(C) 2024, G. Forrest Cook W0RIO
the W0RIO Drake 2B receiver and Q multiplier, click for a larger image.
The Drake 2B receiver is well-known among ham radio operators as an excellent receiver. It is especially useful for CW (Morse code) reception thanks to its 50Khz IF variable passband filter and the optional 2-BQ Q multiplier. The receiver is basically an 80 meter (3.5-4.0 Mhz) double-conversion receiver with one extra triple-conversion stage for the higher frequency bands. The first IF stage operates at 455Khz and the second stage operates at 50Khz.
The Drake 2B also supports five additional user-selectable bands which can be used to add coverage for the 60, 30, 17, and 12 meter WARC bands as well as various short-wave broadcast bands and the Citizens Band. Adding the extra bands require the installation of a few crystals, which may be somewhat difficult to find. Several add-on modules allow the Drake 2B to receive the 160 Meter and 6 Meter amateur bands, see the Resources section below. The receiver shown above is part of my Modular CW QSK system.
This is an advanced-level project that should only be taken on by someone who has experience working with high voltage circuitry. It takes advanced technician skills to work on any vacuum tube equipment. There are lethal high voltages inside of this reciver including 120VAC and 150VDC. The power cord should always be removed when working on the receiver. The circuitry will discharge the electrolytic capacitors when power is removed, but it is always a good idea to briefly short out the capacitors before beginning any work.
A number of modifications can be performed on the Drake 2B receiver. Note that some of these mods may decrease the resale value of the unit and should only be done if one wants to improve the performance of the receiver for regular use. It is a good idea to make any modifications that you do reversible so that a future owner can restore the unit to its original state if they want.
The original 2 wire AC cord was replaced with a modern grounded 3 wire cord. This is the most important modification since it can eliminate a potential electrical shock hazard. Remove the original cord and insert a new cord in the receiver. A standard computer power cord can be used if the IEC connector end is cut off. A few inches of the outer insulation should be removed from the cut-end of the cord, be sure not to cut into the internal wires. The black (hot) conductor should be connected to the fuse socket, the white (neutral) conductor should be connected to the power transformer primary line and the green (ground) wire should be connected to a lug that is grounded to the chassis.
The 6X4 rectifier was removed and replaced with a pair of 1N4007 diodes, these were soldered to the bottom of the rectifier tube socket. Two 1nF/1KV ceramic disc capacitors were soldered across the diodes to protect the diodes from line transients. A 300 ohm/5W resistor was added in series with the B+ line to compensate for the more efficient diodes and bring the B+ down to the correct voltage. A 22uF/250V capacitor was added between the junction of the two 1N4007 diodes and ground for extra hum filtering.
An 0.1 ohm, 5W resistor was installed in series with the non-grounded leg of the power transformer's filament winding (green wires). This drops the filament voltage from 7VAC down to the proper 6.3VAC when running on 120VAC line voltages. Adding this resistor insures that the tubes don't run too hot and will last a long time.
In the original configuration, the power transformer became quite warm hot when running on the 120+ VAC line voltage at my location. With the solid-state rectifiers and filament dropping resistor, power consumption dropped and the power transformer runs at a much cooler temperature.
The receiver has four electrolytic capacitors in a metal can and another on the 6AQ5 audio output tube's cathode. It is generally considered good practice to replace all of the electrolytic capacitors in vacuum tube gear with new parts since they tend to dry out and can sometimes become shorted. Shorted capacitors can destroy the power transformer.
The Drake 2B receiver's 50Khz variable passband filter is one of the main reasons for it's great popularity. Alignment of the filter is not recommended, the manual states: Warning, the passband tuner-selector is factory aligned and no attempt should be made to adjust the slugs. They are aligned in production, using special test equipment not available commercially. This is good advice, unless you happen to own a receiver who's filter is out of alignment. The required special test equipment consists of an RF sweep generator and an oscilloscope.
Before aligning the passband filter, is is a good idea to perform some of the calibration steps that are discussed in the manual. Before calibration, it is recommended that all of the tubes in the receiver are tested, especially the tubes in the RF sections. Any weak tubes should be replaced, preferably with new old stock (NOS) tubes. Calibrate the frequency of the 405 Khz oscillator coil and the 50 Khz oscillator coil and peak the response of the 455 Khz IF transformer using the methods in the manual.
Passband filter coil slug alignment tool
The passband filter's alignment can be off due to the receiver's 50 year old age and/or exposure to a prior owner's "golden screwdriver". Through trial and error, I have devised an alignment procedure that can be performed fairly easily. This process requires an oscilloscope and an RF sweep generator. A modified diddle stick alignment tool is also required. This can be purchased as part of a set of alignment tools. One could fashion a home-made alignment tool by sawing a slot in a brass rod using a fine-gauge metal saw. A small piece of heat-shrink tubing or other plastic tubing can be positioned around the slot end of the tool so that the flattened end of the slug spring does not slip out during adjustment.
Drake 2B passband filter, rear view
In order to access to four slug adjustment springs on the back of the passband filter, a few components will need to be moved out of the way. Unscrew the audio output transformer and put it into a plastic baggie so that it does not short out any circuitry during the adjustment process. The output transformer should have a jumper wire installed between its case and the chassis to enable monitoring with a speaker. A permanent wire between the transformer case and the chassis ground can optionally be added. Unscrew and unsolder the PL-259 antenna connector if one is installed. Unscrew the 4 pin Q multiplier connector and bend it out of the way so that the alignment tool can fit through the mounting hole.
Connect the sweep generator's sync output to the scope's external trigger input and set the sweep generator so that it sweeps from 3.6 Mhz to 3.61 Mhz at a rate of around 10 Hz. Set the sweep generator's output level to around 200 mV P-P, the signal should not overload the receiver's RF stages and peg the S Meter when the receiver is tuned into the signal. The oscilloscope vertical input should be connected between the chassis and the clockwise (high) side of the volume control pot. The scope's vertical gain should be adjusted so that the sweep signal is visible on the screen when it is tuned in on the receiver.
There is 6-32 mechanical centering adjustment screw with a lock nut that is located in the middle of the back side of the IF filter unit. This should be checked and possibly adjusted before aligning the filter coils. The screw should be set so that the passband knob moves the coil slugs in and out across the middle of their mechanical range. The slugs should not hit either extreme position as the knob is fully rotated. Changing this screw involves loosening the lock nut and rotating the screw with a pair of pliers. The knob should have a consistent feel across its range when the screw is adjusted correctly. Once the range is set, tighten the lock nut so that it is snug but not too tight.
Set the receiver band switch to the 80 meter position and set the filter to the 3.6 Khz setting. Tune the receiver to the sweep generator signal so that a wide range of audio tones is heard. Turn the AVC switch to the Slow position. Adjust the oscilloscope sweep rate and sync controls so that one sweep shows up across the screen and the peak response of the filter is visible on all positions of the rotary passband knob.
Set the passband filter to the 0.5 Khz range and adjust the passband knob until you see a peak response on a low tone of around 500 Hz. At this point, the four filter slug screws should be adjusted to get the most narrow and symmetrical pattern on the scope. None of the coil slugs should be at either extreme of their adjustment range. The coils should be set so that the filter has an equal high-frequency response on both sidebands as the knob is rotated. This response can be fine-tuned by re-adjusting the mechanical centering screw as mentioned above.
Once the narrow passband setting has been peaked, observe the frequency response at the filter's 2.1 Khz and 3.6 Khz settings. The passband tuning control should be adjusted so that the full range of the filter is visible on the oscilloscope. A fair amount of passband ripple will be seen in the two wider passband settings, each of the settings should show an increasingly wider response when compared to the narrow 500 Hz mode. At this point, the filter should be correctly adjusted. To verify this, connect an antenna to the receiver and tne in both CW and SSB signals to confirm that the filter is working normally.
Drake 2B passband filter, front view
There are a few things to note about the passband filter. A mis-aligned passband filter can produce a squeeling oscillation on some settings, this is a sign that the filter needs alignment. There are numerous mystery capacitors located around the bandwidth switch, these are tan ceramic parts with no markings other than a drop of colored paint on the top. The capacitors were probably matched in value and marked by color at the Drake factory. The exact schematic of the filter is not widely available, the schematic shown in the manual is only approximate. The filter schematic was most likely considered a trade secret when the receiver was in production. It should be possible to disconnect one end of each mystery cap to measure the value, then reverse-engineer the filter schematic.
The Drake 2B manual contains no instructions for aligning the 2-AQ Q multiplier accessory. Fortunately, a schematic is included and the circuitry is fairly simple. Note that the coaxial IF line cable that connects to the 2-AQ must be a high-impedance type of wire such as coax used for audio applications. The use of 50 ohm coax such as RG-58 will prevent the Q multiplier and receiver from working correctly. Also note that the Q multiplier tuning control will have a minor effect on the receiver's passband when the Q multiplier is turned Off and Peak mode is selected. If Notch mode is selected while the 2-AQ is Off, there will be no effect.
There are two slug-tuned coils on the Q multiplier, L1 is the main coil and is located inside of a black metal shield can. L2 is the notch coil and is not inside of a can.
To adjust the Q multiplier, turn its switch on and set it to Peak mode. The receiver should be set to receive CW with the product detector and BFO switches turned on. Turn the Q Balance control a small amount below where oscillation is heard. Connect the receiver to an antenna and tune into an empty part of the 80 meter band so that you only hear background noise. The receiver AVC switch should be set to the Slow setting and the passband filter should be set to 500 Hz and tuned to a medium pitch. Adjust the L1 slug so that the lowest noise pitch that you hear is in the center of the variable capacitor's turn range, this aligns the notch filter resonant circuit with the center of the 50 Khz IF.
Adjustment of the 2-AQ notch filter coil is more difficult than adjusting the peaking coil. The notch filter coil and capacitor remain connected to the IF line when in Peak mode, but its associated 12AX7 triode section is not powered up unless Notch mode is selected. Note that the notch coil setting can affect the Q multiplier in both the Peak and Notch modes.
To adjust the notch filter coil without special equipment, set the 2-AQ to notch mode and tune into a low-level CW carrier in the 80 meter band, this can be done on the air or with an RF signal generator. Adjust the 2-AQ Tuning and Q Balance controls for the best signal rejection, then adjust the L2 slug for the highest level of attenuation of the signal. Re-adjust the 2-AQ front panel controls and repeat the procedure. Adjustment of the notch coil using this method is somewhat tricky.
A better way to adjust 2-AQ notch filter coil is to use the same sweep generator and oscilloscope setup that is used for aligning the passband filter. The receiver AVC switch should be set to the Slow setting and the Passband filter should be set to 500 Hz and tuned to a medium pitch. Tune in the sweep generator signal until you see it on the oscilloscope, then peak the passband tuning control for the maximum signal. Put the 2-AQ into Peak mode and adjust L2 for the tallest and most narrow peak response. Now set the 2-AQ to Notch mode. Rotate the 2-AQ tuning knob until a dip is seen in the middle of the scope pattern. Adjust both the Q Balance and Tuning controls for the deepest notch in the center of the scope pattern. Rock the 2-AQ tuning control back and forth and make a small adjustment to the L2 slug to get the most symmetrical and deep notch across the 2-AQ's tuning range.
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