Five Step Troubleshooting Method Solves VCR Safety Circuit ... · Fig. 2a: Some safety circuits supply either a digital logic high or logic low signal to the microprocessor. Fig.

Five Step Troubleshooting Method Solves VCR Safety CircuitProblems In Minutes

Five Step Process Saves Time-Verifies If The Problem Is In TheMicroprocessor:

Several circuits external to the microprocessorcan cause system control problems. There is asimple five step procedure to determine if the

problem is in the microprocessor or in an exter-nal circuit. The five steps are:

1. Check the power supply2. Check the microprocessor clock frequency3. Check the data in/data out lines4. Check for a reset pulse5. Check for bad grounds

Your SC61 Waveform Analyzer is the best in-strument to perform this five step procedure,because you can completely analyze the impor-tant waveforms. In many cases, the cause of thesystem control problem will be found in stepthree of this five step microprocessor trou-bleshooting procedure. Steps one and two shouldbe done first, however, since a properly

Fig.1: The system control microprocessor takes data from the safety circuits and front pane/ controls and uses this data to control the operationof the VCR.

operating power supply and correct clock fre-quencies are essential.

Seven Key Safety Circuits Tell TheMicroprocessor How The VCR IsFunctioning

There are seven key functions that are moni-tored by the microprocessor to ensure safeoperation. These functions are:

1. Position of the video tape cassette2. Rotation of the video heads3. Loading of the tape around the video cylinder4. The end of the tape5. Rotation of the take-up reel6. The humidity level inside the VCR7. Ability to record over existing programs

Fig. 2a: Some safety circuits supply either adigital logic high or logic low signal to themicroprocessor.

Fig. 2b: Some safety circuits turn data strobesignals on and off depending on the state ofthe VCR function it is monitoring.

Sensors check these functions and send the in-formation to the microprocessor, which stops theVCR when a problem is detected. There are fourgeneral types of sensors found in VCRs:

1. Optical Sensors2. Hall-effect Sensors3. Mechanical Sensors4. Humidity Sensors

Before you start checking these sensors, youshould first analyze the symptoms. This will giveyou valuable clues into the cause of a systemcontrol problem. In addition, remember to usethe five step microprocessor troubleshootingprocedure (Tech Tip #109).

Five Step Troubleshooting MethodFor Safety Circuits

Troubleshooting of safety circuits can be brokendown into a simple five step process:

1. Check the microprocessor input data lines2. Check the operation of the sensor3. Check the sensor power supply4. Check sensor output processing circuit5. Check the grounds and connections

Lets look at each step and see how to use yourSC61 Waveform Analyzer to perform each step.

1. Check the microprocessor data lines: Thisis actually step three of the microprocessor trou-bleshooting procedure. A system control prob-lem can be caused by a variety of problems,some of which are associated with safety cir-cuits. Your first step is to verify that the problemis caused by a safety circuit, and if so, which one.

System control trouble symptoms often point tothe problem. For instance, if the VCR is dead, itcould be caused by:

a. An end sensor or its associated light tower.b. A defective dew sensor could be inhibitingoperation.c. The mode or cassette position circuits mayindicate an improper condition.

These are some of the first inputs to check. If theVCR starts to load and then unloads, it could becaused by a defect in one of the previouslymentioned circuits or the cylinder lock circuit orreel sensor circuits. In any case, checking thedata being fed into the microprocessor will iso-late the problem down to the defective circuit.Figure 1 shows the various safety circuits used ina VCR.

The signals fed from the safety circuits to themicroprocessor can be of several different types.Some circuits simply send a logic high or low tothe microprocessor. Other circuits either allow ascanning pulse to return to the microprocessor,or inhibit the pulse. Various combinations ofthese signal types can be found in the sameVCR. Your SC61 Waveform Analyzer worksbest for troubleshooting these problems. Its digital

voltmeterfunction letsyouquicklycheckforlogichighs and lows; the CRT and peak-to-peak digi-tal voltmeter functions let you check pulse sig-nals, and the same test probe can be useduniversally for all signal types.

Check each data line: Connect your SC61Waveform Analyzer test probe to the data lineyou want to test. Press the DCV button on theSC61 Waveform Analyzer. Read the DC voltageon the LCD digital display. Compare this readingwith the voltage level given in the service litera-ture. Next, press the PPV button and read thepeak-to-peak voltage on the LCD digital display.If the VCR uses a scanned sensor system, thepeak-to-peak meter will read the amplitude of thepulses on this waveform.

NOTE: In the case of reel sensor pulses, thefrequency of the output signal is typically verylow when the VCR is in the play mode. Thisfrequency is on the order of 1 Hz or less, andlooksmore like a periodicchange in the DC /eve/.Use the SC61 's DCV function and slowly turn thetake-up reel by hand. Watch for a change in theDC voltage reading on the SC61 WaveformAnalyzer digital display as you turn the take-upreel.

If the peak-to-peak digital readout indicates thatthere is a signal present, turn the timebaseswitch on the SC61 Waveform Analyzer until thewaveform is seen on the CRT. In most casesthese waveforms will be low frequency. Set thetimebase on the SC61 Waveform Analyzer to alow sweep rate so you can see this signal.Compare the waveform observed on the SC61

Fig. 3: The DCV feature of the SC61 Wave-form Analyzer lets you quickly determine theoutput level of the signal being fed to themicroprocessor.

Fig. 4: The PPV digital meter and CRT on theSC61 Waveform Analyzer shows you the sizeand waveshape of pulsed sensor outputs.

Waveform Analyzer to those shown in the serv-ice information for the VCR.

One typical system control problem, is the VCRthat begins to load the video tape and thenunloads it. In this example, use the SC61 Wave-form Analyzer to monitor what happens to eachsafety circuit signal as the VCR begins to loadand/or play. In addition, check the amplitude ofthe signals.

Microprocessors operate on logic high and lowsignal levels. A voltage that is not high enough tobe considered a logic high, or low enough to beconsidered a logic low, will result in intermittentVCR operation. Figure 5 shows the voltage lev-els needed for proper operation of TTL andCMOS logic circuits. Use the digital meter onyour SC61 Waveform Analyzer to ensure thatthe signals are large enough to swing beyond thequestionable voltage region.

2. Check the operation of the sensor: Eachsafety circuit is composed of a sensor, intercon-necting wires and connectors, and sometimesan interface circuit between the sensor and themicroprocessor. A defect can occur at any pointalong this signal path. We could walk back, stageby stage, from the microprocessor to the sensor,but in most cases it is faster to make the next testat the output of the sensor. There are at least tworeasons the sensor output should be checked asthe second step of our five step sensor trou-bleshooting procedure. 1.) Sensors have tradi-tionally been high failure items. 2.) Sensors arerelatively easy to find since most of them haveonly specific locations that they can be in. Forinstance, the end sensors are always in the

same place in all VCRs, the take-up reel sensoris always located close to the take-up reel and soon. It only takes a few seconds to locate the testpoints for these relatively high failure rate com-ponents.

Again, your SC61 Waveform Analyzer is the bestinstrument for checking the sensors since itworks universally on whatever type of signal thesensor puts out.

Some sensors, such as the mechanical switchesand the dew sensor, can be checked with anohmmeter. A more dynamic test, however is touse the SC61 Waveform Analyzer to check thevoltage output of the sensor, because micropro-cessors sense voltage level changes, not resis-tance. An ohmmeter does not verify that thevoltage levels going to the microprocessor are ofsufficient amplitude to be recognized.

Safety circuits are tested somewhat differently,depending on the type of sensor used. Thefollowing specifics are given to help guide you intesting the different sensor functions.

Testing the cassette switches and modeswitches: Press the DCV button on the SC61Waveform Analyzer and connect the test probeto the safety switch. For cassette positionswitches, open and close the switch contactsand look for a change in the output signal. If DCsignals are used in these switches, watch thedigital meter on the SC61 Waveform Analyzerfor a change in DC voltage level.

For mode switches, press the play button on theVCR and monitor the output voltage from eachoutput pin as the tape loads around the videocylinder. Compare the voltage on each pin withthe mode switch timing information in the serviceliterature.

If the VCR will not begin the loading process,check to be sure the switch outputs are correctfor an unloaded tape. Sometimes, a VCR mal-functions simply because a gear or lever hasslipped and the mode switch contacts are in thewrong position. This results in incorrect databeing fed to the microprocessor.

Testing the end sensors: Push the DCV buttonon the SC61 Waveform Analyzer and connectthe test probe to the output of the end sensor.Interrupt the beam of light coming from the lighttower by placing your finger over the light inputhole for the sensor or cover the end sensor withblack tape. Read the digital display on the SC61and look for a change in sensor output voltage.

If no change is observed in the end sensor outputvoltage when the sensor is covered, it may meanthat the end sensor is bad or the light tower isdefective. In the case of infrared light towers, youcannot visually inspect for light output. Instead,shine a light on the end sensor and again checkthe output. If you now get a change in the endsensor output voltage, the light tower is notputting out light and should be checked.

In some VCRs, the output of the end sensor willbe a pulse rather than a DC voltage change. Ifyou suspect that the VCR you are working on isof this type, simply press the VPP button on theSC61 Waveform Analyzer and read the resultson the digital display. If you get a VPP reading,verify the results by setting the attenuator andtimebase to lock in on the signal.

Testing the reel sensors: Press the DCV but-ton on the SC61 Waveform Analyzer and con-nect the test probe to the output of the take-upreel sensor. Slowly rotate the take-up reel whilewatching the digital display on the SC61 Wave-form Analyzer. The digital display should alter-nate between a low level voltage reading and ahigher level voltage reading. You can also ob-serve this change on the CRT by switching the

Fig. 5a: Microprocessors that operate withTTL type signals must have digital high andlow signals that fall outside the questionablearea.

Fig. 5b: CMOS type microprocessors requiredigital high and low signals that fall outsidethe questionable voltage level.

input switch to DC and watching the trace on theCRT. If the sensor isoperating properly, the CRTtrace will alternate from one level to another.

Testing drum lock: Press the VPP button on theSC61 Waveform Analyzer and connect the test

Fig. 6: Check the output of each pin on themode switch to determine if the signals arecorrect for the position of the tape loadingpins.

probe to the output signal from the video headdrum FG or PG signal. You will need to check theschematic to determine what signal is used in theparticular VCR you are servicing. Press the playbutton and visually verify that the video drum isturning. If the video drum is turning and a signalis present, the peak-to-peak voltage amplitudeof the signal will be displayed on the digitaldisplay of the SC61 Waveform Analyzer.

Note: A case of no video drum rotation may becausedbya headservoproblem or the micropro-cessor may not be telling the servos to operate.

Testing the Dew Sensor: Press the DCV buttonon the SC61 Waveform Analyzer and connectthe test probe to the side of the dew sensorelectrically closest to the microprocessor. Com-pare the DC voltage displayed on the digitaldisplay of the SC61 Waveform Analyzer with thevoltage given in the service literature. Dew sen-sors currently are designed to change resistancewith increased humidity. This change in resis-tance results in a change in the voltage acrossthe dew sensor.

3. Check the sensor power supply: A low ormissing voltage at the output of a sensor doesnot immediately mean that the sensor is bad.Before replacing the sensor, make sure thatthere is power going to the sensor. This appliesto all types of safety circuit sensors. This isespecially true of Hall-effect sensors. Thesesensors require an additional bias voltage tooperate.

To check the sensor power supply, press theDCV button on the SC61 Waveform Analyzerand connect the test probe to the power supply.Read the voltage displayed on the digital displayand compare with the power supply voltagelisted on the schematic.

4. Check the sensor output processing cir-cuit: Some sensors use additional circuits toprocess the signal before it goes to the micropro-cessor. In the case of a dew sensor, for instance,comparator circuits are often used to sense achange in the output voltage and output either alogic high or low. Other sensors use similarcircuits to ensure that the signal going to themicroprocessor is clean and distinct. If the sen-sor output looks good, but the signal at themicroprocessor is incorrect, check the operationof any intermediate processing circuits.

5. Check the grounds and connections: Wiresand connectors transfer the signals from the

Fig.7: Interrupt the light beam to the endsensors to quickly determine if they areoperating correct/y.

Fig. 8: Slowly turn the take-up reel as youmonitor the reel sensor output voltage. Youshould see the DC voltage alternately switchlow and high.

sensors and auxiliary circuits to the micropro-cessor. If the signal is bad at the microprocessorbut the sensor and auxiliary circuits test outgood, check for a bad connection or a pinched oropen wire within the circuit path.

#159 Form 4316 Printed In U.S.A.