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[LU8EQ]

Hi,

I've been trying to fix a EB27A based amplifier, that is a 28V 2x MRF422  https://www.communication-concepts.com/eb27a-1/

The owner told me that it was working and suddenly a 10uf 63v capacitor on the board where the 28V goes in, blew.

So, I checked the transistors and they where dead. I had to replace the LM723 regulator and the trimpot in the BIAS circuit because they were also bad. I checked many other parts and were fine.

So far so good, all power supply voltages where on the spot, BIAS regulation performed without replacement transistors as indicated on the constructions hints pdf (with a 33 ohm resistor between base and emitter pads, 0,5V).

Then, I soldered the new transistors, PTT, 3w input carrier and puff. They died on the spot.

I am kind of lost right now, everything else seems to be just fine.

I would really appreciate any help. Thanks for reading.

73 de Guido LU8EQ

[AA7IS]

I see you set bias into a test setup. what bias in milli-amps did you set after you actually installed the finals?
If you adjust Bias for X.X volts rather then amps then it's a good way to explode devices as well.

[AA7IS]

Hi,

I've been trying to fix a EB27A based amplifier, that is a 28V 2x MRF422  https://www.communication-concepts.com/eb27a-1/

The owner told me that it was working and suddenly a 10uf 63v capacitor on the board where the 28V goes in, blew.

So, I checked the transistors and they where dead. I had to replace the LM723 regulator and the trimpot in the BIAS circuit because they were also bad. I checked many other parts and were fine.

So far so good, all power supply voltages where on the spot, BIAS regulation performed without replacement transistors as indicated on the constructions hints pdf (with a 33 ohm resistor between base and emitter pads, 0,5V).

Then, I soldered the new transistors, PTT, 3w input carrier and puff. They died on the spot.

I am kind of lost right now, everything else seems to be just fine.

I would really appreciate any help. Thanks for reading.

73 de Guido LU8EQ

The working bias is normally set by applying 28 volts as well as the bias voltage with NO DRIVE!.
The bias is set for a certain amount of idle current, in your case well under an amp.
Once bias current is set then you can apply an RF signal, since the application notes say that 3 watts will push the amp almost to full saturation it seems suicidal to go that route, I would have hit it with no more then one watt usinf a current limited 28 volt supply.
All that is way cheaper then constantly blowing up transistors.