Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
simple low power modification for thr Johnson Ranger and the WA1HnyLR lite mods




 
The AM Forum
August 15, 2018, 09:24:03 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: simple low power modification for thr Johnson Ranger and the WA1HnyLR lite mods  (Read 19541 times)
0 Members and 1 Guest are viewing this topic.
Tim WA1HnyLR
Member

Offline Offline

Posts: 150


WWW
« on: April 03, 2015, 12:58:25 AM »

  Some years back I published an article on high performance audio modifications to the Ranger as well as a proper push to talk circuit as well as power supply modifications. You will find this article on the AM WINDOW . Originally published in the AM Press Exchange in 1991. Since then there have been many new improvements and changes. Many people have done the mods with good results. Then there are those who feel that it is a bit too involved. There are many people who view the Ranger as an exciter class transmitter to drive a linear amplifier with. In many cases if the Ranger were permanently set up as an exciter this would be OK FINE!!!.  

What I have here is a two part article that deals with the appropriate method of power reduction as well as audio modifications that do not involve replacement of the modulation and driver transformer but will yield good quality audio that most operators would be proud to affix their call sign to. The permanent low power modification is easy to reverse as well.


RANGER POWER REDUCTION 101

  There have been many methods of reducing the R.F. output power of the Ranger to drive a typical amateur radio linear amplifier. Power requirements typically vary from a matter of less than 10 watts to as much as 20 watts. A typical Ranger makes about 40 watts. The E.F. Johnson method was to use the Johnson 6DB attenuator pad as well as turning down the grid drive to lower the total output power.  
Lets take a look at the performance of the Ranger at full power. The under sized modulation transformer goes into core saturation at 60 Ma. or so of D C current flow through the secondary of the modulation transformer. The potential audio quality is limited by the parameters that the transmitter is being operated at. If the transmitter itself were modified for low power operation perhaps some of the issues could be corrected.

There are a few hair brained schemes out there to reduce power.

(1) A circuit involving a clamping transistor with an adjustment pot to pull down the PA screen voltage to reduce power. This logic is flawed. True reduction of screen voltage will reduce the output power. What is going on is that the PA tube is starving for screen voltage to make the power that the matching impedance of PA tank circuit which is set up for the optimal impedance that the PA tube wants to see for full power output . When screen voltage is reduced the PA efficiency suffers causing the PA tube to dissipate more energy in the plate. With the screen clamping method, the modulation linearity is affected as well. This is not a good method.

(2) Another method is substitution of the 6146 PA tube with a 2E26. As mentioned earlier, the PA tank circuit components particularly on 160 and 75/80 are not capable of providing  a high enough plate load impedance for a plate current value of about 50 mA. Assuming that the load impedance presented by the linear amplifier is 50 ohms, you will find that an appropriate match can be achieved on 40 meters and up. Even then, the RF output will be in the order of 20-25 watts. This is still, a bit too much output for a number of cases.  

The only correct method to reduce power is by reducing the plate voltage on the PA stage. This method is used in all of the now vintage plate modulated AM broadcast transmitters. There are a few methods of reducing plate voltage in the Ranger transmitter.


PUTTING THE PLATE VOLTAGE ON A DIET

A very simple method of plate voltage reduction can be done at the 9 pin accessory socket on the rear of the transmitter. This connector was used to utilize the Ranger as a combination RF Exciter and audio driver for a larger class C plate modulated transmitter. such as the Johnson Kilowatt. The Ranger could also serve as a power supply modulator for another transmitter, such as the Johnson 6'N'2.  BUT it can also be the portal for reduced power operation.

It will be observed that there are 3 jumper wires on the 9 pin plug. These jumpers set up the Ranger as a stand alone transmitter. A recommended method was to utilize the low voltage power supply for the PA and the full high voltage supply for the modulator.

To accomplish this one would use a single pole two position switch of sufficient voltage rating.

* The jumper wire from pins 2 and 6 is removed on P3.  

* Pin 2 connects to the arm of the switch. Pin 6 connects to the NC Pole.

* The LV supply (pin 4) is connected to the normally open pole of the switch.

* Flipping the switch places the low voltage onto the bottom of the secondary of the modulation transformer.

It will be noticed that the PA plate current reading goes away. If the PA  is tuned up at full power to 100 mA plate current the actual plate current will drop to the order of 60 mA or so. The RF output will be about 15 watts.

While this method achieves plate voltage reduction, it is fraught with issues.

(1) No plate current reading on the meter when in low power mode.

(2) The current rating of the low voltage power supply filter reactor is exceeded. It will run very hot after  a few old buzzard transmissions. If you are one of those that does not see the logic in solid stating of the power supplies will have an unexpected meltdown of the 6AX5 LV rectifier. Its current rating will be exceeded.

(3)  Although the load impedance presented to the modulator is what it would be at full power, there will be more audio headroom than needed under any circumstance. Careful scope or modulation monitoring is very important or you could over modulate at a level that would make a C.B.er green with envy . You could potentially decimate the area of the band that you are on.

(4) An oversight on my part : I have been told that when the mode switch is placed int the CW position and the external power switch is in the low power mode, both the high and low voltage supplies will be tied in parallel. This results in a number of components burning up and/or melting down. For that reason alone this method of plate voltage reduction is NOT recommended.

There is, however, an aftermarket fix. Simply place a two 1 kV PIV diodes in series with the anode of one to pin 4 and the cathode of the other to the low power position on the switch. This will prevent the HV B+ flowing back downstream into the LV supply.

There is another method of plate voltage reduction utilizing the accessory socket.

* Simply remove the jumper wire between pins 2 and 6.

* Run a 2 conductor cable from pins 2 and 6 to a project box that has a multi-position rotary switch and a number of 1K 10 watt wire wound resistors in series with each other arranged around the rotary switch  to progressively short out each resistor successively to change the resistance from a short up tp say 6K ohms.

* Place a 20 MFD  450 volt  electrolytic across the resistor string. The +end to pin 6 and the -  end to pin  2. By clicking through the switch positions one will notice a change in output power. The electrolytic cap is essential to keep the load impedance on the modulator constant, as well as not dissipating audio power needlessly. This method is preferred. It still, however, means one must keep a constant vigil one ones modulation levels.

There are other  methods that have been done, including using a DPDT relay mounted that engages when the function switch is placed into the "TUNE" position. In the stock Ranger the screen of the PA tube is pulled to ground. This control function is utilized to energize the relay which is wired to reapply screen voltage to the modulator s well as switch the low voltage supply to feed the PA stage ahead of the meter shunt.

This method works well but is a bit daunting to some people. It would be more elegant to rewire or modify the mode switch to do the functions of switching between high and low power. I finally did it last year. This will be discussed in a future Ranger modification article.

The crux of this article is to present an easy cost effective reversible method of achieving low efficient low power modification. It has been found that operation at the same level of plate voltage reduction on the modulator does not harm the usable modulation headroom ability. This simplifies matters. Only one power supply is needed for the PA and modulator.

As it is the turns out, the stock Ranger modulation transformer has a decent ratio. It is possible to make 125-130% positive peak modulation with the reduced plate voltage. This is plenty. With the reduced plate voltage on the modulator tube, their source impedance is greatly reduced. This helps to overcome the lack of inductance of the rather small modulation transformer. As measured the low end frequency response was clean down to about 65Hz  @100% modulation. At full voltage the low end response started looking lumpy below 100Hz on the scope.


DON'T TAKE THE HEAT

Another big issue with the Ranger is the amount of heat build-up in the cabinet. By installing an exhaust fan on the rear of the cabinet, solid-stating the power supplies, and installing a more sophisticated PTT keying system, the heat can be reduced and removed. By performing the following modifications, the heat build-up in the cabinet will be markedly reduced when the Ranger is set up strictly as an exciter class transmitter.

Take an 8 pin octal plug from a crapped out tube to build a solid state rectifier replacement for the 6AX5 rectifier. One must use 3Kv PIV per leg (6, 1Kv piv diodes or 2 microwave oven diodes).

* Connect Anodes to pins 3 & 5 , Cathodes to pin 8.

* While assembling the diode replacement plug in connect a short (4"-5") piece of hookup wire to the cathode pin as well. This wire will exit from the top of the octal plug.

* Obtain a second octal plug. All that is needed to be done is to connect the other end of the piece of hookup wire to pin 8 on the second plug. This is one of the filament pins of the 5R4 rectifier.

When finished do the following :

* Plug the 6AX5 solid state replacement into the socket that once had the 6AX5 (V12).

* Plug the other plug into the socket that had the 5R4 (V11).


HOW IT WORKS

If one were to examine the circuit one would notice that the cathode of the low voltage rectifier is now feeding both of the high and low voltage filter reactors. In theory the output voltages from both filter system will be the same. Being that the DC resistance of the high voltage filter reactor is much lower than that low voltage filter reactor there is an increase in the output of the power supply that once was the high voltage supply. The output voltage should be in the order of 350 volts give or take. The Ranger can be loaded up to about 75-80 Ma. This is about 18-20 watts out.

Being that the HV power supply voltage is that much lower there will be that much less heat generation within the transmitter. The factory suggested PTT scheme does generate more heat during standby than it does when transmitting at full power. At the reduced plate voltage there is that much less heat build up. It is not as critical to use the more complex HnyLR PTT relay circuit in this case.


INITIAL SET UP

After fire-up and confirmation that the power supply voltages are what they should be the set up is easy. The tap on R 35  (Screen grid voltage for modulator and goose tube)  Ball park the screen voltage while measuring at the tap to where it should be according to the manual. Key up the transmitter. Do a final tweek to draw  70-80Ma modulator idle current.  The PA tube plate meter shall indicate as always The 6146 will not draw the amount of off resonance plate current that it did at full power. You should see maximum power at about 80 Ma . It will be found that some what more output can be had with 2.5Ma of grid current. At this point you should be cleared for take off.
 

LITE AUDIO MODIFICATIONS

Although in past modifications  the stock audio gets bulldozed and a  much higher  performance resistance coupled circuit is used the limiting factor is the stock modulation transformer. By changing a number of circuit component values as well as omitting various capacitors that kill the high audio frequency response,very favorable results can be had. As with all modification that I h ave published ALL OF THEM MUST BE DONE IN ORDER TO GET THE RESULTS!.

Do not omit any steps in the following changes to the Ranger modulator. Unwanted ultrasonic oscillations may result if all of the procedures are not followed !

Let us begin at the beginning.

* R17 1Meg Microphonium grid resistor, Change to 10-15 Meg ohms . This is necessary for using a D10-4 or other piezo-electric microphonium.

* Replace C50A ( cathode bypass cap with a 25-100Mfd 10 volt min electrolytic cap.

* Change C51 (0.1MFD decoulpling cap) with a 10-20MFD 450 Volt electrolytic cap.

* Change C52 500 pF audio coupling cap to a .005-.01Mfd 600V tubular capacitor ( Orange drop type or equiv.)  

* Remove C53 (200 Pf Mica cap)  

* Remove C 56(470Pf cap from pin 6 of V 7-b to ground).

The modifications so far are pretty straight forward. It was found that the method of application of negative feedback in the original Johnson design could not be made to provide enough negative feedback to flatten out the low frequency response curve below 120Hz. The method of introducing the negative feedback point had to be changed. Rather than apply the feedback to the grid of V8 (12AU7)   application has been moved to feed the cathode of V7-B (12AX7)  second audio stage.

DO THE FOLLOWING:

* Replace C50-B with a value similar with that used for C 50-A  (25-50MFD @10VDC min. ) The negative end of the cathode bypass cap DOES NOT GO TO CHASSIS GROUND.  

* Place a 330 ohm 1/2 watt resistor is placed in series with the negative (-) end of the cap to ground. Negative feedback is introduced at the juncture of the cap and the resistor.

* Connect a 18-22K 1/2 watt resistor from the tertiary negative feedback winding to the juncture of the 330 ohm resitor and the - end of the cathode bypass capacitor(C 50-B).

* Place a 1MFD 100 volt min. non electrolytic cap (orange drop or equiv) in parallel (across) the tertiary negative feedback winding of T2 (modulation transformer). This step is important. Without the parallel cap there will be ultrasonic audio instabilities. This method of introducing negative audio feedback if far more effective.

Back to the rest of the audio section:

* Measure  C57 (.02MFD) audio coupling cap for leakage. If there is ANY leakage , replace cap with anything from a.01-.05 MFD 600V cap ( Orange drop  or equiv)

* Change R27 (150 K  grid resistor) with a 470K-680K 1/2 watt resistor. Being that in the original schematic one end of the resistor went to the tertiary negative feedback winding. Being that the negative feedback winding is feeding the cathode of  V7-B the end of the grid resistor now is connected from grid of V8 to ground.  

* Change C59-A (15MFD cathode bypass) is replaced with a 25-100 Mfd 25 VDC min. electrolytic cap.

* Change R28( 820 ohm 1/2 watt resistor) with a 680 ohm 1-2watt resistor. The higher dissipation rating improves reliability.

* Remove C60(.005 cap)  across secondary of T-3 audio driver transformer)

* Replace with a 10-12K 1 watt resistor. This is a swamping resistor. This step must not be omitted.

At this point the speech amplifier mods are completed. Now on to the modulator itself. Dynamic stabilization of both screen and bias voltages is important!  

* Replace C59-B with a 100-300Mfd 50 volt electrolytic cap.

* After setting modulator idle current between 70-80 M a measure the bias voltage. Place a zener diode or combination of zener diodes in series with each other from the center tap of the secondary of T3 (driver transformer to ground. This will stabilize  the bias voltage.  

* Place a 10-20MFD 450 volt cap in parallel with C71(.1MFD mod screen bypass cap).

The following are capacitor removals:

C69-.02MFD-C66-C67-C68-C 83 all.002@1.6Kv Disc ceramic caps.  

The following caps to be changed are in the RF section:

* C47 & C48 (.002 Mfd @1.6Kv disc ) Replace with 470 Pf-.001 MFD @ 3Kv disc ceramic caps. These are bypass caps at base of rf plate choke.

* Change C46  (.002 MFD screen bypass) with a 470-500Pf 1Kv disc ceramic cap.          

At this point you have completed  all of the transmitter modification pertaining to the modulator and modulated stage  The last phase of modification is the power supply.


HOME STRETCH ITEMS

 The power supplies need attention. The  bias supply must be solid-stated. Use 2-1kv diodes to take place of the 6AL5 (V 13).

* Replace C90-A & B  with a 100-200MFD 150 V electrolytic..  

* Replace C78(30  MFD 450 V  LV supply filter cap) with am 100-200Mfd 450V electrolytic cap. If the Ranger is going to be operated strictly as a low power exciter, the HV filter cap (C77-10MFD@700VDC ) may be replaced with a 100-200MFD 450V electrolytic.

* If eventual full power operation is contemplated, simply use two identical capacitors in series with a 100K 2 watt resistor across each capacitor for voltage equalization.

You may want to use the Ranger with a high quality audio chain which requires line level input. Simply install an appropriate connector. A RCA connector or a 1/4 phone jack one jack on convenient location on rear of chassis. Run a length of shielded audio cable from the line level input jack to across the audio gain pot ( R21).


CONGRATULATIONS!
 

  You have just completed the low power Ranger transmitter modification as well as  making huge improvements to the audio section.

Enjoy,
Tim WA1`HnyLR
APR-3   2015                                                        
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.081 seconds with 18 queries.