EXTTR still not working. Voltage at TP4?

[Ulf Tjerneld]

A while a go I blew something in the External PTT. I suspected that Q5, NUD3124 might have fried and I got very good help in this forum to source a replacement. Now, I have exchanged it. But I still have no working external to actuate amplifier relay.
So, with Q5 taken out, I measure at TP4, which is the gate of Q5, in RX I have 2.27 volts and in TX 1.85 volts. This is obviously wrong since I guess I should have close to 0 at RX, or at least below the turn-on voltage of the mos-fet. I would expect 3V+ at TX to open Q5.

 

Any ideas where I could search next?

 

73’s – SM0NOR - Ulf

 

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[Steve Haynal]

Hi Ulf,

You may have fried the pin on the FPGA. With no power applied to the board and no Q5, what resistance to GND and 3.3V do you measure at TP4? This is a 3.3V logic signal. High should be near 3.3V and low should be near 0V.

73,

Steve

kf7o

[Ulf Tjerneld]

Hi,

At TP4 to GND I measure 29,7 ohms and to 3.3V I measure 61 ohms. The GND and +3.3V test points are from pin 4 and pin 1 on DB 17. I don’t know what the expect here. I have a pretty good idea of how the HL works in general, but the inner works of the FPGA is a bit beyond my current level of understanding…

/Ulf – SM0NOR

 

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[Juergen Hussels]

Hi,

if you have fried the fpga, it is not that bad, as it still shows a voltage swing from 2.27 to 1.85 Volts. You can add a comparator with e.g. a pot for the reference voltage and adjust it that it will switch on RX/TX . This comparator can drive a transistor to 
have on open collector output to drive a relay again. Next time you may want to add an opto coppler to be safe.

73 Juergen

[Ulf Tjerneld]

Interesting solution, Jörgen! But I Think there might a simpler solution if the FPGA is fried. It seem K2 has an unsed pole actuated by INTTR which potentially could be wired to the RCA jack instead of  Q5.

73’s – SM0NOR - Ulf

 

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Från: Juergen Hussels
Skickat: den 19 december 2021 12:28
Till: Hermes-Lite
Ämne: Re: EXTTR still not working. Voltage at TP4?

 

Hi,

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[Steve Haynal]

Hi Ulf,

Yes, there is an unused pole on K2 which you might be able to use. The difference between EXTTR and INTTR is that EXTTR always switches for TX but INTTR switches only when the PA is enabled. This is so that full duplex when using low power TX is possible. This may or may not matter for you.

73,

Steve

kf7o

[Ulf Tjerneld]

Good Point. At the moment I can live with it.

What did you make of the ohms measurement I did? IS a fried FPGA output the most likely explanation?

 

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[Steve Haynal]

Hi Ulf,

The measurements are very low. I see 4.5KOhms betweeen TP4 and ground, and 5.4KOhms between TP4 and 3.3V.

73,

Steve

kf7o

[Ulf Tjerneld]

Alright. So, that is that then and we probably have the explanation!

Thanks!

Ulf

 

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[u...@sm0nor.com]

And now the conclusion....  pin 5 on K2 is lifted from ground. I had to mill around the pin on both sides of the PCB. I destroyed the relay in the process. With no 3V relays on the shelf I had to wait over Christmas for a new relay. But now in place, the work-around works as expected. The black cable connects the relay pin to the exttr RCA-jack and I can now key the amplifier again. The whole procedure is reversible if I decide to change the FPGA.

[Steve Haynal]

Hi Ulf,

Glad you were able to make it work!

73,

Steve

kf7o