The problem appears to be related to the caps inside the BFO tank, most
likely one or both of the 50pF caps. I've never seen inside a 390 BFO
assembly, so I don't know what type of caps those are but I'm looking at the
schematic right now.
Are they the unsealed mica type...? Is there an online pic of the assembly
internally..?
73...Jordan VE6ZT
----- Original Message -----
From: "Renee K6FSB" <k6fs...@gmail.com>
To: "Jacques Fortin" <jacque...@videotron.ca>; "R390A List"
<R-390@mailman.qth.net>
Sent: Tuesday, August 19, 2025 11:13 AM
Subject: Re: [R-390] R390 IF deck 70j-3/Z502 BFO interesting issue-
follow-up saga finally..Success.......original BFO issue will not resonate
at or near 455kc, tube not the issue. this is 70J-3, most likely should
have been a 70J-1.
I do believe this is not the original BFO as screws are 4-40 w/6-32 lock
washers ( what?) on the bracket to chassis, shaft is quite short, plus all
the poor reworked connections.
A. took the BFO apart, re soldered connections and tested caps - all
withing reasonable spec, generally on low side, hmmm
fix poor connections, reassemble test no change. dang.. A friend gave me
an unknown ARVIN unit...hmmm similar issue 453- 478.
Soooo, lets scrap all the external stuff, start over. then iff no go redo
the internals and add some inductance.
I did keep the 5 pf dogbone C533 as it was fine at 300V all else is now
new or NOS. however all bypass Vit Q caps are good and reused(note the gnd
is BFO not chassis like the A, something I may change later).
I rewired per a R390A with C533 and R545 (150K, I had a CC 1% measured
155K) from V508 pin 1 to the Gnd lug on chassis not the gnd on the BFO.
I did this for ease of BFO change if needed.
so R546,7,8 all measured high just at the 20% mark replaced them with
Carbon film.
Results-
*now tunes from 451.17 to 481.98 khz...........Hooray! *not sure of
original spec but think should be closer to ~448khz was the bottom. may be
due to inductance change due to time?*
*I need to add an extension to the shaft as the needed range is about a
turn from the bottom, (short shaft BFO to begin) so can use the proper
clips (absent) on the control shaft and bellows. next to machine a small
shaft extension.
the only thing I can think is C532 (100pf meas 103pf) and R545 (150k- meas
174K) combo were at fault even though tested fine and original voltages
were reasonable.
I am leaning towards the postage stamp mica internal fault??.. I have
found some bad ones in the past.
now for the 6BN6 PD in the 3TF7 hole.....
Renée, K6FSB
On 8/1/25 8:15 PM, Renee K6FSB wrote:
Merci Jacques.
the BFO going wonky is probably why this deck had an rg58 going external
with a series 50 pf to the detector. ( nice extra holes!)
for grins I added a 50pf externally and it moved slightly (3 khz) so
maybe internal caps are at issue.must be more than 100 pf
So no one has encountered this previously? strange! I always get the odd
ducks...that is the way it was at work...we cannot fix it, give it to
Renée!
Alright, next thing is to re-open and actually check the caps. solder
joints looked good...but......looks taint everything.
simple Hartley, caps and variable inductance.
will do so next week....I am out of town for performances.....
I was just hoping someone had encountered this previously.....better to
ask and not reinvent the wheel.
more to come next week
Renée, K6FSB
On 8/1/25 7:35 PM, Jacques Fortin wrote:
Like one of the 50pf is open…
Jacques, VE2JFE in Montréal (more or less).
Le 1 août 2025 à 18:59, Barry<n4...@knology.net> a écrit :
Hi Renée,
I'm not sure I follow this. Are you saying it will _only_ tune from 463
to 479 kc?
Barry - N4BUQ
R390 70j-3 BFO has an interesting issue- one I have not seen before.
Z502
tunes from 463 to 479 kc, stable and good signal per spectrum analyzer.
checked all external parts- all withing spec, opened unit, all parts
present ( someone has opened it the past seal was broken). "Things"
look ok.
moving core appears to be intact does not appear to have been tampered.
Before I go any further anyone with thoughts?
- any known loss of permeability of the core? possibly due to temp?
caps internally go "bad" 1600pf, 50pf NPO, 50pf N600.......there is not
much in there as per schematics...simple hartely osc.
the core does not go completely to the end of the form, maybe couple mm
from end. but does cover the coil length.
How do get the core out for inspection? I saw where someone else had
epoxied one back together.....but no notes on full disassembly.
So....If it is non repairable...anyone have a 70j-1 or a -3 available?
Renée, K6FSB
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