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Looking for a tech,please help zenith r56w36 chassis sp02pc

7K views 42 replies 2 participants last post by  evilpea 
#1 ·
I have a zenith model r56w36 chassis sp02pc, I dont have a manual nor can I find one, sorry. I picked this tv up for a cool 20$ spot with a convergence issue..... I thought it would be a simple ic exchange but it turned into everything but. Found some swollen caps on the convergence board replaced them, esr meter said only one was bad but 4 others were swollen and leaking electrolyte so I replaced them as well. also found some resistors burned and replaced them with close to the same thing about .5 ohms more than what was there pluged in and turned on and they light up like a christmas tree jerked plug out removed connections to board and tested the 26+ and 26- volts in, which were where the caps had swollen, got about 50-VDC on both..... so I traced the power back to the main PWR board and happend to notice by accident more swollen caps on the 26+ and 26- output trac on the main PWR board, they were swollen on the bottom. The caps are c849 c850 c852. This is where I need a little help.... I took PWR board out so I can work just on it, when I plugged it in I wouldnt get any voltage so I looked at the board and found relay RL802 and think it is responsible for power on sequence so I shorted the gate terminals on it but am only reading 2 or 3 VAC on the secondary side of the transformer for 26+ and 26- and only 15VAC on the primary side. Am I shorting the wrong thing or why would I not be getting the voltage to the primary side? I tried tracing it out but its a little beyond me. It seems like everything on the primary side gets to D801-,4 terminal marked RBV-606, and stops. If anyone could help me I would be eternally greatfull.
 
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#27 ·
High output on a switching supply is usually a failure in the feedback circuit or the regulator itself. Almost never a transformer. The fact that both sides, +/- are high makes me think something on the primary side with the regulator or related parts.
 
#32 ·
I have come to a revelation the primary pins that drive the 26 feeds also drive the 12 ,8, and 5.5 volts I think if it was a primary problem wouldn't it mess with these voltages as well? I think it is picking up the extra voltage on down the line somewhere on the secondary side of the 26 feeds. I tested the voltage from the secondary pins of the transformer for the 26 feeds, they were both constant AC current, one was 8VAC and the other feed was 15VAC. I see in the schematic that diodes D817 and D818 rectify the voltage to DC I will get a reading on the cathode of these diodes tomorrow and see what the rectified voltage is.
 
#33 ·
Here are my strange findings. On the 26+ secondary terminal from the transformer it goes from a constant 8VAC/0VDC (before the diode/anode) D817 to 105VAC?/48VDC (after the diode/cathode). The 26- goes from 15VAC/0VDC (before the diode/anode) D818 to 48VDC/0VAC (after the diode/cathode). I Will look at the board tonite and see if I can figure out how the 26+ is being feed with mains power!?! Untill then any thoughts?
 
#35 ·
I have a general idea. I have not been in such an in depth circuit before. My first semester of EE is next semester or the one after that, depends on when my equipment comes in. If I were more experianced I would not be here looking for someone with experiance to help me, it pains me to be ignorant. I can not see why people with more experiance here can not see it as simply as I do. It does not seem that complex. I am supposed to be getting 26+ output I am not, I am getting double. I realize that after the coil/choke much of that high frequency AC current will disappear, probably to the voltage that is doubled 48V, here is my question why is there AC voltage after the diode period. The circuit is a bit to complex for me at this point, that is why I am here, to seek the guidance of someone more experianced that is willing to help me. The answer is not as simple as the problem. I have searched extensively for information that would relate to my problem and how to diagnose it, no luck. You are my last hope for intelligence.
 
#36 ·
There is nothing wrong with not knowing and seeking help. What we can do is limited, but sometimes we can be informative.

So the (very) basic idea is this...you have a.c. mains rectified by the main bridge diodes. That d.c. is applied to the switching transformer and switched on and off at a high rate (anywhere from a few tens to a few hundreds of kHz). That creates a field that rises and collapses in the secondary, thus the current that will be used by the set. The controlling IC on the primary side controls the rate of switching, either by varying the rate of a constant width pulse, or by varying the width of the pulses (PWM). In this case, the current is switched on/off by the FET in the IC. Note that the feedback from the optocoupler is applied to the input of the IC (also controlling the oscilator and providing the latch for overvoltage) as well as to the comparator that drives the oscillator. Control is effected by a combination of switching of the FET and saturation in the windings of the transformer ( control windings with the 2sc3852 transistor referenced with a zener diode). This is why it is unlikely to get an overvoltage problem on the secondary unless there is a problem in the feedback (like the error amp malfunctioning, but they usually will shut it down).

You are correct that on the secondary the diode rectifies, and the cap and coil filter what is left of the switching pulses.

Check the continuity of the grounds on the secondary side.
 
#37 ·
HA you just made me feel dumb, thank you it has been a long time since someone was able to do that. I just came to the realization that FET-Stands for field effect transistor, having known this since I was 10 it did not really occur to me that the FET was used to run the primary side in order to energize the secondary side. I knew that DC voltage alone would not energize the secondary side because the only change in field was when it was turned on and off, and thought that is why they used ac to power the primary...but I always seen rectified dc going into them and never could figure out why. Now I know its because of the (Field Effect Transistor). duh it makes since to me, NOW. I was not aware that the FET accomplished this by turning on and off rapidly. The reason I do not hink it is a primary side problem is because the primary, I think, drives four different voltages (26+&-V, 12V, 8V, 5.5V) If I am thinking correctly they wound the secondary windings to accommodate these different voltages from the primary windings, so the primary puts out constant and the secondary takes what it can get,with a certain amount of windings, to create the out put voltage that it is supposed to. If it were a primary voltage problem would it not affect all of the voltage outputs on the secondary instead of just the 26+&-? On the secondary ground continuity here are the results of it, all results with 200 ohm setting, 35V/115V GND to 40V GND is nothing, 40V GND to 26+&-V GND is nothing, 26+&-V GND to 115V/35V GND is infinate continuity or 0.1. Out of all of those the only one that does not have infinate continuity with the chassis ground is the 40V GND it reads nothing with the board unplugged from the rest of the chassis and laying on my desk for the test.
 
#38 ·
I am having a very hard time following your terminology. I don't know what you mean by "infinate continuity" though I believe it to be low ohms from your .1 value. I also don't know what "it reads nothing" means. Usually, an infinite resistance reading on a meter is an open circuit. Any other reading is whatever the resistance is, with 0 ohms being a perfect connection (which really does not exist). Most meters are not very accurate reading below an ohm or so, so you would consider fractions of an ohm to be a pretty good connection.

Yes, if only one voltage in a secondary is off, you would not suspect the primary. Look for a bad transformer, a bad connection, a cap installed backwards, IF everything is as described and you are using the proper references for your measurements.

It is very hard to give specific troubleshooting advice without having the set in front of me. When users are not familiar with the circuit design, do not use the proper or usual terminology, and time intervenes between posts, it is very difficult to keep track of what is going on. This is a perfect example of why we do not provide step by step troubleshooting in the forums. This is something that an experienced tech with the set present would have solved in a few checks. It is impossible to impart the observation skills and understanding of this kind of experience in a few posts dispersed over weeks.

I don't mean to be rude or to criticize your inexperience, but with a forum of over 70,000 members to administer, and a job and family, it simply is not practical to help everyone through every step of a problem. I hope that I have been informative and helpful, but I have to limit any more time on this. I must stick to our policy regarding specific troubleshooting advice and limit help to more general questions about the technology.
 
#39 ·
I thought infinite was universally standard, meaning forever, accompanied by continuity means forever continuity. That is why I also put the actual reading from the meter, just in case their is some confusion in the meaning of infinite. For the "it read nothing" That means the meter did not change so it did not give a reading meaning it read nothing. I apologize for the delayed post's by the time I get done with all my school work and home and work responsibilities all I want to do is relax, the week end is the only time I have a day to work on this. Their are no caps backwards but I am still using the swollen caps that I mention earlier because I do not have quick access to replacements so I was going to buy all parts at the same time ounce I get everything working right, I see no since in buying expensive convergence ic's if I can not fix the power issue that likely blew the other ones. Could these capacitors on the 26 feed that are swollen cause this voltage problem even though my esr meter said they were within tolerance? My esr meter reads esr&dc resistance.
 
#40 ·
I would not expect swollen caps to cause these high readings. The swollen caps are likely because of excessive voltage across them. There is obviously something that is being missed here, but I do not know what it is. I simply don't have the time to dig in to the circuit nor to go back and parse the info in the thread to see what might be going on.
 
#41 ·
Well thank you for your time anyway. I will not give up on it yet. I am still searching for an answer and will continue to untill I, get so fed up I just throw the board in the trash and kick the rest of the set across the yard or, find out what is causing the problem. There is not much between the diodes on the 26 feed and the output connection, this is where I am sure my problem is coming from. I looked at it a little bit today but did not get anywhere on it because of school, yet another seemingly endless week of it. I even went to the local electronic shop, if you can call it that, to look for replacement caps to ensure they are not the cause but they were closed for the holiday, but like you I think they are a result of the extraordinarily high voltage. I will still keep you/everyone else updated on the progress if I make any.
 
#42 ·
Well I can say with certainty that it is not the capacitors on the SMPS board, I never really thought it was but what the hey might as well I have ran out of things to test on the SMPS board. On to the convergence output board maybe something is causing some kind of weird feed back issue, I am thinking maybe a shorted transistor somewhere. All the ones I tested on the SMPS board came out good but it could still be intermittent which I can not test for because I have nothing to read them like a curve tracer.
 
#43 ·
Well I figured it out, all is well. There were no chips to pull the voltage down so it appeared as over voltage. Replaced everything and it works great. Now all I need is a way into the service menu so I can make some small adjustments to the convergence, any help with task would be awsome.
 
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