From csus.edu!wupost!howland.reston.ans.net!noc.near.net!sunfish.hi.com!brainiac.hi.com!user Wed Aug 18 23:04:21 PDT 1993 Article: 25914 of rec.radio.shortwave Path: netcom.com!csus.edu!wupost!howland.reston.ans.net!noc.near.net!sunfish.hi.com!brainiac.hi.com!user From: steve@hicomb.hi.com (Steve Byan) Newsgroups: rec.radio.shortwave Subject: Re: Icom R70 mods wanted Followup-To: rec.radio.shortwave Date: 18 Aug 1993 21:26:35 GMT Organization: Hitachi Computer Lines: 70 Distribution: na Message-ID: References: NNTP-Posting-Host: brainiac.hi.com Here is the R-71 version of the Drake R7/R8 type passband tuning system: Add a jumper wire between the cathodes of diodes D42 and D45 on the main unit circuit board. The cathode end of the diode is marked by a band. The diodes are located near Filter Switch 1 (the switch you set when you add the optional narrow filter). Turn Filter Switch 1 on. That's it. Usage: select LSB mode and narrow filter position, and tune to zero-beat. Rotate the pass-band tuning control to select between LSB and USB, or anywhere in-between. Select normal or wide filter position and you get the original ICOM PBT. The mod for the R70 should be just as easy, but you'll have to find the diodes on your schematic. Just jumper the cathode of the diode that selects the optional narrow 2nd IF filter to the cathode of the diode that selects the "through" postion in the 2nd IF. This modification will bypass any narrow filter you may have installed. It forces the narrow filter selection to bypass the 2nd IF filters, while still selecting the narrow filter in the 3rd IF. The standard ICOM pass-band tuning shifts the 3rd IF passband against the 2nd IF passband: No PBT shift LSB shifted USB shifted __|__ __|__ __|__ 2nd IF / | \ / | \ / | \ / | \ / | \ / | \ ___/ | \___ ___/ | \___ ___/ | \___ __|__ ____| |____ 3rd IF / | \ / |\ /| \ / | \ / | \ / | \ ___/ | \___ _/ | \_____ _____/ | \_ __|__ __| |__ result / | \ / |\ /| \ / | \ / | \ / | \ ___/ | \___ __/ | \_____ _____/ | \___ | | | ^ ^ ^ ^ ^ ^ full b/w narrowed b/w narrowed b/w This mod bypasses the 2nd IF filter, so you have just the 3rd IF filter, which the PBT control can move around relative to the center frequency: No PBT shift LSB shifted USB shifted __|__ ____| |____ 3rd IF / | \ / |\ /| \ / | \ / | \ / | \ ___/ | \___ _/ | \_____ _____/ | \_ | | | ^ ^ ^ ^ ^ ^ full b/w shifted b/w shifted b/w I haven't performed this mod on my R71 (I have to install PBT tuning first - I got one of the ones without PBT), so you're on your own with this one. I expect that the close-in dynamic range will be worse when using this flavor of PBT on the R71, since the 2nd IF filter is bypassed Steve Byan internet: steve@hicomb.hi.com Hitachi Computer Products (America), Inc. 1601 Trapelo Road phone: (617) 890-0444 Waltham, MA 02154 FAX: (617) 890-4998