[R-390] R-390A trouble

Thu, 28 Mar 2002 20:23:32 -0500

Hi Jim,
I believe I know what the problem may be with your radio....the front end is
oscillating due to an open C227 (.047 uF)  6DC6 cathode bypass cap. The
cathode of V201 is eventually connected to the RF gain pot on the front
panel (via cathode bias line), and when that bypass cap opens the whole
wiring harness gets hot with RF and feedback can occur almost anywhere.

I had the same problem and symptoms in my 390A, only I noticed the
instability above 20 MHz. The ANT Trim control acted more like a VFO than
anything else :^(

 C227 is that little gray stud mounted cap that sits on top of the deck next
to the 6DC6, I don't think anybody EVER checks or replaces it during a
refurb. I didn't even realize it was there until I went troubleshooting!

Best thing to do is just snip the lead to it and wire in your own
replacement ceramic cap under the chassis. Try to keep the lead lengths
short, especially the ground return of the new cap. But it's not super
critical; once you examine the original wiring you'll notice that the stud
mounted cap was placed some distance away anyways. For my replacement cap I
used five small .01 uF ceramics in parallel 'cuz I had a bunch of them.

IF this doesn't work, I still agree with the other guys that it's a
regeneration  problem and to go after the other bypass caps in that stage.
There's a screen bypass, and also a plate bypass cap on the B+ side of all
the RF coils for that stage. And last, but not least, there's that C103 50
uF bathtub electrolytic that is the audio range bypass for the RF gain pot;
it has been known to cause a screech or two when it opens.

Let us know how you do,

John

----- Original Message -----
From: Jim Temple <[email protected]>
To: <[email protected]>
Sent: Wednesday, March 27, 2002 5:42 PM
Subject: [R-390] R-390A trouble

> Hi Roger and list,
>
> OK, I want to fix this receiver.  Here goes.
>
> 1.  R-390A bought as a "massacre" yellow line from Fair Radio.
> 2.  After cleaning, and replacing many of the "problem" caps (listed on
many
> of the resident web sites), I corrected a below 8mhz problem, by finding
> that the ground lug under T-207 was loose and insulated from ground by a
> shellac layer.
> 3.  The unit, at first alignment, aligned well.
> 4.  At some point, within a couple of hours of operation, the receiver
> developed a screeching/motorboat putt putt sound.
>
> Symptoms:
>
> 1.  On bands 00,01,02, and 03, the screeching/motorboat sound is not
> present.
> 2.  Above band 03, the motorboat sound will vary in amplitude and putt,
putt
> frequency with adjustment of the antenna trim control, and becoming more
> intense as band increases.
> 3.  The carrier level meter indication increases as the amplitude
increases,
> the strongest coinciding with the antenna trim peak prior to the problem
> developing.
> 4.  The AGC control will change the frequency of the putt,putt, depending
on
> the position of the control.  Fast=fast.  Slow=slow, or right on the edge
of
> putt,putting.  Medium=medium frequency.
> 5.  The AGC voltage measures about 8-9 volts during the motorboating.
> 6.  The screeching/motorboat sound disappears with removal of V201/'6DC6.
> 7.  I replaced the antenna trim assembly with a spare from a spare RF
deck,
> with no change.
> 7.  The ohm reading of the antenna trim vertical shaft is 1.5meg, and
> doesn't vary with rotation of the control.
> 8.  With the RF deck removed the antenna trim wires measure a smooth
> increase and decrease of the variable caps within the enclosure.
>
> I have inspected and examined the bottom components of the RF deck several
> times and can find no shorts or broken wires.
> I am positive the RF deck timing is correct.  I have triple checked the
> timing and am positive it is correct.  The deck did work fine for a couple
> of hours.
>
> My troubleshooting skills are lacking, so sometimes I do not approach a
> problem in a logical fassion.
>
> My approach will be to thouroughly examine the RF deck components.
Measure
> the values of components I reasonably can.  I thought the voltages in the
> Y2K manual were measured with the tube in place, but I am unsure at this
> point.
>
> There is clearly a great deal of knowledge and expertise on this list, so
I
> hope my lack of troubleshooting skills will not get in the way of a fix to
> this receiver.  I may need a little more "hand holding", detailed
coaching,
> than the average owner.
>
> Thanks for listening.  I hope I have given enough detail to get me started
> on the right road.
>
> Sincerely,
>
> Jim
> 73, KF4ICZ
>
> ----- Original Message -----
> From: "Roger L Ruszkowski" <[email protected]>
> To: <[email protected]>
> Sent: Wednesday, March 27, 2002 12:01 PM
> Subject: Re: [R-390] Fw: Alignment help.more
>
>
> >
> > Jim,
> >
> > What are you doing to an R-390/A.
> snip....
> > List your problems here and get some specific help.
> > Quit trying to solve and reinvent it all by your self.
> > It is more fun that way. I do agree.
> snip....
> > What is your real objective?
> > Do you want some tube extenders, or do you want to fix a receiver?
> >
> > Roger KC6TRU San Diego.
> >
>
>
>
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