The following article is presented with the intention of offering
tips and traps distilled from repairing hundreds of phone equipments
to my fellow engineers who repair TVs,VCRs and Audios and who wish to
add Phones to augment their income for a little effort.If you have
experience mending radio equipments,then it should be easy to tackle
cordless phones.Please send your comments,suggestions,bouquets,or
brickbats to me at the following:
send mail to [email protected]
Snail mail: M.P.Prakash R.Lewis
56,Rukumani Road,
Kalakshetra Colony,
Besant nagar,
MADRAS.600090. INDIA
Cordless Phones
There are hundreds of models from dozens of manufacturers
operating in radio frequencies to communicate between the base unit and
the handset in a two-way fashion.Usually,these machines are reliable like
most of the modern electronic equipments are,but there are exceptions
which mean,we make a living ....
Like the remote controls of VCRs/TVs ,the handset suffers a bit of rough
treatment,but unlike the remotes,these handsets are bulky and heavy which
result in physical damage,dislocated battery etc when dropped.
Faults can be broadly classified into two categories:
1.Completely dead: No sound or no "sound & light " show.
2.Phone shows signs of activity:Does not dial out or dials wrong
numbers,or noise in the background. Phone dead:Check the handset for activity (Visual or aural).If
not check the Nicad battery,which in most cases will supply 3.6 V.
Check under load for at least 3.0 V to rule out the battery.If there is
low voltage,before condemning the battery,recharge it with a bench power
supply,at roughly C/10,where C refers to the capacity of the battery in
mA/h for about 10 hours.For a quick checkup,fit another 3.6 V Nicad,taking
care to observe the polarity,to the handset for any sign of revival.If
the "dead" nicad springs to life after the recharge,connect a 4.5 V
torch bulb and leave it for 30 minutes at the end of which the bulb
should still be on.If this is the case,you can safely surmise that
the battery is still useable and look for sluggish charging
characteristics of the base unit or the customer.
While you are at it,inspect the base unit and its main power
supply.Most of the present day base units sport an external mains
adapter which often gives up the ghost.Connect your bench power supply
to the base unit and turn on the handset to establish connections.
If the radio link is intact,there will be a LED to indicate this fact
and you should hear a dial tone,if the base is connected to the phone
line.Since the rf portion of the base unit and the handset are
operating at low voltages and at small currents,they rarely fail.
Resist any temptation to tweak any presets or coil cores,in the rf
stages.Generally speaking,most of the cordless phones have only about
100 ft or so as operating range.There are some models which put out
higher power (around 90 dB micro V/m),which would extend the operational
range to about 300 ft.Older Sanyo phones like CLT35,use about 1.8 Mhz
from the base and about 49 Mhz from the handset.The lower frequency
uses the mains lead as the antenna and the handset uses a telescopic antenna
(so does the base for the "return" link)for VHF operation.The lower
frequency makes the phone susceptible to mains borne interference
(like kitchen food processors,tube lights & motors ).Recent models use
mostly about 49 Mhz for two ways,and some even offer a choice of
channels for trouble free operation in a rf active area.There are
some models which even use a microprocessor to scan an "empty"
channel for interference free usage.
You can build a converter to receive these channels in a
normal FM radio receiver to check their operation.However a RF
sniffer probe hooked to your 'scope should indicate the presence
of rf transmissions.
If you have established that the radio link is intact,and
the quality of voice is poor in the line,proceed as follows:
Hook up a voltmeter to the phone line,which should display about
50 V,when the phone is off.With the phone on-line the reading should
drop to about 10 V or so.On the other hand,if it drops to less than
5V,suspect a short in the line stages (Usually the coupling
capacitor for the ringer,or the bridge diodes in the front stages
can go leaky).However,the line matching transformer is reliable and
if they fail,they go open.And check for continuity in the RJ11
socket and the cable that links the base to the line.The presence
of a polarizing dc voltage can cause a sort of black oxidation
in the copper leads,especially in the presence of moisture.
Poor quality audio would confuse the answering machine's micro
-processor which needs clear changes in the voltage level to retain
control over the answering mode.Absolute values are not important
for their normal operation.Hence secure connection to the phone line
is essential for a satisfactory performance.Again check the dc
output from the mains adapter under answering mode.As observed
earlier,the small unregulated power supplies rarely live up to
the expectation.Build an appropriately rated regulated power supply
using the ubiquitous 78XX power regulator chips with plenty of
capacitance.If you dabble in hi-fi machines,you know the value
of electrolytics and generously rated power supplies.
Although this article has started as "Repairing tips
about Cordless phones",the discussions have meandered to the
connected topics as well.Yet no circuit diagram or any product
specific information was included,because of wide variations in
circuit design.Future articles would show some constructional
projects which would accelerate the fault-finding process.
