Johnson Navigator meter

[WQ9E]

I am starting to go through a Navigator I picked up and like many other Navigators the meter coil is open.  Looking at the schematic the meter (10 ma movement) is either directly in series with the grid or is in parallel with a 33 ohm resistor for plate current measurement.  The 33 ohm resistor is spot on and not switched so the meter should never have full plate current flowing through it.

I found a cosmetically matching meter movement that I will transfer the scale from and I will use a resistor to adjust the sensitivity to match.  But I am not seeing any clear circuit reason why so many Navigator meters have failed.  For example two were recently listed on ebay and both had replacement meters.  I have not had a failed meter in any of my other Johnson rigs so either it is the movement (probably since it is a different type) or something I am missing in the circuit.  The schematic is on edebris for anyone interested:  http://bama.edebris.com/manuals/johnson/navigatr/  and the schematic file is NAVFIGAD

[k9jri]

I am starting to go through a Navigator I picked up and like many other Navigators the meter coil is open.  Looking at the schematic the meter (10 ma movement) is either directly in series with the grid or is in parallel with a 33 ohm resistor for plate current measurement.  The 33 ohm resistor is spot on and not switched so the meter should never have full plate current flowing through it.

I found a cosmetically matching meter movement that I will transfer the scale from and I will use a resistor to adjust the sensitivity to match.  But I am not seeing any clear circuit reason why so many Navigator meters have failed.  For example two were recently listed on ebay and both had replacement meters.  I have not had a failed meter in any of my other Johnson rigs so either it is the movement (probably since it is a different type) or something I am missing in the circuit.  The schematic is on edebris for anyone interested:  http://bama.edebris.com/manuals/johnson/navigatr/  and the schematic file is NAVFIGAD

This was a common problem on the Heathkit AT1.  In that case, and possible this case too, any incidental contact to ground with either meter terminal when the meter is in the plate current mode will run B+ from the other meter terminal through the meter and it will go "puff".  In the case of the AT1 it would often happen when someone was working on the rig with the power on.

Maybe  -  Mike

[KA2DZT]

Not sure exactly what the circuit is, but maybe it's something with the switch contacts.

Rereading your post, I figure that the 33 ohm shunt resistor is soldered right on the switch terminals.  Switch contacts can be, make before break or break before make.

Just a few thoughts to give you something to examine.

There is some reason why so many of those meters failed.  Johnson may have made the xmtr with the wrong type of switch.

Fred

[AB2EZ]

I haven't looked at the Navigator schematic, but on my Ranger I made the following change to protect the meter:

On the Ranger, there is a capacitor bypassing each side of the meter to ground. While this offers some protection for the meter... every time the plate current position is selected or deselected,  a transient will flow through the meter if the meter switch contacts to not break and make at exactly the same time... and for other reasons as well.

I removed the two existing meter bypass capacitors, and I placed one of these capacitors between the meter terminals. This provides a path for transients that bypasses the meter.

Doing this also reduces sparking of the switch contacts when the plate current position is selected and deselected

SDP

[WQ9E]

Thanks to all for their thoughts on the Navigator meter.  When I did an autopsy on the movement there were a couple of strands that were pulled out and broken.  I think this probably came from the previous owner's attempt to repair (or maybe exercising his frustration) instead of from some explosion in the coil core.   

I found a replacement movement that was a bit more sensitive but it fits the case perfectly.  I shunted it to provide a 5 mil full scale grid current reading (easy to remember as .02 X plate scale) and changed the plate shunt to match the new movement.   The plate shunt is permanently in the plate lead.  I think the switching might be the issue so I will not switch the meter when the key is closed.  I also moved the capacitors around as suggested.

The rig is working fine and hopefully the new meter will have a long life.  Later this spring I plan to repaint the case and find some matching screws for those that are missing, probably one of the jars of small screws will yield enough exact duplicates.

[N8ETQ]

  A couple "back to back" diodes in Parallel may prove usefull.
1 st thing I do. I saw an SB-200 meter on E-pay at 90 bucks.
Sheesh, you could replace the jugs for less.

/Dan

[WQ9E]

Dan,

Thanks and I did install diodes across the meter.  I do this on most rigs that cross the repair bench after losing a Hallicrafters S meter due to a circuit failure.  I was tempted to put in a slide switch with a center off position so the pointer wouldn't constantly bang around during keying but I couldn't find anything that had the right setup and would fit without looking modified.  When I take it out to paint the case later this Spring I think I will install a little meter shorting switch activated by the fake crystal cover knob.

If a real original RCA AR-88 meter showed up on ebay it would probably go sky high given all of the AR-88 receivers sold without meter movements.