r-390a sensitivity below 8 mhz

[ve8xj]

I bought a R-390a about a year ago and love the receiver except for the fact that below 8mhz it has little to no sensitivity . I am going to take it out of the rack here soon and play around with it an check some tubes and voltages . I have done a little research to see if other people have had similar problems . The only thing I have found to date is this from Chuck Ripples notes.

Date: Tue, 26 May 1998 09:14:26 +0000
From: "Chuck Rippel" <crippel@exis.net>
Subject: [R-390] R390A Tech Tip
I was finishing up a Collins RF deck yesterday and upon beginning the
alignment, found why the radio was sold.  The bottom bands, those below 8
mHz were down in sensitivity about 20 db.
A quick analysis showed the output of the first xtal oscillator as measured at P-
221 to be approximately 3/4 volt P-P  instead of the usual 5.2V P-P or so.
Additionally, when performing the variable IF alignment, T-207 would only
show a peak at the very bottom of the slug travel.
The problem turned out to be C-327, located across the primary of T-207 had
changed value.  Once replaced, the proper injection levels to the mixer were
restored.
-----------------------------------------------------------------------------------------------------

If anyone has any other ideas about what it could be let me know . It will be a week or so before I get around to it.
Also I am intimidated as hell by this mechanical monstrosity and have not been able to find any step by step on how to get the IF deck out to get at the underneath of it. Any source data.  on how to remove the IF deck would be appreciated  .

Thanks in  advance.

Tim Ve8xj

[w3jn]

There are tons of R-390A manuals on the net.  One of the best is the "Y2K R390A manual" or something like that.

Removing the IF deck is really easy.  Loosen the shaft couplings on the BFO and bandwidth controls, using the correct Bristo (Bristol) spline tool.  Pull the shafts out and free from the deck.  Remove the Amphenol plug at the back corner and the yellow mini coax (it's a mini BNC, so a quarter turn twist will do it).  Using a Phillips screwdriver loosen the two green screws from the top.  Pull the IF deck free, that's it.

As far as your problem below 8 MHz, could be the bandswitch Geneva mechanism is out of sync.  The front end coils are switched in a 0-1, 1-2, 2-4, 2-8, 8-16, 16-32 MHz as you step thru the 1 MHz bands.  A little Geneva mechanism does this; follow the bandswitch shaft underneath the radio towards the back and you can see this thing at work.  If it stays on the 8-16 MHz band when it should switch to the 2-8 MHz band, that would certainly cause no sensitivity.  As could a bunch of other things.

[WD8KDG]

Anyone who has a R-390/A needs to download the Y2K manual, print it, read it! The following link has all the stuff
http://www.r-390a.net/

Plus it doesn't hurt to read the R-390/A e-mail reflector:
http://mailman.qth.net/pipermail/r-390/

Another thought on the poor or no reception below 8MHz, a crystal could be the issue.

Craig,

[flintstone mop]

Worst case is the problem is under the RF deck and the 1st oscillator??
Mine was some bummed out mica caps that killed it...maybe the 1st mixer sorry been a while. But a troublesome area for the 390A, in mine anyway. Most of my low sensitivity problems are in that area. Bad tubes or components.

The other bands above 8Mhz I never touch and the sensitivity is always close to spec.

fred

[W2PFY]

I had a similar problem where the receiver would only have sensitivity on one end of each band. It turned out to be the gear slipped off the shaft and the slugs were not tuning correctly. You can see the gear laying near the S meter.

[Ed/KB1HYS]

Vitamin Q caps?  some folks swear by  'em, some swear AT them...

[WD8KDG]

As to gears slipping, read the Y2K manual: Mechanical alignment comes first, then the electrical alignment.

Vitamin Q caps are for the most part, real gud! It is those brown beauties of death, BBOD's that have seen better days.

All in all, they (R-390/A's) are a good battle mode receiver, much bang for the buck.

Craig,

[ve8xj]

All good places to start. I have downloaded the Y2k manual for the r390a ,will have to buy more paper to print it . I'm hoping that it is something simple like the capacitor. I did read somewhere else that c327 (a vitamin Q ) eats itself through a acid reaction of some sorts.
 I will check over the gear assemblies when I get it out too. Please let it not be a gear. I am not Swiss.
 It does receive better sometimes after a long warm up ,but only a little bit better. Luckily it all seems to be aligned accurately below 8mhz ,just very low sensitivity from 8mhz to 1mhz.
Question is the Bristol spline tool the same as torxs bits??

Thanks for the advice on getting the IF deck out . That doesn't sound too bad.

[WD8KDG]

Bristol (Bristo) spline wrench is not the same as a torx. Only one size bristol is needed. On the audio board, for lack of a better description, there is a wet tantalum cap. It has acid in it and will leak over time, tuf on the board! It can be replaced with a dry tantalum. Wet tantalums are pricey.

Craig,

[ve8xj]

Don't have any bristol spline wrenches ,so I just ordered them off of Ebay . So won't be able to start on this project till I get them . I'm sure there will be extra posts from me when I get to ripping the old girl apart.

Thanks for now

[ve8xj]

Well I finally got my Bristol wrenches in the mail the other day . So yesterday I got brave and took the R-390a out of the rack and put it on the bench.

It turns out to get at the capacitor that I suspected I had to take out the whole RF deck which is a daunting task the first time you do it. One problem I did find right off the bat even before I opened it up was that my AR-88lf which is mounted above the R-390a had sprung a leak from one or 2 if the bathtub caps and leaked oil all over the R-390a. Some had gone inside and had collected in the IF section around the base of the 2 5814 (12au7)'s there. The oil was in the sockets of those tubes and probably wasn't helping the situation.  So after cleaning that up I found C327 and replaced it ,though it turns out that was not problem as it had only lost 2% tolerance. Took a very good look around for any problem but could not find anything screaming out to me .

  After a bit of frustration I got it all put back together again and turned it on . STILL low sensitivity below 8mhz. Finally came across the problem . P221 which I think injects the IF for the Rf deck below 8mhz had a bad center socket on the mini BNC and was pretty oxidized on the silver exterior after a bit of tinkering I got it fitting snug and making contact and she came to life . I also decided to go thought the RF alignment procedure  below 8 mhz and found it also made a lot of difference.

  Been doing comparison tests all afternoon on the bench between a modern receiver and the R-390a and found that it is performing the way that I like now.

  Thanks all for the help and suggestions for this thread.

Next is that leaky AR-88Lf . DOES IT EVER END !!

 

[WD8KDG]

ve8xj,

Good to hear you brought the R-390/A back to life. Yes, the RF deck is a real sweatheart to remove! One one of the links that I posted, gotta really search, the methods to test S+N vs: N ratio is given. Easy to do with a VOM and sig gen.

Play with it till 20:1 is achieved & then u got a gud play toy.

Craig,

[N8ETQ]

Hey Craig,


 Just from memory I think the 390A is "Dual Conversion" Below 8mc.
Theres a rock in there (17mc in an oven?) and a mixer tube kicked
in below 8mc. It's been a while since I messed with one but I would
check the easy stuff first.  I think the osc tube for the 17 mc rock is
a 6c4, the mixer may be a 6be6. It may be a cap but it wouldnt be
my first guess.

  Make sure the dial on the top of the crystal osc agrees with
the Veeder-root and you don't have broken clamp. There is a
diagram in the manual where the "Cams" align at a specified
frequency (can't remember now but look in the book). Should be
an easy fix since the whole rest of the rig is working.

   Also inspect the "Cams" for the IF as I think it adds a stage
of that as well. Prolly a 6ba6 too.(5714?)

Keep us in "The Loop" please and GL!

73

/Dan

Well I finally got my Bristol wrenches in the mail the other day . So yesterday I got brave and took the R-390a out of the rack and put it on the bench.

It turns out to get at the capacitor that I suspected I had to take out the whole RF deck which is a daunting task the first time you do it. One problem I did find right off the bat even before I opened it up was that my AR-88lf which is mounted above the R-390a had sprung a leak from one or 2 if the bathtub caps and leaked oil all over the R-390a. Some had gone inside and had collected in the IF section around the base of the 2 5814 (12au7)'s there. The oil was in the sockets of those tubes and probably wasn't helping the situation.  So after cleaning that up I found C327 and replaced it ,though it turns out that was not problem as it had only lost 2% tolerance. Took a very good look around for any problem but could not find anything screaming out to me .

 

[ve8xj]

Well it seems that I haven't fully solved my issue yet !

I went into the shack this morning and turned on the R-390a and still very low sensitivity below 8mhz. Thinking that it could be the poor BNC plug P221, i unplugged it and then plugged it in again and it came to life once more. But be damned if I can get it to show any signs of poor conductivity. I have wiggled it back and forth and not even a crackle .  

  I am beginning to think that it could be some strange electronic voodoo.  
Could it be that by unplugging P221 waiting a second and then plugging it back that I am giving some component the jolt it needs to come back to life and work ?? A defibrillator so to speak.

P221 is the cable that feeds the 1st crystal osc to the 1st mixer. From T207 in the OSC through a resistor in parallel with a capacitor directly to the cathode of the 6c4 mixer. Both tubes check good,and I replaced the 100pf mica across T207 yesterday. Perhaps T207 needs tweeking ?  

When the sensitivity is down  it still receives the stations just poorly so I figure the 17mc xtal is working . But as soon as I unplug it and re-plug it shazzam she springs to life.

  Suggestions welcome gentlemen.

[WD8KDG]

ve8xj,

My darned Y2K manual is out in the shack. So, from memory, the crystal deck also has a 17MHz crystal. Swap it with the 17MHz crystal in the little round can on the RF Deck.

I'd have to have the page in front of me to figure out what crystals are used in the Crystal Deck below 8MHz.

Another question, have you tried ALL the bands below 8MHz??

Craig,

[ve8xj]

Hi Craig

I wrote in my last post;


"When the sensitivity is down  it still receives the stations just poorly so I figure the 17mc xtal is working . But as soon as I unplug it and re-plug it shazzam she springs to life. "

Poor writing skills. I should have specified that what I was unplugging and re-plugging was P221 not the xtal.

Yes the condition was the same on all bands below 8mhz.

 I did however since I last posted gone through the procedure to peak the 1st OSC ,which obviously needed it . She peaked a fair bit. I will leave on for a while and see if low sensitivity comes back at all. Then turn it off overnight and cold start it and see if all works as it should.

  Could it be that the 1st osc was not tuned enough to hold oscillation and simply fell out of oscillation  to the point that it was barely functioning . By unplugging P221 then re-plugging it started the oscillation again .

We will see .

[WD8KDG]

Been down this road before.  Track'en down bands with poor sensitivity is a bummer.

If it goes on the fritz again: That big round knob on the lower right is hard to turn, but check all the upper bands as well. I've seen where some of the lower bands go silent, plus one or more of the upper. If that is the case a crystal in the Crystal Deck is kaput.

There ain't nuthen in a R390/A that can't be fixed.

Craig,

[ve8xj]

Good morning

  Well a cold start of the R-390a reveled the same problem. Low sensitivity below 8mhz. I checked all the higher bands and they worked fine.

  The only solution I have found so far is that by unplugging P221 and then re-plugging it she springs to life and works VERY well. BUT ..... it does not seem to be  P221 as the connection is solid and I cannot get it to show signs of poor conductivity. P221 feeds the 1st Xtal OSc to the 1st mixer . 

  If i can I will swap out the 17mc xtal in the 1st xtal osc with one from the xtal deck and see if it is that . As well recheck the tubes ,though I did that once before but can't hurt. I have already tweeked the 1st oscillator ,done a rf alignment,and replaced c327 across t207 of the 1st xtal osc.Checked P221 very well. Hoping not to have to take the Rf deck out again but will if I have to.

 I will keep you appraised ,but as always suggestions welcome.Worse comes to worse I can put a switch on the P221 cable mounted on the back and call it the low band jump start  Just kidding but tempting.

[W4AMV]

I assume you have no O' scope. I have a similar problem and suspect that the xtal may have aged to the point that the its equivalent series resistance is just high enough to prevent starting. However, a noise impulse, as you are doing with P221 removal-and-insertion is enough. A swap with the xtal should be a good clue. 

[WD8KDG]

As a heads up; if the cause is the 17MHz crystal on/in the RF deck, the two crystals are both 17MHz, CR-36/U as per the Ye Olde Y2K manual.

I went out to the shack, looked and made changes to this post. Fair Radio sales has crystals, don't know what type. If this is the problem, call and ask. International Crystal without a doubt can make the correct type.

Craig,

[ve8xj]

Well I swapped out the xtals and does not seem to have much difference . Thanks Craig for looking up that info for me . Good to know they are still available.

Didn't have much time today to play with it though and will continue tomorrow if possible. Going to do some more  reading of the Y2K and plan my next move.

W4amv no I do not have a scope ,but it is on my wish list as I seem to be doing more and more of this stuff all the time.

[WD8KDG]

You're getting to a point where test equipment or the lack of is going to slow things down. So, for grins & giggles take a small screw driver and stick it into the hole, rear of the receiver where the shaft of the crystal deck is located. Give it a slight twist both directions. Just looking for dirty contacts at this point.

Then there is the Geneva drive adjustment. Scott Seickel's excellent "R-390/A Gear Train Rebuild" Tutorial found @
http://www.r-390a.net/faq-refs.htm
scroll down the page..........pictures show how this is adusted. The pics are better than what is in rev2 of the Y2K.

Might be a break in the cable you have been plugging and unplugging?

With a frequency counter & probe, you would know if the oscillator was working.

Craig,