HOT keeps breaking down

Hello!

First of all, please forgive me my English, it is not my mother tongue...

I have a television set (Philips MD1.2E AA) of which the HOT was a short ciruit. I replaced it and the fly-back capacitor as well. The man in the store gave me a BU1508DX instead of a BU1506DX which was not in stock (but which was in the TV); he said it was compatible. (Is there anyone who knows whether this is true?) After inserting it, it immediately broke down again and I took it out: it is as short-circuited now as the original one. I can't measure a short-circuit in the primary circuit of the HV-transformer. Is it likely that there is one on the secondary site? If chances are high that after replacing it (and maybe some small other parts) the TV will be okay again, I would not mind spending the money. Does anyone have an idea about the price of such a transformer? The power supply is making hick-up sounds. Another peculiar thing: You know the sound a light-bulb makes when shaking it? The same sound comes from the picture tube if one moves the TV gently e.g. when putting the back side on again. It is also similar to the "tsjinggggggg" sound my monitor makes after pressing the degauss-button (without the first "click"). Is there a chance that there is a short-circuit in the tube so that it demands too much energy from the transformer and thus from the HOT?

I hope someone can give me some tips, thank You very much in advance!

Yours sincerely, Rene

Reply to
Rene
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"Rene" ha scritto nel messaggio news:wYGdnXfB448 snipped-for-privacy@infopact.nl...

SPPPPPPRRRRRRRRRRRRRRRR!!!!T There's something that isn't ugual on those two xistors... they aren't interchangable, BU1508DX has VCESAT of 1.0V and BU1506DX has VCESAT of 5.0V and there are some diverse parameters...

I.

Reply to
Inty

"Art" schreef in bericht news: snipped-for-privacy@comcast.com...

Dear Art,

Thank You very much for Your reply! There is a signal on the little transformer that gives the signal to the base of the HOT, so I think that won't be the problem. I ordered a new LOPT, I will post the results later.

Sincerely, Rene

Reply to
Rene

"Inty" schreef in bericht news:Kl9xe.22344$ snipped-for-privacy@news3.tin.it...

Dear Inky,

Very funny, in another store they also said that their computer proposed the

1508 as a 1:1 replacement. I do not trust their computers. Well, I mean, I do not trust the people who put this info into their computers. I ordered a new LOPT and also two 1506's. When I have results, I will post them here.

Thank You very much for replying!

Sincerely, Rene

Reply to
Rene

That signal on the base of the HOT has to be pretty-much perfect, or the HOT dies.

Tom

Reply to
Tom MacIntyre

A low saturation voltage is actually better. I think of the BU1506DX as a cheap alternative, and seem to remember having replaced those with BU1508DX's in the past.

--
Met vriendelijke groet,

   Maarten Bakker.
Reply to
maarten

"Rene" schreef in bericht news: snipped-for-privacy@infopact.nl...

Comparing the datasheets of both types reveals they virtually be the same although the 1508s specs are slightly better. The VCESAT Inky mentioned is a maximum so worst case. The actual VCESAT may be below 1V for both types.

The LOPT is a pretty expensive device. You can find many tips for checking the it at:

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a relatively easy to build tool at:
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Although the LOPT might be defective, the control signal of the HOT is at least as suspective. You hardly can check this out without an oscilloscope. You need to check the frequency and the pulsewidth. The pulsehight may give you an idea whether or not the pulse has enough power to drive the HOT correctly. A positive DC level (p. e. caused by a leaky driver transformer) will be disastrous for the HOT. For maximum certainty you'll need the specs of the manufacturer.

petrus bitbyter

Reply to
petrus bitbyter

"petrus bitbyter" ha scritto nel messaggio news:42c8350b$0$776$ snipped-for-privacy@reader20.nntp.hccnet.nl...

a

Ahem, my name's Inty, not Inky :-) They aren't 1:1 xistors !

I.

Reply to
Inty

A perfect match for a HOT is hardly the most important aspect of its survival, especially short-term. Slight disruptions in the drive signal can be disastrous.

Tom

Reply to
Tom MacIntyre

But after such defect has been fixed, a good match is necessary. Maximum voltage ratings should be the same or higher, saturation voltage should be the same or lower, switching speed needs to be the same or lower, beta needs to be in the same ballpark. So don't go replacing a 2SD869 with a BU208 in most applications. It will run hot and blow.

--
Met vriendelijke groet,

   Maarten Bakker.
Reply to
maarten

Switching speed needs to be HIGHER of course, the number of milliseconds lower...

--
Met vriendelijke groet,

   Maarten Bakker.
Reply to
maarten

"petrus bitbyter" schreef in bericht news:42c8350b$0$776$ snipped-for-privacy@reader20.nntp.hccnet.nl...

Thanks for the links, I already ordered a new one at Lejos (30 euro's). Off course they are useful for the future and the schematic of the tester is great! (A pity You and Maarten did not reply to the post in the Dutch group, I had been really hoping for that.)

Hi,

I have the official service doc in which the shape of the signal on the transformer that drivers the base of the HOT is displayed. I do have a scope, I wrote that in the Dutch thread (sorry for multiposting instead of crossposting, I normally never do that, but then, these groups have different languages). The picture is not exactly the same. Unfortunately I did not know where to get the mass for the scope, I chose to measure over the transformer, as the side of it that is not connected to the base of the HOT, is connected to "gnd 1". I hope the picture I get, is OK then (can't think of a reason why not, but then, I am not an expert). The test point is called "L2". Frequency and pulse-width are OK, the shape is not exactly the same. I will have to check again for the DC-component as I was not aware of it's importance when I was testing, I do understand it now. If the peak level is not as high as on the doc (I remember it was lower because the scope was to be put in 20 V/div mode while mine's maximum is only 10V/Div), can it cause the HOT to die?

Thanks very much for replying.

Greetings, Rene

Reply to
Rene

One particular Thomson chassis of the past comes to mind, the TX-85/86. A 100 ohm, 35 VDC capacitor goes bad in the drive circuitry (which used no drive transformer, by the way). The DC voltage starts to float up from the desired -2.25 or so VDC, the negative spike's peak rises to about -8 from -11, the HOT runs hot, and eventually dies.

Tom

Reply to
Tom MacIntyre

Dear Tom,

Thank You again. I have ordered several new capacitors to replace the ones in the drive circuit. Perhaps I will replace some more (why not all of them (well, not on the processor board maybe but I mean the ones on the large signal board), they're not that expensive). I will also re-connect my scope to the machine and see how much the shape of the wave differs from the one in the service doc. And check whether there is a DC-component behind the transformer.

Greetings, Rene

Reply to
Rene

Hi again!

Good news: The TV is Okay! I'm very happy about it and so are my parent, the owners. It really was very educating, with my scope I studied a lot of the signals in the line-circuit. A special message to Art and Tom: I had some doubts about Your remarks about the importance of the shape of the signal that drives the HOT (the doubts were caused by lack of knowledge on my side): I must admit that You were very right. The line-drive transformer turned out the "bad guy". On the base of the transistor that drives this transformer, the signal was perfect, the shape of the signal on the transformer wasn't steep enough by far.

Thank You all very much for helping me!

Sincere greetings, Rene

Reply to
Rene

Always nice to be involved in a successful venture...that's what this group is all about. :-)

Tom

Reply to
Tom MacIntyre

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