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142 gtl alignment help

kaos513

Sr. Member
Apr 10, 2014
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Putnam county,NY
Hello guys i have a 142gtl, also aquired hp3478a bench multimeter here is my issue when i test my final in Madc scale its good but when i test my finaldriver meter shows Madc and when i keyup goes into Adc any help would be great.
 

Here is my update i have another 142 gtl and that one works great when i test my final and driver seems like they might be an issue on final driver stage?
 
A high reading like that suggests either you left the radio in AM mode, or someone turned the bias too high, or the bias trimpot has gone bad.

It's fairly uncommon for the bias temperature-sensing diode to go bad in that radio. Because it feeds into a buffer transistor TR40. The current output from TR40 feeds base-bias current into the base leg of the final transistor. This serves to protect the bias diode from surges when a final transistor breaks down.

Did you try turning down the bias adjustment VR9?

Could be someone adjusted it by watching the wattmeter. Not the correct method.

73
 
I think it's time to check.

There are three basic things that can cause this, if the trimpot won't behave like it should.

Bad final,

Bad TR40

Bad trimpot.

The MV13Y bias diode should get checked, but remember that it will show a forward-voltage drop that is double what a normal silicon diode will show, around 1.3 Volts.

But it almost never goes bad.

Almost

73
 
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Umm, maybe your sentence construction could use some refinement.

I see that what you really meant above was that the driver's reading was way too high.

But seeing it written as "
when i test my final in Madc scale its good but when i test my finaldriver meter shows Madc and when i keyup goes into Adc any help would be great.

I read that to mean the *final* reading was out of hand.

As for the driver, the picture is simpler. If you take the driver's bias diode D46 loose and test it, I suspect you'll find that it's bad. The only other part in the circuit is the trimpot VR8, and it would usually cause a low-current reading if it fails, not a high reading.

I thought you meant the final.
Oops.

Oh, and the driver's bias diode D46 is type MV1Y. It's different from the MV13Y diode on the final transistor. D46 should show only the 2/3 of a Volt on a meter's continuity scale, and only with the black test lead on the banded (dot) end. Should show open circuit with the red lead on the banded, or dot end.

I'll guess that your D46 will show an open circuit in both directions.

If it does, a new diode may fix the problem.

73
 
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I assume you have no output power on AM or SSB?
I assume you have good reception and receive audio?
I assume you cannot hear any output that can be heard on a monitor radio sitting on the same bench with the 142GTL that you are working on?
I assume you have the proper collector and base voltages on the driver and the final?
And last but not least I assume the readings you are talking about are taken by properly hooking the meter up in USB mode, NO MODULATION, in BIAS current measuring/adjustment procedure mode?
Now what exactly are the measurements and if you can show us an image of the reading on the meter?
There that should clear the air a bit!
 
i do have output power and modulation on all bands just seems something will not let me test the driver for adjustment.i did over 7 radios i own all worked perfect is just this one that is that is giving me issues.
 
’Twas brillig, and the slithy toves
Did gyre and gimble in the wabe:
All mimsy were the borogoves,
And the mome raths outgrabe.

“Beware the Jabberwock, my son!
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch!”
 

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