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Author Topic: Correct biasing procedure and replacing MRF429 finals with MRF448 in an TS-950SD  (Read 4306 times)
ZS5WC
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« Reply #15 on: October 29, 2017, 04:39:14 AM »

 ;)Hi Mircea,
First of all very well done with the new callsign!..
I really enjoyed reading your very well described technical reply, something we don't see very often anymore nowadays.
Wow those must have been expensive sparks!. Shocked
In my previous comments I stated-"make sure bias transistors are OK"..I normally test all passive components and then test active bias circuitry BEFORE connecting finals.
It is also a good idea to use perhaps a lower rated fuse to PA initially until it all checks out.
If I get more than 0.5v at BASE pads without Finals I will not connect them until the biasing can be adjusted properly.
Good Luck and hope to hear you on HF, enjoy the fruits of your labour.

73 de William
ZS4L / ZS5WC
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DC1MC
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« Reply #16 on: October 29, 2017, 05:30:09 AM »

Hi William and thanks for the good wishes and previous advices, I should have followed them  Cry

Yeah, the laziness and impatience got the best of me, I'm actually one of the few HAMs that could solder relatively good Wink, I can only hope that the former finals were actually bad as well, anyway they would have not survived with that bad bias, but it's my first PA and I live and learn.
Also the violet flames inside the transistors were pretty cool, the rest of damaged components didn't cost that much for the lesson they've told me, and the with BLW95 I've just got lucky on an EBAY auction, some German OM was disposing of them.
All in all I call this first TRX revival a success.
Let's see how the antenna will work, another important stepping stone.

 73 de Mircea DC1MC

 
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G3RZP
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« Reply #17 on: October 29, 2017, 06:25:58 AM »

Mircea,

When working on a tx, always keep an analogue multimeter available - digital ones, even expensive digital ones, can lie like an HR manager when there's RF around.
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ZS5WC
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« Reply #18 on: October 29, 2017, 10:51:17 AM »

 Wink;)Just as a last comment--I do think an equivalent 1.7A for driver pair is way over spec.
I would say 50mV is more than sufficient.
I normally set the TS430 drivers to 500mA, so I do think the service manual is wrong.
At this high bias, you are looking for runaway problems.

73 de William
ZS4L / ZS5WC
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