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Drake R-7 vs. Drake TR-7 Receive



 
 
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  #1  
Old November 14th 07, 03:04 AM posted to rec.radio.shortwave
john
external usenet poster
 
Posts: 52
Default Drake R-7 vs. Drake TR-7 Receive

Hs anyone compared the receivers in these two radios side by side - I
am interested in hearing all comments from hams and swl' operators. If
the TR-7 is basically the same receiver as the R-7 - i would opt for
the TR-7 so i can transmit as well. Is the TR-7 as sensitive and
selective and does it have the same continuous coverage as the R-7??
also how are these radios on LW and MW BCB - very interested in
hearing - thanks so much in advance to all on this great site

john - ki4ucw

  #2  
Old November 14th 07, 09:08 AM posted to rec.radio.shortwave
Pete KE9OA
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Posts: 271
Default Drake R-7 vs. Drake TR-7 Receive

I've got both the R7 and the TR7. Both of them receiver fairly well, and
both of them use the same 1st mixer. The differences? The TR7 is a dual
conversion unit while the R7 is a triple conversion unit, the difference
that the R7 has a 50kHz final I.F. The TR7 uses an envelope detector for AM
while the R7 uses what they call a Synchro Phase detector. Actually, this is
a Quasi-Synchronous detector of the same type that is used in the Racal 6790
receiver. What is done in this approach is that the I.F. signal is broken
into two paths. One path feeds the RF input of a product detector while the
other path is squared up through a limiting stage and applied to the LO port
of the product detector. Demodulated audio is extracted from the I.F. port
of the product detector. If you are getting the impression that a product
detector is actually a mixer, your impression is correct!
Anyway, further on.............the R7 has a switchable 10dB RF preamp while
the TR7 has none; the R7 has RF bandpass filters for every range, down
through the VLF band while the TR7 only has filters down to the 1.5MHz
range. In order to have LW/MW coverage on the TR7, you must use an antenna
connection that bypasses the RF preselector (input bandpass filters).
Both of the units are ok out of the box, but I have improved the performance
of both of my units by replacing the
Vari-L CM-1 1st mixer with a Mini-Circuits SRA-3MH. This is a Level 13 mixer
that has only 4.77dB of midband conversion loss vs the 6.5dB conversion loss
that the CM-1 has. The CM-1 is rated down to 500kHz (or is it 1MHz?)
while the SRA-3MH is rated down to 40kHz.
Drake got around this problem by interchanging the RF and I.F. ports, since
the I.F. port on this mixer is rated down to DC. Still, the replacement
mixer has lower conversion loss, so you can improve the noise figure of the
unit by 1.73dB. and that is without having to use an RF stage ahead of the
mixer. The result is better performance, since any RF amplification ahead of
the mixer would degrade its IP3 performance by the amount of gain that this
RF stage is providing. As an example, if your IP3 is +20dBm, if you place
10dB of amplification ahead of the mixer your IP3 will degrade to +10dBm.
Another improvement that I did with my units was to replace the 1N4148
diodes that Drake uses in the 2nd mixer with a matched quad set of 1N5711
hot carrier diodes. This will improve the IMD performance of this stage
After all of these mods, the MDS improves from 0.15uV to 0.07uV. Not bad for
a few hours work!
For all out DXing, I would purchase the R7...........if you also want to
transmit, get the TR7. Both of the units are respectable for LW/MW
reception.
The R7 is going to run you around 1000 dollars, while the TR7s run between
400 and 700 dollars, depending upon what is included in the deal.
If the seller pulled out the extra filters (which many sellers seem to do),
expect to pay another 100 dollars to purchase an
AM filter from INRAD.
Expect the fact that you might have to do alignment of the BFO injection
oscillators. My TR7 needed a bit more than that. I purchased one on eBay
from a fellow in Ohio that was selling it because "it was excess to his
needs". Whenever I would try to transmit, the circuit breaker on my Drake
PS7 power supply would trip. Somebody defeated the ALC in order to get a
little bit more power on 10 Meters. The unit would put out 250 watts on 80
Meters, but the power would droop as you went higher in frequency. When I
noticed that the BFO injection level was low, I traced the problem to a cold
solder joint on the exciter board. When I e-mailed the seller about this
situation, he said "oh, I was wondering why I had to turn up the mic gain so
high on this rig".
The point of what I am saying is that you really need to be careful when
purchasing one of these things. Come to think of it, almost every radio that
I ever bought over the web needed some kind of repair, except a Sony 6800W
that I bought from Mike Magakian and a Lowe HF-250 that I bought from Dave
Zantow (N9EWO).
Good luck on your search............no, I am not selling either of mine. You
will understand why, when you get ahold of one!

Pete

"john" wrote in message
ups.com...
Hs anyone compared the receivers in these two radios side by side - I
am interested in hearing all comments from hams and swl' operators. If
the TR-7 is basically the same receiver as the R-7 - i would opt for
the TR-7 so i can transmit as well. Is the TR-7 as sensitive and
selective and does it have the same continuous coverage as the R-7??
also how are these radios on LW and MW BCB - very interested in
hearing - thanks so much in advance to all on this great site

john - ki4ucw



 




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