Troubleshooting Chapter Ten.

Chapter 10 Things That Have Never Worked.

10.1 Power Supplies.
10.2 Audio Amplifiers.
10.3 Radio Receivers.
10.4 Simple Test Equipment.

Chapter 10

Things That Have Never Worked.
As a trouble shooter you will occasionally encounter something that has never worked. This may be a prototype constructed in an industrial or university research and development laboratory. It might be a DIY (do it yourself) project that was brought to your shop. It night be a kit that was constructed by someone else or yourself. What ever it is the fact that it has never worked can make the job of troubleshooting much harder than with something that used to work and stopped. The malfunction might be caused by a defective component but it is more likely to be cause by a wiring error or wrong component value.
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10.1 Power Supplies.
Power supplies can range in complexity from simple transformer-rectifier-filter circuits to multi output with electronic voltage and current regulation. If the power supply is one of the latter the regulator circuits should be separated from the basic rectifier filter and the basic circuit tested separately.
Although a simple power supply has little to go wrong, as pointed out earlier in this book failures can cascade turning a simple problem into a more complex one. An electrolytic capacitor may have been taken from NOS (new old stock) and may not be formed up to its rated voltage. Such a capacitor may show good using an ohmmeter but will blow fuses and diodes when the power is turned on. Since the capacitors have never been put into use they should be tested at their rated voltage.
For a brief discussion on the construction of an electrolytic capacitor, how to test it, and its behavior in a circuit, refer back to 3.2 Electrolytic Capacitors.
For a discussion on testing a power supply which may have unformed electrolytic capacitors and how to reform them refer back to 4.3 The Power Supply Section.
If the basic rectifier filter section is working properly The problem is in the electronic regulator circuit. If the unit uses IC regulators there is little to do but replace the one that seems to be causing the trouble. If the unit is blowing fuses it will be necessary to isolate the individual regulator circuits probably by removing the ICs. If the circuit is built with discrete components refer to section 5.2.
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10.2 Audio Amplifiers.

10.2.1 Transistor amplifiers.
Beware of treating the symptom. Many amplifiers I have opened up have revealed a burned or scorched resistor. Replacing this resistor will not fix the amplifier. The replacement will just burn out as the old one did. However, this will point you to the area of trouble.
Transistor audio amplifiers usually just blow fuses. This requires testing with power off. Most DMMs only apply 200 mV or less to the test leads. This may not reveal the problem. A VOM is a much better instrument for this kind of testing. Test from power supply positive to ground, from power supply negative, if the amplifier has one, to ground, and from power supply positive to negative. Be sure the polarity of the leads is such that the proper polarity is applied to the test point, positive to the positive rail and negative to the negative rail. This is likely to reveal that there is a short, which you already knew.
If the amplifier has multiple PC boards you can unsolder the power supply leads to each one in turn and determine which board the trouble is on. If there is only one board isolating the trouble is more difficult. Removing the power transistors from their sockets one channel at a time may, at least, show which channel the problem is in.
Testing Transistors.
Now that you have the power transistors out you might as well test them. A transistor tester is handy but not essential. The most basic test is the junction test. A DMM cannot be used for this test because it does not apply enough voltage to cause the junctions to break down. Set the VOM to measure ohms on the RX1 range and connect it to the collector and base. It should show a diode with the anode to the base in an NPN and the anode to the collector in a PNP transistor. The polarity reversing switch on the Simpson 260 is a real time saver when making this test. The transistor should also show a diode between base and emitter. Anode to base in NPN and anode to emitter in PNP. It should show open in both directions from emitter to collector.
WARNING: APPLYING POWER TO THE AMPLIFIER WHILE THE OUTPUT TRANSISTORS ARE REMOVED MAY DAMAGE THE DRIVER AND PREDRIVER CIRCUITS. THIS IS NOT RECOMMENDED.
A voltage breakdown test might also be a good idea. The schematic will tell you how much voltage is applied to the transistors. If for example the power supply is plus and minus 40 volts then in the static condition 40 volts is applied across each output transistor. Connect an external power supply with a series connected current limiting resistor of 10 k ohms to the collector and base of each transistor one at a time in turn. Positive to the collector of NPN transistors and negative to the collector of PNP transistors. Connect a voltmeter of any type across the 10 k ohm resistor. The meter should not indicate a voltage any greater than a few millivolts. If the reading is more than 1 volt the transistor should be replaced. Note: a reading of 1 or 2 volts might be observed if the transistor is a good germanium.
If the outputs test good.
If the transistors test good the problem may be caused by a transistor or other defective component earlier in the circuit causing the output stage to draw too much current. The high power transistors in the output are usually in sockets which require the removal of two screws to take out each transistor for testing. On the driver and predriver level the transistors will have to be unsoldered from the board. To junction test low power transistors set the VOM to the RX 10 or RX 100 range. When doing the breakdown test use a 100 k ohm resistor instead of a 10 k ohm.
If all transistors test good you need to start looking at the passive components. Shorted capacitors are the most likely suspects. Test electrolytic capacitors at their rated voltage not with the low voltage applied by an ohmmeter. Open resistors are also a possibility. For example an open resistor connected from the positive power supply rail to the collector of a predriver transistor could allow the transistor to go into saturation pulling the amplifier's output to the negative rail resulting in an apparent short.
If the defective component can't be located it may come down to shotgunning.
10.2.2 Vacuum Tube Amplifiers.
The first assumption when a non working vacuum tube amplifier is encountered is to assume a tube is defective. Although failure statistics tell us to check the tubes first in my personal experience with tube radios, TV sets, and instrument amplifiers is that the problem was caused by a passive component as often as it was by a tube. That said, the tubes should be checked first because they are so easy to replace. Even if the tubes are brand new they still should be checked. If they were taken from NOS (new old stock) there is a higher probability one is bad.
Unless you have a high quality vintage tube tester which is in good operating condition the best way to test the tubes is by substitution of tubes that are known to be good. There are two conditions in which this should not be done. One is if the amplifier is blowing fuses and the other is if one or more tubes has a red hot glowing plate. In these two cases the known good tubes may not be good after the test.
If the unit is blowing fuses and it has a rectifier tube, unplug the tube and see if the fuse blowing continues. If it does the most likely defect is a shorted power transformer. There is one thing you should check before condemning the transformer. Check to see if there are any shorts in the heater wiring or bypass capacitors in the heater circuit. Check the capacitors for shorts. If there are no capacitors unplug all the other tubes making sure you know which tubes go where. If there are no numbers imprinted on the chassis and you don't have a layout drawing, make one so you don't get the tubes mixed up. Heater shorts in a tube are rare but possible. If the amplifier still blows fuses the power transformer is the culprit.
If unplugging the rectifier tube prevented fuse blowing then the filter capacitors become the prime suspect. If the amplifier has silicon diodes check the power supply capacitors and diodes. Remember that a shorted diode is likely to be part of the symptom rather than the cause. Check the capacitors at their operating voltage before assuming the diodes are at fault.
Another way of testing the power supply is to unplug all of the tubes and use the dim bulb test. Remember you can't possibly burn out a 120 volt bulb by applying 120 volt line voltage to it. Start with a low wattage bulb and move to higher wattages. For full details on the dim bulb test refer to 9.1 Steps to First Power on.
If the fuse doesn't blow immediately after the power switch is turned on but the plate of one or both of the output tubes begins to glow red hot and then the fuse blows the outputs are drawing far too much current. In the case of a new design or kit oscillation is a good possibility. This may be taking place at a frequency above human hearing and will only be detectable with an oscilloscope. The primary leads of the output transformer may have been reversed turning negative feedback into positive feedback. Be certain there are no oscillations going on before proceeding.
Capacitors are the next suspect. If the amplifier uses cathode bias a shorted or reversed cathode bypass capacitor may be at fault. If fixed bias is used there may be a shorted or reversed capacitor in the bias rectifier/filter circuit which is preventing any negative bias from being applied. Shorted or even leaky coupling capacitors from the plates of the driver tubes to the control grids of the outputs could be applying a positive voltage to the grids causing too much current to flow. If the screen grid voltage is electronically regulated the regulator may be defective allowing too much voltage on the screen grids. This can cause too much plate current to flow.
If fuses are not being blown but the sound is weak and distorted coupling capacitors earlier in the circuit are suspect. Plate load resistors which may be defective or the wrong value can also cause this symptom.
If static testing does not reveal the problem it may be necessary to disconnect the negative feedback and use signal injection or signal tracing in conjunction with signal substitution techniques to localize the defect. Be cautious when negative feedback is disconnected. The overall gain of the amplifier will be 10 times or more greater than it was with the feedback connected. Oscillation is a possibility under these conditions. Keep input and output leads separated as much as possible and by all means use shielded cables between the oscillator and input of the amplifier. Set the volume control on the amplifier very low so the oscillator can be operated at a higher level.
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10.3 Radio Receivers.
A communications or even a simple AM broadcast radio that has never worked can be a difficult problem. There are all those tuned circuits that have never been adjusted because the receiver doesn't work.
The first step is to half split at the detector. In most receivers the volume control is just after the detector. This is usually true even in short wave or ham receivers. Inject an audio signal at the top of the volume control and if you hear it the trouble is in the RF or IF circuits. If you don't hear the signal then the problem has been reduced to a simple audio amplifier problem and should be easy to solve. Once you have the audio working chances are you can align the IF and RF stages.
To troubleshoot and align the pre detector section an RF signal generator is an absolute must. It might be possible to trouble shoot without one but we are talking about a receiver that has never worked and has not been aligned.
Some service shop RF signal generators such as those made by Heath and Eico included DC blocking capacitors in the output. Higher end generators do not have a blocking capacitor and one should be added on the outside when making the following tests. Use a coaxial cable with the appropriate connector on one end and clip leads on the other. If you don't have one, make one. Clip the ground lead to the chassis* and clip one lead of a 0.001 microfarad capacitor in the hot lead clip. Use the other capacitor lead as a probe to make the tests.
* If the receiver being worked on is line operated and has no power transformer, the chassis is connected to one side of the power line. These radios, commonly known as All American Fives, should only be worked on while powered from an isolation transformer. Question. How did you get hold of an AA5 that has never worked?
Tune the generator to the frequency of the IF and turn on the modulation. Turn the generator's output up full or to 1 volt if it goes higher than that. Touch the test lead to the collector or plate of the last IF amplifier stage. You should hear a tone in the speaker. If not, find out why because you should.
If you heard a tone move the probe to the base or grid of the same amplifier stage. The tone should get much louder. Reduce the output of the generator and tune the adjustment or adjustments in the last IF transformer for maximum signal.
If the receiver is a simple tube type AM broadcast radio it most likely has only one IF amplifier stage. If a transistor AM radio it will have two IF stages. If it is a sophisticated communications receiver it will have several IF amplifiers and will likely be a double or triple conversion receiver. Work your way back through the amplifiers following the same procedure. Only align the transformer in the output of the amplifying device while the generator is connected to the input of the device. When you come to a mixer connect the probe to the plate or collector to see if you are getting a signal through all stages between there and the speaker.
Next check to see if the oscillator signal is present. If not, find the problem and fix it. If so tune the signal generator to the frequency of the preceding IF or if the receiver is single conversion to the received band. Tune the generator around until you hear it in the speaker. If this is a multi conversion receiver and the IF is off frequency adjust the oscillator frequency until it is correct. A transistor receiver may have amplifying stages between the two mixers. A tube receiver most likely will not.
As you work your way back through the receiver stages you will be aligning the tuned circuits and upon arriving at the antenna you may find the receiver is working properly. The alignment may need a little touching up.
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10.4 Simple Test Equipment.
There is a problem with test equipment. To test and repair a VOM, you need a working VOM. To test and repair an oscilloscope, you need a working oscilloscope etc etc. Since the heading is simple test equipment we will not try to repair oscilloscopes and let's not even think about digital multimeters.
If the problem is a VOM the first question is, "Is it completely dead or works on some ranges but not on others?" If the answer is completely dead the chances are the meter movement is defective. Unless you have the skills of a jeweler or watch maker repairing the meter movement is beyond consideration.
If it works on all voltage and current ranges but not the resistance ranges, check to see that batteries have been installed.
If it works on some ranges but not on others the probabilities point toward a wiring error or misplaced resistor. Since this device has never worked the chances that you will see a cooked resistor are small.
If the device is a VTVM or SSVM and it is completely dead check to be sure the amplifier is getting voltage from its power supply. Such meters have a rather complicated switching circuit between the amplifier and meter movement to allow for measuring negative voltages. Make sure this switch is wired correctly. If the meter will not come into calibration on any of its ranges check the amplifier for proper wiring and component values. If it works on DC but not on AC check the AC to DC converter (rectifier) for proper wiring and component values.
For simple things such as an RF probe the RF probe for the Signal Tracer described in section 2.7 if one of the diodes were to be installed backward there would be no output. For a wave meter, not described in this text, if the meter moves backward either reverse the diode or the connections to the meter movement.
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