[R-390] New Guy

[K2CBY@aol.com]

"I don't plan on firing the radio up until it is recapped."

If I were you, I'd go a little slower. 

Recapping an R-390A is not a job to be undertaken lightly -- especially in the IF subchassis where there isn't much elbow room. What's more, I've never been convinced that wholesale recapping is strictly necessary. 

The greatest danger of working on everything at once is the risk of making an error. If the receiver was working to begin with, a mistake during overhaul merely creates a troubleshooting problem. That is difficult enough when you are dealing with a "patient" whose idiosyncrasies you don't know, but that's the easy case. 

Now suppose there was a pre-existing fault that prevented the receiver from working "out of the box." Add a second error during overhaul, and you've got real trouble. Now you have two troubleshooting problems, and you've got to solve BOTH of them before the receiver utters a peep. That is a real nightmare unless you have "known good" modules on the bench to swap.

R-390As are pretty hardy beasts. In more than 20 years of owning one, the only times I have had a completely "dead box" are the result of tube failure, plus one instance of a mechanical filter with one side of the coil shorted to ground (only the 4 kHz bandwidth didn't work.) and an open selenium rectifier in the antenna relay circuit.

That's not to say that I haven't been inside the receiver on countless occasions and given it one "from the ground up" overhaul. But that was to optimize performance, not to restore life to a corpse.

I think the first order of business ought to be to see whether you have a working receiver. If so, concentrate on one module at a time. Clean it up; get it going; and be sure it works before going on to the next module. 

If confronted with an R-390A of unknown condition, I would proceed as follows:

(1)  Be sure the receiver has all its tubes in the right sockets. You don't necessarily have to test the tubes -- just be sure that there's really a 6AK6 in the V603 and V604 sockets, etc. Be particular to check that there is a 3TF7 in the RT510 socket on the IF subchassis. (If not, there should be a resistor wired between pins 2 and 7 of that socket.) (Alternatively there should be a short between pins 2 and 7 of RT-150 AND BOTH V505 and V701 should be 12BA6s not 6BA6s.)

(2)  Take a good look at the power supply and be sure that you have two 26Z5's in the sockets, OR (more commonly) that these have been replaced with silicon diodes. Be sure the diodes are properly connected. (Arrow to pin 1 and/or 6 of the tube socket, bar to pin 3 and/or 8.)

(3)  Pull both the plug-in electrolytic capacitors (C603 and C606) on the AF subchassis one at a time (so they go back in the right socket). Inspect these for deformity or physical leakage. If they look OK, measure each section with an ohmmeter. They should read at least 50k. If they are physically damaged or fail the ohmmeter test they have to be replaced before powering up. Otherwise, just stuff them back in.

(4) A word about the notorious C553 on the IF chassis. It is not especially prone to go bad but it has developed a nasty reputation because, when it is shorted, it applies B+ to the mechanical filters – killing them dead. If I were inexperienced on the R-390A I'd play the odds that mine is one of the 99.9% of all R390s in which C553 is as good as the day it was made. If I were more neurotic, it is the ONLY capacitor I would replace as a precaution before firing up the receiver. It is fairly easy to get to and should be replaced with a BRAND NEW .01uf 400 wv ceramic or mylar capacitor.

(5)  Be sure all the interconnecting power and RF cables (including the devilish little mini-BNCs) are properly connected. (Note, P218 goes to J518, P213 to J513, etc. so there's some logic to it.)

(6)  Check the fuse(s) on the rear panel. The AC line fuse F1 should be 3 amps and is present in all models. Later models also have two B+ fuses. F102 should be 1/4 ampere and F103 should be 1/8 ampere. These can be a troubleshooting aid. If F103 is blown, a component in one of the subchassis B+ circuits is drawing too much current. If F103 is OK and F102 is blown C606 is probably bad. If both F102 and F103 are OK and F101 is blown, suspect a short in the power transformer, rectifiers, chassis wiring, filament circuits or oven heaters.

(7)  There are two barrier strips on the rear panel. Be sure that jumpers are installed between the following terminals: 1-2, 3-4, 11-12. The receiver won't yield any output unless these are in place. A jumper is also normal between 14-15, but that affects the Line Output mode only. (See Figure 2-2 in the Y2K Manual.)

(8)  While you are at the rear panel, set the OVENS switch to OFF.

(9)  Inspect the AC line cord for shorts.

(10)  Ground the chassis to a reliable AC ground.

(11)  You are going to need a 600 ohm speaker or high impedance headphones. Connect the speaker between terminals 6 and 7 on the rear panel barrier strip. (Use of a low impedance speaker won't damage the receiver, but you aren't going to hear much audio.) The headphone connection has a series resistor of 6800 ohms, so don't expect to hear much of anything if you jack a set of 4 ohm "hi-fi" phones into the front panel.

(12)  Connect an antenna to one pin of J104. Ground the other pin.

(13)  Plug in.

(14)  Power on.

With the RF GAIN and LOCAL GAIN (audio) wide open, the FUNCTION switch set to MVC and the LIMITER OFF, you should hear something at this point – even if it's only shot noise. 

If not, check for B+. If that's good, check the audio subchassis.

If there is noise but no signals are heard: (1) rotate the bandswitch to be sure that you are getting a noise peak in the detent position – if not, the crystal oscillator switch indexing is off; (2) do the same with the BANDWIDTH SWITCH – if the wafers don't line up, the switch won't reliably "make"; (3) check the oscillator tubes first  V207, V401 and V701 substitute known good tubes; (4) then check the 6C4 mixers V202, V203 and V204.

A set of tube socket adapters is invaluable for troubleshooting a dead receiver. Just remember that if you are using the kind with exposed terminals, be sure they aren't shorting against an adjacent IF can.

Once you get the receiver going and gain some familiarity with it you can tackle the big projects like recapping or overhauling the gear train. But these jobs are not for the faint of heart, nor can they be accomplished in a weekend. And don't be tempted to work when you are bleary-eyed with fatigue. That's an invitation to disaster.

Finally, you should regard the mechanical filters, the switches, the crystals and the mechanical parts as IRREPLACEABLE. Be extra careful not to damage these or you are going to have to spend a lot of time and not a little money looking for replacements unless you have charitable friends with well stock junk boxes.

Miles Anderson, K2CBY