Sherwood Engineering Incorporated Sherwood Engineering Incorporated

1268 South Ogden Street Denver, Colorado 80210 USA

EMAIL SEI     Phone: 303-722-2257     FAX: 303-744-8876

9 a.m. - 5 p.m. (Mountain) Monday - Friday

VISA, Mastercard, & American Express Welcome

Drake R-4C

Drake R-7 & TR-7

Racal 6760 GM

Atlas Radio Replacement SSB

Distributed Roofing:
Icom IC - 765

First-IF Roofing Filters:


Drake R-4C Filters:

  • 16-pole Receiver cascaded filter selectivity

  • Reduce QRM leakage, overload and eliminate blanker false triggering

  • Ideal for DX and contest work

  • Maximum skirt selectivity with maximum intelligibility

  • Variable bandwidth

  • Total bandwidth with CF-2K/8 & FL2400 is 1900Hz @ -6dB & 2900Hz at -60dB

  • Ultimate rejection better than 120dB

  • Two-filter switching with PC board & relay sockets: No longer available

  • Three-filter switching with 2 PC boards & sockets: No longer available

  • Four-filter switching with 2PC boards & relay sockets: No longer available

  • LSB & USB CF-2K/8 filter pair: No longer available

*  The prices of the switch kits above
have increased due to an price increase in the TO-5 relays


Drake R-7 & TR-7 Filters:

    AM Filters:

  • 2.85 kHz CD - 2.85K / 8: No longer available!

    Enhanced SSB Filter:

  • 2.85 kHz CD-2.85K/8: No longer available!

End of an era


Racal 6790 GM Filters:


    AM Filters:

  • 16 kHz CR - 1600 / 15: $179.00 US

  • 8 kHz CR - 8000 / 9: $179.00 US

    SSB and CW Filters:

  • 2.1 kHz CR - 2100 / 8: $249.00 US

  • 400 Hz CR - 400 / 8: $249.00 US


Atlas Radio Replacement SSB Filter

Part number CA - 2.2K / 8:
$55.00 US

Replace the wide 2.7-kHz crystal filter in your 210 or 215 transceiver with a more desirable 2.2-kHz 8-pole crystal filter. This is a direct mechanical and electrical drop in replacement filter for the original unit. Requires adjusting carrier oscillator capacitor to compensate for the narrower bandwidth. This filter is on 5645 kHz. There may be some early 210/215s that were on a different IF frequency. Fully refundable if it doesn't work with your radio.



Distributed Roofing Filter for the Icom IC-765

Part number CI-3K/8:
$149.00 US

The IC-765 has an IF shift feature that allows one to move the passband around for the best SSB audio quality or to reduce interference.  The same type of IF shift is offered in the IC-706 series and the R9000 receiver.  This should not be confused with the variable bandwidth modification often performed on the 765, which allows one to reduce the IF bandwidth by offsetting the 9 MHz and 455 kHz filters. 


When the IF shift button is pushed “IN”, the 9 MHz crystal filter is completely removed from the circuit, to allow the 455 kHz filter to shift its passband position.  Unfortunately the elimination of all selectivity at 9 MHz degrades the performance of the radio significantly, as noted in the QST review of December 1990. In that review the ARRL reported the 5-kHz dynamic range is reduced from 85 dB to 73 dB.  The 2-kHz dynamic range is degraded even more.


This problem can be completely eliminated by adding the Sherwood distributed roofing filter.  Instead of removing the SSB filter at 9 MHz, we insert a 3.3 kHz crystal filter that allows the IF shift to function without degrading the radio’s dynamic range.  This allows the FL-96 2.3 kHz filter (or optional FL-80) to move around inside the Sherwood 3.3 kHz filter, yet still keeping strong adjacent signals out of the 9 MHz IF chain.


The only compromise with this upgrade is the IC-765 will no longer function on 10 meter FM.  The 3.3 kHz filter that is in the circuit on FM is too narrow for that mode.  Addition of this filter does not degrade blanker performance. This kit is compatible with the Inrad 69 MHz roofing filter modification.


Sherwood Engineering Incorporated

Sherwood Engineering Incorporated
1268 South Ogden Street Denver, Colorado 80210 USA
Phone: 303-722-2257     FAX: 303-744-8876     EMAIL SEI

15 February 2009
T Cantrell, webmaster