EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: decoder_82 on December 28, 2022, 09:34:52 am
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Hello,
I am working on a Panasonic AG-500 TV / VCR player. The device doesn't power on any more.
Inspecting the power supply area revealed that Q801 was shorted. Further I discovered that the zener diode D820 was also shorted.
Two questions:
- what would be suitable replacement for D820 ? It is a EQB01-05 but I can't
find the exact same reference. Would any Zener 5v 1W work ? I was thinking to
use a 1N4733.
- are there any other parts I should put more attention on in the PSU section ?
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When power transistors are gone in a SMPS, 99% the driving circuit is gone too.
Check for ESR the caps in both primary and secondary sides.
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Thanks I will double check the capacitors. What is strange is that none of the fuses (primary or secondary side) were blown.
I guess there is a component that failed open somewhere.
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Ok so electrolytic capacitors have acceptable ESR value.
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Dry joints on the switching transformer, and resolder every diode on the secondary side. Replace C9818 and C9808, and replace R9813, as it is likely either drifted high or open circuit. Check R9803 and R9804, R9805, D9817 and D9809 as well, as if any of those go open circuit you will blow that transistor again. Secondary side replace all the electrolytics after the rectifiers, they need really low ESR, anything above 0.5R is a fail ESR wise.
Also while you are there resolder the LOPT on the TV board, along with the driver transformer for it, the line transistor, the driver and all the large resistors in the HT side, as they tend to get dry joints, especially on a set that is meant to be moved around.
Nice units, well made, and pretty reliable as well.
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Thanks for the suggestions. I will check and report progress.
Indeed this is a nice unit and those units rarely fail from what I read.
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Alright so I have replaced C818, C9808, C816, C819, C812. The ESR was still within acceptable range.
I also checked all the diodes and resistors suggested.
Solder joins have been reflowed.
Now I see something new. Voltage at the filter cap is very low (9v). Further check revealed that R9801 power resistor failed high. Will order a replacement but I wonder why this happened
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R9801 sacrificed itself to protect the fuse.
They put a note on the schematic: ‚dead circuit‘ but probably in the wrong place 🤓
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Yes strange that none of the fuse blew.
Not sure what is the way forward here is to further trouble shoot.
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I checked usual suspects like the HOT and doesn't looks like there is a short there.
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You can also verify the power supply by interrupting the B+ rail and loading it with a 40W incandescent bulb if still available.
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Hopefully I still have a couple of incandescent light bulbs.
So would interrupt D9810 and plug the light bulb there ?
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Check R9813. May now be much higher resistance or even open circuit.
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R9813 should be 0.45 ohm and I observed 0.7 ohm. Might be indeed wise to replace it as well. Thanks for the suggestion
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R9813 should be 0.45 ohm and I observed 0.7 ohm. Might be indeed wise to replace it as well. Thanks for the suggestion
Should be 0.47 Ohm. Unless you are quite certain that your measured 0.7 is accurate then it is probably OK. Many multimeters will show 0.1 or 0.2 even when the probe tips are held together. You need to subtract that from whatever the meter shows when measuring low value resistors.
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Yes good point the DMM might not be 100% accurate on such small values. I checked on a replacement one and I also got higher reading.
Will provide more update after testing with the light bulb on the B+ line
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Alright so I got my replacement resistor for R9801.
I did the light bulb test on the B+ line. Unfortunately no glow was observed. It means the power supply is not working. Will check every components once again on the primary side.
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Q9801 is self oscillating using feedback winding B1-F2. Output voltages are stabilised by sensing the voltage developed in winding F1-F2 which via Q9803 and Q9802 control dc at Q9801 base. Q9804 senses the voltage across R9813 and robs Q9801 of drive when current is too high.
I would disconnect the outputs on the secondary by removing the fuse F9802, put your lamp in series with the mains input and connect voltmeter across C9806. Then apply power and observe voltmeter without touching anything. Voltmeter will show negative voltage if the primary side is oscillating. Disconnect power and wait several minutes for C9805 to discharge before touching anything.
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Ok so I disconnected F9802 and put the light bulb in series with the mains input. When I put the power on, the light bulb glows quite strongly. At C9806 I get +12v
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Ok so I disconnected F9802 and put the light bulb in series with the mains input. When I put the power on, the light bulb glows quite strongly. At C9806 I get +12v
I assume that positive 12 volts is with meter plus and minus leads on the plus and minus ends respectively of C9806. If so looks like the primary side is oscillating. At least when no load on secondary.
Replace the fuse. Unsolder the cathodes of D9810, D9814 and D9815. Voltmeter between either end of the fuse and cathode of D9815. Apply power. Any voltage reading? If yes, do not worry if markedly different from stated 14 Volts as the measurement is without capacitor on output of rectifier. If yes repeat test on cathode of D9814 and then cathode of D9810.
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Yes exactly positive 12 volts is with meter plus and minus leads on the plus and minus ends respectively of C9806.
Here are the voltages I get with the fuse back in place:
D9815: 7v
D9814: 14v
D9810: 127v
So looks like the power supply is working, at least without load.
Now I have to figure out on which line there is an issue.
The test was still done with the light bulb in series. Maybe I can try without ?
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Reconnect only one diode at a time and check if it still gives voltage.
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Alright so I connected again the diodes, one by one. I got voltages on all lines. I then made a test without the light bulb in series and the monitor started.
Not sure why R9801 blew maybe it was already damaged at the first place ?
Anyway my monitor seems to be working fine now. Thanks all for the help.