G'day all
I noticed there has recently been some stuff on the reflector with regard
to the modifying the Drake R4C. My friend Dr Frankenstein (Phil VK6APH)
and myself (Igor) have recently completed phase 1 of a rebuild of an R4C,
with cheap modern-ish components. John G3PQA and Tom W8JI have been of
great assistance in this.
Adding fuel to the recent comments about the lack of performance of modern
radios, I am forced to admit that the modded (we call it Harmanised) R4C is
at least as good as my FT1000MP for weak signal operation and nicer to
listen to.
The Harmanised R4C in question has been re-built about 50%. The most
crucial addition for 160m CW operation is undoubtably the the Sherwood
600Hz roofing filter (and complimentary IF gain redistribution). The
Sherwood 600HZ 1st IF CW filter is simply amazing - doesn't seem to ring
and sounds clear as a bell.
In addition, the modded R4C has:
1. LM3175T audio amp
2. 12V regulated supply for solid state circuitry
3. NE602 product detector
4. NE602 third mixer
5. SBL-1 single balanced 2nd mixer (to be replaced with a SBL-3 double
balanced mixer, which runs OK at 50KHz).
6. MAV-11 after the 2nd mixer to bring the gain up.
Currently the receiver has no AGC.
At this stage the R4C only has the stock Drake SSB filter, plus the
Sherwood 600Hz 1st IF filter and a Drake stock 500Hz 2nd IF filter (both
independently switchable). The shape factor of the 25-plus year old Drake
500Hz filter doesn't sound great, but it still makes a really great
selectivity aid in a pile-up coupled with the R4C's Passband Tuning - which
is far more effective than IF Shift/Width controls on the FT1000MP in my
opinion. I think the R4C notch filter kills the FT1000MP one stone dead.
The FT1000MP has been set-up with the R4C so it can be switched into
circuit using the FT100MP's antenna switch. As an illustration of its
effectiveness on weak signals, I listened to a K5 station calling CQ until
he faded down to the noise on the FT1000MP - switching to the R4C I could
still hear traces of his signal.
I guess the reasons for this are fairly straightforward - less internal
receiver noise on the R4C because of no phase noise (obviously) and no VHF
IFs, plus quiet mixers in the Harmanised R4C. I think it is quieter than
the FT1000MP when tuned between signals and sounds more natural/less noisy
on signals.
Phase two, which we hope to start in a couple of weeks, is:
7. Replace the single balanced TUF-1 second mixer with a +7dBm double
balanced SBL-3 mixer
8. Replace the valve 1st mixer with a +7dBm TUF-1 double balanced mixer
9. Complete the gain distribution so all AGC voltage is applied after the
narrow filters and everything from the 3rd mixer forward can run wide open.
10. Build a new three position AGC system - one for fast CW, one for slow
CW and one (hang-type) for SSB.
11. Fit a 6:1 reduction drive on the front of the R4C.
The intention is to ultimately leave only one valve in the signal line -
the RF amplifier - to preserve the R4C's permability tuning.
Once the rebuild has been completed and works to our satisfaction, it will
be written up for publication.
Vy 73,
Igor (alias Steve, VK6VZ)
PS I won't be getting rid of my FT1000MP - its ease of operation is far too
good. The R4C will simply be run in parallel with it.
PPS As Igor I find the components we need, any heavy lifting and the
mechanical stuff. Phil does all the design work - the real stuff.
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