[Johnson] Solid State Power Supply in a Ranger
Sherrill Watkins
Sherrill.Watkins at dgs.virginia.gov
Mon Oct 18 14:13:45 EDT 2004
Dear Jerry K: Excellent comments and right on target. There was also an
excellent article in ELECTRIC RADIO MAGAZINE about this exact same problem a
number of years ago; I don't remember which issue but a search of the Index
may find it. Question: when making this resistor replacement, would a wire
wound 10 watt resistor be a good choice? How about installing a fuse in
series with the resistor? - 73- Sherrill W. k4own.
-----Original Message-----
From: Jerry K [mailto:w5kp at direcway.com]
Sent: Monday, October 18, 2004 1:51 PM
To: Johnson at Mailman. Qth. Net
Subject: RE: [Johnson] Solid State Power Supply in a Ranger
The real reason to do something with R3, the 18K VFO dropping resistor, seems
to have been sort of lost over time in the telling of the VFO meltdown story.
Actually, R3 won't frying the boards when it stays at 18K like it started
out. But it's a carbon resistor that was marginally rated for what it has to
do, wattage wise. Since it runs hot all the time, even when new, it usually
steadily decreases in value over time. I have found two or three that were
down to 8-10K, although I was lucky and no board damage had yet occurred. As
R3's resistance decreases, the voltage the VR tube has to drop steadily
increases (after all, R3 is a voltage dropping resistor), requiring it to
conduct harder to do it's job, which causes the current through R3 to
steadily increase, which causes the resistance to decrease even faster, which
again causes the VR tube to work harder, etc. etc. It's a vicious circle, and
eventually, a mini-Chernobyl will happen. The resultant circuit board damage
will be caused by an overheated and severely overworked VR tube, not by the
resistor itself. I have seen several with a 1/2" circle of circuit board
around the VR tube completely charred and burned away. The real underlying
culprit is the under-powered R3, which, if it had been a 5 watt or more to
start with, would have had a fighting chance of remaining at 18K for the long
term. Removing R3 from the VFO chassis might help some heat-wise in the VFO
cabinet, but the most important thing is to REPLACE it with a new power
resistor of 5 or 7 or more watts (I use 7.5 watt Dale wirewounds in mine).
End of problem. I haven't experienced any noticeable increase stability or
other benefit from relocating R3 under the chassis, even though I've done it
on a couple of my Rangers. The Ranger VFO appears to be quite happy running
"warm". A small muffin fan lying on top of the front left top of the cabinet,
directly over the power supply tubes, will, however, give significant
improvement in stability, especially on long key-down AM transmissions, and
will greatly extend PS tube life.
I just wanted to clarify the issue a bit if possible for new Ranger owners.
Relocation of R3 is not necessary. Replacement of it with a higher wattage
new resistor IS. End of my two cents...
73, Jerry W5KP
-----Original Message-----
From: johnson-bounces at mailman.qth.net
[mailto:johnson-bounces at mailman.qth.net]On Behalf Of Bob Sullivan
Sent: Monday, October 18, 2004 7:25 AM
To: Ross Stenberg; Tim Anderson - AG4XM; Johnson at Mailman. Qth. Net
Subject: RE: [Johnson] Solid State Power Supply in a Ranger
Ross,
I hope you have moved that dropping resistor out of the VFO enclosure! If not
I suspect you are slowing cooking the VFO boards. I always relocate that
dropping resistor to under the chassis. Some of the Ranger VFO's I have seen
are pretty bad due to the heat generated by this resistor.
73, Bob
WØYVA
My station: http://www.isquare.com/personal_pages/hardware-14.htm
At 07:19 AM 10/18/2004, Ross Stenberg wrote:
>Tim, I know the purists would object, but I have been running my Ranger
>II with SS rectifiers and the resulting higher voltages for the last
>four
years
>without any problems. I did however increase the value of the 18K
>dropping resistor supplying 0A2 regulator for the VFO and used
>electrolytic caps
with
>appropriate voltage ratings throughout.
>
> 73 Ross K9COX
>
>
> >>>I've solid stated the power supplies and beefed up the
> >>>electrolytics in
>my
>Ranger I and it's working fine but of course the B+ voltages are now
>too high. (Around 800 on the high B+ and 600 on the low.) I've been
>using a variac set at around 90 volts on the mains to drop them down to
>a safe range. Anybody have a more elegant solution to drop the B+? 73
>Tim Anderson, AG4XM
>
>
>Johnson mailing list
>Home: http://mailman.qth.net/mailman/listinfo/johnson
>Help: http://mailman.qth.net/mmfaq.html
>Post: mailto:Johnson at mailman.qth.net
Johnson mailing list
Home: http://mailman.qth.net/mailman/listinfo/johnson
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:Johnson at mailman.qth.net
Johnson mailing list
Home: http://mailman.qth.net/mailman/listinfo/johnson
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:Johnson at mailman.qth.net
More information about the Johnson
mailing list