To all vintage enthusiast & FM experts, I had this unit on my bench for a while now.
When I feed the unit (via Variac) 60 VAC the FM band is perfect, low noise, sensitive station tuning even without an antenna.
No stereo light (sound is only present when pressing the mono button).
As soon as I plug it directly to 120 V there is distortion & noise all across the FM (stations are still there but much lower sound quality & garbled).
AM & all other inputs are working fine.
The 76 Khz signal on PA1001A demodulator is present until the a certain voltage threshold then it disappears.
The majority of the elect. capacitors have been replaced, perhaps it is a power supply issue or one or more components that are failing beyond a certain voltage. Looking for other opinions. Thanks
When I feed the unit (via Variac) 60 VAC the FM band is perfect, low noise, sensitive station tuning even without an antenna.
No stereo light (sound is only present when pressing the mono button).
As soon as I plug it directly to 120 V there is distortion & noise all across the FM (stations are still there but much lower sound quality & garbled).
AM & all other inputs are working fine.
The 76 Khz signal on PA1001A demodulator is present until the a certain voltage threshold then it disappears.
The majority of the elect. capacitors have been replaced, perhaps it is a power supply issue or one or more components that are failing beyond a certain voltage. Looking for other opinions. Thanks
Measure the DC power supply voltages at full AC line, and compare with the service manual info.
Especially look at the voltage regulators.
https://www.hifiengine.com/manual_library/pioneer/sx-680.shtml
Especially look at the voltage regulators.
https://www.hifiengine.com/manual_library/pioneer/sx-680.shtml
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The DC values are close not exact, there are no voltage regulators in this circuit just transistors.
Please see attached voltages taken when the unit is at full AC
Attachments
There certainly are voltage regulation circuits in the receiver.
Perhaps someone else can help you.
Perhaps someone else can help you.
In AM mode those chips have no IF signal applied, however the voltages look about right.
My 780 uses the same chips. You may have to scope the IF signal going into the PA3001
It could be a bad capacitor or something starts oscillating at higher supply voltage.
There is a 13.8 v regulator that is a zener and npn transistor. This will be on a small heatsink that will get hot.
My 780 uses the same chips. You may have to scope the IF signal going into the PA3001
It could be a bad capacitor or something starts oscillating at higher supply voltage.
There is a 13.8 v regulator that is a zener and npn transistor. This will be on a small heatsink that will get hot.
The voltages on 2SD712 are very close to SM (within half volt), most of the capacitors (electrolytic) have been replaced.
Something puzzles me on the FM front end: the voltage on the Drain of the dual gate mosfet is 11.6 V (it supposed to be 10.2 V) & only when I reduce the AC, to obtain such voltage the FM works fine. My analog scope bandwidth is up to 100 Mhz.
The fet may be weak, there may be a jumper or resistor supplying it that can be increased so the voltage there is lower.
Try a 220 ohm where R104 (47 ohm ) is.
Try a 220 ohm where R104 (47 ohm ) is.
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I can certainly try that, better today I received the parts from little diode, that includes a new Mosfet, will post results, thanks stock
I changed the DG-Mosfet and there are no improvements.
I will try checking and re-aligning the IF discriminator.
I will try checking and re-aligning the IF discriminator.
What is the regulator voltage when it works at half ac power? There could be oscillation on the power rail.
At 60 VAC...........2SC1384 (B)=22.9, (C)=24.6, (E)=22.4
2SD712 (B)=10.6, (C)=12.8, (E)=9.9
If I put the scope on AC mode on C404 for example it is showing excessive ripple, I may have to re-examine the capacitors in the power supply circuit.
If you have a zener around 9 to 10v try that where the 14v zener is. A 0 .1uF cap across the
B E pins of the 2sd712 may be required.
B E pins of the 2sd712 may be required.
I have stock of many components, I will investigate & try that tonight, thanks for the support, not giving-up
Tried swapping the 2SD712 for another beefy transistor without any significant results.
I need to understand if the local oscillator is working properly.
I will try feeding a signal into the PA3001A & checking the output.
I need to understand if the local oscillator is working properly.
I will try feeding a signal into the PA3001A & checking the output.
Just tested my sx -780, the front end works well from 6 to 18 plus volts. I did not go higher for fear of killing it. The pa1001 and 3001 should not go higher then 13 but will work on less.
The front ends look identical.
Further along you have a npn IF amp mine has a HA1201. If you scope the output from the transformer you should see a clean 10.7mhz sine with fm mod on it.
The front ends look identical.
Further along you have a npn IF amp mine has a HA1201. If you scope the output from the transformer you should see a clean 10.7mhz sine with fm mod on it.
There is a power wire to the front end that goes on when the selector is on FM. I put power here after pulling the wire leaving the IC's on the 13.5v regulator. The IF amp transistor could be oscillating at full power. This is after T1 which is fed by the 2SC535 mixer.
One of my tests consisted on pulling the red 13.6 VDX from the FM front end completely & feeding the FM front end board with a clean DC battery, but again that did not solve the problem. Yes there could be problems with the local oscillator and that transistor involved that you mentioned. But if I read carefully the specsheet of the PA3001A & PA1001A I notice that if the input signal is not high enough or that signal is detuned, the chip will actually shut-off and not let stereo pass altogether. More tests will be required. I have also a spare SX-580 that I can compare to. I will try scoping & probably post some pictures to share.
Bypass the IF amp transistor, it should still recieve strong stations. This transistor is on the main tuner pcb around the ceramic filters.
If you have a ksa1815 you can try that there.
There is a pot to set the local osc at 76khz to lock in stereo reception. If that is the problem mono is still clear.
If you have a ksa1815 you can try that there.
There is a pot to set the local osc at 76khz to lock in stereo reception. If that is the problem mono is still clear.