[Central-Electronics] RE: Central-Electronics Digest, Vol 46, Issue 1

[Steve Hobensack]

I have found these sudden frequency shifts can be caused by a bad silver mica cap. Capacitor checkers usually have an adjustable hi-pot tester hooked to an eye tube. I would check the capacitors that effect frequency change. Also, I once made the mistake of using a vfo that was plugged to the same 120 volt circuit as the MR Coffee coffeemaker. Thus, it might help to voltage regulate the filament current. Before doing that, try a Sola constant voltage 120 volt ac power supply. Low wattage units are cheap on Ebay.  I drive my 453-vfo with an outboard Collins pto canister. It will phase-lock to the 1626 oscillator. It is very stable but a bear to qsy. I run my CE-10B on 60 meters. Slight drift on that band is against the law.  
..73..Steve..N8YE


> From: central-electronics-request at mailman.qth.net
> Subject: Central-Electronics Digest, Vol 46, Issue 1
> To: central-electronics at mailman.qth.net
> Date: Mon, 31 Dec 2007 04:00:58 -0500
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> Today's Topics:
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> 1. Lake Shore Band Hopper update (Jim Candela)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 30 Dec 2007 18:47:28 -0600
> From: "Jim Candela" 
> Subject: [Central-Electronics] Lake Shore Band Hopper update
> To: 
> Message-ID: 
> Content-Type: text/plain; charset="us-ascii"
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>
>
> Hi Group,
>
> You may recall that a few months ago I posted about obtaining a Band
> Hopper VFO. After my first round of cleanup and repairs I put it into
> use with my Central Electronics 20a. At that time I was very happy with
> it, and it was by far the most stable VFO I had tried with my 20a to
> date. That was then, and over time it started miss-behaving.
>
> The troubles I see ranged from:
>
> * normal warm up drift, about 700hz upward drift from cold start
>
> * short term narrow band drift where the frequency would shift around
> 50-150 hz over several minutes and yet not go any farther than that
>
> * sudden jump of 200-300 hz from where it was set and then remain as
> stable as with short term narrow band drift
>
> * I also noticed that when listening to the beat note on a receiver and
> tuning the VFO, the frequency change was erratic.
>
>
> So far I have found and corrected four things:
>
> The OB2 regulator tube for the 6CB6 VFO was extinguished, and then it
> would flicker outside the anode sleeve. When this would happen the VR
> voltage would vary from 105 to 117 volts DC and the VFO large 200-300 HZ
> frequency shifts would occur when I saw the VR tube flicker. Replacing
> the OB2 seemed to fix this as the VR voltage was a steady 105.3 volts.
>
> The oscillator tuning capacitor ROTOR to ground connection is done with
> a forked wiper that is screwed to the side of the condenser frame. I
> removed the forked wiper, cleaned it, and then adding a little silicon
> grease lube. On the Band Hopper you can easily remove the side cover to
> get access to the mounting screw. On the CE BC-458 VFO you can't do
> that, but maybe you could drill a small 3/16" hole to get access with a
> small screw driver. Anyway cleaning the forked wiper eliminated the
> erratic tuning when trying to zero beat.
>
> I then put the VFO on Variac power and when varying the AC line voltage
> from 110-125 volts the frequency would slowly shift 250 hz. I tried
> another 6CB6 and the amount was 200 HZ. I again tried another 6CB6 and
> the shift was reduced to 50 HZ. OK, good enough. I reduced my drift
> from 17 HZ per volt to less than 4 HZ per volt. I have found similar
> variation with tube to tube swaps with the CE BC-458 oscillator tube,
> the 1626.
>
> I noticed that if I wiggled the 6CB6 the frequency would move around and
> jump a little. I used a scribe and 'tightened' the tube socket female
> pins, and scuffed up the 6CB6 tube pins with steel wool. After doing
> this work and then moving the tube resulted in less frequency shift, and
> the erratic jumps were gone.
>
> At this point I tested the VFO (no case) for stability. The dang thing
> after about 10 minutes stayed nearly dead zero beat to my ICOM R-75
> receiver. I was tuned for about a 5 HZ beat note. Over 30 minutes later,
> no change. This was great I was thinking..
>
> So I put it in the case, hooked up the CE 20A, and let it warm up. The
> zero beating was so much nicer now. Then I noticed the frequency vary
> around like before in the 150 HZ window. About 30 minutes later it
> shifted up 300 HZ suddenly and then was very stable until I turned it
> off.
>
> I need to go through this again. Does anyone have any ideas other than
> replacing components? My next move was going to replace the 6CB6 grid
> leak resistor and make dam sure the OB2 VR tube is still behaving.
>
> 73,
> Jim Candela
> WD5JKO
>
>
>
>
>
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> End of Central-Electronics Digest, Vol 46, Issue 1
> **************************************************

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