a moody television set

[a_vip]

I have an old, '80s era 13in Sharp TV with a strange problem. It shuts off for no apparant reason, and then wont turn back on again, sometimes for a few minutes, sometimes for days. When it shuts off, it cannot be turned back on either by the remote, or the button on the set, for an arbitrary period of time. Sometimes it will operate all day without a problem, other times it wont stay on 5 minutes. The picture is perfectly fine (when the set actually works) and the Tv doesnt seem to have anything else wrong with it. Also the problem seems to have gotten worse over time. I would think its problem with a relay (latching, or refusing to latch) but Im not terribly knowledgable about televisions.<p> I would like to repair the set, and was wondering if anyone had ever heard of problem like this, and/or had any suggestions. Any help would be much appreciated.<p>Thanks

[cato]

This is a wild guess...I'm sure more reasoned responses will follow. However, it sounds to me like a circuit breaker is kicking out and then eventually resetting. I'm guessing that one or more of the large electrolytic caps in the power supply is starting to dry out and fail...therefore something is drawing more current that it should, not quite a short, but on the way...THEM CAPS HOLD LETHAL CHARGE so be damn sure they are discharged before poking at them....

[Edd]

First jest /“querstion”…whatever is a VIP doing with<<an old, '80s era 13in Sharp TV>>? or is this just the one that eventually was relegated down to the pets quarters. <p>That vintage of Sharp didn’t use ckt breakers and Ivory Soap odds (99.86/100ths) on the fuses not being intermittent/loose, since you get recurrent operation..
Seeeems like that vintage of Sharp used a pulse width modulated SCR as its raw power DC rectifier with a feedback loop enacting that corrective action.
What you actually will need to do is get the units model number or chassis number and come back with it. I can then research its circuitry from schem data in our LRC/Tech Lib.
With no further information now, in the interim, if you will get an insulative probe……a dry dowel or plastic rod of 12-24in length, and always keep one hand in your back pocket for safety. Start with the set cold, free from AC power and only visually inspect for power resistors that might have crazed solder joints that have deteriorated from thermal stress with time. Also, look for darkened/discolored areas of the PCB that would be indicative of hot operating components/devices.
Have the back off the set and a large viewing mirror set up in the front. There will typically be one of two conditions, an operative set, or it being in the fault mode that you have been experiencing. Take the probe and initially only lightly jar the suspect component lead areas in the previously ascertained sites. Then you could move on up to heavier semiconductor devices like the SCR and the Hoz output Xstr and its nearby drive transformer as well as any of the physically larger sized electrolytic capacitors that are PCB mounted. A localization of a fault should typically result in your sets giving you a clue in your pictures operation or failing to your experienced fault mode.
That’s about all for now without further model info feedback.
Also adhere to Cato Cato’s caveat…..we were going to suggest that you keep both hands safely in your rear pockets and enact an orally manipulated probing…..but that might just prove to be too cumbersome. <p>73's de Edd
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[Chris Smith]

Ive got a almost brand new 24 inch [1980's] that does the same thing. Turn it on, and it flips off, wont stay on. <p>Im saving it for some ones grand kid to see what TV's used to look like? <p>And let them figure it out.<p>[ May 16, 2004: Message edited by: Chris Smith ]</p>

[a_vip]

First off thanks for the suggestions, and as to why a vip has an 80's era television, well, being a vip is no reason to let things go to waste <p> It seems I was a little off on the year for this set, it was manufactured July 1991, but it sure looks 80's. The model number is 13SB50.<p>Thanks again.

[dacflyer]

sounds like bad solider joints from heating up and cooling down,, usually around the power resistors..i bet if you jap slap the tv one time it will come back up..check the solider joints.i have had many tv's like that...low temp solider sucks....

[Timothy Rasch]

I have the answer for your Sharp 13sb50. I have it in my service tips program: replace d1713 12v zener diode and c1705 10uf capacitor. If I remember right is is 10 uf @ 250v in the power supply. I think I have the service manual too so you can write me directly if needed. Tim Rasch [email protected]

[Edd]

VIP:
Back to you with some T/S feedback for you on your Sharp 13SB50 …and….. in your sets particularly quirky case ….we know what SB stands for.
Am I to assume that you have already enacted a components/PCB inspection along with a thermally hot running components connectivity probing ?
With an unplugged set, pull the back cover and slide the chassis back for a top and bottom inspection. Two chassis landmarks for reference will be the tuner on chassis rear left corner and the flyback transformer at chassis right rear corner. The initial area of interest will be centered in between those refs. Take note of the white 7W 2.7 Ω WW fusible/current inrush pwr resistor at center chassis rear. Just to its top left corner is your units power relay, black case. Sq pancake housing. The metal film pwr resistors of most import are the 4 just forward of that relay 2x of 270Ω and 2x of 2.2kΩ values. Those units cook 24/7 to provide the sets standby power supply Found that cluster?...…then take the front 2 and look linearly right almost to the chassis edge where a pair of 2.2kΩ metal film res are seen and take note of them and also of the proximity of your black hoz driver transformer at their apex’s corner. Shift backwards towards the rear of the chassis from that point where U will then see the plastic Hoz output pwr xstr on its quasi-rectangular “U” heat sink located at the edge of the chassis.
Move to the bottom side of that heat sink and at its open end will be the black SIP casing of your power controller IC, labeled IC701.The next item for U to ID is the
Pwr SCR regulator (D711) which is in its heat sink and located about ½ way between prior IC701 and the raw BT filter C706 (470 µfd/200VDC) [it’s beside the pwr relay from whence you did start]. Take note that there was one more power film resistor at the closest corner of the flyback xfmr in the area where you just were.
The next step in order would be to take a Sharpie and circle on the bottom of the chassis, EVERY component lead on EVERY one of the suspect components which I have sequentially pointed out to you. All Power/metal film res, the pwr relay,4 leads of the Hoz driver xfmr, Hoz out xstr’s 3 pins, IC701’s 7 leads, SCR D711,3 leads Also there are two (#11 & 12) protruding wire connections coming from the bottom of flyback xfmr’s buck winding, but I am unsure if they are numbered/marked , so mark all flyback bottom pins (~12x). (Typically all laid out in a quasi-circle).
Next step, you need to reflow the solder joints on EVERY one of those prior selected solder connections inclusive of a supplemental bit of fresh solder to every joint.
Needless to say the soldering iron should be of adequate wattage (or excess) to bring a joint to a liquid state prior to a “Two Mississippi” count. One can always enact a surgically precise joint forming with a higher temperature iron .But woe the potential damage/delaminating of a PCB’s pad/land that is subject to a marginal temp iron used for excess time on a fixed area. Also, the tip being perfectly tinned and continually maintained in that manner, with a damp sponge wipe between every joint with a supplemental fresh solder tin as required. Also the use of a rosin flux on each joint, even if it’s the amber rosin of Radio shack in the short white toothpaste tube. Works OK for me and is econmical and readily available
Final test would be reassembly and possibly daily test thru a timer/clock radios switched AC outlet etc with the detection of the turned up audio, located within earshot, to remind you each day at that time, all was working well on set start up to date.
Tim’s reference mentioned suspicion of the sets D1713 12VDC standby supply Zener clamp diode to ground which is at the end of the 2.2kΩ film resistor (cluster) I had mentioned at this posts very beginning. I just don’t find crunched Zener’s reviving. The set should just be dead/no turning on …ever with no 12VDC Standby. On the other item, the C1705 10 µfd/160VDC initial standby PS filter cap. That would be a PRIME/AUTOMATIC/#1 suspect if that elect cap was fed from a fast switching diode with its pulsed power being derived from a ~40-65khz switch mode power supply or even a piggyback flyback winding at its 15khz rate. Those freqs and currents just absolutely hammer electrolytics with time. However, the situation here, is that electrolytic is fed with common/bucolic rectified 60~AC. That caps relatively low µfd value is only responsible for ≥ 25ma-≤100ma max of supply current. Its worst case scenario is that the cap is located within ½ inch of the quad of warm running 4 metal film resistors, mind you, not HOT.
Well, try this, if it doesn’t nail it, a hanging out of test leads from critical voltage test points for timely monitoring would be next in order for further T/S analysis.<p>73's de Edd
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[email protected].........(Firewalled-Spam*Cookies*Crumbs)<p>[ May 25, 2004: Message edited by: Edd Whatley ]</p>

[a_vip]

Thanks for all the suggestions everyone. I haven't had time to do much with the set, but i'll be able to give it a good going over this weekend. <p>I did give the set a brief inspection though. I plugged it in, and the set would not power up, so I took the case off and had a look. The solder connections seem to be OK, and there were no visibly damaged components. It was quite dusty inside, particularly on and in the area of the of the flyback transformer. I got the bulk of it off and out of the chassis. About then I noticed a three position switch labeled "S1701 aging". That sounded promising to me. I couldnt find what component it refered to, but i assume its a switch,(i would think the power switch) as all the switches are senoted by S****, or S*****. Looking at the chassis from the rear the swith was in the far left position. Unable to resist the urge to play with it i set the swith to the middle position, replaced the back cover, and plugged the unit in.<p>Presto! The unit powered right up, but without me turning it on. The LED channel indicator on the front was not lit up, and I could not turn the unit off with either the button on the set or the remote. All I got was the black and white ants fighting. I unplugged the set, and moved the switch to the far right position. This time when I first plugged it in nothing happened, but to my suprise the unit now seemed to turn on and off! Does anyone know what the purpose of S1701 Aging is?<p>I used to have this set hooked up to a VCR and a small audio amp w/speakers and now I remembered why. The audio on the set is all muddy and garbled. This is probably really important, but I handnt remembered about it until then. There is a regular, somewhat low pitched occillation that occurs. This muddles the audio pretty badly. This distortion/interference is pretty constant, and does not change period, pitch, or intensity.(Like i said im not too knowledgeable about any of this) . I would think from the periodic nature of whatever it is that the likely culprit is the audio amplifier , but I checked the speaker anyhow. Hooked up to anouther source it worked fine, and aside from some sun discoloration it was fine. I replaced it anyway, with no effect.<p>Unfortunatley I do not have an occillioscope. In fact the only test equipment I have are a multimeter and a continuity tester. I was wondering, Could the winscope program offered on the website be used in this case? I havent tried it yet.<p>I took a look at zener D1713, as Tim suggested. It looked ok, connections look good, and i went at it with the diode tester on my multimeter and it checked out. <p>But the moody behavior of this old goat TV wasnt through with yet. After working perfectly (aside from the audio) for 2 days, it up and shut off on me. I was able turn it back on 2-5 minutes later.
This happened a couple more times in the next few hours. Then it worked fine for a half a day, and shut off for 3 hours. In my ignorance of what the TV's actual problem was, I retreated back to s1701 aging, but got no help this time. The middle switch still produced the always on, no LED numbers, but neither right or left positions allowed me to turn it on. After this three hour bout it returned to operation, worked for a day, and had another 2-3 hour bout. Very moody still. <p>I plan to give it a thorough going over this weekend, thanks to Edd for the diagnostic procedure ideas. I'd really like to figure out what is going on with this thing. Again I dont know much about TV's, but Id really like to learn. i will surly learn something in my fight with this one.<p>This is my first time posting here and I'm really suprised and grateful for all the help and suggestions Ive recieved so far. <p>Thanks everyone. <p>a_vip

[Edd]

Well, wonder if you have done the prescribed reflowing of the solder joints of specific potential trouble areas of your set that I had referenced to. That will eliminate those areas as being further possibilities of the problem.
The typical visual inspection that you subjected them to could have deceived their condition. I well realize how deceiving that they sometimes can be. In my times of failure analysis at NASA I had the use of state of the art stereoscopic microscopes with a variance of lighting media sources/types. As well as shifting to IR thermal micrography, if on a device in an operating state. Finally, if needed, then stepping up to use of a scanning electron microscope, then potentially being in the realm of up to watching molecules "drift".
One of two common modes of failure or marginality are dissimilar metal oxide disassociation, In such case, there is a poor melding of the soldered joints components. Primary cause ?.....an initial insufficiently proper solder "wetting" of the component , typically impeded by oxide presence on the lead or secondarily, excess storage time duration such so as to have unitiated an oxide buildup on a previously initially acceptable unit .
To see a classic case of this refer to the photo at the end of the post topic initiated by Hacklesup at:
http://206.131.241.58/ubb/ultimatebb.ph ... 1&t=001813 <p>Second failure mode is layer fracturization, where there are developments of minute fissures/crazes, usually quasi circular in structuring. These typically occur in joint areas where subject to higher heat than normal, mechanical tensioning, or stress by repeated hot/cold cycling. At a certain grade of degradation the joints remaining conductive surfacing area can be depleted progressively more if the joint is subject to moderate to high levels of current passage or high current pulses. Resistive hot spots can then vaporize off even more of the conductive solder materiel within the fissures.
When I see those last types of joint failures I would think that a comparative real world physical condition might be having a steel post or wooden post set into concrete and chain hoisting it and its bottom mass of concrete fill in and its associated earth clumping up and out, and then releasing it back down into its own footprint where it would then resettle in and seem normal to casual observation,….unless one should lean against it.
On your finding of the Sharps aging switch in the lower left quadrant, typically that unit would be of no consequence whatsoever as to your problem. However, if you bumped it into another position when cleaning up the chassis, a puzzling different story. Taking that switches manner of function into consideration, you might as well use it in the first T/S step after all the joints are reworked.
Seems like your set uses a u-control microprocessor in the standby mode and is always at the ready as long as the set is AC wall powered. One of its varied inputs is the serial digital data stream from your remote control hand unit. Another of its inputs is a sequential matrix scanning of the sets front manual pushbuttons for any action.
When you initiate a POWER ON command from either the remote or manually, the U/P decodes and enables a high output state on one of its interface output pins. This is routed direct to the base circuit of the sets npn sil relay driver transistor Q 1701, [hidden topside within the U/P metal shielded box at chassis top left corner quadrant] whose collector load is your sets afore designated POWER RELAY Ry1701. That .6-.7 VDC on its base received from the U/P switches on the relay to turn on the TV's main circuitry. Now, with that info at hand to relate to, it seems that the sets aging switch is a single pole 3 rotary positions unit with a protruding plastic tab as the mechanical positioning agent. Seems like...relying on 10yr deeep pipeline cache memory....that unit had only one active position , with the two other positions being blanks. Being sure of that data, but the fuzzy part was which the engaged position that was the aging function and seems like it was the top center position. To confirm, just have access to the Aging switch and plug set into AC power but don't turn on the set. One position of the Aging switch should turn on the set as it is switched into it. You see, the sw goes from the collector of the Relay dr xstr to ground in that one Aging position and would produce a response as if the relay dr xstr had been continually activated by the U/P power on command.
If you will place the set away in a secure place where it can safely left open case and AC powered then you can start random trials on it starting up from cold, and whenever it fails to come on, sneak over to the Aging switch and put it in its on position and see if the set will then come on as soon as it hits that position. If so, then that would be indicative of fault back up the U/P trail.[I Don't really expect it]
As for Sharps specific "Buzzword" designation of Aging switch, the only correlation I see is that if the sw was left in the Aging position, the set would definitely be aging......24/7.
Typical Aging sw use: Place the set in the Aging position if the remote set is to be utilized in conjugation with a switched/timer AC outlet, clock radio AC outlet, cable box outlet.
RSVP<p>73's de Edd
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