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Old 28th October 2010, 03:23 PM   #551
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Attach the pictures of 1 MJ15003 Motorola (broken) and replaced with MOSPEC MJ15003.
I think this is the same of the other 3 pcs bought in the same shop at the same time
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File Type: jpg 03012008881 [800x600].jpg (102.8 KB, 877 views)
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Old 28th October 2010, 03:55 PM   #552
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I clean with Acetone top of the MJ15003 transistors and the printing comes off....
FAKES ....
But could be the cause of ZTX overheat?

Sorry for many post fot the same topic but...i can't edit
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Old 28th October 2010, 05:26 PM   #553
jaycee is offline jaycee  United Kingdom
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It is most likely the cause of your problems. I would check also the driver transistors, if these are fakes they may be causing an oscillation, which would cause problems.
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Old 28th October 2010, 05:41 PM   #554
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could someone send me the RIGHT temperature value of the ZTX (without signal applied)

very very thanks
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Old 28th October 2010, 05:52 PM   #555
jaycee is offline jaycee  United Kingdom
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They should become only slightly warm... you should be able to put your finger on them and not say "ouch!"
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Old 28th October 2010, 06:22 PM   #556
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mmm....there isn't this risk...i can touch the transistors ZTX without any problems for few minutes...Maybe i worried too.I replace on the channel 2 the Motorola(fakes) with 2 MOSPEC (i think genuine) with the same results.Without signal applied i can read on the thermometer about 48-52...after 10-15 min power on
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Old 30th October 2010, 08:14 AM   #557
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i bought MJ15003 - MJ15030 - 15031 "ONSemi" on Distrelec website.
Thursday should give them to me...
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Old 30th October 2010, 08:39 AM   #558
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Ciariello,

I see a very bad thermal contact between the heat sink and transistor mounting plate. This can be the cause of the problem. Measure temp on the transistor mounting plate and then the heat sink. They should be the same or at least very close.
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Old 30th October 2010, 08:41 AM   #559
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the temperature value on the mj15003 transistors (on both channel)is near to 25-26 (cold...) my problem are the ztx....
the surface of the heat sink and the mounting plane were polished and made smooth.
between the 2 parties, I put a good layer of thermal paste. then the two parts are securely tightened with 4 bolts placed 1.5 cm apart

Last edited by ciariello; 30th October 2010 at 08:54 AM.
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Old 5th November 2010, 01:00 AM   #560
Ruwe is offline Ruwe  Europe
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Hi ciariello,
First check the current through the 68 ohms resistor in the emitter of ZTX 653. If it is about 8.5-10 mA you're good.

If your bias current is not adjustable most likely the Vbe multiplier doesn't work (the transistor between the ZTX653 and ZTX753). Check the pin orientation and the resistor values around. The voltage for the right bias current is about 2.35-2.38V between the collector of ZTX653 and ZTX753.

One of your channels has DC offset problem. Probably something at the inputs resistors or the feedback resistors. Check the values of the resistors.

... And get rid of the fakes, but I don't believe they are causing the problems.
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