[Johnson] Solid State Power Supply in a Ranger

Jerry K w5kp at direcway.com
Mon Oct 18 13:50:46 EDT 2004


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
>
>
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