balance.
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I had previously replaced Q601.
I did not have that replacement for Q603 so I lifted R601 and I now have balance.
34v and -34v.
Pin 1 19v Pin 0 32v
I did not have that replacement for Q603 so I lifted R601 and I now have balance.
34v and -34v.
Pin 1 19v Pin 0 32v
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merlynski
Curmudgeon Electronicist
That is the rail voltages on the dbt, if those are STK pin 0 and 1.
What is the dcv on the + side of C601?
On the + side of C602 (the other channel)?
merlynski
Curmudgeon Electronicist
Is that the voltages with R601 lifted?
merlynski
Curmudgeon Electronicist
OldZofTorC said: ↑
Thank you for the time. My computer came back on. Thank goodness. I was trying to use my gf I pad. That thing is impossible. I have to go out to help someone on a ranch tomorrow so that is going to eat up a bunch of the day just in travel. But I do want to keep trying. I am starting to see how this circuit works and this is a good lesson on how amps are designed. Can you tell why I am having the problems around Q601. Is there any way to fake it out. I see that is what you are trying to do with R601. Is there another way to isolate the circuit between AN7060 and Q601?
No way to isolate because the problem is subsumed in the feedback loop. If you break the loop you can't tell where the problem is by measuring voltages. When the loop is closed the voltages are wrong everywhere. What I try to do is keep eliminating outside influences, like pulling up that resistor, removing the output fuse, etc. Active components, transistors and ICs, are easiest to die, so are prime targets for replacement. Transistors first, they are easiest to get subs for. Electrolytic caps age poorly, so they are also prime targets, but easy and cheap to replace. Resistors that have high current through them, or get hot are next, except they become first line targets if they show damage. Regular resistors are a pain because, for the most part you have to lift one end to measure, with one exception: nothing in an unpowered circuit will make a resistor have more resistance than its spec. Film caps are next to last, then ceramics. Hard to find ICs are last to be replaced, by definition. FWIW, I found some AN7060s just by looking online.
Ohm's Law and Kirchhoff's Laws need to become your friends. Measure voltage, think current.
Whew
Thank you for the time. My computer came back on. Thank goodness. I was trying to use my gf I pad. That thing is impossible. I have to go out to help someone on a ranch tomorrow so that is going to eat up a bunch of the day just in travel. But I do want to keep trying. I am starting to see how this circuit works and this is a good lesson on how amps are designed. Can you tell why I am having the problems around Q601. Is there any way to fake it out. I see that is what you are trying to do with R601. Is there another way to isolate the circuit between AN7060 and Q601?
No way to isolate because the problem is subsumed in the feedback loop. If you break the loop you can't tell where the problem is by measuring voltages. When the loop is closed the voltages are wrong everywhere. What I try to do is keep eliminating outside influences, like pulling up that resistor, removing the output fuse, etc. Active components, transistors and ICs, are easiest to die, so are prime targets for replacement. Transistors first, they are easiest to get subs for. Electrolytic caps age poorly, so they are also prime targets, but easy and cheap to replace. Resistors that have high current through them, or get hot are next, except they become first line targets if they show damage. Regular resistors are a pain because, for the most part you have to lift one end to measure, with one exception: nothing in an unpowered circuit will make a resistor have more resistance than its spec. Film caps are next to last, then ceramics. Hard to find ICs are last to be replaced, by definition. FWIW, I found some AN7060s just by looking online.
Ohm's Law and Kirchhoff's Laws need to become your friends. Measure voltage, think current.
Whew
Would you please study this drawing. I don't understand why there is no voltage drop across R619 and R617. The IC and Q601 are out of circuit. 
merlynski
Curmudgeon Electronicist
You asked why there is no voltage drop across R617 and R619 with IC603 and Q601 removed (temp resistors installed):
The current source for those two resistor comes through Q601 from the emitter of Q603 (+V supply), then through pins 0 and 1 of IC603, then through those resistors (-V supply). When Q601 is removed there is no path for current to flow, Ohm's Law, no current flow, no voltage drop. In this case it would come through the temp resistors IF Q601 was present. Q601 does two things in the circuit, it provide voltage amplification for the audio, and dc current to control the dc offset. The feed back for this comes from IC603 pins 4 and 7 going (eventually) to pin 2 of IC601 (diff amp inverting input). The temp resistors are there to close the feedback loop because IC603 is removed. The circuit behaves as it would (almost) normally EXCEPT there is no current gain (no IC603).
The current source for those two resistor comes through Q601 from the emitter of Q603 (+V supply), then through pins 0 and 1 of IC603, then through those resistors (-V supply). When Q601 is removed there is no path for current to flow, Ohm's Law, no current flow, no voltage drop. In this case it would come through the temp resistors IF Q601 was present. Q601 does two things in the circuit, it provide voltage amplification for the audio, and dc current to control the dc offset. The feed back for this comes from IC603 pins 4 and 7 going (eventually) to pin 2 of IC601 (diff amp inverting input). The temp resistors are there to close the feedback loop because IC603 is removed. The circuit behaves as it would (almost) normally EXCEPT there is no current gain (no IC603).
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Thank you. Tomorrow I will swap those Differential amps around.
I am going to have to put this aside for a couple of days until a new desoldering tool arrives.
I got the tool in Sat. So I just moved the An7060s around and the problem is the same as before. These are my reading on DBT with the resistors in place of the STK. The Left channel is fine.
1. 19v
2. -34v
4. 25.5
7. 25.5
9. 34v
0. 32.29v
I need to get my head back into this circuit so I will probably wait until tomorrow to try again. I'll start measuring traces looking for opens.
1. 19v
2. -34v
4. 25.5
7. 25.5
9. 34v
0. 32.29v
I need to get my head back into this circuit so I will probably wait until tomorrow to try again. I'll start measuring traces looking for opens.
Circuit is unloaded..
merlynski
Curmudgeon Electronicist
The next steps are to test every component in the negative feedback path, from pins 4&7 of IC603 to pin 2 of the AN7060 IC601. Look for (measure resistors) bad resistors, leaky caps and or open traces, or solder bridges. Along the green line and in the blue outlines.
GrundigXpert
Active Member
So you feel desperado because your Technics is faulty: Poor, poor, pitiful me... You're no good ! you seem saying while removing the cover. But after your effort (It's so easy !), you can hear a voice asking Try me again ! 
I know you have given me this lesson before, but the conditions are the same for both channels. Right channel works correctly.
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The next steps are to test every component in the negative feedback path, from pins 4&7 of IC603 to pin 2 of the AN7060 IC601. Look for (measure resistors) bad resistors, leaky caps and or open traces, or solder bridges. Along the green line and in the blue outlines.[/QUOTE]
Thank you I am on it.
Thank you I am on it.
So you feel desperado because your Technics is faulty: Poor, poor, pitiful me... You're no good ! you seem saying while removing the cover. But after your effort (It's so easy !), you can hear a voice asking Try me again !
Ha ha you said "Cover" .
I checked all the resistors in the circuit, but not the caps. I have found R507 to read 4.6ohm in circuit (120K) so I am going to follow that. R507 is correct out of circuit. Keying in on the Bass tone circuit. with the bass pot in the center I get a near short between the center and the outside taps on both sides and on both channels. If I take the pot all the way in each direction I wlll get 12k on one side from center and a short on the other on both channels. I will wait here for suggestions.OK it turns out that 4.6ohms is normal for the pot when it is centered(out of circuit).I have checked all the caps in circuit. Most do not register on my tester other than not being shorted. Do I need to pull them and get a reading? I am still suspicious of the readings around the bass pot.
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