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Pioneer sx1500t tuner woes. Help.

anderslober

anderslober

Yikes...not again!
My sx1500t is acting up at the moment.........it now has low FM output now. All other inputs, phono1/2 , tape hd, aux, etc, work normally.
With my trusty DIY signal tracer, I can hear the FM signal leave the MPX board..I guess this indicates that the signal'decoding' is working.
With the volume at 12o'clock , I can also hear the FM signal through speakers....a bit scary with the volume so high.....so, I do have output, just very, very low.
All switches are squeaky clean......deoxited multiple times......
Which tr controls the amplification of the FM signal? Any ideas would be hreat...thanks...:thmbsp:
 
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I may have found something.....I measured all the voltages on q201-204 on the FM IF board. Q201-203 measured normal like whats indicated on the scat. Q204 did not...where I should have 10v, I have 0.47v.......also where I should have 1.5v, I have 0.48v....the voltage going to the IC is 12.5v...as it should be.
Could the q204 be fried? Where the heck do you get a LM703L???
 
I am trying to source some LM703's. Are ECG703 and NTE703 good substitutes? Or are they they same? The datasheets are very similar.

I still think this is where my FM problem is.........a defective LM703........

(I know about NTE products.................)
 
try swapping q203 and q204 and see if the problem moves. Put them back when done.

and yes, uA703, ecg703, lm703, and (shudder) nte703.

Round package. If you get dips, get a socket and adapt it in....

Please post your voltage measurements at all pins,
especially if the problem stays with the location, i.e. the bad voltages don't move...
then I would look at c216 0.01uf as a rf ground point "decoupling"...

It sure can't hurt to swap in another C216 0.01uf

we HAVE on occasion seen ceramic disc caps short out...
 
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Hello Mark.....cool......will do........regarding dips.......are they actually mountable? I would have to drill a bit.....?
I will check the cap too........
 
anderslober said:
Hello Mark.....cool......will do........regarding dips.......are they actually mountable? I would have to drill a bit.....?
I will check the cap too........
Click to expand...

Hovering dip socket with wires to the correct holes. not pretty but serviceable. last case resort... get the can type if possible.

try swapping C210 and C216 (same value) first? avoids soldering on the IC's.
 
Ok....will do.

Here are the voltages.....starting at the black dot......and going clockwise on the legs.
Black dot:12.4v.
Next leg. :-0.48v
Next leg. :-0.53v.
Next leg. :0.0v
Next leg. :-0.52v.
Next leg. :-0.48v.

C216 voltages: -0.48/0.0v.
 
The NTE device may be your only option, unless you are lucky enough to find NOS parts or good donor equipment. Finding a linear package makes it even more difficult.
 
I found some uA703's today......gold legs.....metal can....real nice......freakin' expensive
......10sgd EACH! OUCH!
I will pop one in and see what happens.........should be good......
 
I have FM again.....changed q204 to a new uA703.........now all seems ok.....will run unit for some hours........
 
In an effort to getting this unit working optimally, I started checking all my voltages on the the FM IF board and MPX board. I found one big discrepancy on the MPX board.
According to the scat, q702/703 should have 5.1v on their collectors.......I have 12.5v. Am I reading the scat correctly?
 

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Also q708 collector should be 2.7v.....I have 12.48v............collector of q701 has 12.5v......should be 600mV........
What could be wrong?
 
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Ok...I had a sx1000tw on hand....as they have the same tuner/mpx section, I started to compare voltages........the sx1000tw works fine.......the voltages for both units were quite similar. Except when tuning the sx1000tw, the IND pin voltage goes up and down as you tune in and out of stereo(as it should). The sx1500t remains constant at about 21v. I have really strong signal and output on both units...........just no stereo light on the sx1500t. What controls the voltage on IND pin? All tr's are brand new ksc1845's on the MPX board.
Could q705 / OA79 diode be bad on the sx1500t?
Any info is good........thanks.
 
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I am willing to try and change the oa79........what is a good substitute? 1n34? 1n60? others.?

Below is info from 'english electronica':

Search (manufacturer PN): OA79
Results 1
63223 References in database
Max list size 10
For Audio IC please check Audio IC Circuits
PART NUMBERREPLACE PARTSUMMARYCASEPINOUT
OA79

Diode GermaniumAA113, AA119, 1N34, 1N54, 1N60
45V, 35mABox OA79 Pinout OA79
 
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