[R-390] R390 IF deck 70j-3/Z502 BFO interesting issue-

[Renee K6FSB]

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<n4buq at 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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