[Elecraft] XV-144 Receiver RF Amp Bias Adjustments

[Don Wilhelm]

David,

First - those diodes across the relays do not control anything, they are 
there only to absorb the reverse polarity voltage kickback when the 
relay opens.  That means when a relay is active, you will have 12 volts 
on the cathode and close to zero on the anode.  So K4 is activated, but 
K5 is questionable.

6 volts on the anode of the diode is not good.  That would say there is 
some current going through K5, perhaps enough to lift the armature off 
the NC contact, but not enough to close the NO contact.

The control of relay K5 is Q2 on the front panel board.  I suspect that 
transistor may have a collector to base short or leakage.  If there is 
near volts on the base, the collector should be open circuit (K5 not 
activated), but when the base voltage rises to 0.6 volts or more, Q2 
should conduct fully and pull relay K5 in (transmit condition).

There is an error on the IF and Control Schematic.  The input to U4 
connects to the 12PRI voltage source instead of 12V as indicated on the 
schematic.  There is no way that power can get to the firmware chip if 
it as shown.  I just traced the PC connections on an image of the board 
and U4 connects to the cathode end of D14 (12PRI)

With your JP8 jumper in place, closing the front panel switch pulls in 
relay K3 which puts 12 volts on the regulator and in turn sends 5 volts 
to the firmware chip and wakes it up - the firmware chip raises the 
voltage on pin 12 which causes Q3 to conduct, pulling K4 in.  K5 should 
still be open (both the cathode and anode of D15 should be at 12 volts) 
and should remain so until transmit is asserted by closing the KEY IN 
signal.

73,
Don W3FPR



On 7/30/2011 4:22 PM, KJ4YQK wrote:
> Hi Don,
>
> Reposting Here I don't think my post are making in via outlook.  Anyway here
> is where I am at.
>
> 1.  Yes I have 12 volts on the cathode of D14, but no voltage on the anode.
> D19 had 12 volts on the cathode and 6 volts on the anode.
>
> 2.  Been over the jumpers about 10 times now.  My settings are as follow on
> page 9.
> JP1: 1-2  JP2:2-3  JP9: 1-2
> 1 milliwatt  JP3: 1-2  JP4:1-2  JP5 2-3 JP6 2-3
>
> JP7 Shorting block has jumper on it and no shorting block on JP8
>
> Front Panel Board has a jumper on JP1.
>
>
> 3.  Not sure if you got my  last email.  Just in case.  I was thinking Q3
> might be bad because I had to move it from the front side of the board to
> the back side.  I pulled Q3  and it check good.  I went ahead and replace it
> with a new one.
>
> 4.  Retouch most of the solder joints around the relays and the pre-amp
> section.  Including the R12.
>
>
> Question (not sure if I have this right in my reading of the diagrams):  Q3
> (front panel) sends a ground to D14 (main board) when the radio is power up
> by a positive signal from U4(front panel).  The PWR En signal maybe, just
> wondering if I am reading this right.  If a ground appears on the anode of
> D14 the diode causing forward bias, and the voltage should drop below 12
> volts to like 6 volts causing K4 to activate.
>
> Thanks David
>
>
> --
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