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 Another build conundrum 
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Post Another build conundrum
This is really getting to me now...this is one of those jobs almost ready to be popped under the bench for ever...

This little Tx ...I am trying to set the bias, but there isn't any. At rest I see 40mA. I am supposed to be able to set up to 200mA with no signal present and key up...well I can't set ANYTHING.

On the diagram attached voltage is fed at point V, in red.

In the blu box two things have changed. In Lieu of the power adjust pot (on another board, so this is for ease) points D and DH are shorted by a wire. The rest trimmer of 10k is replaced by a fixed resistor of 5k for test purposes (the proper trimmer was missing from the kit and instead a 1k was fitted before I noticed it. I pulled it out yesterday and added the 5k as a ball park figure for testing.New 10k in the post)

Point 6T has got pretty much full supply voltage of c12v on it. I don't think this should have.

[color=#FFFF00]ALL transistors are tested and working according to my Chinese tester. ALL resistors are correct. ALL diodes are correctly oriented.[/color]

The designer, Tim, has just moved house so has no easy access to any voltages in his prototype so I can't ask him again.

For the life of me I cannot see what would stop the bias current changing.

Anyone got any ideas?


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David. G4YVM
Fri Feb 01, 2019 11:34 pm
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 Another build conundrum 
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Post Re: Another build conundrum
6T should have around 6.3V on it, why not just use a 6V1 zenner?

Which is the power pot and which is the bias pot?

Are the devices chinese or brand names?
Some chinese devices need a far lower bias, otherwise they just turn full on (ie many are not proper spec devices, I ended up buying a whole load of 510s locally)

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Sat Feb 02, 2019 7:39 am
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 Another build conundrum 
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Post Re: Another build conundrum
The bias is a 10k preset on the board. Pout is a 10k pot off board in the front panel but they are connected. At the mo ths Pout is bypassed.
I didnt source the devices.

From.your questions though you can't see why im not getting bias. I wondered last night if there's a component missing...its happened before im this kit. Something I've not twigged is on the circuit but not the placement scheme.

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Sat Feb 02, 2019 8:37 am
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 Another build conundrum 
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Post Re: Another build conundrum
Hi David, D105 is should be clamping 5.1VDC.
TR105 removes the bias (Class C) when it is keyed for CW, check the polarity of the Zenner D105 and D106, D107.


Sat Feb 02, 2019 8:52 am
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 Another build conundrum 
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Post Re: Another build conundrum
G4YVM wrote:
The bias is a 10k preset on the board. Pout is a 10k pot off board in the front panel but they are connected. At the mo ths Pout is bypassed.
I didnt source the devices.

From.your questions though you can't see why im not getting bias. I wondered last night if there's a component missing...its happened before im this kit. Something I've not twigged is on the circuit but not the placement scheme.



Adjusting the bias will affect the power output, but it will also affect the linearity, so I can't see having the two controls connected being a good idea.
You would normally adjust the power by adjusting the drive.

The circuit is not easy to see, hard to read, but without the rest of the circuit, it is had to say.
ie is the voltage at the red square constant, or only on TX?

There is no point in trying to go any further until you have the correct voltage at T6, the zenner and diodes act as a regulator for the bias adjustment, Then if the device under the red square (can't read the part number) is duff, the bias can not be correct.

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Sat Feb 02, 2019 8:58 am
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 Another build conundrum 
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Post Re: Another build conundrum
Ok so t6 has got to be 5.1v



Ok...off we go.
Thanks

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Sat Feb 02, 2019 9:19 am
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 Another build conundrum 
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Post Re: Another build conundrum
What should make t6 5.1 v? Is it that zenner alone? I'm thinking I can isolate sections of board to test them, even if I cut tracks. The zenner is the right part number, but in not sure how to test it is working

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Sat Feb 02, 2019 9:36 am
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 Another build conundrum 
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Post Re: Another build conundrum
I think it should be 6V3, 1V2 from the two diodes and 5V1 from the zenner,


and I should have realised it was CW :)

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Sat Feb 02, 2019 9:47 am
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 Another build conundrum 
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Post Re: Another build conundrum
6.3.

Steve, if I'm seeing supply voltage as I am, does that tell me anything at all about those diodes?

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Sat Feb 02, 2019 9:57 am
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 Another build conundrum 
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Post Re: Another build conundrum
G4YVM wrote:
6.3.

Steve, if I'm seeing supply voltage as I am, does that tell me anything at all about those diodes?

Are you measuring wrt to ground?
Even if the zenner is goosed you should still see a voltage less than vcc due to the drop across the resistor and the two diodes below the zenner.


Sat Feb 02, 2019 10:37 am
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 Another build conundrum 
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Post Re: Another build conundrum
M0XXQ wrote:
G4YVM wrote:
6.3.

Steve, if I'm seeing supply voltage as I am, does that tell me anything at all about those diodes?

Are you measuring wrt to ground?
Even if the zenner is goosed you should still see a voltage less than vcc due to the drop across the resistor and the two diodes below the zenner.


And if the zenner is open circuit? or the diodes are the wrong way round.

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Sat Feb 02, 2019 10:40 am
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 Another build conundrum 
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Post Re: Another build conundrum
Sangoma wrote:
M0XXQ wrote:
G4YVM wrote:
6.3.

Steve, if I'm seeing supply voltage as I am, does that tell me anything at all about those diodes?

Are you measuring wrt to ground?
Even if the zenner is goosed you should still see a voltage less than vcc due to the drop across the resistor and the two diodes below the zenner.


And if the zenner is open circuit? or the diodes are the wrong way round.

Which is what I mentioned earlier?
If the zenner is short it won't clamp and assuming vcc is 12V - 1.2V and whatever the voltage drop over resistor is it won't be measuring 12V.


Sat Feb 02, 2019 10:44 am
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 Another build conundrum 
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Post Re: Another build conundrum
M0XXQ wrote:
Sangoma wrote:
M0XXQ wrote:
G4YVM wrote:
6.3.

Steve, if I'm seeing supply voltage as I am, does that tell me anything at all about those diodes?

Are you measuring wrt to ground?
Even if the zenner is goosed you should still see a voltage less than vcc due to the drop across the resistor and the two diodes below the zenner.


And if the zenner is open circuit? or the diodes are the wrong way round.

Which is what I mentioned earlier?
If the zenner is short it won't clamp and assuming vcc is 12V - 1.2V and whatever the voltage drop over resistor is it won't be measuring 12V.


Sorry, I took goosed to mean faulty, not short circuit :)

Agreed, if it is shorted, you would see around 1V2
If it was open circuit, you would see the supply voltage, assuming a high impedance meter

Of course if there was a bad joint on any of the diodes resulting in an open circuit, you would also see the supply voltage.

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Sat Feb 02, 2019 10:51 am
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 Another build conundrum 
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Post Re: Another build conundrum
I took goosed to mean broken as well.

Steve, those voltage hints...brill.
Qrx

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Sat Feb 02, 2019 10:58 am
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 Another build conundrum 
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Post Re: Another build conundrum
Ok I don't believe this.

I tested all three of those diodes, the zenner and the 2 1n4188s (or whatever) and they ALL tested as resistors (4188 were 12 ohm the zenner was 20 ohm. I couldn't believe that so I got other diodes from the stores and they tested correctly with my tester saying, quite clearly, DIODE and showing me the cathode, anode and voltages. I used my DVM and got sensible readings across them. Back to the failed ones and again, quite clearly, ALL just resistors.

Seriously, HOW??????

Research has told me the if I inadvertently overheated the diode I would get this result. But all three? Phew, have I screwed up.

I NEVER did this with my Weller. The cheaper precision Gold has been the only change.

I have my iron set to 350 degrees for placing parts and 400 for rapid desoldering, but these diodes weren't subject to that, only the 350 placing temp.

Im considering chucking the iron and buying a Weller again

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Sat Feb 02, 2019 11:32 am
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