[HBR] My "75S-3W"

Mon Dec 24 14:16:32 EST 2012

Congrats on your receiver project, Ian! It sounds like a very nice project!
                     Holiday wishes and Vy  73
                     Ron  n4gjv

From: Ian Wilson <ianmwilson73 at gmail.com>
To: HBR Receiver List <hbr at mailman.qth.net> 
Sent: Monday, December 24, 2012 10:55 AM
Subject: Re: [HBR] My "75S-3W"

Hi Don,

The product detector that I am using is very close to that published in the
ARRL
Sideband Book. This in turn is very similar to the typical operation setup
given
in the RCA data sheet under "Balanced Mixer". Some details:
  - the final IFT is center-tapped
  - deflector plates are driven push-pull from the IFT
  - center tap of the IFT is biased to about +20V & bypassed
  - 1200 ohm cathode resistor
  - 68k screen grid resistor
  - 50k plate resistors (my B+ is a little low so I reduced from the
original 100k)
  - there should be a (DC) balancing control but I forgot to add this

The output is at fairly high impedance but no problem if you are going into
a
500k or higher audio gain potentiometer.

I discovered accidentally* that a 6JH8 performs indistinguishably from the
7360
in this circuit (BUT you must ground the heater pin that connects to the
internal
shield). Somewhat of a surprise since the recommended operating conditions
for
the two tubes are apparently quite different. Have to wonder if RCA did a
reverse
engineering job on the 6AR8....

* Initially I was using a NOS NIB tube from National Electronics labelled
"7360".
This had massive hum on the output. Replacing with an RCA 7360 fixed that
problem. On closer inspection I noticed that the tube actually sits on a
small
PCB skirt that holds 9 pins, and reroutes the original tube pins! The actual
tube pinout corresponds to the 6JH8 family. Once I grounded the appropriate
heater pin this worked very well. Now I don't know whether to keep the fake
tube as a curio or to put my RCA 7360 back in its (expensive) box :)

Happy holidays,

73, ian K3IMW

On Sun, Dec 23, 2012 at 2:57 PM, donald haworth <donmhaworth at gmail.com>wrote:

> Hi Ian,
> I have been following your efforts with great interest, thank you for
> sharing!  Could you post details about your 7360 product detector? BTW,
> N3ZI (Doug) makes a 'digital dial' for about 30 bucks, that's hard to
> beat-and it  has programmable offsets as well.....
>
> Hope you have a Merry Christmas!
> --And A Merry Christmas to all on the HBR list!
>
> 73 Don (wb0rai) Nashville
>
>
> On Sun, Dec 23, 2012 at 1:21 PM, Ian Wilson <ianmwilson73 at gmail.com>
> wrote:
>
> > Though I would write up the results of my attempt to build a dual
> > conversion
> > receiver modelled on the Collins 75S-3C architecture. I chose this
> because
> > it allows me to use the 1.3-3MHz PTO that is working beautifully, and
> > because
> > it uses a BPF to provide 0.2MHz of 1st IF bandwidth without requiring
> > tricky
> > tracking tuning. I have a few 3.05MHz +/-175kHz BPFs here that I picked
> up
> > at some swap meet. These fit the frequency range well enough.
> >
> > The receiver is working FB right now, thanks to those who helped with
> > suggestions about getting the IF strip working well. At present this is
> > just
> > a 7MHz receiver but I plan to add a synthetic box-o-crystals using the
> > Si507.
> > The only problem at the moment is a large birdie. This may be because I
> > don't
> > have a crystal of the proper Collins frequency. The lineup is:
> >
> >  - 6EJ7 RF stage
> >  - 6U8A xtal oscillator
> >  - 6DJ8 Pullen mixer
> >  - 3.075MHz BPF
> >  - PTO
> >  - 6U8A mixer (triode used as buffer to frequency counter)
> >  - Collins 455kHz mechanical filter
> >  - 2x 6EH7 IF
> >  - 7360 product detector
> >  - 12AT7 BFO
> >  - G4OEP audio-derived delayed AGC (solid state)
> >
> > I left space on the IF board for an IF AGC amplifier but the
> audio-derived
> > AGC is working well enough for the moment.
> >
> > The audio stage, PSU, etc, is all a lash-up at the moment.
> >
> > The tuning rate of the PTO is 50kHz/turn. I have a National 6:1
> > reduction drive
> > in front of it, giving 8.3kHz/turn. This seems to be a very good
> compromise
> > and it is easy to tune SSB transmissions.
> >
> > I have no mechanical skills, so none of the schemes that I considered for
> > driving a frequency indicator from the PTO shaft were practical. Since I
> > need
> > to use a uP to control the Si570 anyway, I may as well add an LCD display
> > and frequency counter to display frequency directly (can add in the
> > required
> > offsets since I know which 200kHz band is being used).
> >
> > I hope to offset the negative karma accrued from using sand state by
> using
> > a magic eye signal strength indicator. Currently looking at the choices
> > available in the boxes-o-tubes here.
> >
> > Happy Holidays to all!
> >
> > 73, ian K3IMW
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