Date: Fri, 20 Jul 2012 22:02:53 -0700
From: "Jim W7RY" <jimw7ry@gmail.com>
Subject: [Amps] Drake L4B 50K Resistors HOT!!
I was just repairing a Drake L4B that I picked up from my want listing on
QTH.com. I just happened to measure the temperature of the 50K 50 watt
resistors in the power supply with my infrared thermometer and WOW!!
The middle of the resistors run 510 degrees f !! I could not believe it!
Needless to say there outta here! I won?t need them anyway. I?m using the amp
to prototype my QSK board for the Drake L4-L7 amplifiers that will use resistor
cutoff bias instead of the +120 volts that the resistors provide.
73
Jim W7RY
## I have four of the drake L4b amps. each of those 50 watt bleeders is
dissipating 35 watts. The pair of 50 k resistor,s is in series..and also in
series with a 7 k @ 5 watt below the power supply chassis.
The V drop across the 7k is +130 vdc ( only when on the 2650 vdc position).
The vdrop is 90 vdc when on the cw ( 1900 vdc) position.
## toss all 3 resistors ! I then paralleled ( for redundancy) two 100k
@ 3 watt MOF’s = 50 k. Wire that in series with the CT of the fil xfmr.
That’s the yellow wire hanging off the tr relay.
## I then simply rewired the 3pdt mech relay such that it just shorts out the
50k on TX. The 50 k dissipates zero watts anyway. then u have ZERO heat
from the L4PS power supply cab !
Your no load B+ will NOT rise either on RX. Mine did not budge at all.
I also replaced all the 100 k @ 2 watt carbon EQ resistors across each lytic
with 100k @ 3 watt MOF”s
(mouser has em in 1% tolerance) . replace the lytics as well. Use 4 x
diodes and 2 x lytics to make a doubler..then u will have +63 vdc instead of
the oem +24 vdc relay supply.
oem supply used a simple half wave rectifier, with one diode and one cap. I
replaced the 3pdt mech tr relay with 3 x new gigavac GH-1 relays.
One for input, one for output, and the 3rd one does the bias. U can get em in
either 12 or 26.5 vdc coils. If u want blazing fast qsk... install a 50 uf
cap across the dropping resistor
Dropping resistor installed between the + 63 vdC and the t-r relay
coils..... with all 3 x coils in parallel. + 63 vdc is plenty for a 26 vdc
vac relay coil....and also works good on a 12 vdc coil.
## I used 3 x vac relays, so that each amp can easily handle 2 kw on bypass
mode. ( u have to be able to do that to calibrate the internal wattmeters). I
string all the amps nose to tail, and then
tune em up on different bands. ( or stuff like 2 on same band). Each amp is
placed in operate mode. A simple rotary switch ensures that only ONE amps
key line is activated at one time..simple interlock. key line from xcvr to
rotary
switch, then 4 outputs, one to each amps key line.
## Bypass swr is dead flat going through all 4 x amps..even on 10M. BTW,
drakes +130 vdc was a stupid idea. the + 130 vdc will eventually eat the
middle contact on the oem 3pdt mech tr relay.
Later....... Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|