TenTec RX321 First Look

[Frank Gentges]

AMRADers and RX320ers,

Some surplus TenTec RX321 receivers are showing up on eBay.  I bought 
one from the eBay seller to see what it is like.  Many thanks to Rory, 
N7CR and Tom Curtola for listing some of the characteristics on the 
Yahoo RX320 email reflector.  I have used some of their information below.

Here is my first look.

The RX321 was based on the RX320 and was built to specifications for 
Globe Wireless for a shipboard project..  Globe Wireless abandoned the 
project and new units are being sold on eBay.  The seller is 
"liquidating a large number" of these receivers so we may be seeing them 
for a while and around hamfests and the like.  "Large number" is open to 
interpretation and we can only guess.  For now some 22 units have been 
put up for sale.

The RX321 is slightly larger and more robust than the RX320 to 
accommodate a larger RF front end board that is different than the 
RX-320 RF front end.  It includes a mounting bracket that would 
facilitate mounting on a bulkhead or table.

The intermod specs are improved.  The RX320 is spec'd at 3rd order at 
+10 dBm while the RX321 is spec'd at +15 dBm.  The second order 
intercept of the RX321 is spec'd at +60 dBM while the RX320 is not 
specified.

The oscillators are ovenized in the RX321 and are spec'd at +/- 10 Hz 
over 0 to 40 degrees C.

The little active antenna included inside the RX320 is gone with the 
RX321.  That circuit could have been a source of intermods.

The antenna connector is an SO-239 on the RX321 instead of the RCA plug 
on the RX320.  This is a welcome change for most hams.

The DC power is shown as 12-24VDC on the RX321 instead of the 
unspecified power on the RX320 using a wall-wart.  We did find the RX320 
worked fine on vehicle 12 VDC power.  The RX321 includes a whole power 
distribution PC board that was added most likely to clean up dirty ships 
power.  This should make it work on mobile power better although we had 
no problems.

DC power and the speaker connect via a terminal strip on the RX321.  The 
RX320 uses phone plugs and DC coaxial connectors.

Remote audio line out on the RX321 is a balanced, center tapped transformer.

The RX321 seems to have a mute connection.  That will need to be tested.

The RX321 comes with a nice little 120 VAC to 12 VDC switching power 
supply.  So far I have not heard interference but time may reveal problems.

The RX321 has a power LED and a more rugged power switch.

I have been running the RX321 on my RX320 software and it seems to work 
just fine with all functions doing what they should.  The DSP board is 
slightly different with a different part number and different EPROM 
identifier.

The IF is not brought out like on the RX320D for digital radio.  I 
suspect the cable can be spliced and an IF output added.

It works down to 500 kHz but not down below 100 kHz.  I will have to 
take a look at that and see if an AMRAD LF modification is in order.   
The RX320 is a real LF/VLF performer, I suspect the RX321 could be too.  
The RF board layout is enough different it will take some reverse 
engineering to find the circuitry differences.

Overall, I see the RX321 having a more professional look and feel 
compared to the RX320.  I think I might prefer the RX321 to the RX320 
for my ham work. 

They are currently selling for around $200 on eBay and are a nice buy at 
that price.   I want to scrounge up the schematics so I can better work 
on and modify this design.  If I find them, I will scan them and make 
them available electronically.

Frank K0BRA