[Central-Electronics] 10B and 20A BC-458 VFO issues

[George Maier]

Hi Jim:
It seems that the low voltage electrolytics always go bad before the high
voltage caps do.  

Anyway, when I finally got tired of drifting around with the "CE armored"
458, I replaced the selenium bridge with a silicon bridge, and put in a new
4700 uF filter cap.  I now run both of the tubes on DC, and the added
current drain keeps the filament volts around 12.64.  The new bridge
rectifier is mounted to the inner wall of the chassis, on the underside, in
the corner opposite the 1626, and close to where the 1629 used to be.  It
runs cool as a cuke.  I'll take a few pix this week and send them along to
anyone that's interested.  

73, George - W1LSB





-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net] On Behalf Of Jim
candela
Sent: Saturday, November 11, 2006 10:02 AM
To: central-electronics at mailman.qth.net
Subject: [Central-Electronics] 10B and 20A BC-458 VFO issues



   Hi Group,

   I did some work on two BC-458's this week after my primary transmitter
went off the air due to a VFO problem. All of these issues pre-date my
ownership of Central Electronics gear, and I'm sure some of you have went
through this stuff before. I have some questions too, so those OM's out
there with the answers please step forward.


My non 'CE armored' VFO stopped operating because the 1626 filament voltage
had dropped to ~ 8.5 volts dc for a nominal 12.6 volts. The arrangement here
is a 12vac transformer, full wave selenium rectifier, and a can type 3000
mfd filter condenser. The condenser was shot, and not filtering much. With
my DVM on AC, I read about 4 volts AC RMS ripple. I bridged that capacitor
with a 1000mfd/16v capacitor, and the voltage went up to 12 volts DC, and
the ripple decreased to 0.5v AC. I removed the wire to the old capacitor,
added a terminal lug, and made it permanent with the 1000mfd/16v capacitor.
Back in my rig, the carrier null was deeper than before. This suggests that
as that old can capacitor was dying, I had AC ripple (hum modulation) on my
VFO output.


While I had that VFO on the bench, I was using another BC-458 in 'CE armor'
to run my 20a. This VFO drifted more, and zero-beating was accompanied with
frequency jitter which seemed like tuning knob backlash. This VFO had
several modifications such as silicon diodes soldered across the selenium
rectifier, and a few other mods in the CE VFO mod bulletins. The 1626
filament voltage was about 10 volts DC with nearly 3 volts ripple. I did the
same capacitor replacement as above, and now I had 13 volts! I ended up
cutting out two silicon diodes from the FW bridge rectifier, leaving the
rectifier half silicon, and half selenium. This gave me 12.4 volts DC with
about 0.5v AC ripple. The filament transformer on this unit was only putting
out 11vac with 118 vac line voltage. I think this is a little low, but with
the two silicon diodes I got the 1626 DC voltage about right.


I also noticed on the 'CE armored' VFO that there was a copper braid
attached to the VFO tuning capacitor between the case and rotor. The solder
job to the aluminum rotor shaft had become un-done. Anyone know how to
solder to aluminum?? As a temporary fix, I attached the wire secured by a
small tie-wrap. The zero beating problem where the frequency control was
erratic was now gone. Is this 'strap' fix in any CE VFO bulletins?? The VFO
I started with above without the 'CE armor' also had a strap on the tuning
capacitor, and VFO zero-beating worked beautifully.

Several years ago with a 3rd VFO I have, I tested several 1626 tubes where I
varied the filament voltage down from 12 to 10 volts DC using a variac on
the filament AC power to check for frequency shift. It was surprising that
some 1626 tubes showed almost no drift, and others drifted several Khz. I
figured that the ones that drifted a lot were low on reserve emission, and
were mostly used up.

I hope to start a little dialog on these BC-458 issues. Maybe my experience
can help someone, and maybe somebody out there can clear up some of my
questions, or add new experiences.

Regards,
Jim
WD5JKO
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