General Circuit Troubleshooting

[dw85745]

When troubleshooting an electrical circuit are there some general rules of how to proceed:

For example with a TV:

1. SAFETY: It is my understanding by going from the HV anode (rubber cap on CRT) to Kine (Chassis) ground that this will discharge all HV caps. Is this correct or does each Cap need to be discharged separately?

2. If you check the B+ voltage coming from the Low Voltage Power Supply (LVPS) and it is:

a. LOW going into the flyback do you test back toward the supply looking for a bad component or do you go toward the load (ie load is pulling voltage down)?

b. Conversely if it is HIGH which way do you go?

2. Can you safely disconnect one or more circuits (boards) from the Chassis and power it up for testing without destroying the TV or item under test?

 

[elecmec]

Some of these questions bring back memories.
The HV Anode charges a capacitor which is made from a conductive coating inside and outside of the glass picture tube. If you discharge the capacitor, and wait several minutes, you will see voltage on the connections. Some charge remains in the glass and with time will appear on the capacitor plates. I usually disconnect the wire and place a jumper between the connection on the tube and some ground point on the picture tube. As for the chassis, older tv's had other capacitors in the system called "door knob's" which usually did not store much charge, but should be discharged. Today's tv's are designed as cheep as possible. The power supply is tapped anywhere it can to supply voltage to one of may circuits. The good news is the charge on capacitor drains off quickly, (tube systems may have some charge, because when the filaments are out, the tubes do not bleed off the charge of power supply capacitors quickly). If you remove a circuit, you change the load of the power supply circuit, and reduce loading which could sent a higher voltage to other circuits in the system, causing another failure. It is best to have a schematic with voltages identifed, so that appropriate loads can be substituted for the removed circuit, before applying power. If you are trying to power one circuit, you may want to use a seperate power supply, using voltages shown on the schematic. Be sure this is the only circuit powered.
I hope this helps.
Elecmec

 

[Cappels]

dw85745 (Visitor) Mar 18, 2002
When troubleshooting an electrical circuit are there some general rules of how to proceed:
For example with a TV:
1.ÊÊSAFETY: It is my understanding by going from the HV anode (rubber cap on CRT) to Kine (Chassis) ground that this will discharge all HV caps.ÊÊIs this correct or does each Cap need to be discharged separately?

A: Some displays have bleeders or high voltage sense reistors that will bleed the supply in a few seconds, but you should never assume that they supply is safe to touch. Always ground the anode cap with a grounded screwdrive (grouned to the contact on the CRT's quadag coating -that black sutff on the outside of the bell), and never hand the anode lead unless it is grounded (suggest a clip lead or something similar). After the CRT has been sitting for a while with the anode lead disconnected, the soak charge, often to several kilovolts will accumulate between the anode connector and the outside dag, so never touch the anode connection (one day my boss was pointing his finger at an anode connector of a CRT under the workbench. I said "watch out", and Raymond said "This? Its been here for a year...AAAAKKK!!!!!!&quot. A clean CRT is a high quality capacitor and will hold a charge for years.


2.ÊÊIf you check the B+ voltage coming from the Low Voltage Power Supply (LVPS) and it is:
ÊÊÊÊa.ÊÊLOW going into the flyback do you test back toward the supply looking for a bad component or do you go toward the load (ie load is pulling voltage down)?

A: Yes, both. Depends on the particular circuit. Always check the power supply first. Sometimes, there is an "antibreathing resistor" in series with the flyback circuit, in which case, it sometimes opens.

ÊÊÊÊb.ÊÊConversely if it is HIGH which way do you go?
A: Check the power supply first.

2.ÊÊCan you safely disconnect one or more circuits (boards) from the Chassis and power it up for testing without destroying the TV or item under test?
A: Probably not.

 

[Dan76]

1:"When troubleshooting an electrical circuit are there some general rules of how to proceed:"

Simple stuff: ALWAYS CHECK SIMPLE STUFF FIRST!
-Power supply; Is the circuit getting power? (This includes external things like cords, plugs, outlets, fuses, and breakers)
-Obvious problems; Burned or Bare wires. Does anything look, smell or sound unusual? USE ALL OF YOUR SENSES. Is anything unusally Hot?
-What has changed and what is the same? Has the machine been subjected to any events that could have caused this?
-Ask someone who has worked on the device (or a similar one)before. Old timers can often point you in the right direction.

Guidelines:
-If possible get a GOOD, DETAILED schematic of the machine.
-Technical mauals often have troubleshooting guides, use them.
-Divide the machine into functional blocks. EX: Step-Down Transformer, Rectifier, etc... (Circuit boards are sometimes a good way to make the division) After dividing into blocks start in the middle and check outputs. (The tech. manual may provide appropriate values waveforms etc..) based on results you can continue until you find the faulty Board or even component! Remember a good output means the problem is farther down the line.

Safety: DON'T BE STUPID!
-Be very careful when you work on energized equipment
-Use adequate lighting
-When in doubt TURN IT OFF!
-Discharge stored energy (Capacitors, springs, etc..) And MAKE SURE it is discharged.
-Make sure no one can energize the machine while you are working on it (This means USE LOCKOUT/TAGOUT!)
-DON'T RUSH, that is when you make mistakes.

Other:
-Be careful with screws, bolts, etc... don't drop them in the machine.
-Keep small easily lost parts in a tray or bag.
-Use the right tool and use good tools, it's not worth wrecking a $200+ machine because of the lack of a $6 screwdriver
-If you get stumped and fed up, Take a Break! Return with a calm and open mind.


As far as your second question "Can you safely disconnect one or more circuits (boards) from the Chassis and power it up for testing without destroying the TV or item under test?" Some devices Yes, some No. If you aren't sure DON'T!
However you can remove the board or component and test it individually.

By The Way this system can work for almost any machine, not just electrical!

Good Luck!