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#1 baddboybill

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Posted 27 March 2012 - 10:36 AM

I have a mini Sharp where it comes in stereo but the LED lights up very dim. I changed LED and resistor but still only getting less than a volt at LED :hmmm: any ideas :hmmm: I have no scheme :sad:

#2 char

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Posted 27 March 2012 - 11:41 AM

YOU must use 1 volt smd led :yes: :surf: :thumbsup:

#3 baddboybill

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Posted 27 March 2012 - 11:49 AM

YOU must use 1 volt smd led :yes: :surf: :thumbsup:


The original LED was same. Little under 2v :hmmm: Theres just not enough voltage so I need to find out why :huh:

#4 Superduper

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Posted 27 March 2012 - 12:21 PM

YOU must use 1 volt smd led :yes: :surf: :thumbsup:


WRONG!

Bill, the stereo indicator lamp circuit is NOT as simple as you would think. This circuit normally operates by sinking the LED to ground through a control device. The cathode of the LED, if an led is used, usually grounds through either an MPX decoder chip (95%) or transistor. The Anode side of the led is usually fed a positive voltage. The cathode side, however might also see positive voltage, but less so. The difference, determines whether the led is sufficiently forward biased to light. The voltage at the cathode side will vary depending upon the MPX controller. Obviously, depending upon the appurtenant circuitry, and the algorithm for which the MPX decoder is designed, the threshold for when it lights and how brightly it lights will vary. In fact, the dropping resistor for that sinking circuitry probably isn't even the one you changed. Although this is how the stereo LED circuitry usually works, it varys depending upon the boombox, the mpx decoder chip used, etc. This simply isn't a situation where one can tell you change this and it's fixed. The voltage at the led anode can range from 3-9 volts. Who knows what it's supposed to be for your boombox? Also, what's the cathode voltage supposed to be? Yep, that's right, you probably will see positive voltage at both legs of the led. All I can suggest is that you acquire a manual and do voltage checks. What boombox is this anyhow?

#5 baddboybill

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Posted 27 March 2012 - 12:53 PM

YOU must use 1 volt smd led :yes: :surf: :thumbsup:


WRONG!

Bill, the stereo indicator lamp circuit is NOT as simple as you would think. This circuit normally operates by sinking the LED to ground through a control device. The cathode of the LED, if an led is used, usually grounds through either an MPX decoder chip (95%) or transistor. The Anode side of the led is usually fed a positive voltage. The cathode side, however might also see positive voltage, but less so. The difference, determines whether the led is sufficiently forward biased to light. The voltage at the cathode side will vary depending upon the MPX controller. Obviously, depending upon the appurtenant circuitry, and the algorithm for which the MPX decoder is designed, the threshold for when it lights and how brightly it lights will vary. In fact, the dropping resistor for that sinking circuitry probably isn't even the one you changed. Although this is how the stereo LED circuitry usually works, it varys depending upon the boombox, the mpx decoder chip used, etc. This simply isn't a situation where one can tell you change this and it's fixed. The voltage at the led anode can range from 3-9 volts. Who knows what it's supposed to be for your boombox? Also, what's the cathode voltage supposed to be? Yep, that's right, you probably will see positive voltage at both legs of the led. All I can suggest is that you acquire a manual and do voltage checks. What boombox is this anyhow?


Norm this is the QT27. I see what your saying about positive voltage at both cathode and anode.

#6 Superduper

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Posted 27 March 2012 - 01:06 PM

Norm this is the QT27. I see what your saying about positive voltage at both cathode and anode.


Exactly right. And the difference in voltage between the two legs will determine if there is enough forward bias to light the LED to proper brightness. Unlike a simple 2v+ on cathode and anode to ground, this is something that is not as easy to grasp. So when you say you are seeing less than 1 volt at the led, once again..... I don't know what that means. Are you measuring at the cathode? Anode? What's the voltage at the other leg... see what I'm saying? Conventional lighting circuits suggests that anode positive, cathode ground. But this isn't a lighting circuit. It's a dynamic indicator circuit and it's behaviour is dictated by the circuitry around it and what is happening in that circuit.

#7 baddboybill

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Posted 27 March 2012 - 01:59 PM

Norm this is the QT27. I see what your saying about positive voltage at both cathode and anode.


Exactly right. And the difference in voltage between the two legs will determine if there is enough forward bias to light the LED to proper brightness. Unlike a simple 2v+ on cathode and anode to ground, this is something that is not as easy to grasp. So when you say you are seeing less than 1 volt at the led, once again..... I don't know what that means. Are you measuring at the cathode? Anode? What's the voltage at the other leg... see what I'm saying? Conventional lighting circuits suggests that anode positive, cathode ground. But this isn't a lighting circuit. It's a dynamic indicator circuit and it's behaviour is dictated by the circuitry around it and what is happening in that circuit.


Yes I'm getting. .64 at cathode and 2.34v at anode so that must be to much positive voltage at cathode to not allowing it to light brighter :yes: I understand. Thanks Norm

#8 Superduper

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Posted 27 March 2012 - 02:03 PM

Yes I'm getting. .64 at cathode and 2.34v at anode so that must be to much positive voltage at cathode to not allowing it to light brighter :yes: I understand. Thanks Norm


Not necessarily. Do we know for sure that anode voltage is correct?

#9 baddboybill

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Posted 27 March 2012 - 03:19 PM


Yes I'm getting. .64 at cathode and 2.34v at anode so that must be to much positive voltage at cathode to not allowing it to light brighter :yes: I understand. Thanks Norm


Not necessarily. Do we know for sure that anode voltage is correct?


Unfortunatly without scheme no :no:

#10 baddboybill

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Posted 13 October 2017 - 04:20 PM

Help again, needed with FM stereo led not lighting. I’ve replaced 1 capacitor that was bad and VR1 with no luck. Stereo separation is still working fine. Was told led might be bad but I think chance is pretty slim on that diagnosis. Of course can’t locate service manual for Montgomery ward 3995a

#11 caution

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Posted 13 October 2017 - 05:13 PM

Never seen a Wards service manual, you might have to trace the circuit out a bit. How are your drawing skills? :-)

 

It shouldn't be much, the stereo separation function and stereo LED output are usually just pins on the MPX IC with a few passives in between.



#12 baddboybill

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Posted 14 October 2017 - 08:15 AM

Never seen a Wards service manual, you might have to trace the circuit out a bit. How are your drawing skills? :-)

It shouldn't be much, the stereo separation function and stereo LED output are usually just pins on the MPX IC with a few passives in between.


Thank you I’m gonna replace led and check path

#13 caution

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Posted 14 October 2017 - 07:35 PM

Good deal, if that doesn't work take closeups of both sides in the tuner area, maybe we can figure out how many parts are at play



#14 Superduper

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Posted 14 October 2017 - 09:39 PM

Montgomery wards actually do publish service manuals.  I have a few including the GEN 3998a, but no 3995a or the popular 3996.  As for the LED, did you do any testing?  Did you test for voltage at both cathode and anode side to confirm that there is at least 1.6v or more difference measured from either side to ground?  Also are you sure that it is actually locking onto stereo mode?  In other words, is there a noticeable difference in sound quality between stereo and mono modes?  Usually the difference is unmistakable.  



#15 baddboybill

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Posted 15 October 2017 - 07:56 AM

Good deal, if that doesn't work take closeups of both sides in the tuner area, maybe we can figure out how many parts are at play


I will try that thank you

Montgomery wards actually do publish service manuals. I have a few including the GEN 3998a, but no 3995a or the popular 3996. As for the LED, did you do any testing? Did you test for voltage at both cathode and anode side to confirm that there is at least 1.6v or more difference measured from either side to ground? Also are you sure that it is actually locking onto stereo mode? In other words, is there a noticeable difference in sound quality between stereo and mono modes? Usually the difference is unmistakable.


Norm I have not measured because I have to try and hook up so it will work but I can actually hear the difference when in stereo mode.

#16 baddboybill

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Posted 19 October 2017 - 10:22 AM

Ok was able to get led to light so it’s working. Replaced one bad resistor and resolderd a jump bar back in place but still didn’t fix issue. Pulled out this resistor which tested ok but this clear what sort of looks like capacitor thing. Anyone know what it is? Thank you

Attached File  9A3D65F9-EDC7-4B6E-9C8D-F580A138ABB7.jpeg   43.23K   0 downloads

#17 caution

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Posted 19 October 2017 - 10:25 AM

Like these?

cond6.jpg



#18 baddboybill

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Posted 19 October 2017 - 10:43 AM

Ok found out it’s a polystyrene capacitor. 1000j 50. Reading is zero on ohm meter. So can I replace it with a different cap or does it need to be polystyrene?

Attached File  5B8BFF13-3EDA-446A-BC09-62BAEE019FAE.png   559.05K   0 downloads

Guess I just want to know if I can use different type for this

#19 caution

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Posted 19 October 2017 - 11:33 AM

It shorted, good job finding the bad part!

Use the same type if you have the option. They are more expensive than ceramics and so they probably spent a little extra on it for good reason, either in a tuned circuit or to improve the quality of the audio signal. Ceramics, polystyrenes, polypropylenes, tantalums, polyester... they all have different characteristics at different frequencies. Some start possessing more inductance than others at higher frequencies, blah blah...



#20 baddboybill

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Posted 19 October 2017 - 11:41 AM

It shorted, good job finding the bad part!
Use the same type if you have the option. They are more expensive than ceramics and so they probably spent a little extra on it for good reason, either in a tuned circuit or to improve the quality of the audio signal. Ceramics, polystyrenes, polypropylenes, tantalums, polyester... they all have different characteristics at different frequencies. Some start possessing more inductance than others at higher frequencies, blah blah...


Thank you but I can’t seem to locate any. I have a 1000uf electrolytic but they have +- leads where the polystyrene do not. Not sure what to do

#21 caution

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Posted 19 October 2017 - 11:48 AM

I can send you one if you want, I have one I'm sure of it.

But if you have a dead box you could always pull a ceramic, poly or mylar cap off that and use it, as long as you can find one that says 1000 on it (letter at the end doesn't matter, it's just a tolerance value). But this value is picofarads, not microfarads, so it's actually a .001uF cap. It's also printed as 102 on them (10 + 2 more zeroes).

 

Poly film and mylars look like this

 

s-l300.jpgECQ.jpg



#22 baddboybill

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Posted 19 October 2017 - 12:11 PM

Can you check please to see if you have one. I appreciate it. ;-). Thank you brother

#23 baddboybill

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Posted 19 October 2017 - 01:23 PM

I just can’t seem to find anything that will work

#24 baddboybill

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Posted 19 October 2017 - 01:48 PM

Ordered some new ones. Thank youAttached File  B6757894-D7F4-4A6A-8793-197322565C22.png   315.77K   0 downloads

#25 Superduper

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Posted 19 October 2017 - 02:03 PM

It needs to be polystyrene.

#26 caution

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Posted 19 October 2017 - 02:24 PM

In the mail! Should be there Saturday or Monday

There's some wax on it from the radio circuit it was in, I didn't test it with a meter but it should be fine

37766730352_6b80ea65b6_b.jpg



#27 baddboybill

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Posted 19 October 2017 - 02:36 PM

It needs to be polystyrene.


Oh great

In the mail! Should be there Saturday or Monday
There's some wax on it from the radio circuit it was in, I didn't test it with a meter but it should be fine
37766730352_6b80ea65b6_b.jpg


Very cool Eric is that same one as mine? You have my address cuz I’m in Florida now?

Thank you bro

#28 caution

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Posted 19 October 2017 - 04:15 PM

Yeah I still had it from when I sent you those M70 antenna bases that I thought were original :lol:



#29 baddboybill

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Posted 19 October 2017 - 04:52 PM

Yeah I still had it from when I sent you those M70 antenna bases that I thought were original :lol:


Thank you so much Eric :-)

#30 Superduper

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Posted 19 October 2017 - 06:10 PM

Bill, did you test that cap in circuit before removing it?  In general, if you are testing a capacitor with an ohmmeter, then I presume you are checking to see if it's shorted, because that is the only meaningful test really for a cap of such low value, and tested in circuit, a short will definitely be obvious.  The only other thing that you can check with an ohmmeter on larger electrolytics is to see if the value counts up before going to infinity.  That is the cap ramping up it's charge as you supply current from the meter.  However aside from demonstrating that the cap is charging, it's hardly an exacting test.  On such small values as these, the charge is so small that it ramps up almost immediately, probably faster than the DMM could even register.  Anyhow, next time you see a styrol (polypropylene) cap in a tuner -- I highly suggest that you DO NOT randomly remove it for testing.  IN GENERAL, they hardly ever fail.  The MPX decoder fails far more readily.  I would say 100, perhaps 1000 MPX decoders will fail before one of those caps fail.  On the other hand, if there is 1 capacitor that is heat sensitive enough to be easily damaged through the soldering/desoldering process -- it would be the styrols.  You should probably heatsink the leads before removing and installing.  Once it's been removed, there's no way to know for certain if it failed due to the soldering.  Circuits that use styrols only do so because the circuit is highly sensitive and in tuners, especially tuned circuits that require stability, styrols are the way to go.  Not only do they have far better tolerances but they also are very stable temperature wise.  Value drift is something you do not want in a tuner.  Let's hope this fixes your problem and if it does, you lucked out in that #1, you found a defective cap that hardly ever goes bad and #2, you hopefully didn't throw your tuner out of alignment.  There may be other reasons for a stereo indicator light to not illuminate but you really need to study the circuit better to understand how it's being lit.  Is the MPX decoder chip doing the actual sinking/driving of the led?  You can tell by simply finding the MPX decoder chip and studying the datasheet.  Or does your circuit even utilize a MPX decoder chip?  Or does the indicator LED require external drivers (transistors?).  Obviously, if those are bad or not working properly (bad transistor is exponentially more likely than a bad styrol), then the LED will not light.  For example, the GEN3998A uses transistors to drive the tune/stereo indicators.  Although not unheard of, it is far far less common than circuits normally found in boomboxes.  If this still does not fix your problem, you might want to take good photos of the area top and bottom and maybe Eric can help you attempt to draw out a small schematic of the relevant circuitry and see how the LED is actually "driven."  Just my 2-cents.