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Troubleshooting Techniques, Tips

A Universal Convergence Technique - submitted by Edd Whatley

When confronted with a Big Screen TV and not having its manual to confirm the locations of and adjustment functions of the pots nor the presence of I.D.'s silk screened on the board . I utilize the following technique

Initially make small reference "tics" on the pot shaft and its case on all the affected pots with a micro Sharpie permanent marker (blue seems to work best). This way you can go back to the starting point(s) if necessary. Next I utilize a test aid made up from a test lead ~ 2 ft long with a standard size E-Z hook on each end. Cut the lead in half and solder a momentary contact n.o. spst push button switch to the cut ends. My unit has the mini switch enclosed in the round plastic shell salvaged from a standard size phone plug (guitar...not telex). This permits all the fingers to grip it and the thumb to "blip" the switch rapidly. Hook up your cross hatch generator to the set and set up a large service viewing mirror back far enough so that you can look back to it and get a full panoramic view of the whole TV screen. Take one of the E-Z hooks and affix it to the center wiper pot of the specific pot you are about to evaluate, place the other hook on either of the pots outer terminals. A quick "blip" should reveal the color you're working with and it's displacement should clue you in as to its function e.g. hoz or vert bow, skew, trapez, lin etc. Then you can make a sheet for reference until you locate the needed controls that need alignment. Should you not get a pattern displacement you probably chose the outer pot terminal that already had its wiper element set close to that end , so swap to the other terminal. Not too many sets utilize variable inductors on the board anymore, but when they do, you can suspicion it's a hoz type of adjustment. If the coil utilizes a ferrite screw core I use a mini pocket screw driver that has a small magnet on the end of the handle. Placing this in the inductor shell will skew the inductance greatly and you can surmise its function. BTW this technique is very useful in rf circuits if you will use an allen wrench that just pass thru your slug, this way you can see the tuning effect on your circuit or identify a coils function without disturbing its initial setting.

 


Repairing LCD's - submitted by Jeff Roberts. Jeff is a member of our email discussion groups.. All subscribers have access to his, as well as over one hundred other technician's comments and expertise.

If the lamp is broken of course - replace it. Otherwise a bad lamp will still light. It just won't be a nice white color. More of a reddish orange, only lit on the ends or flickering just like any other fluorescent lamp. You can, of course, try to measure the output of the inverter but with the loading effect of the meter/scope and the fact that it's a high freq AC output I wouldn't know what to look for. Lamps are only about $10 so that's where I start. Even if it doesn't solve the problem you can still charge the customer, you just tell them there was no sense putting a new inverter onto an old lamp so you replaced it while you were "in there". I personally wouldn't want to warrantee a laptop with a new inverter and a used lamp. If it comes back next month with the old lamp burnt out you'll be replacing it for free under warrantee. The customer doesn't know invertors from lamps so they'll assume because the display went black last time and the symptom is the same that you'll cover it. Lamps and invertors can both be had from JKL through Digikey. The biggest problem with invertors is their physical size and mounting holes. I read the warrantee card on a KDS LCD screen a few months ago, in fine print down at the bottom it said that the lamps AREN'T covered by the warrantee. This means that if the lamps go out while it's under warrantee the unit goes to an authorized service center and they change them but they get to bill the customer for the work. This may increase the revenue of those that are authorized because they will now get paid for a lot more units by the customer instead of KDS but it's also going to make for a LOT of very unhappy customers. I currently get $135 Cdn to change the two lamps in an LCD. We always change both, no cheapskate onsies thank you very much. Lamps are about $10US - $15Cdn ea so labor shows $100 which is comparable to what I used to get for a monitor repair. My advice is to go to the places that sell these and get in touch with the manager, show/explain to the manager that the lamps aren't covered and that if his customers come in with this symptom they will be better served by sending them to your shop for faster service. A lot of the big box outlet stores take the returns and send them off to a depot. This could leave the customer without his unit for up to a week and they'll still get a bill. In the next 5 years a lot of these will be failing, the lamps only last a couple of years if left on all the time. Although I strongly disagree with the idea that LCDs will replace the monitor any time soon (read in the next 10-15 years) I don't let opportunities slip by either.

 


Testing TV/Stereo/VCR infrared remote control - submitted by Calvin Smith

Using a camcorder, look in the viewfinder while the camera is looking at the output of the remote. The camera is very sensitive to infrared and works as a great converter so you can see the pulses of data sent by the remote.

 


Testing remote control using ordinary AM radio receiver - submitted by Rhonn

Use an ordinary AM radio for testing remote control by simply pushing some remote button near the radio and moving the dial near the 500 Khz. If the remote is working you can hear the tone burst from the remote oscillator.

 


Remove IC's with tiny pc runs and clearances

Plated-thru holes with small clearances are the worst. In these cases I sometimes use a dremel tool with a cutoff wheel to cut the leads free of the chip. The leads can then be individually heated & removed by using just an iron. The holes can be be sucked clean after all the leads have been removed. This is probably a slower process, but it guarantees that no foils will be damaged in the procedure. This works well for SMD devices too.

 


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Repairing speaker amps - submitted by Scott Ross

I have a '90 Legend LS with the Bose individually amplified speakers. Three of the four speaker amp combinations were very low volume with some motorboating (low freq. oscillation) in the rear speakers. I replaced all the electrolytic capacitors (the can-shaped ones) with new ones, as I have heard they leak out their electrolytic fluid over time, causing the gain to go way down, as well as the motorboating and, in some cases, a whining sound. I used a signal generator to inject a 1000hz signal into the input, as well as my bench power supply to give it 12vdc. After each cap was changed, I flicked on the power and verified that the tone was coming out of the speaker. I observed that after replacing a few of the smaller caps that the gain greatly increased, and after replacing the larger caps that the motorboating vanished. I also replaced the four output transistors on one amp, in the case of this amplifier, they were n-channel mosfets. The device number on the existing transistors is apparently a proprietary Bose number, so I substituted an IRFIZ24N, which seems to work fine. If you're not proficient in soldering, get a friend who is, as it's easy to ruin your amplifier.

 



Capacitor Alert - submitted by Bill Stark

Nicachon Capacitors leaking in 1991 and 1992 TV's Mitsubishi, Zenith, VCR Supplies. One Mitsubishi had 2200mf caps in supply and ones in computer output-to-video chip for tint color 47mf etc. that caused video noise in picture on a 35 in. Look for black solder on bottom of board, black specked traces on bottom and round brown cicles around caps. Caps are blue and black in color. Just look for Nicachon.

 


Replacing a tyre for cassette players - submitted by Russell Burns

Having a problem finding a suitable tyre for a cassette player? Most times I can get the correct width but not the thickness. You can build up the size of the wheel by applying small strips of adhesive label carefully cut out to fit inside the rim of the plastic wheel then put in place the replacement tyre. Have restored many cassette players that would otherwise been scrapped.

 


Repair plastic cracks and rebuild any surface - submitted by Gavin Beverely

Take SuperGlue / CrazyGlue and apply to surface that needs rebuilding or gluing together. As soon as possible take a pinch of baking soda and sprinkle on the Crazyglue. This causes the Superglue to set instantly, approx. half a second, and also builds up the surface. This leaves the Superglue and baking soda as hard as granite -no exaggeration. I have not yet found anything that can compare to the strength and holding power of this stuff. Very useful around plastic.

 


Infrared Remote Control Test - submitted by Michael Eminescu

I found that the best way to test infrared remote controls from all types of equipment is to place them in front of any handheld video camera (VHS or VHS-C or 8mm) and push any button on the remote. You will see a series of rapid pulses in the viewfinder or LCD panel indicating that the remote control is working. Also on remote controls that have more than one infrared LED you can easily see if an LED is not working. This is also an easy way to test for buttons that are stuck because of dirt on the remote control. This method could also be extended to test any device that uses an infrared LED such as infrared barriers. That is because the CCD device used in camcorders is sensitive to infrared. And even better, if you have a digital camera with an LCD screen (I am referring to the ones that are used with PC or Mac computers and are able to store images on floppy disk or PCMCIA card) they can be used as well. As they become less expensive and smaller, the advantage is obvious - portability to a customer's site. Try it, have fun and be productive!

 


Repairing VCR's - submitted by Steve Wright

The one thing I have found that causes the most problems in VCR's is the mode switch. They come in all shapes and sizes and are usually very cheap, low-quality items. Do yourself a favor and at least clean and lube the mode switch in every unit that shows any kind of mechanical wierdness. It just may save you a whole lot of time and effort.

 


Clearing CRT Shorts - submitted by Jim Kocmoud. Jim is a member of our email discussion groups.. All subscribers have access to his, as well as over one hundred other technician's comments and expertise.

If the CRT is shorted Focus to G2, one technique that we use is to disconnect the G2 wire from the CRT PCB. Then, power up the set, and when the focus voltage backfeeds into the G2 and builds up a potential, it will arc thru the G2 circuit protector to ground and Voila, short is gone! If the CRT is shorted HV to Focus, we remove the CRT PCB, and hold a grounded screwdriver about 1/2" away from the focus pin. Again, after power up, the resulting arc usually clears the short.

 


Capacitor Leakage - submitted by Peter Gottlieb with an addendum by Jeff Roberts

Editors Note: A series of messages on this topic were posted to AnaTek's ELREPAIR email discussion group. The postings of Mr. Gottlieb and Mr. Roberts were so helpful and authoritative that got their permission to post them here for everyone. Thank you Peter and Jeff.

 

I have seen quite a few mentions of capacitor leakage, and some references to the "acid" that leaks out. I would like to correct this misconception and explain what I have found out about the construction of electrolytics and the dangers of leakage.

 

As you may remember from physics, to get high capacitance you need either a large surface area or very close spacing of the electrodes. Since large surface areas, even in a jelly-roll type construction like in an electrolytic cap, are impractical, the solution is to get very close spacing between the plates. If you open up an electrolytic you will find a plain aluminum plate, an "insulator", and an aluminum plate with some sort of coating. This coating is actually the insulator and is very thin. What seems like the insulator is actually a porous holder for an electrolyte solution.

 

The electrolyte used in most electrolytic capacitors is a salt solution in a solvent. This solvent is not water (it would freeze too easily), it is some hydrocarbon (frequently DMF, dimethylformamide). The salts used are proprietary to each capacitor manufacturer but are not table salt!! The DMF is itself not corrosive and will evaporate once the capacitor seals are breached or the DMF escapes the can. It is the salts that are so corrosive to the circuit traces, especially in damp or humid environments.

 

Now for the problems. A dry salt is not corrosive or conductive, but add moisture and major problems start. Any high impedance circuits can be shorted out or pulled to the wrong state. Power circuits will corrode, short out, and can even catch fire (it was a fire investigation where I learned of this "interesting" possibility). If you find a capacitor that has leaked, you MUST thoroughly clean the residue from the board or you may end up with a call-back. This has happened when a repair is made when it is dry out and then when the humidity increases the camcorder or whatever no longer works properly.

 

I saw a comment that corrosion can occur some distance from the leakage. This is because the solvent can carry the salts further than you may think. You really need to do a lot of cleaning. A few of the salts are UV visible so if you have a UV lamp handy give it a shot, you might get lucky. If a lot of caps have leaked the unit may be Beyond Economical Repair unless you can do a bulk cleaning of the board.

 

Jeff Roberts added:

 

Just to add to your explanation a good way to bulk clean. We have a particular power supply where the electrolyte leaks out and makes a real mess, not only does it corrode the traces but it can be conductive which makes for "extra" traces and shorts and a whole mess of problems.

 

We remove all of the caps and use a Q-tip and alcohol to start the clean up process, we then repair/replace any damaged traces and then we put them into the dishwasher and let it wash and rinse them. We don't use the dry cycle but stop it and just let them dry up overnight or with a hair dryer, then replace the caps and it's done. This works great!!!

 

The boards clean up like new and all of the electrolyte is cleaned off so there is no future corrosion or problems. Dishwashing soap is VERY powerful and combined with the high pressure water jets the job it does is great, after all if it'll clean the lasagna pan - a little chemical spill is nothing. Best of all it's cheap and much faster than trying to hand clean a board.

 

NOTE: We have an old dishwasher that belongs to the shop. I would not advise that you wash any dishes along with any circuit boards since you may end up with electrolyte on your dishes - YECH!

 

The following question was posted by a subscriber:

 

If the solvent or DMF has not evaporated, is the electrolyte conductive? I would assume so, as I thought the electrolyte serves to bring the capacitance up by allowing charges to migrate very close to the next layer. Perhaps you could clarify your description of the construction, as I am confused as to what you are calling the "insulator" and the "coating", and why the electrolyte is used.

 

Peter Gottlieb's answer:

 

To answer your question, yes, if the solvent has not evaporated the electrolyte is conductive. The "solvent" is no more complicated than when table salt is dissolved in water. When wet, it is conductive and corrosive (due in part to plating/electrolysis) but when completely dry the salt becomes an insulator. Of course, salts tend to absorb moisture from the humidity in the atmosphere so once you get a salt solution on something there will always be a leakage until it is cleaned. A good example of how salts absorb moisture is the salt calcium chloride, which is used as sidewalk deicer.

 

If you leave a cup of these granules out in a humid area they will absorb moisture until they all melt together into a wet mass. Dessicant packs are made of this same salt, and you can make dessicant packs by sewing a bunch of this salt into a fabric bag.

 

As for the construction of an electrolytic cap, one aluminum electrode is coated with a thin oxide, and this nonconducting oxide is the insulator in the capacitor. However, this oxide has a rough surface. To get the other plate of the cap mechanically close to this rough surface requires a trick - make the other plate a liquid! Thus, the electrolyte is really the other plate and the electrolyte is in contact with the bare aluminum plate connected to the other terminal. There is a porous (usually paper) separator between the two plates and this serves two purposes: hold the liquid electrolyte solution, and keep the two metal plates from touching each other.

 

You may ask, why can't the two plates touch each other if one is coated with insulating oxide? The answer is that they can touch, but vibration or shock (mechanical) or a sharp bit of dirt could break the oxide coating and then the cap would short out.

 

So, in short, the oxide coated aluminum plate is one electrode and the insulator, and the electrolyte and bare aluminum plate is the other electrode.

 

Regarding cleaning, I had read that Tektronix puts their scopes in a spray booth and uses hot water and mild detergent to clean them when they are sent in for major service. This is followed by a day in a 160 degree F oven. I remember that their comment was that the water was least damaging of all cleaning methods.

 

I used to wash off PDP-11 computer boards in the sink when they became dust encrusted and this really freaked everyone out. One of my bosses called DEC to ask if this was OK and was told that this was actually the preferred cleaning method. They still thought I was weird, but that is another story...

 

Nowadays, with the concern for the ozone layer, all PC board manufacturing uses water-based fluxes and hot water cleaning. However, there are some components that cannot withstand this and they must be installed after the "wash." Some parts have little plastic seals that are removed after the wash, like DIP switches and piezo buzzers. Before you just wash any old board you have to be able to identify whether any components will be damaged and remove them first. You can sometimes tell by those components not having clean connections, an indication they were installed manually after the PC board wash cycle. Relays will have an epoxy seal if they can handle washing. Do not wash a relay that is not sealed, you will ruin it.

 


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How to determine a Zener Diode's voltage - submitted by Richard Thomas. Richard is a member of our email discussion groups.. All subscribers have access to his, as well as over one hundred other technician's comments and expertise.

Take the good diode and solder a 1K resistor to one end, then apply about 24V across it with the positive of the voltage source to the banded end of the diode. Now measure the voltage across the diode only with your meter. That will tell you its zener voltage for replacement. The 1K resistor is necessary to drop the excess voltage i.e. if you apply 24V and it is a 10V zener then the resistor will drop the other14V and will limit the current to 14/1000=14Ma. Power will be140mW across the diode-more than safe for a 1/4W zener or above.

 


Service tip on Philips 9" portable TV - submitted by Rick Hille

I have serviced several Philips model 09PS10C portable color TV sets, and have discovered what I believe to be a manufacturing or design flaw that can lead to a premature demise of the set. There appeared to be a common thread to the failures in all three sets. On closer examination, the fuse on the line voltage converter module was blown as if an overload had occured. The main DC fuse on the TV chassis was not blown on two of the sets, but evidence of overvoltage on several electrolytic cans was visible (tops domed-out to almost bursting) in all. The line-converter main switch transistor was blown in all cases. Separating the line-converter module from the set and repairing it, I noticed that its DC output at no load was up around 35V, and well over 25V under load, which was peculiar since it was feeding a common DC point that can be externally powered from 12V. There is a 25V electrolytic on this line, so I would guess the DC level should be no more than 16 to 18V if properly designed. Since I have no schematic of the unit, I relied on common sense for this estimation and assumed that more than just the switch transistor was faulty with the converter. The design is a typical flyback type with a crude regulation loop formed from a negative voltage that sets the switch bias and controls its duty cycle. The negative voltage is produced by a separate winding on the flyback transformer, and is rectified, filtered, zener clamped, and tied to the switch base through a bias network. The negative voltage filter cap (C609) is a rather small size 47uF 16V electrolytic, which I would guess has a relatively high ESR at the frequency of the switcher (40-50kHz). My theory is that it heats up from the fast, tall spikes coming off the regulation winding and dries out, allowing fairly large current spikes to be driven into the zener. The zener fails, or becomes leaky or resistive. When this happens, the DC bias drifts up towards ground, which increases the duty cycle and hence the DC output voltage of the converter. With the zener open (as found in one of the sets), the DC output hit over 35V with no load. In this particular set, about 6 or 7 transistors and fusible resistors had to be replaced to get it working again, since the 35V killed the secondary regulator and was briefly presented to the 10V DC bus on the TV chassis. The fact that the converter is always powered when plugged in allowed the high voltage to develop while the set is turned off. The simple fix here is to replace the cheap, obviously under-speed capacitor on the converter module with a better one, such as a 47uF 63V with a 105C temperature rating. A quick external diagnostic is possible. The DC jack at the back og the set presents the converter output on the spring contact when no plug is present. By CAREFULLY poking a sharp, slender probe tip to the spring without disengaging it from the bypass contact, the DC output of the converter can be measured without opening the set up. If you see 25V or more, unplug the set, and replace C609 and the zener (5.6V) before powering it up again. Note that because I did this without a schematic, the zener voltage was a trial and error affair. The TV-on DC voltage appears to be 15V with 115VAC mains, and around 18V when off.

 


Quick CRT Test - submitted by submitted by Johannes Jongbloed

Years ago I was on the road repairing color tv's and also training would-be television technicians. Firstly I would not be carrying CRT testers, Oscilliscopes etc up 20 flights of stairs to service somebodys TV. I had to make some sort of diagnosis without all this. I taught my trainees that in most situations you do not need all this equipment apart from looking as if you known what your doing and impressing people. We were at the job to fix it, not to look impressive. In the case of a suspect CRT, if the filaments voltage is ok and glowing, the EHT(HV) is ok and there is a couple of hundred volts on grid two. Then by shorting out momentarily the cathodes of the three guns one by one with your dummy load mains lamp (all real techs do have one do not they) to deck (earth/ground). The result you should get is a very bright red, green or blue raster with retrace lines momentarily. If you do the picture tube is 99 percent ok. And you should start looking at drive or major grid one problems. I usually short the collectors momentarily of the RGB output transistors via the light globe if the unit uses transistors so as not to accidentally short the wrong pins of the CRT. Try this tip on a known good monitor (or tv) to see the effect and thus have that extra experience to allow you to judge the condition of a CRT. I hope this makes sense to those of you out there that do need to learn.

 


Troubleshooting a unit that blows fuses - submitted by Clint Hamilton

Something that may be a little more convenient for some; circuit breakers. I have a set I keep around for consistently fuse blowing problems from about 1 amp to about 7 amps. Just reset every time they trip until you correct the problem. Connect to existing fuse base with alligator clips.

 


How to quiet noisy transformers - submitted by Jeff Roberts. Jeff is a member of our email discussion groups.. All subscribers have access to his, as well as over one hundred other technician's comments and expertise.

Take a can of shellac or varathane. Solder 3-4" wires onto the four corner pins of the transformer so you can hold it by the wires. Holding onto the wires so that the transformer is inverted and level lower it into a can of shellac or varathane so that it is submerged up to the pins but do not get the pins wet. Remove and hang to dry for a day or so. This has quieted quite a few noisy coils and transformers.

 


How to clear CRT Shorts - submitted by Mike Fischer

Remove the CRT card from ther CRT. Ground the G2 pin with jumper clips. Connect the focus pin to a screwdriver shaft. Apply power momentarily to get the HV up. Unplug the power and quickly touch the screwdriver to the CRT anode. Discharging the HV from Focus to G2 often removes internal shorts.

 


Checking resistors - submitted by Greg Stark

I can check 10 resistors on a monitor pcb in just under 2 minutes whereas pulling a leg out method and getting it back in the hole, remembering to soldering them all as I have missed a few in the past takes me almost a 1/2 hr. Now what I do is cut one of the leads/legs of the resistor about 1/2 way between the resistor body and the point it enters through the PCB. Measure it, join it back up and drop a bead of solder at the cut. Its very fast and I can now tell which resistors I have tested. Never had a problem with the 3 watt and larger resistors separating from excess heat at the solder joint either. I figured if the solder does melt at this joint, it would probably be for the better than otherwise burning up something or starting a fire.

 


Locating the component that is causing the fuse to blow - submitted by Derek Cook. Derek is a member of our email discussion groups.. All subscribers have access to his, as well as over one hundred other technician's comments and expertise.

This is a sure fired way of finding your fault: Set your watt meter to 1.5 amps and 0 volts. Turn up the AC until you start drawing 1 amp for 25 seconds. Turn off AC feel around for the hottest component and there you have it. These are the easiest to find. Do not let the AC go higher than 16v.

 


Looking for bad connections - submitted by Wade Staggs

When looking for that elusive bad connection on a circuit board a plain old toothbrush is your best friend. Simply run the toothbrush over the board till your push the bad connection into working. This works great on cold solder joints.

 


Troubleshooting uc3842 - submitted by Johannes Jongbloed

When you have a Monitor power supply blown up that uses the old favorite uc3842 chopper driver ic. ie, blown mains fuse, shorted fet, etc., a quick way to determine if the uc3842 ic has survived without taking it out of circuit is to measure pins 5,6 and 7 as if pin 5 was the base of a npn transistor, using an analogue meter on rx1. If it reads forward with negative lead on pin 5 and o/c with positive on pin 5, chances are 99% ic ok. Have found this a useful check and saved replacing uc3842 for no reason many times. (Murphy does play a part 1% of the time.)