lunedì 3 giugno 2013

Unable to fix a youkits tj2a

Last year I've received a dual band (20 meters and 40 meter) qrp transceiver named TJ2A partially assembled.

The transceiver kit is made from and distributed mainly on Ebay.

I've just build a TJ2A in the past without problem, mine worked well from the beginning.

The ham in this video is me with my original TJ2A

Due to a misunderstand with a Ham I've changed my working TJ2A with this partially completed.
This is another, bad history.

This TJ2A was half completed and I tried to complete it.  Soon I've found that the transceiver was badly assembled with a lot of mistakes as, trace cutted, bad solders, inverted LM386, components swapped and so on.

I must say that at that moment I was really disappointed because instead of a working TJ2A I had received un-repairable one.

After a lot of investigations I've corrected nearly all the problems, now the TJ2A receives but it does not transmit with the correct power.  The expected power is, from the specifications about 3-4 watt, but at the moment I obtain only half a watt!!

Now I'm really frustrated because i feel that the solution is near but I'm not able to fix this last section.

On the picture the part of the schematics where is located the PA.

I've checked and re-checked all the fundamental parts and all seems at the correct position with the correct value, bu the power is still low.

On this guide of Pa1ap I've found his measured values and they seems near in value of the mine:

On the 40 meters band I have:

R60 80 mV pep
R65 1400 mV pep
C92 800 mV pep
ANT 4 V pep

The only anomaly that I've found is that at C92 I've a voltage less that the voltage at R65.
I'm not really sure if Q14 is used as an amplifier or as a buffer.

Unfortunately the Pa1ap guide does not report the values at c92 so I've not a reference value.

I've checked Q14 and it's working in my opinion.

To obtain 4 watt at the antenna I must have nearly 40Volts on 50 ohm at the antenna.
So the voltage level must rise from 80 mV pep at the input of  Q12 to 40 Volt to the antenna.

So please help me, now I'm really blocked!!

The complete schematics could be found here:

TJ2A complete schematic

3 commenti:

  1. Q14 operates as an amplifier.

    The DC voltage at the base of Q15 should be around 700mV.

    Sorry I do not have one of these to give you my voltage levels.

    1. Hello Dave, seems that the c2078, that I've bought on ebay are Fake and they not give RF power.
      I've bought some replacement made by they products copies better than originals of obsolete components like 2078

    2. I confirm that the c2078 that I've bought on ebay from an Italian seller are Fakes.
      Today I've tested a transistor from Eleflow that works.
      Unfortunately I've a lot of unsoprressed carrier. I must investigate..