Black Box IC515C Service Manual download pdf (Page 8)

3. RA-3/3A Power Supply Troubleshooting

Once the FET has been changed along with R607, you will need to disconnect the secondary voltage line by

lifting D667. This is very important since switch-mode supplies draw very small amounts of current while running

unloaded and the load from the 5-volt line must be kept from drawing any current. Many technicians are fearful

of running switch-mode supplies unloaded. Most are able to run unloaded, but this test is not going to supply

more than 40VAC. You will need to keep the AC low with the variac and watch for current draw. While turning up

the AC voltage, monitor the drain of the FET with an oscilloscope to ensure it is starting to oscillate. This will

need to be done because even if the oscillator starts running, you will probably not be able to detect any current

on most amp meters that come equipped with the variac. If you do not see any oscillation by the time 40 or

50VAC is reached, there is a problem.

Oscillator starts with very little AC current being drawn: Re-connect the load to the secondary and bring up the

supply with the variac while monitoring both AC current AND the voltage at D667. You want to ensure that

current draw continues to remain low and that the input voltage to IC655 does rise dramatically above 7 volts.

Rising current would indicate a short on the secondary or the output of IC655. A continuously rising voltage at

D667 indicates regulation problems. Regulation problems will be discussed later. If the supply runs with full AC

and minimal current, the repair has been completed

Oscillator starts with excessive current: This is an obvious indicator as to what caused Q601 to fail. This situation

is rare in this type of power supply in an unloaded condition but when it does occur, it is almost always caused by

a leaking or shorted feedback coupling capacitor (C614) or shorted winding(s) in T602. Ringing the transformer

or viewing the waveform at Q601D will expose problems in the windings. Listen for any ticking sounds and watch

the oscilloscope for bursts of oscillation, which would indicate the over-current and/or over-voltage stages are

doing their job.

Dead Standby Supply – Q601 not Shorted

Assuming very little AC current is being drawn and there are no chirping or ticking sounds, you are dealing with

a circuit that is not starting or is not able to sustain oscillation. Verify the presence of B+ at Q601. Next, read the

voltage on the gate of Q601. The same voltage present on the drain should appear at the gate

No voltage at gate: No voltage indicates either an open start resistor (R609) or the gate is loaded down. Read

the resistance at the gate of Q601 relative to hot ground. It should read infinity. If it does, lift one side of R609

and read its resistance. You must lift it since it is a 22MΩ. If resistance is read at the gate, you will need to find

the component(s) at fault. As mentioned earlier, due to the high impedance of the FET gate, many components

can be damaged by incoming transients. Even a small amount of leakage can overcome the pull up function of

R609. All you can do here is to “ohm out” parts or, more effectively, lift components to get the voltage back on the

gate. In the case of line transient damage, you may find several components loading the area down. Do not

forget about feedback capacitor C614 since it is connected to the feedback winding with a low resistance to

ground.

Voltage present at gate: This indicates the start resistor, R609, is OK and nothing is loading the gate. You now

have a narrow choice of components to check. Either the feedback path is open (C614, R616 or the feedback

winding itself), or the “tuned” portion of the oscillator has a problem. This leaves us with leaking or, most likely,

shorted C617 or D623 in the snubbing circuit or T602 itself.

Voltage Regulation Errors

This failure can be difficult at times due to the closed loop needed for this circuit to operate. It becomes even

more difficult when the problem is excessive B+ since components can be damaged. Listed below are some

troubleshooting techniques:

Low B+ -

This is easier to work on since you will not risk damaging components. Excessive current is not being drawn

since components are not smoking. It is safe to assume the regulation feedback loop is not open since this

would cause B+ to run too high. That leaves two possibilities: 1) The oscillator is incapable of producing adequate

B+ due to frequency shift or distorted oscillation; or 2) the regulation control components are not performing their

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Black Box IC515C Service Manual Page 8

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