Sherwood Engineering Incorporated Sherwood Engineering Incorporated
1268 South Ogden Street Denver, Colorado 80210 USA
email     Phone:
303-722-2257     FAX: 303-744-8876
9 a.m. - 5 p.m. MST Monday - Friday

Receiver Test Data
First-IF Crystal Roofing Filters Electronic Kits Owners Manual
Band Crystals Custom Front-Panel Switch Filter Brackets
Crystal Cover Mode Switch Cover Cleaner & Lubricant
PRE-4 Preamp FS-4 Interface Kit Receiver Modification Service
Sherwood Engineering Inc. Home Page Short Wave Listener Catalogue SE-3 Synchronous AM Detector

Sherwood / Drake R-4C
Still the Finest Competition-Grade Receiver
Receiver performance tests were taken on current, popular receivers and transceivers. Dynamic-range measurements were made using two equal-strength test signals 2 kHz apart to approximate a "real-life" pile up, plus then more normal 20 kHz spacing. A few radios had to be tested at 50 or 100 kHz because of synthesizer noise. (See below for additional details). Some units, due to limitations in the i-f filters or board layouts, had excessive test-signal leakage, requiring close-in measurements be made at 3- or 4-kHz spacing. AGC threshold, the point below which audios falls off rather linearly with signal, is listed in microvolts across the antenna input (assumed to be 50 ohms). Also listed is the drop in audio-output level at the AGC threshold, as compared to a 5 mV (typically S9 + 40 dB) signal. Tests were made using a 500- or 600-Hz CW filter, when available.

Receiver noise floor can be degraded by the reciprocal mixing of an adjacent strong signal with phase noise from local oscillators, LO amplifier stages, phase-locked loops, or synthesizers. Noise from the receiver LO chain is listed for a normalized bandwidth of 1 Hz at a given kHz offset in dB below the LO carrier (dBc). To calculate actual noise from the table for a given filter: Integrated Noise = Table Value N minus 10 log (filter bandwidth in Hz). Thus an LO with noise 100 dBc and a filter 500 Hz wide would degrade the noise floor at least 3 dB (at the listed signal spacing or less) whenever a signal was 73 dB above the reciever noise floor (about S9 + 15 dB). 120 dBc or greater is desirable.

Drake R-4C

  Testimonial on SEI Modifications  
  to the R-4C  
A rating of front-end selectivity is provided, with broad delineations of A through F, rating the ability to reject out-of-band signals. A tracking pre-selector is graded as A+, while no input selectivity would be F.

Receiver ultimate rejection rating is not standard. It is NOT what the rejection is 10, 20 or 50 kHz off frequency. This shows whether signals two to four filter bandwidths off frequency will be heard as undesired leakage. Ultimate rejection of 120 dB will not show leakage until approximately S9 + 60 dB.

Also on this web site is information on our complete line of products, plus certain circuits to improve specific subsystemns in the R-4C. Most of our modifications have been published in Ham Radio magazine. We offer assembled PC board update kits, along with in-house installation services.
For further explanation of our receiver testing procedure, see: "Present-Day Receivers - Some Problems and Cures". ham radio, December 1977, page 10. PDF copies of our ham radio articles are available for download from the web site front page.

R-4C SSB 8-Pole First-IF Crystal Roofing Filters

Improve the early-stage selectivity of your Drake R-4C by adding eight additional poles with an internally-mounted, switchable set of first-IF roofing filters. This provides 16-pole total receiver cascaded filter selectivity. The filters will reduce QRM leakage, overload, and eliminate blanker false-triggering from adjacent stations. Ideal for DX and contest work. The overall shape factor is better than 1.5. Maximum skirt selectivity with maximum intelligibility. Variable bandwidth can be obtained by offsetting first and second-IF filters with passband tuning control. Total bandwidth with CF-2K/8 and FL2400 is 1900 Hz at -6dB and 2900 Hz at 60 dB. The ultimate rejection is better than 120dB.

Other Available Filters

  • Two-filter switching with PC board, relays & sockets: No longer available*
  • Three-filter switching with 2 PC boards, relays & sockets: No longer avaialble*
  • Four-filter switching with 2 PC boards, relays & sockets: No longer available*
  • LSB & USB CF-2K/8 filter pair: No longer available
*  Increased prices of switch kit parts have made these products uneconomical to sell.

  Installed Component Picture              Alignment Tips            


600 Hz 6-Pole First-IF roofing filter for Drake R-4-C
    • Low Loss for Full Sensitivity!
    • Improve the early-stage selectivity of your Drake R-4C with the time-proven otpimum-bandwidth, low-loss CF-600/6 first-IF CW roofing filter.
    • Eliminate those high-pitched beat notes from signals thatleak around the switchable second-IF crystal filter.
    • Improve ultimate rejection to better than 130 dB, unlike competing units.
    • Eliminate the chance of strong signals overloading the second mixer, causing intermodulation and desensitization.

Compare Stock Filter to CF-600/6
  Stock CF-600/6
BW -6 dB 8 kHz 600 Hz
BW -60 dB 32 kHz 2 kHz
Poles 4 6

The CF-600/6 is optimized for beat notes of 400 to 800 Hz on the lower sideband of zero beat, maintaining transceive CW capabilty.

Both the stock filter and our CF-600/6 can be mounted in the receiver and relay switched, allowing both SSB and AM as well as optimized CW operation.

An optional filter switch kit is no longer available. It consisted of a PC board, two TO-5 relays, sockets and hardware.

Selection of the CF-600/6, independent of the mode switch, allows it to be used with any second IF filter. This flexibility is best accomplished with our custom front panel switch which replaces the AGC switch. This upgrade also gives the user two additional AGC speeds, for a total of 5 plus OFF. See full details under "R-4C Custom Front-Panel Switch."

  • 600 Hz 6-Pole First-IF filter: No longer available

Electrical Kits
R-4C Filter Capacitor Replacement Kit R-4C Audio Amplifier Solid State 3rd Mixer! R-4C Product Detector R-4C New Replacement Power Supply Board

R-4C Filter Capacitor Replacement Kit

Old chassis-mounted capacitors dried out? New ones are no longer available. Replace the two electrolytic cans with our four individual filter capacitors on a PC board. Requires removing the two can-type capacitors, and installing our board in its place. Also includes a fifth capacitor for the 150 V. DC supply modification.

  • FC-4 Filter Capacitor Replacement Kit: $30.00 US

R-4C Audio Amplifier
AMP-4 Improvements in the Drake R-4C receiver, up to now have been mainly confined to the i-f system. One area that needs improvement is its audio strip, which suffers from buzz and higher-than- desirable distortion. It also dissipates 7 to 10 watts of heat near the PTO. The AMP-4 replacement audio amplifier eliminates these problems. While intended as an R-4C retrofit, this circuit performs so well that we recommend it for other communications uses. (Overall gain is 40 dB.)

Installed Component Picture

  • AMP-4 wired PC board amplifier: $49.00 US

Solid State 3rd Mixer!
Having problems with noisy 6EJ7 tubes in the R-4C third mixer? Introducing MIX-4 solid-state third-third mixer kit. Eliminates frying, crackling and microphonics. This PC board replaces the high-gain tube mixer stage with an FET amplifier followed by an IC mixer. Also cures power-line and R-4C power-supply-induced buzz that couples into the 6EJ7 grid.

Mounts on top of existing FL-2400 filter.
Installed Component Picture

  • MIX-4 wired PC board replacement third mixer: $59.00 US

R-4C Product Detector
pd-4 The product detector in the R-4B /C has serious dynamic headroom problems. Under transient conditions (AGC attack), the i-f signal level can actually exceed the BFO input level, repeatedly rendering the detection very distorted for a fraction of a second. The PD-4 IC product detector eliminates this deficiency, plus reduces detected audio that gets back into the AGC detector.

  • PD-4 wired PC board detector: $35.00 US

R-4C New Replacement Power Supply Board
Like our other kits, this is a pre-built PC board ready to install in the radio. It replaces the existing power supply board. the old board is discarded, and all the wires reconnected to the new board. As with all our other R-4C kits, this is a high-quality plated through hole tin coated board of the highest quality. It also includes the special buzz-filtering AC bypass capacitors that have long been included in the PS-4 parts kit. (Don't be mislead by cheap imitations.) RPS-4
Installed Component Picture

  • R-4C New Replacement Power Supply Board: $49.00 US

Drake R-4C Owners Manual

  • Professional reproduction
  • Color printing with clear plastic covers for better durability
  • Includes full size fold out schematic. Schematic for serial number 21,000+.
  • Drake R-4C Owners Manual: $20.00 US

R-4C Band Crystals

  • All ham frequencies 160 through 10 meters including WARC bands are currently in stock.
  • We currently stock all 10 meters segments 28.0 through 30.0 Mhz.
  • HC-6/U and even HC-25/U crystal prices have been significantly increasing. Long term availability is uncertain.
  • R-4C Band Crystals: $25.00 US

R-4C Custom Front-Panel Switch

Our custom front-panel AGC/First i-f filter switch is available again. This dual-concentric switch replaces the present AGC switch, providing not only AGC, but DC control for our roofing filter switch kits. In addition, two more AGC positions are provided to better optimize AGC speed to varying conditions and modes.

  Installed Component Picture         Operation of the FPS-4      

  • R-4C Custom Front-Panel Switch: $69.00 US

R-4C Filter Brackets

Are you missing the brackets that hold the plugin accessory filters in place? They are now available.

  • R-4C Filter Brackets: $8.00 US

R-4C Crystal Cover

Are you missing the rear crystal cover? This cover protects the 15 crystals that can be plugged into the rear of the radio.

  • R-4C Crystal Cover: $15.00 US

R-4C Mode Switch Cover

Used on radios with serial number 21,000 and up. This is an RF shield for the 3rd mixer oscillator.

  • R-4C Mode Switch Cover: Discontinued

Caig Labs Contact Cleaner / PTO Lubricant

The DeOxit spray is the closest thing to magic on switches and pots. Use it on all Drake rotary switches and pots. Cleans up intermittent contact on bandwidth and filter switches, and eliminates noise on AF and RF gain pots. Of course this works on any brand of electronic equipment.

The copper-doped grease is the best lubricant for the PTO lead screw. Not only lubricates, but provides a better ground on the PTO slug assembly.

  • Spray can of DeOxit: $19.95

PRE-4 Preamp

10 dB high-dynamic range preamp. Relay switched when DC supplied. +30 dBm intercept. Similar to preamp in Drake R-7 receiver. Operates on 12 to 14 volts DC at 100 mA. Rated at 500 kHz to 50 MHz, though usable above and below that frequency.

  • Amplifier board only: $75.00 US
  • Add 30 MHz low pass filter: $30.00 US
  • Amplifier in mini-box: $125.00 US
  • Amplifier in Pomona cast aluminum box with BNC connectors: $189.00 US

FS-4 Interface Kit

The Drake FS-4 band synthesizer provides the R-4C (and some other Drake models) with general coverage. The stock connection to the radio was a kluge, requiring the user plug a short cable into an accessory band crystal socket. Drake recommended unplugging the connection when the normal band crystals were used, as the feed loaded down the injection oscillator. For over 20 years we have offered the FS-4 Interface Kit that solves this connection problem. The kit consists of a TO-5 RF relay, and several RCA jacks. When installed, turning on the power to the FS-4 keys the relay, switching out the band crystal bank, and injecting the FS-4 signal.

  • FS-4 Kit: $79.00 US
It is field installable. Installation takes about 2 hours.
If we install the kit while servicing your R-4C, the labor charge is $49.00 US


Receiver Modification Service

Sherwood is now offering installation of our electrical and filter modifications. Advance authorization requested for scheduling.

  • AMP-4, PD-4, RPS-4, MIX-4, and FC-4 update installations in R-4C: $49.00 US per kit
    ($245.00 for all five kits)
  • SSB switch kit installation for CF-2K/8 pair in R-4C: $49.00 US
  • CF-600/6 relay kit installation R-4C: $49.00 US
  • Four-filter switch kit for CF-2K/8, CF-600/6 and 8-kHz filter in R-4C: $98.00 US
  • Custom front-panel switch installation: $49.00 US
  • R-4C/FS-4 interface kit installation: $49.00 US
  • Full alignment of Drake receivers: $95.00 US
  • Needed parts extra.:
  • JRC NRD-515 & NRD-525 modification: See separate page on Japan Radio products.

Sherwood Engineering Incorporated Sherwood Engineering Incorporated
1268 South Ogden Street Denver, Colorado 80210 USA
email     Phone:
303-722-2257     FAX: 303-744-8876    
9 a.m. - 5 p.m. MST Monday - Friday

Updated 1 November 2016