9
3. RA-3/3A Power Supply Troubleshooting
Troubleshooting
Once a few standard procedures are followed, this power supply is not too difficult to repair. The greatest
obstacle is the precise timing required for the two transistors inside of IC601 to work together. They form a
classical AB-type amplifier design. This means the transistors need to be biased near the upper end of the cutoff
point and this is provided by resistors R610 and R611 for the upper half and R621 and R618 for the lower.
Anything upsetting the biasing of the feedback lines can cause an overlap. Once both transistors are on at the
same time, even for a brief moment, they will fail. Incoming transients from the AC line, excessive noise in the
feedback circuit or leakage of the coupling capacitors C620 or C621 can cause this to happen. The other cause
may be excessive current that exceeds the capacity of the transistors, such as shorts on the secondary line.
Problems in the feedback circuitry are the most difficult for the technician. In a typical situation, the technician
finds IC601 shorted and checks the secondary lines for shorts. He then fires the unit up and loses the newly
installed IC.
This problem can be solved by using a proven method to actually run the supply with the original
cause still there, and yet not lose the switching IC. Also covered will be regulation problems, which are quite
easy to solve with a couple of simple tricks.
Since the most common failure is shorted switching transistors, that scenario will be covered first. Power supplies
that will not run is not very common, but will also be covered. Regulation troubleshooting will then follow.
Dead Power Supply, IC601 Shorted
The important rule to remember when this happens is to assume that something caused the failure. Occasionally,
IC601 will simply fail
and will turn out to be an easy repair. However, you should be ready for the likelihood that
something else is still wrong. Rule number one is: Never fire up the set to see what happens. Always use a
variac to bring it up slowly. There is one problem to deal with before this is done. The main relay will not engage
until there is at least 60VAC coming into the set. This will supply enough unregulated B+ to the switching
transistors to generate full secondary voltages and they will be lost if a dynamic load (such as a defective flyback)
or a shorted line still exists that was missed during the performance of static resistance checks. It is going to
involve jumping the main relay while controlling things with the variac. This is a safe method so long as care is
taken and the current is monitored closely.
Some technicians like to use a light bulb in series with the AC input to protect against failures. This worked fine
on analog supplies, but all it did was sit there glaring at you to indicate a problem existed. It is not a good idea to
use this method on switching supplies. The light bulb will substantially reduce the incoming AC voltage and,
consequently, the unregulated B+. The power supply will now go into low frequency mode and try to get the
proper secondary voltages. This will produce greater “on” times for the transistors and they will soon overheat
and fail. Use a variac.
Once IC601 has been replaced (it is assumed that fuse resistor R612 will also need replacement), some other
checks are required before power is applied. A quick visual check of the components around the switching IC
should be done to look for signs of overheated parts. Watch the small capacitors for signs of cracking or
swelling. This is particularly important if lightning activity has occurred recently. Check all secondary voltage
lines for shorts and if all is OK, it is time to apply some power.
Plug into the variac and set the meter for AC current if it is an integrated type. Jump the relay RY601 and start
bringing up the AC voltage. Watch the current draw closely. At around 40VAC, about 750ma to 1 amp will should
appear. Continue to increase the voltage and allow up to 1.5 amps to appear, but no more. As you near 60 to
70VAC, the amperage should actually start decreasing as the voltage input rises. As the switching supply gets
more voltage, it will increase its frequency due to the transistors having a shorter on time. By the time you reach
full 120VAC, the amperage should have dropped to somewhere between 600 and 800ma. There is a good
chance you now have a repaired unit, but there is still one more check to perform. The soft start circuit could
have caused the IC failure and if this is so, IC601 will fail at turn-on. This may happen immediately or even
weeks later. Move forward to the section dealing with soft start. Unfortunately, it does not always go this easy
and a couple of different scenarios might occur from the ideal outcome above. Following are some suggestions
for resolving them:
