The AM Forum

Hi all ,I just brought a R-390A back to life but I ran into a problem.
I loose signals when I go down in Freq. like 40 and 80 meters
The bands above work just fine,.
I plan to look at the tubes first....

It may be the lower frequency crystals.

6C4 tube?

Gone thru a couple R-390A receivers before and one thing I noticed, for some reason they have a high rate of bad tubes. Not shorted or open filaments but low emission.  So, although a pain it always a good idea with that receiver to go thru and check all the tubes and you may be supersized.
Don’t know why it happens, maybe because when they were used in military service they were always left on and that just wore out the tubes? Also, the last couple I did found at least one or two bad crystals in the crystal pack. The design of that radio is basically a thirty-band receiver with a crystal pack that converts all incoming signals down to the first variable IF, you have to go band by band and check each one to find out if all the crystals work. And like I said, the last two receivers I went thru had at least one bad rock.
Fair Radio was selling crystals for that radio for $7 each but with them going out of business that’s going to be an issue in the future.

A gentleman who went through one of my R-390As with a fine tooth comb found that almost all of the tubes were 'dead'. lit up, no shorts or gas but just plain wore out. He told me that these things were left on or standby continuously for decades and that's why so many have bad tubes.

The reason I mention the 6C4 is that is exactly what happened to my R-390A. Talking with Chuck Ripple, K8HU, (ex-WA4HHG), years ago he told me about that one.

A common issue is that sensitivity falls off below 8 MHz on all bands. Is that what is happening?

8 MHz band is the point where the circuitry changes.  At that and below you will be using the 17 Mc/s crystal inside the round crystal oven on the RF deck. It is assumed you will have checked the tubes already. The crystal can be weak.  There is a 17 MC/s crystal in the main crystal deck also that you can borrow to test.

You then measure the voltage at P221 per the manual. It should be about -10V.  If not, does T207 peak sharply at a point other than at an end?  If not, injection will be low along with sensitivity.  There are three causes for this. One or both mica resonating capacitors at T207 change value or go open. The third, which I find bizarre, is that T207 changes value and no longer resonates with the original capacitors.  This is most easily found with a NANO VNA to see where T207 is tuning.  Friend W6SSP advises using a series resistor at the VNA to keep from spoiling the Q of the tank circuit. T207 seems to go lower on the self resonant frequency, so the 100 pf mica gets changed to an 82 pf. This might be a good ad for the NANO VNA.  And my theory that no matter how much test equipment you have, you always need more.