Another advantage of having more than one of these. besides not being in a
rush to fix it, is that I can put another one side by side and compare
measurements. Yes, the cutoff bias is too low. A good amp was using about
45v in cutoff and 8.7 on tx. So back to troubleshooting. Found some more
discrepancies in the board schematic and ended up tracing it back to the
power supply board, which I had just sealed up because everything had tested
ok on it. What I am seeing now is that the power supply is loading down the
bias supply. The only connections to ground in the power supply are the
multi-megohm divider for plate voltage, and a diode/resistor combo that is
meant to protect against B- going negative with respect to the chassis. so
the only possibility is that the diode is breaking down, even though it
passed the standard ohm meter diode test feature. I can at least duplicate
the problem by putting 12vdc on the B- wire that goes to the control board
with it disconnected from the control board, with 12v it is drawing about
6-8ma which would be more than enough to drag down the bias supply that
supplies 60v through 20kohms to supply the bias.
I am considering creating a collection of schematics for these amps. I have
been waiting for responses from AN Wireless or their spinoff Dedicated RF
whom I have asked about the rights to the old Command Tech documentation
before I went off and copied them. They appear to be the last owners of that
line. So far I haven't had a response from either of them.
David Robbins K1TTT
e-mail: <mailto:k1ttt@arrl.net> mailto:k1ttt@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@aol.com [mailto:TexasRF@aol.com]
Sent: Saturday, April 20, 2013 15:46
To: K1TTT@ARRL.NET; amps@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@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@arrl.net> mailto:k1ttt@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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