For convenience I have grouped this section into the following:
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Communication is the essence of life. Modern means
of communication begins with a Telephone. If you have a cordless phone (some of the tips
are applicable to Mobile phones as well ), then you will be better off taking a peak at
this page before you summon help.
For detailed tips on servicing Cordless phones:
Normally you can expect a
working life of about 3 years or more for a Nicad battery in a cordless phone,which
depends on the usage pattern of the customer. I have had to change a Nicad battery within
a month of an earlier change,because the customer had the habit of "parking
" the handset always on the base unit.So to prolong the working life of battery
,check here: Usage Tips on Using Cordless phones
If you are like me who share a single telephone
line with many extensions (in which all sorts of phone instruments are connected ) and a
modem , you need to build this simple circuit to keep them separated.This little circuit
keeps the modem happy whenever online , enabling the modem to negotiate the highest
possible speed it is capable of .
modem Isolator Circuit
These zener diodes also protect the instruments against surges ( though
modern phone instruments have a built in surge protectors in the line side ).
Incidentally, this circuit can also be used in all the extensions of your phone line to
enable privacy. The moment someone lifts up the phone ,all other extensions go dead.
Commercial gadgets cost a lot of money ( some even use a relay and an external power
supply) and tend to be bulky.
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Most of the time the small mains power pack that comes with your cordless
phone and even some of the external modem unit fails for no apparent reason . These cheap
far east adapters contain little more than a mains transformer and a couple of
diodes with an electrolytic capacitor. They offer no regulation against mains voltage
variation or offer transient protection. The following circuit offers transient protection
and a regulated power supply even in the absence of mains voltage. Uninterrupted Power supply for Cordless Phones
The following project was developed to record the complete conversation
of the phone automatically using a cheap cassette recorder. You can use this circuit to
monitor your line in your absence .Circuit diagram of the
Phone Recorder
As we have seen earlier, if you have the right information, you diagnose a fault relatively fast and your productivity is high. On the other hand, if you are an ordinary mortal , you look everywhere ,get nowhere and spend tedious hours hunting down the elusive fault ....
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mongst other things , I upgrade VCRs and TVs. With the advent of satellite TV/cable TV,plenty of Channels have come into existence,which called for "cable ready" tuners and selectors. We retrofit the new breed of hyperband Tuners and CITAC ( Computer Interface for Tuning and Control) panels to the TV.This calls for a certain amount of redesigning which can be laborious and painstaking. Circuit diagrams are essential before such a foray.Still many a times, you would be forced to undertake a "grand Tour"of the pcbs.Eventually, the idea is to make the modifications in such a way that the safety of the equipment is not compromised. In this field it is all too easy to forget to dress up the wire loom away from hot running components,which over a period of time, under very hot ambient condition can result in a fire accident. Or as a local "Engineer" had repaired a microwave oven, where he had removed the door switch which would prevent the operation of the unit when the door is open, with disastrous results to the customer... I spend atleast 15 minutes at the end of the repair to check things over,which might sound wayward ,but at the end of the day, I have total satisfaction that the job won't bounce.As you deal with dangerous voltages,it pays to
take all possible care and precaution while at work. Remember "It is the mills
(milliampere) which kills and Volts that jolts " ! It has been estimated that as
low as 8 mA is enough to cause a fatal cardiac arrest under certain conditions.
Most of the TV/monitor FBTs produce lots of volts, but the current capacity is rarely more
than a few milli amps. But the mains has enough potential to pass highly damaging currents
through bodies,including your own !
They say that "Don't Smoke on your bed - For the ashes falling may be
your own !" . The most dangerous situation is checking the voltage in a
microwave oven, because the current involved is 350 mA at about 2000 V ,which can cook you
. A carelessly held meter prod can blow out the meter in no time. I use a specially built
negative HT meter to measure the magnetron voltage. Before I turn on the power, the meter
is connected in site , saving me an increase in blood pressure!
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Observe all precautions whilst repairing chopper power supplies and non isolated chassis of TVs.Use isolation transformers if you must. I prefer to work direct with my feet off the ground.There are some choppers whose working conditions change if fed from an isolation transformer.But use variacs to avoid premature balding and/or excessive consumption of alcohol . I like to watch Diwali fireworks -not the antics of a SMPS.If you have no variacs, use a suitably rated incandescent bulb in series with the load to check. Years ago, I had been reduced to the level of crying silently when a Philips CTV continued to blow its chopper transistors for no apparent reason,until I chanced to change the mains on/off switch. But fortunately,these days most of the designers have tended to wean away from sadism and prefer to use independently running chopper chips with enough protection built in. No more of the CYCLOPS or other evil designs which utilize the chopper circuit as part of the line output stage. To balance things, they have invented smd to show their spite.Talking of designers, I have come across several products which take ages to take them apart. For example I had to dismantle about 50 screws before I gained access to the cassette deck in order to replace the belts.Reassembly was nerve wracking for the simple reason that I had to reconnect half a dozen plug connectors with no identifiable tags.A wrong connection could send the rows of well populated pcbs (several of them with 80 pinned smd chips) into orbit. I wonder if arthritis is also part of the occupational hazard. As it is, it is awfully trying - to make a living with the vapours of industrial solvents and lead/resin vapours .
I admit this might well be a thankless job, yet the satisfaction you feel after a particularly trying fault is cleared, is heart-warming indeed ! My idea of relaxing is ,apart from curling up in bed with a paperback, to have a go at the stock faults.
I am reminded of a slogan from somewhere : "Those who study History will not commit the mistakes of the past,but always will find new ways of committing the same mistakes . " Similarly, If you are long in the tooth, and unfortunate enough to be in this trade, you would have developed many of these labour saving devices and yet spend countless hours ....
The first in this line up is the Dynamic FBT tester..I have ordered/stocked many FBTs which had been a drain on our scant resources. Most of the time you tend to find the culprit by substituting the suspected component. A good move if the suspect is an electrolytic capacitor,but a bad move if it was a costly FBT. I cooked up this circuit after much experimentation. The criteria of the design,included the creation of near identical situation of a line-output stage in the test setup.
Description: You will be mildly surprised to see the ubiquitous 555 timer chip in this circuit. For sheer familiarity I have used components that have been with us for ages. The timer chip is configured as the monostable multivibrator ,which runs at around 20 kHz. This frequency can be adjusted to some extant by the preset in the setting up stage to get the highest pulse across the output transistor.Since the collector voltage is only 18 V, the peak collector pulse does not cross into the danger zone.To "pulse " the transformer , we deliberately use a comparatively short pulse with about 5 µS duration.( The normal line pulse duration is 12 µS.). This , coupled with the absence of harmonic tuning capacitor can produce sufficiently high eht voltage to test the transformer in question.Though numerous designs have been published in many trade journals, this design was cooked up and built in a matter of hours and have served us well for the past 5 years or so. I have checked even chopper transformers and many pulse transformers with this.
Setting up : This circuit is simplicity itself, as it has little more than a preset to set up. First check the B+ voltage at the red crocodile clip.It should be 18 volts when checked with a digital voltmeter. Next connect a known good transformer ( any make will do ! I prefer to start with a split-diode transformer rather than the old fashioned onion type ) in the test terminals (crocodile clips) . If you connect the primary to the terminals, you can hear the eht rustle up ( Keep your hand, face, etc away from the eht cap !) and the meter should deflect to approximately half the scale. The idea is to check the current drawn by the transformer. If the transformer is good, it draws about 125 mA to 250 mA depending upon various factors. You can draw an arc with a long bladed insulated screwdriver from the anode cap, which should increase the current consumption by a little. At the same time look for corona discharge from the body . If the current drawn is more than 300 mA ,the transformer is faulty. This circuit does not tie up your 'scope and takes a few seconds to check. Over a period of time you will have acquired enough idea about the "normal "current consumption of various transformers and you rarely need to order another transformer with out confirming the fault.
These pages have been up for a very long time in terms of the Internet. A little over 5 years perhaps. I have received countless mails requesting help,sharing views about electronics in general and thousands of spams. I have decided that something ought to be done about this. Hence some changes have been made in this site as elsewhere in my other sites. Please share your views,comments,suggestions and criticisms in my forum dedicated for the technology. Whilst at it, you might also like to browse Bestnetcraft for its cutting-edge technology implementation. Host your website,add functions and features from a technology-savvy service provider - Bestnetcraft.
