[R-390] R-648

[ToddRoberts2001 at aol.com]

In a message dated 10/2/2005 9:07:17 AM Eastern Standard Time, 
jamminpower at earthlink.net writes:
Anybody know anything about the R-648? There is one you-know-where (item 
5814012399). It looks a lot like our good friends - slug racks, PTO, 
mechanical filters. It looks like someone took a lot of parts from a 51J4 
and an R-390 and put them back together without a manual. Interesting 
receiver.

James A. (Andy) Moorer
www.jamminpower.com

The R-688 is a nice receiver but it does have a few limitations. It does not 
cover the broadcast band. I believe the first band is 190-550KHz and the next 
band up normally starts at 2MHz but there is enough overtravel that you can 
get it to tune down to almost 1.9MHz.   Also the one on eBay does not have the 
case but one could be made up for it or possibly you could find an old R-648 
case somewhere from a real junker R-648.  It is best to replace the noisy 
dynamotor power supply with a small 120VAC power supply and one can be fitted inside 
the chassis easily and there was an old construction article in 73 magazine I 
believe showing how to build an AC power supply for it - that makes it a very 
nice table-top receiver with a built-in AC supply. Another thing about the 
R-648 is that it uses 500KC filters similar to the ones used in the 51J-4 except 
the ones in the R-648 are solder-in types and the 51J-4 filters are plug-in. 
The R-648 comes with 2 stock bandwidths - 1.4KHz and 6.0KHz. The 1.4KHz BW 
filter is not really too practical as it is too wide for CW and a bit too narrow 
for SSB or voice. If you replace the 1.4 with a 3.1KHz filter from a 51J-4 
then the receiver plays great with 3.1KHz  for SSB/Narrow AM and 6.0KHz for wide 
AM. The R-648 filters will play nicely in a 51J-4 also if you file off the 
solder tabs on the pins,  then they will plug into the 51J-4. You may need to 
file down the threaded mounting studs a little bit also to plug into the 51J-4 
but I believe the studs have the same spacing as the locator pins in the 51J-4 
filters. One could mount some standard HC6-U crystal sockets in the R-648 IF 
amplifier box so you could have it compatible with the 51J-4 plug-in filters and 
file off the solder tabs on the original R-648 filter pins so everything 
would be compatible to swap filters back and forth between the 2 radios. 73 Todd 
WD4NGG