I have an R390/URR and I noticed the Dial lock has a cap over it. It looks factory. SN 344. Does anyone know why it is not there. I havent seen another like this.
Thanks

Not sure. maybe it was not needed or was a later mod.

If you do not have a manual for your R-390 you should get one. For the R-390 the manual you need is TM 11-5820-357-35.

If your R-390 has not been restored yet you should go ahead and restore it. There are four 47 ohm resistors in the audio chassis and also four 47 ohm resistors in the power supply that must be replaced. Tubes should be tested and any marginal ones replaced. Also an alignment needs to be done on the receiver as well.

One word of caution. Do not under any circumstances remove the RF assembly or try to do an alignment without the manual.

There is a way to increase the sensitivity of the receiver if you're using an unbalanced antenna. It requires swapping the plugs on the antenna relay. I think it is either swap P209 and P210 or swap P209 and P211 then short the two terminals of the balanced antenna input together. IIRC it puts 1/2 of the balanced antenna input circuit in series with the other 1/2 of the balanced input circuit which increases sensitivity of the receiver and allows all the benefits of the balanced antenna input to be used with the unbalanced input. On mine I replaced the unbalanced antenna jack with a PL-259 jack which fit the hole perfectly. That way I do not need the original hard to find antenna connector.

My antenna is a 38' length of wire from an old AA5 loop antenna strung around the inside of my 12' X 12' building connected to a balun which connects to the receiver and I get excellent reception.

Thanks for the information.

You're welcome. Really a fine receiver when restored properly.

To connect to a speaker you will need a Hammond 119DA 600 ohm to 4 or 8 ohm impedance matching transformer available at Antique Electronics Supply.

A good speaker to use with the receiver is a vintage 15" Jensen Concert Series speaker. You will need a tweeter with that speaker (vintage Magnavox 5" tweeter works well) along with a proper 6 db/octave passive crossover as the Jensen does not sound too good in the upper midrange. I used a 7.8 uF capacitor for the tweeter and a .5 mH inductor for the Jensen and it all sounds very good.

One mod that can be done if your 26Z5 rectifier tubes go bad is to substitute them with diodes. Just use four diodes and connect the banded ends of the diodes to the 47 ohm resistors with the unbanded ends of the diodes going to the plate terminals on the rectifier tube sockets.

Also for more bass response replace a couple .001 uF caps in the audio amp with a couple .1 uF caps.