[R-390] Intermittent Calibrator Signal !

[David Wise]

As Roger sagely advises, divide and conquer.  If your problem is the BFO, this message I sent last November may be of interest:

Some years ago my BFO gradually weakened and died.  Replacing the tube did not help.

The tube drives the coil as a cathode follower.  The cathode follower is low impedance but not zero.  I believe the design is marginal, with very low coil impedance at the driving point; a good 6BA6 can just manage it.  Q deteriorated, and eventually the tube couldn't maintain oscillation.

I improved the tube's driving ability by adding an emitter follower stage.  There's a sweet spot where the DC and AC requirements coincide, so you can do it with just two resistors plus power source.

Disconnect pin 7 from coil;
Pin 7 to 2N2222 Base and 100 ohms to coil;
Emitter to 22 ohms to coil;
Rectify and filter 6.3VAC to power the Collector.

BFO comes on strong and clean every time.  Good tube, weak tube, doesn't matter.  I call it the Bionic BFO.  :)

HTH,
Dave Wise
________________________________________
From: R-390 <r-390-bounces at mailman.qth.net> on behalf of Roger Ruszkowski <flowertime01 at wmconnect.com>
Sent: Wednesday, February 22, 2017 6:46 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Intermittent Calibrator Signal !

Rick,
You ask,

I’m having an issue with a R-390A I'm working on - TheBFO/ calibrator signal is random - sometimes it's there, other times it's not andsometime it's there and then just fades away. Any suggestions on the best place to start digging ??
Thanks ! Rick Popovich


You have two different oscillators in operation here.
Which one is intermittent.

Tune up some CW with the BFO.
IF it stable or does the receiver drop the CW as the BFO fails.
If CW is good then fix the calibrator oscillator.
If not then fix the BFO oscillator.

First pass these days is to deoxit all the tube sockets in the suspect circuit.
 The problem is looking like The BFO switch and function switch wafers come to mind
as oxide problems. Last but not least every little ground lug is starting to give
intermittent operations.

None of it is serious. But yes it may be time to just shot gun all the hardware
connections in the IF or Oscillator section of the RF deck.

Your looking for fried out of spec resistors.
A tube died
It almost killed a resistor before the tube was changed.
No one has looked at the bottom of these decks in a long time.
The receiver worked for 30 seconds after the tube was changed and the
receiver declared fixed.

Regular PM may have gotten a tube pulled and left marginal charred parts behind
that have now finally gone over the edge.

Eye ball Eye Ball Eye Ball finds most problems.

Good luck and
Keep with it. The receiver can be fixed in a couple hours of bench time.

Roger Ruszkowski  AI4NI KC6TRU old timer now and 33C4H in the past.








-----Original Message-----
From: Rick Roxx via R-390 <r-390 at mailman.qth.net>
To: r-390 <r-390 at mailman.qth.net>
Sent: Wed, Feb 22, 2017 7:15 pm
Subject: [R-390] Intermittent Calibrator Signal !

I’m having an issue with a R-390A I'm working on - TheBFO/ calibrator signal is random - sometimes it's there, other times it's not andsometime it's there and then just fades away. Any suggestions on the best place to start digging ??  Thanks ! Rick Popovich______________________________________________________________R-390 mailing listHome: http://mailman.qth.net/mailman/listinfo/r-390Help: http://mailman.qth.net/mmfaq.htmPost: mailto:R-390 at mailman.qth.netThis list hosted by: http://www.qsl.netPlease help support this email list: http://www.qsl.net/donate.html
______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:R-390 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html