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Old 15th Jun 2019, 11:11 pm   #1
delaitt
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Default Power supply transistor blows up

Hello

I'm trying to repair a Philips TV with H chassis.

The BU326A in the power supply (T2082) keeps blowing up. I replaced the transistor once and the TV was working. Sometimes when powering it down & up again after replacing it, the power supply has some difficulties to come up and voltages were low. Trying again a few times got the power supply to deliver the correct voltages. At some point i tried again and it killed the transistor (T2082) and also blows up the fuse 2105 (1A).

Is there a way to protect the transistor while troubleshooting?
What could cause the transistor to blow up?
What could cause the power supply to sometime not start properly?

many thanks

thierry.
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Old 16th Jun 2019, 1:43 am   #2
Andrew2
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Default Re: Power supply transistor blows up

Hi Thierry. I don't know this particular chassis, but I would replace the base drive electrolytic to T2082 and R2128 and D2076. Also check the clamp/snubber components in the collector circuit.
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Old 16th Jun 2019, 1:43 am   #3
Otari5050
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Default Re: Power supply transistor blows up

Are you using a load lamp in series with the mains input? It doesn't always stop semiconductors from letting the magic smoke out, but it can help to tame things down sufficiently that you can work on it without nailing the transistors.
I'd check all the components inside the dotted line, especially the BY206 diodes and the 0.47 uF cap on the base of the transistor.
Shorted turns on the switching transformer could cause the transistor to fail, or failure of the snubber components (the various parts within the dotted line).
Could also be a bad batch of transistors, depending on where you sourced them from.
I haven't repaired TVs since the early 2000s but we tried using transistors sourced from china and they'd either not work at all, short out on start-up, work for a few minutes then go bang or work for a week then fail.
As for the supply not starting properly, I suspect it's related to whatever is causing the BU326 to die.
I'd start by going through the whole supply and testing the parts as outlined above, and then I'd start looking at other less likely parts (resistors etc) as it's possible that one of them going out of spec (or high / open) could cause a similar problem.
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Old 16th Jun 2019, 6:50 am   #4
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Default Re: Power supply transistor blows up

The most likely cause of failure of the transistor is that its Collector voltage gets exceeded. So I agree check the BY206 diodes and the 680pF and 10nF capacitors they connect to.These come into play when the transistor is switched off and tune down the frequency (and amplitude) of the peak voltage that the transistor's collector experiences when the transistor is cut off. When the transistor is switched on its protected in a saturated state, or should be.

Another thing, if the drive voltage time period, which the transistor is turned on for is too long, more energy is stored in the output transformer's magnetic field, and the peak collector voltage will increase. So in this sort of circuit an abnormal drive waveform can destroy the output transistor for that reason too.
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Old 16th Jun 2019, 9:02 am   #5
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Default Re: Power supply transistor blows up

Quote:
Originally Posted by Andrew2 View Post
Hi Thierry. I don't know this particular chassis, but I would replace the base drive electrolytic to T2082 and R2128 and D2076. Also check the clamp/snubber components in the collector circuit.
Yes I would go along with that as well. 0.47uF is a very low value for an electrolytic and it's very likely to have dried out.
In fact I would be suspicious of a lot of the electrolytics in something of this age and it may be worth checking the others as well.
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Old 16th Jun 2019, 10:08 am   #6
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Default Re: Power supply transistor blows up

Better to replace all the small electrolytic caps in the switch mode power supply. They cannot be tested reliably especially with an intermittent fault.

Worth a check is the value of R2023 to the base of T2003 [BSX79]. It should be 120K and may have risen in value. Nasty faults these PSU blow ups.J
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Old 16th Jun 2019, 10:30 am   #7
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Default Re: Power supply transistor blows up

Check for poor solder joints around transformers and connectors as well.
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Old 16th Jun 2019, 10:54 am   #8
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Default Re: Power supply transistor blows up

Check the value of R2121 in the resistor module. It's the resistor which is marked with a circle. Might be one of those critical components known to cause power supply blow-ups. It might be possible to disconnect base components to the BU326 and monitor the waveform across the secondary of the driver transformer. The switch mode oscillator start up supply resistor is R2122. Once the SM output stage is working diode 2084 supplies a stable voltage to the SM module.
Well worth checking the snubber components across the primary of the SM output transformer.

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Old 16th Jun 2019, 11:02 am   #9
Argus25
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Default Re: Power supply transistor blows up

The 0.47uF capacitor is an optional design feature. With the diode it charges up a small amount, with a polarity negative at the base, when base current flows when the output transistor is switched on. When the output of the driver transformer reverses polarity, the stored charge and voltage assists in the transistor turning off. Another method to achieve the same effect puts a diode & capacitor in the output transistor's emitter, which you will see in some circuit variations, to generate what is known as a "stand off" voltage.

In any case the circuit will work regardless of the presence or absence(if it has dried out or not) of the 0.47uF capacitor, so that is not what is causing the transistor to fail.

In addition I could add that it is a little unusual/irregular not to have a clamp diode across the emitter and collector of the output transistor, to prevent the collector voltage going negative with respect to the emitter, which can also destroy the transistor. In some switching circuits, you can get away with this, provided the damping/loading on the output transformer is adequate. But, in this scenario, what it also means is that another possible mechanism for transistor failure is that the total load on the supply is too small. If the supply was not being fully loaded, to prevent this issue destroying the transistor, you could tack a BY228 or similar, on the transistor's C-E connections with the cathode of the diode connected to the collector.

Last edited by Argus25; 16th Jun 2019 at 11:14 am.
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Old 16th Jun 2019, 11:30 am   #10
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Default Re: Power supply transistor blows up

The control circuit is really similar to that of the K9/K11 power supply, which are very reliable generally and won't blow up under any load/no load conditions. I wonder whether the rest of the circuit is similar to that of the Aus/NZ version of the K9/K11 (mains isolated version).

In any case, I would check the base drive electrolytic first.

Last edited by Maarten; 16th Jun 2019 at 11:35 am.
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Old 6th Jul 2019, 5:08 pm   #11
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Default Re: Power supply transistor blows up

Thank you for your replies.

For info, before this post and the BU326A blowing up i had replaced the following:

- R2121 (150K) which was open circuit
- C2163 with a 1uF ; have now replaced it with the correct value of 0.47uF
- R2126 (stubber resistor) which was open circuit
- checked the stubbed diodes

--

Since the post, I've now replaced the following

- replaced the small electrolytic capacitors outside the SM module
- replaced all small capacitor values in the SM module (inside the dotted line) including small electrolytic
- replaced C2160, C2161, stubber diodes, D2076, C2159
- replaced R2132 (which was slightly out of tolerance)
- added a BY228 between the collector and emitter of the BU326A
- checked R2122, R2123/R2124, R2026, R2046 which appear to be ok

I've applied the mains with a variac to see the waveform at the B point of the SM module and also at the base of the BU326A whilst the BU326A is disconnected. The frequency appears to be 58us with 100V+ at the (B) point of the SM module. The voltage appears to be 10V or so at the base of the BU326A.

Is there anything else i need to check whilst the BU326A is disconnected and before plugging the BU326A back ?

Thanks

Thierry
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Old 27th Jul 2019, 5:16 pm   #12
delaitt
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Default Re: Power supply transistor blows up

Hello

It seems to be working!

Thanks

Thierry
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Old 27th Jul 2019, 6:10 pm   #13
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Default Re: Power supply transistor blows up

The 150k if I think it is where it is, would cause a blow up on any sw/mode PSU.
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Old 3rd Aug 2019, 1:50 pm   #14
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Default Re: Power supply transistor blows up

I have a 100-watt bulb in a holder mounted on the wall behind the bench with a length of flex supplying it. I disconnect thr feed to the line output stage and instesd connect the flex from the bulbholder to the HT output of the power supply. I monitor the HT voltage supplied by the power supply, whilst feeding the mains input to the set via a variac and slowly increasing the voltage. I find that this is a simple method of checking whether the power supply is in orderwithout risk to other parts of the set.
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