Explorer II Preliminary Review


From: chuck adams (adams@chuck.dallas.sgi.com)
Date: Fri Oct 13 1995 - 04:25:36 EDT


As I may have mentioned years ago I have a son who is now at
the ripe age of 27. He is in the final stages of his PhD in
Astrophysics with the same major prof that I had at Texas A&M
University. He sent me email today saying that he had a short
wave receiver that I had given him some time ago on and was
listening to BC in German and was thrilled. There may be a
ray of hope left after all. I had tried incentives like
money, inheritance, and all to get him interested in radio
but alas all to no avail. He is excited that all that
E&M (Electricity and Magnetism) and Maxwell's Equations
really work in the real world. :-)

OK, so no progress in the area of ham radio so I thought I
would trade him off for something that I could use. Kind
of like they do in the NFL. So I called Dick at OHR several
times and faxed him a resume of the kid and a picture to see
if we could strike up a deal. Well my exceptional power of
persuasion paid off. This may be on the cover of the
National Enquirer next issue - "Father sells kid into slavery
for use of small radio for a few days!!". ;-)

Well I stretch the human imagination quite a bit with this
story, but it makes fun reading at the checkout counter at
the grocery store. I can just see the urban legends starting
now. :-)

I awaited anxiously at the door after getting home today
looking for the brown truck. I then go off to watch the news
and see who did what to who today and sure enough the
door bell rings at 5:45CDT. Yep, it's a small box from
OHR. WOW. Off to the shack to see if there really is
a Santa Claus.

So, I have in my hot little hands the ONLY OHR Explorer
II in captivity!! I mean the prototype. I mean one of
a kind. I mean priceless.

OK, this puppy is on 40M, my favorite band in the whole
world. And it is the night of the fox hunt to boot.
So I unpack it from the box and set it on the desk and
then take the two page schematics (the only other thing
in the box) off to the couch to compare with the old
explorer schematics. More on this later, the schematics
not the couch. Now back to the shack to really power the
Explorer II up and listen and work some stations.

First put on dummy load with WM-1 wattmeter to check output.
Get the 7.5AHr gel-cell and measure the big V at 12.39.
Turn on the rig and it drops to 12.38. So far so good
and I hear noise in the earphones. Get the MFJ-249 and
check endpoints of the dial and get 7.000 to 7.080MHz.
Fairly wide range and can be adjusted. Dick says 75KHz
for 40M and 20M and 50KHz on 30M. Builder can probably
adjust range to further increase this, but at the risk
of too fast a turning rate even with reduction in variable
capacitor. Rig is rated at 2-3W output with 13.8V, but I
don't have that high V so adjust drive for 0.95W output into
50ohms. Dick I promise to set it back..... Really.

For those of you who own or have seen the previous version
of the Explorer, the case is the same and the little red
LED for the power indicator is replaced with a knob on
the upper left hand side of the front controlling the
variable bandwidth filter in the IF. Guess it is about
1.4KHz at the widest to about 350Hz on the narrowest
width. All this done with a Cohn filter with varicaps.
Neat and effective. This allows one to set the width
to a wide value for tuning across the band and making
sure you don't miss anyone. After selecting a station
you can narrow it down from the front panel control to
a comfortable width and we probably all have our favorite
width for filters at different times.

So now hook up long wire, well random wire if you wanna
get technical on me. And wow, lots of signals tonight.
Tune from the low end up and see how the receiver sensitivity
is. It is great. I can hear some weak signals just peeking/
peaking above the noise level. Also hear some strong signals
too. I set the filter to wide and wander up to around 7.063MHz
and sure enough hear some SSB. A group of VE3s running some kind
of net. I can understand them but not real high quality with
the restriction on the bandwidth, but at least understandable.

Go back down to low end of band where a group of stations are
pouncing on some DX and sure enough I can separate out the stations
with the narrow filter. So for $99.95 list price you can have the
best of both worlds, selectivity and sensitivity.

I move on over to 7.040MHz to see the start of the fox hunt at
8pm CDT and sure enough there is a group there. About 30 seconds
into this my "friend" about a mile away starts up the KW about
800 cycles down. Had I been listening on any other radio the AGC
would have not kicked in fast enough and I would have been in
agony with a blast to sensitive ears, but I'll be darned. This
AGC is the best I have seen. I remember posting and having
seen others post that AGC in a CW rig was a killer for sensitivity
and effectiveness. Not so here. Dick has a version of Wes Hayward's
AGC from a 20M SSB rig that was in QST a while back and it works great.
It's audio derived but is the best I've seen. This is one of the mods
that Dick came up with between the Explorer and the Explorer II. This
feature alone is worth the whole kit and kaboodle, but hold on gang
there's more. When I move down and up from the local I don't hear
the intermod and overload that I usually hear on other rigs. A real
neat improvement. Thanks Dick. Being here in Dallas where there are
a number of QRO guys within a couple miles of me this feature
of the rig will really help. And I no long have to worry about
getting a blast in the ear doing weak signal work and have a
real thunderboomer of a signal tune up on freq without a warning.
We've all been there done that. Dick has improved the front end
quite a bit on this new rig.

I am listening as I type in this report at 0559Z and am hearing
G3XAQ, ZL1DYC, UL2YH/KC4 who is really generating a pileup :-),
VK2RKE, ZL3BSH, so I'm going to stop right here for a minute.
Listening for N6ULU. :-) NOT. All the above with signal levels on
receive at 559 or better. Good opening to EU and Pacific.


OK, so let's summarize the above and then after a few more days of operating I'll come back, but I wanted to get this out since this happens to be one of the rigs that John has gotten into the Friday 13th 20% off qrp-l group deal with OHR. See previous postings over the last 24 hrs to get details or contact John.

Things relative to the old Explorer in Explorer II (40M)

o New front end for rcvr o Same local vfo but moved to 2085 to 2165 KHz o Cohn filter with variable bandpass for IF at 4.915MHz o Simplified output from MC1350 to NE602 detector o New AGC circuit o Elimination of one audio amp stage o Noise reduction in receiver o Elimination of power LED and addition of variable bandpass control on front panel o Parts reduction with an increase in performance and a reduction in price

Overall performance from first 6 hours of operation

o No noticable drift after first five minutes. About 200Hz drift in first few minutes. May be the protype as not in original Explorer that this author remembers.

o Same size and approximate weight of original Explorer. Not a backpacker to some people, but a darn good rig.

o Excellent sensitivity and low internal noise.

o Great variable bandpass adjustment from front panel.

o The usualy OHR great QSK circuitry.

o Fantastic AGC!!

o Good audio drive for headphones as not used above 35-40% on 8ohm headphones even for weak signals. Yaesu YH-77 headphones.

For the $80 discounted price through Friday Oct 13 1995 it has to be the deal of the year. IMHO.

Expect shipments to begin in about 3 weeks or maybe 4 if a large order comes in from this group. So if you are interested contact John and get on the list that you saw posted.

As I got an already assembled unit to play with, I did not see the manual, but it should look very similar to the old version with new board changes incorprated into it. OHR's manuals are very very good.

-------Usual Disclaimer----------------------

I am not an employee of OHR or a stock holder or being paid for any endorsements or reviews of the above. Just a satisifed customer from previous purchases and have a 30M version of the Explorer II on order with OHR. This is going to cost me gang as they come in 40M, 30M and 20M versions. :-)

Good luck, especially today.

dit dit -- Chuck Adams (K5FO CP-60) adams@sgi.com Box 181150, Dallas, TX 75218-8150

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