[Amps] Drake L4B 50K Resistors HOT!!

[Jim Thomson]

Date: Fri, 20 Jul 2012 22:02:53 -0700
From: "Jim W7RY" <jimw7ry at 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