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Killed TX this time...


Peter McCracken
 

... so my fun continues!

Running some tests with antenna tuning on the MJE I seem to have blown the finals or I thought I had, no watts on any band except 40m into a dummy load. Replaced the finals with two RD16's as I had wanted to do this anyway. Now testing not getting any output on any band. Note v6 has a second set of holes for the reversed pins.

Checked have 12v on final legs, on TX one gets 3.8v across the legs and the other about 4.2v across the legs.

Confirmed D8 gets 12v on TX on 28MHz and the others D11,D12,D13 all show zero volts which is expected.

Checked drivers and all show circa 0.65V and pre-drivers show 12v.

Reception is working fine but definitely not getting any output, any thoughts on what else I should be checking ?

Thanks!


Reed N
 

Did you retune the biases?
https://www.hfsignals.com/index.php/ubitx-tuneup/


Reed


MadRadioModder
 

Are they REAL RD16HHF1’s?

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of peter.mccracken@...
Sent: Thursday, July 30, 2020 3:17 PM
To: BITX20@groups.io
Subject: [BITX20] Killed TX this time...

 

... so my fun continues!

Running some tests with antenna tuning on the MJE I seem to have blown the finals or I thought I had, no watts on any band except 40m into a dummy load. Replaced the finals with two RD16's as I had wanted to do this anyway. Now testing not getting any output on any band. Note v6 has a second set of holes for the reversed pins.

Checked have 12v on final legs, on TX one gets 3.8v across the legs and the other about 4.2v across the legs.

Confirmed D8 gets 12v on TX on 28MHz and the others D11,D12,D13 all show zero volts which is expected.

Checked drivers and all show circa 0.65V and pre-drivers show 12v.

Reception is working fine but definitely not getting any output, any thoughts on what else I should be checking ?

Thanks!


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…_. _._


Peter McCracken
 

They seem to be, they have the dimples at the top corners.


Peter McCracken
 

Thinking about that but surely I should see something on the meter without doing that ?

Doesn't appear to be voltage across R97 which has me worried.


Peter McCracken
 

U2 is putting out 5v though on TX.


 

Hi,

Why should there be voltage across R97?  Unless the gate on the final is blown, there should be no substantial current flowing through R97.

If you didn't turn the bias voltages down to zero before powering up, you may have blown one or both of the new finals already.  The bias voltage is unique to each transistor because the gate cannot be made exactly the same on every single one.  If the bias was set higher that was needed for a transistor, very high current would be allowed from source to drain and this can cause the channel to burn out.

73,


Mark


Peter McCracken
 

I adjusted the bias as per instructions, 470ma baselines and 670ma on key, if I roar into the mic it goes up to 710ma. They did not seem to be massively misaligned.


 

Hi,

What was the current draw before re-adjustment?  That would indicate how far off the bias was (but not how balanced they were).

What is an "MJE"?


Evan Hand
 

Peter,
First off, I will assume that it was working at one time.  Since the issue started have you tried CW?  Also, I noted that you have power on 40 meters.  Is it above 7 watts?

If you truly have power on 40 meters and not on any other band then I would look at the LPF relays to verify that they are working.

Just some suggestions and questions.
73
Evan
AC9TU


Peter McCracken
 

So I may have found the source of the issue...

Q912,Q911,Q93,Q92,Q96,Q97 all seem to be happy campers.... Emitter = 0.5V Base = 1.2V Collector =12V
... this seems to be all very uniform and on spec from what I have read.

Q90 however seems to have taken one for the team.... Collector = 12V Base = 0.34V Emitter = 0.02V... it seems poorly....

R82 = 100ohms R83 = 2.2 ohms (looks ok)

R81 = 188 ohms R80 = 25 ohms (looks dodgy especially as R81 is supposed to be 1K)

Not sure what has went on here but it looks like I need to change these resistor and Q90.


Evan Hand
 

Peter,
The readings for Q90 do look suspect.  I would suggest removing it as a starting point, then measure the resistors again.  Q90 has been know to fail, mostly from nearby transmitters when the uBITX is connected to an antenna.  I have not heard or read of it going out because of low or high SWR on TX.  Back to Back 1n914 or 1n4148 diodes across K1 pins 12 and 16 can protect the BE junction of Q90 when not transmitting.  The threshold voltage of the diodes is high enough that it will not affect receiver sensitivity yet still protect Q90.  Alternative is to make sure that the antenna is disconnected or grounded when using other transmitters nearby to it.

Just my thoughts,
73
Evan
AC9TU


 

The gate does not draw any current in mosfets.

Raj


At 31/07/2020, you wrote:
Thinking about that but surely I should see something on the meter without doing that ?

Doesn't appear to be voltage across R97 which has me worried.


Peter McCracken
 

... replaced Q90 with a BFR106 and R81 with a 2.7k resistor and still no joy. Getting 0v out on the emitter of the BFR106 so the issue must be elsewhere. I tested the Q90 I removed and it was working fine. I'd really appreciate some advice on what to check next, really willing to learn to hit me with any and all recoemdnations!

Thanks!


Evan Hand
 

Peter,
You did not answer the question on CW output.

As to where I would start, I would be looking at the signals starting at TP2 working my way to the Antenna (TP 3 to TP 8).  This would be with an oscilloscope, as I have one.  The scope does not need to be accurate at the frequency being measured, just looking for a signal and that it is increasing at each stage.  Alternatively, you could build and use an RFProbe.  Here again, looking for an increase in the signal level.  All of this is with CW as the source.  There is no real level using SSB unless you are feeding a constant tone, which is really the same as the CW for the power sections.

Do an internet search for RF Probe to see how to build one.

73
Evan
AC9TU


Peter McCracken
 

Thanks Evan, I have a Siglent 1202 I bought a while back but haven't made time to to really understand how to use it properly... I guess now would be a good opportunity!

So fire up the scope attach the probe ground and TP2 and see what I see ?


Evan Hand
 

That is pretty much it.  I will look later today for the thread that I took pictures of the scope traces for each of the TP that I listed.  Need to take the wife to visit with the grandkids right now.

You are looking for the carrier frequency at each of the test points, starting with mVolts getting to almost 60 volts PP if my memory serves.

73
Evan
AC9TU


Peter McCracken
 

Thanks Evan, Testing with CW I get about 0.1W output on 20M.


Peter McCracken
 


Peter McCracken
 

In previous message it's R83 not R86... apologies.