[R-390] For your consideration, Imperial Electronics Inc. Receiver restart today

[Jim Whartenby]

John
You are being cautious so that is a good thing.  You didn't mention that you had tested the tubes to be sure that poor performance of the R-390A is not due to weak tubes.

That EIB-895 mod you mention has always bothered me.  I have the gut feeling that it is based on bad data.

To start, the 26Z5 tube manual data indicates that the typical voltage drop is approximately 35 volts at 200 mA per plate.  So with the two plates in parallel, the nominal tube voltage drop should be around 18 volts since the internal tube resistance are now halved.  I don't see why an 18 volt increase in B+ would be an issue.  A line voltage increase of about 7 volts will do the same thing.

As for the 6AK6 having a problem with a high B+, I think that there is something else going on.  The tube manual data is misquoted saying that 180 volts is a maximum B+ value.  But it is clearly stated in the tube manual that this is a typical operating voltage.  The maximum operating plate and screen grid voltage is given as 300 vdc.  The R-390A maintenance manual indicates that the screen grid of the 5AK6 is nominal with 215 volts applied.  So I don't think that this is the real problem.

Field Change 1 addresses an oscillation problem with the 6AK6.  It changes how the suppressor grid is wired for V603.  It directly connects the suppressor grid to the cathode instead of to ground.  One assumes that this breaks the feedback path so as to prevent the UHF oscillation.  Why V604, another 6AK6, does not get the same treatment is a mystery.

I would think that better B+ filtering at RF would also break any feedback paths.  Paralleling C603B with a 0.01uF ceramic bypass capacitor would be a smart thing to do.  Might as well do the same for C603A and C603C as well.  Electrolytic capacitors are known to act more as inductors then capacitors at a few megahertz so any additional filtering at RF frequencies would help keep things stable.
Comments?
Jim
Logic is a systematic method of coming to the wrong conclusion with confidence.  Murphy 

    On Saturday, July 18, 2026 at 12:58:41 AM CDT, jkharvie via R-390 <r-390 at mailman.qth.net> wrote:   

 Greetings After about 10 on and off years, I finally have one of my R390A's to the point where I plugged it in to check it out.  These receivers weigh more than I remember when they are all assembled!
Following the various manuals along, very helpful to have the multiple editions of all the terrific community papers, schematics, and manuals. I found that a number of the resistors I replaced in 1999 needed to be replaced again.
As I decided that you only live once, this meant I needed to make this my curtain call receiver rebuild.  RF deck, IF deck and the rest of the works.

I slowly powered the receiver up last night, 3 meters active monitoring, enjoyed looking at the tubes slowly turning on with the lights down low to see if all started at about the same time, and at a similar brightness.  Good so far.  Good smell, no sparks, no arcs, no smoke, no fire. Receiver started and it was nice.

Nominal voltages and currents on start up using the variac.  B+ at about 147.5 v.

Major mods other than new resistors and caps include a move of this unit off of a dependency on tube diodes onto some solid state diodes that plugged into the tube sockets, and I did add per the EIB-895 the new power resistor, value of 220 ohm, used a 10W tucked under the blank-off plate in the AF module in an aluminum stand-off I made, held down into place with one of the existing blank-off plate machine screws.
I spent a lot of time and effort with the IF deck, one in and one out and this deck now has zero carbon composition resistors.  

So...the receiver is now back together, mechanically aligned, cams are in good placement and tracking, RF switch appears to be aligned, crystal deck appears to be aligned, 10-turn stops in place, all smooth and continuous and proper rotation etc.  

Hooked up speaker to the unit via a small xfmr that was 120v in 12v out as a good enough audio impedance match for the 600 ohm to 8 ohm speaker

Electrically the following demonstrate functions that resemble alive and maybe well, I picked up a Direct Digital Synthesizer electronic signal generator as a source of frequency, have a 4-channel Tek TAS-475 scope that I had to rebuild the switching power supply to get it going, but it is now operational, as is my trusted Triplet 630 meter and my newer DMM's.

In general the receiver appears to be on the way to be functional...but needs some work...Following is OK:
Power SupplyLampsElectrical Bias so far as measured, good
Tube v501 in IF deck has measured good electrical characteristics using DMM on Plate of 183.1, Screen 97.3, Cathode 5.4, Grid ~0.1 v DC
Mechanical Band pass filters check out
Detector circuit lives
AGC - vice MGC - TBD
Wide narrow filter - checks out
Meters - both Line and Carrier are operational
BFO - works
RF gain - appears to be working (some stage lack of gain concerns)
Local Audio Gain - appears to be working


The key issues to resolve1)  Calibration onboard not yet working, nothing here yet
2)  Gain in IF and RF does not appear to be where it needs to be

I intend to identify and fix why these are not working
Using E211 on the RF deck, it now takes more signal injection than should be needed to hear on the speakerWhen I plugged a 3 foot long wire into E211, interesting that the receiver received many spurious emissions from the signal generator, these were picked up and amplified, this was encouraging.

I ran a couple of tests measuring DC bias on diode detector with signal input+BFO at levels of:Input       Diode Load Measurement 
0.002v =  -0.509 volts
0.003v =  -0.530 volts0.005v =  -0.582 volts0.006v =  -0.632 voltsThe outcome shows a gradual increase in negative bias but not at the gain sensitivity level indicated in the manual, i.e. -7.0 volts with 150mv of 455kHz applied to the IF Deck

With the BFO on and input of 455kHz at 0.04 volts into E211 measured -12.5DC v at the diode output (rear terminal strip)
Does the group have knowledge on the specific response per stage of the IF and the RF sections for applied signal gain at each tube?

What did I miss?  As I read the manuals and look for specifics to resolve.

thanks in advance for the words of wisdom.

JohnN3JKE



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