Intermittent High Voltage
Arcs are common in transmitters of this era.
The Viking 2 sockets for the
5R4 rectifiers have rivets. The sockets are not spaced far enough
from the chassis. The easiest fix is to replace the mounting hardware
with insulated standoffs and longer screws to increase the path.
Corona dope (from GC) also helps in addition to this fix if used to
cover carbonized areas on the socket and rivet, but do not depend on
it alone. I heard that tool handle dip also can be used, and is
available at good hardware stores. Change the hardware to increase
the spacing. Also sleeving may be necessary on the HV filter choke
leads all the way back into the bells of the choke enclosure. This
sort of thing can cause problems in the DX100 high voltage circuitry.
Click on thumbnail to see sleeving indicated by arrows. The circles call attention to the ceramic standoffs. The VR tube dropping resistor is circled to show where it has been moved to provide better mechanical stability.
Valiant One and 2 also have
problems. Inspect carefully for any arc points in the
radio before doing any mods.
I include
photos of arcing on the terminal strip for the filaments of the 866.
Also, the wiring for the HV plate filter choke are insulated with
additional sleeving. The damaged terminal strip had to be replaced
with ceramic standoffs. The chassis shows carbonized areas due to a
long time problem with this that was not addressed by the previous
owner. This was a persistent intermittent that was hard to track down
the first time.
Click on thumbnail to enlarge.
There is
another terminal strip near the LV fuse for the 866 filaments which
can cause trouble.
Click on thumbnail to enlarge.
Sleeving may be
required on the 2.5 VAC filament wires into the LV transformer. The 4
pin sockets may arc. They are often phenolic, not ceramic. The
jumpers for the filaments between the 866 sockets can be unbundled
from the wiring harness and replaced with wire having better
insulation. Or switch to Silicon Alley rectifiers and eliminate all
2.5 Volt filament wiring if the insulation in the transformer has
already failed. It could save you having to replace a transformer.
Replace the
866s with 3B28 tubes to avoid mercury warm up issues with resulting
flash over upon application of HV power. I have commercial solid
state replacements in my rig. You may find the Silicon Alley units
work for you. I like to use a solid state relay to key up the high
voltage transformer primary, which switches the AC at zero crossing.
This prevents transients which can flash things over or damage solid
state replacements; it also prevents a scary thump when you press the
PTT switch.
The modulator
transformer terminal strip is a possible problem. Ceramic standoffs
help.
Click on thumbnail to enlarge and note the purple circle around the added standoff to correct arcing of the plate lead of the modulator.
The back panel
jumper connector J8 is a possible problem. So is anything with
modulated B+ on it. Keep this in mind if you want to go to +130%
modulation peaks and clip just the negative side to reduce splatter.
The photos
included with this small article are self explanatory, and a grim
record of the battles I have had with this problem.
All these
potential problems are the reason I did not attempt the “East
Coast” mods that raise the HV B+ to 1000 V on the modulator.
Some people have had success with this approach, but looking at the
wiring harness and all these known failure points, I sought another
solution. This one worked out OK for me. Look it over, and see if it
will work for you. A new $25 driver transformer goes a long way
toward improved performance. This one makes full modulation and
decent frequency response without exceeding ICAS tube ratings or
risking failure of old wiring to “hot rodding” the HV
power supply.
Everyone has
their own solution; just because they differ does not mean that one
or more of the approaches is wrong. I present my reasoning here, and
the alternatives to my approach; you are free to explore options. I
hope you found something useful.
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