[R-390] Got my R-390A working great.

[David Olean]

Well the first thing I did was study the manual and see how the circuit 
worked. There are two stages involved, the 4th IF and the AGC  tube, 
V-506A, with the carrier meter between them. I did some resistance 
checks around there. I first thought that maybe the cathode resistor on 
V506 was open as that would route current thru the meter (backwards) and 
to ground on the other side of the meter. I checked and the V-506A 
cathode resistance was about 28 ohms which is correct. Next I checked 
with an ohm meter on the other side of the meter at the 4th IF. I saw 
the 680 ohm resistor, but when I checked up at the cathode terminal I 
saw about 3 ohms to ground, which is not right. The only thing it could 
be was a bad cap at that point. I had first swapped out tubes for both 
V-506A and V-504, the 4th IF 6AK6. New tubes did not help, so I was down 
to a bad 0,1 MFD cap, C-528. I took it out and it was shorted.  A new 
cap fixed it right up.

I will try some tricks to loosen the slugs. I was worried I would break 
them. It was hard to do a good alignment with them so tight. I need to 
spend more time and check the overall RF gain settings vs ultimate 
sensitivity. I'll provide some more results as I do them as well as what 
I see on 7 MHz. My oscillator is weak on that band also.

73

Dave K1WHS

On 12/1/2019 1:55 AM, Steve Toth wrote:
> Dave
> Congratulations on getting the R390A up and running!
>
> Personally i would like to know how you found the bad bypass cap.  I 
> have a "blue striper" R390A sitting in the closet that I have to 
> eventually get to.
>
> And, IIRC, you can use small amounts of talcum powder to lube the 
> tuning slugs. Check the R390A info page and Wei Lee's "Pearls" to 
> verify that - I'm reaching way back in the gray cells here.
>
> P.S.  I've always been tempted by the R392 - they are so funky they 
> are absolutely cool!  But then I would have to buy an old military 
> truck/command car/jeep (personally I like old Dodge Power Wagons) to 
> put it in.  I have no problem going down that rabbit hole, but the 
> wife..............;-).
>
>
> - Steve  W7SJT
>
> "Always look for a positive solution then Improvise, Adapt, and Overcome"
>
> - adaptation of the U.S. Marine Corps motto
>
>
> "Every day is an opportunity to challenge the achievement of your dreams"
>
>
> "Success is not final, failure is not fatal.  It is the courage to 
> continue that counts."
>
> - Winston Churchill
>
>
>
>
>
>
> On Saturday, November 30, 2019, 06:43:03 AM MST, David Olean 
> <k1whs at metrocast.net> wrote:
>
>
> Hello BA enthusiasts
>
> I have been slowly working on getting an R-390A running.  I was side
> tracked for a short time with some carpentry, but had everything working
> except the carrier meter. The meter was pegged to the left at all times.
> Funny thing because when I first started work on the R-390A, that meter
> worked. (?)  Well I finally got some time to work on it again, and found
> the problem with the meter,  a bad bypass cap in the cathode circuit of
> the 4th IF amplifier. I did not suspect it as this was a 1967 EAC
> contract unit and it had modern poly caps in it. Of course I assumed
> those new looking parts were fine and so I looked everywhere else at
> first.   The shorted cap looked like a 3 ohm resistor.
>
> After getting everything running, I decided to tweak up the IF
> transformers. Just about every slug was badly stuck. I managed to get
> them sorta loose, but there must be something that can be put on them to
> make them turn more freely.  Any ideas? Tuning the 455 kHz IF strip
> improved things a bit.  Going on the higher bands, at 26.5 MHz, I was
> seeing -125 dBm at 1 kHz BW, and -123 dBm at 2 kHz bandwidth for a 10 dB
> S/N ratio. This is better than 0.3 uV.
>
> I listened on the BC band and with an 8 kHz filter, that carrier meter
> was always up near 100! It hardly ever moved. I had a good beverage
> antenna on it and could not believe the signal strength. So now I have
> an R-1051B, an R-390 and a 390A finished. The next step is to work on a
> couple of R-392s. When I get those running I have to choose which one I
> will keep as I don't have much room for all of them. What a quandary! I
> am leaning towards keeping the R-390 from the first Collins contract.
>
> Dave K1WHS
>
>
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