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Author Topic: BELT & SUSPENDERS PROTECTION : JOHNSON VALIANT PANEL METER  (Read 202 times)

N4MQ

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BELT & SUSPENDERS PROTECTION : JOHNSON VALIANT PANEL METER
« on: December 17, 2022, 07:25:29 AM »


Belt and suspenders:  Meter protection on VALIANT transmitter

While reconditioning my Valiant 1 transmitter a HIGH VOLTAGE short circuit fried my meter due to high overcurrent.  Here I will describe my approach to prevent this from ever occurring again.

The meter was in the plate position, while calibrating the vfo for 160 meters.  With a pronounced bang my attention was drawn to the meter reading full scale. Powering off the rig immediately I noticed the meter was still full scale and more. 

No fuses were blown and an inspection above and below the chassis found that the 0.2 ohm shunt resistor grounding the transformer center tap was damaged.  The metal film resistor had 80% of its exterior evaporate exposing the body material.

I replaced the 0.2 ohm resistor with a 10 OHM ½ watt resistor and observed the underside of the chassis when I applied power again.  As expected there was a pop and the 10 ohmer split in half, but I saw in the corner a small twinkle of light by the rear jumper plug.  There was an insulation breakdown in the socket (inside the chassis rear apron ) which initiated the hard short of the plate high voltage supply.

Here is the problem!  In the MODULATOR or PLATE meter position, the front panel meter is connected across a shunt resistor in the main current path.  If the shunt resistor opens OR is blown up by a short circuit fault, ALL of the current will try to pass thru the meter damaging it instantly.  This can be prevented in with the simple addition of one component.

In testing I determined that the meter is a 4 milliampere movement, 20 ohm winding with 0.080 volts required for a full scale reading.  To prevent meter damage I added a SCHOTTKY DIODE in shunt across the meter terminals. The diode cathode is connected to the NEGATIVE terminal of the meter. 

Should another high current short occure, the excess current will be carried by the diode limiting the voltage applied to the meter. My testing showed the diode used had a conduction voltage beginning at 0.084 volts, sorta of close to the meters full scale voltage.  Higher currents result in a greater conducting voltage, however the winding has a much better probably of surviving the fault condition.  The diode selected was one of my “solar diodes” used in a 2 axis 400 watt solar panel I built.  The designation of the diode is : 20SQ045 available on Ebay.

Not all diodes are the same, in a sample of 5 SCHOTTKY diodes in one package I found the forward test voltage on my Fluke “diode”function ranged from 0.084 volts to 0.127 volts.  Typical silicon diodes are useless as the cut-in voltage is above 0.500 volts and can range up to 0.700 volts!

In conclusion I am in need of a Valiant 1 meter for my other valiant that is awaiting restoration in the “back” workbench.  If you have one please email me thru my qrz email, thank you. Enjoy W  ;D  :) D Y  Christmas colors............

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W3SLK

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Re: BELT & SUSPENDERS PROTECTION : JOHNSON VALIANT PANEL METER
« Reply #1 on: December 18, 2022, 06:27:40 AM »

Woody, I had a similar instance with my Viking I. Whenever I would switch to the 'Plate' position, I would blow a fuse. When I did it while it was out of the box, I noticed some arcing going on inside the rectifier. Come to find out that over the years, the rubber grommets that protected the posts on the meter deteriorated and it was shorting out over to the 'can cover' of the meter. Also, check that spaghetti around the any and all Johnson transmitters. They are notorious for breaking down and doing the same thing!
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