[Amps] hf-2500 again

[K1TTT]

That is essentially correct on how this one works.  I did replace the
transistor that shorts out the 330k resistor and that seems to be working
now, the last thing I found was a capacitor across that 330k resistor was
shorted and keeping it in tx mode.  I have also replaced the zener and the
transistor it was regulating.  The voltage driving the bias is a 60v supply
through 20kohm.

 

David Robbins K1TTT
e-mail:  <mailto:k1ttt at arrl.net> mailto:k1ttt at arrl.net
web:  <http://wiki.k1ttt.net/> http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or  <telnet://k1ttt.net/> telnet://k1ttt.net

 

From: TexasRF at aol.com [mailto:TexasRF at aol.com] 
Sent: Saturday, April 20, 2013 15:46
To: K1TTT at ARRL.NET; amps at contesting.com
Subject: Re: [Amps] hf-2500 again

 

David, it sounds like the cutoff bias is not operational. Most amplifiers
have a resistor in series with the zener bias that develops cutoff bias when
in standby. Then a relay or solid state switch shorts that out when in
transmit leaving only the zener bias.

 

Maybe whatever took out the original zener bias also damaged the cutoff
switching circuit like welded relay contacts or shorted switching
transistor.

 

Without cutoff bias you can expect some broadband noise while receiving.

 

This is a total guess on my part as i have no direct experience with your
amplifier model.

 

73,

Gerald K5GW

 

 

 

In a message dated 4/20/2013 7:33:30 A.M. Pacific Daylight Time,
K1TTT at ARRL.NET writes:

Ok, got parts in and replaced most of the bias circuit.  Now I have 9.7v on
the cathode on rx and 8.7v on tx.  Would those be reasonable for bias when
cutoff and tx in ab2 operation for the pair of 3cx800a7's??  I have a
feeling the cutoff voltage is a bit low as I am seeing a bit of plate
current, at least its just barely moving the needle instead of the 400ma I
had before.



David Robbins K1TTT
e-mail:  <mailto:k1ttt at arrl.net> mailto:k1ttt at arrl.net
web:  <http://wiki.k1ttt.net/> http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or  <telnet://k1ttt.net/> telnet://k1ttt.net



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