Help with FM Signal path circuit debug needed Technics SA-800

Discussion in 'Tuners' started by rocknroller, Jan 11, 2018.

  1. rocknroller

    rocknroller Active Member

    Messages:
    323
    I believe I am mistaken about there being good audio coming into AN363 Pin 2. I now don't see anything there (or out Pin 6 from AN377). This may be my ignorance of using a scope but I don't see any obvious signal there at all. With my scope set to 5V/div, 1ms, DC coupling (? should this be on?) and probe set to 1X, I see a audio signal at pin 13 (meter circuit?) of AN377 (same audio I confirmed with audio tester at that point) . I don't see any other obvious audio on any other pin now with the scope (swore I did in earlier tests with the other scope but maybe I'm confusing receivers)

    What should my scope setting be on with respect to voltage, time etc? Should I be 1X or 10X on my probe?

    Shouldn't I be seeing something at Pin 1 and Pin 8?

    Another interest point, I remeasured all voltages at that chip
    Pin1 2.2
    pin2 2.2
    pin3 2.2
    Pin4 - ground
    pin5 1.5 (muting off)/1.57 (muting on)
    pin6 5.8
    pin7 4.2
    pin8 5.7
    pin9 5.7
    pin10 5.7
    pin11 17 (***This subjectively seems very high)
    pin12 5.1
    pin13 2.8
    pin14 ground
    pin15 5v
    pin16 0
     

     

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  2. rcs16

    rcs16 Super Member

    Messages:
    3,496
    Location:
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    17V on pin11 of AN377, you have a PS fault, I think the limit for these parts is around 16V. Fix this issue first, hopefully the chips will survive this OV condition.
     
  3. rocknroller

    rocknroller Active Member

    Messages:
    323
    Ok - I need to track that voltage problem down.

    But I also need to make sure this new (used) scope is actually working correctly in case I have to return it. I was under the impression that with a 100Mhz scope I would be able to trace these signals down. Yet when I look at pin 1 of chip AN377, which should be the IF input, and the tuner is tuned to a station around 89 Mhz (that I can see with the scope on pin13 of that chip and hear with an audio tester) I see nothing but a straight line (1x probe or 10x probe). As I decrease the volts/div dial it ever so slightly increases until I reach 20mv at which point it disappears altogether. See attached pic of my scope and settings (I have touched nothing other than the volts/div dial and the time dial. I don't even know what anything else does!) The pic has an artifact of my camera speed vs display and looks like the first two grids are less bright but that is not the case when viewing with the eyes. Even assuming the chip is dead, which it likely is, Shouldn't I see an obvious signal coming into the chip or is the IF freq still outside the range of a 100Mhz scope? Or am I not using the proper settings somewhere on the scope controls
     

    Attached Files:

  4. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,418
    Location:
    Cleveland, Ohio
    First of all the scope probe needs to be a 100MHz bandwidth probe. Check that. Next, it needs to be set at 10X. If it is not a Tektronix probe with the plastic knob around the connector the scope will not automatically read the correct voltage when you switch the probe to 10X, so just be aware that what you measure on the scope is 10 times larger than what it says it is. So 20mV is really 200mV, which may be ok for an IF signal.
     
  5. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,418
    Location:
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    [​IMG]
    It's getting to be a PITA to flip back and forth to look at this, so here it is again. Pin 2 is the input, not pin 1. Looks like you should see audio at 10 and 11.
     
  6. rocknroller

    rocknroller Active Member

    Messages:
    323
    It is a P6100 probe labelled 100Mhz (not textronixs so I now understand the reading is really 10X larger. But why when i change the dial to the next lower setting I get no signal visible at all? Or anything below that 20 (200mv at 10x) I only see the line at 20 or higher.
     

     

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  7. rocknroller

    rocknroller Active Member

    Messages:
    323
    You are talking about AN363. I was referring to the chip prior that feeds it AN377. I didn't see pin 8 on that chip (source for pin 2 on yours) have any obvious signal or at least an substancial increase from what was on pin1 of that chip. In the original post(s)/other reciever that was a diag to see that the AN377 was dead. I was told the signal coming in at pin 1 should be much larger at pin 8. That is not the case on this receiver (unless i am not seeing the signals properly on the scope why is why I want to make sure the scope is working ok. - though clealry the AN377 chip could be dead here too because of the high 17v power)
     
  8. rocknroller

    rocknroller Active Member

    Messages:
    323
    whoops I stand corrected - Pin 8 IS much larger on AN377. it was off the screen I had to zoom out. So now I'll check AN 363
     
  9. rocknroller

    rocknroller Active Member

    Messages:
    323
    OK - I think I have a (semi) complete handle on this now AN377 data sheet pinout below
    an377.jpg

    I see something on the scope at pin1, larger something at pin 8. I see/hear and audio signal at pin 13, but virtually none at pin 6. My assumption was that pin 8 (IF output) feed AN363 Pin2 (IF input) but according to the schematic it does not. In fact, it is Pin 6 (audio output) that feeds pin 2 of AN363.
    an377_circuit.jpg
    Since there IS no audio output from pin 6, there is no audio input to AN363 etc. So the problem seems to lie around AN377. There is a Audio mute circuit (pin5) which winds through C104/R106 (both checked good) then down to D103 labelled as a "OA99 Switching diode" on the parts list then onto the front panel FM mute switch. This diode reads 0L and 0.22 when tested, which doesn't seem right, though I've never seen a diode of this type before. Shouldn't that be more like 0.5-0.6v?? If this part is bad, then I assume it is somehow muting the audio within AN377. If it is bad diode, what is a suitable replacement?
     
    Last edited: Apr 20, 2018
  10. rocknroller

    rocknroller Active Member

    Messages:
    323
    As to the power supply problem, that too is a mystery. The voltage coming from the power supply is slightly off, but not terrible. Possibly due to R708 reading 194 vs 180
    Technics sa-200 power_supply.jpg

    But where things go wrong is when it gets to the regulator circuit on the fm board. it never appears to get reduced, saved a couple volts.

    FM Poer regulator.jpg

    I checked the resistors, diode, swapped the lytic cap out, pulled the transistors and tested.
    Q752 (schematic says 2sc828 but parts list says 2sc1328-t ?) tested fine on a old B&K tester tester on low
    Q751 (2sc1398 on schematic, parts list says 2sc1398-Q) tested fine on low. When I accidentally turned it on high it failed the test and registered as two different type of transistors. So I'm wondering if there's a leak going on when used in circuit. I did not repull and re-test Q752 on high.

    Do those voltages shown validate that Q752 and/or Q751 are bad?
     
  11. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
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    Location:
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    What voltage do you read on collector of Q752?

     

     

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  12. I LIKE MUSIC

    I LIKE MUSIC Super Member

    I have not re-read this thread, but in general unless you are looking a DC voltage specifically you should use AC coupling. If there is a DC voltage present along with the signal that you want to look at, when you increase the sensitivity of the oscilloscope, the DC voltage may drive the trace off the screen.
     
  13. rcs16

    rcs16 Super Member

    Messages:
    3,496
    Location:
    Baldwin, Ontario, Canada
    The longer you run at elevated over voltages the more risk you take at burning up a comp.
    with power off, measure ohms, D751 in both directions, I think it is open. replace it. the zener can be any axial 14V/500mW zener diode. Also check R752 for value, isolated from the ckt.

    As said above, use the scope and DMM on the collector of Q752, it says 14.3V on the schematic.
    They should compare in value.
    There are times I only measure +ve DC voltages so you can ground the scope probe, move the x position to the bottom graticule, unground/measure, then you have more DC voltage range to display.
    Set the X position back to center to measure AC voltages for pk/pk readings.

    Do not worry about Q752 type, there are 100's of possible subs, it is just a small signal bjt. Usually one uses a 2sc945(ksc945c) for this job but a ksc1845 will do the job. Watch/check your pin outs.
     
  14. rocknroller

    rocknroller Active Member

    Messages:
    323
    I showed it on the diagram but it's hard to see - it's 21v at the collector of Q752
     
  15. rocknroller

    rocknroller Active Member

    Messages:
    323
    THANK YOU! That was driving me crazy and also explains why I couldn't "zoom in" past a certain point. I couldn't get the trace to appear on the screen no matter the the vertical setting. Very helpful tip!!
     
  16. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Glad that I could help.

    Good luck with your repair.
     

     

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  17. rocknroller

    rocknroller Active Member

    Messages:
    323
    As mentioned, I had tested those parts, though the Diode was done on the diode setting, not ohms. I just rechecked both. R752 showing 268 (schematic says 270 so ok), diode D751 on diode check shows 0L/.66 . On ohms it shows 0L/27.5Mohms. Do you still think the diode is bad?

    So also mentioned I get 21v there. Obviously high. Given the passive components all seem to check out, should I replace one or both of those transistors?
     
    Last edited: Apr 20, 2018
  18. rocknroller

    rocknroller Active Member

    Messages:
    323
    I also found they changed this regulator circuit in later series (starting at sa-400) from this
    FM Poer regulator.jpg

    to this

    SA-400 regulator.jpg

    They got rid of the 2sc828 altogether. I wonder why (maybe it's a systemic problem?) All other components are the same. Should I pull 828 out and 0.01 cap and replace the C752 cap with bigger value like the later rev? It certainly simplifies the troubleshooting
     
  19. rcs16

    rcs16 Super Member

    Messages:
    3,496
    Location:
    Baldwin, Ontario, Canada
    The first version did look rather weird, the 2nd is a regular design.
    You can wire up as they show in the 2nd version, it will work fine.

    A zero means a short , same as when you short your probes together. if the diode measures zero in any direction, it is shorted out.
    A zener should measure on the DMM the same as any other Si diode.
     
  20. rocknroller

    rocknroller Active Member

    Messages:
    323
    Sorry - My incomplete shorthand - not 0. 0L (infinity) I believe that diode is fine. However I am concerned about the other diode I mentioned that is driving the FM muting circuit (D103 labelled as a "OA99 Switching diode". This diode reads 0L and 0.22 when tested - the 0.22 concerns me unless that is normal for a "switching diode"
     

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