EF Johnson Invader Mods

[WD4KCM]

I just completed the ugly VFO mods (thanks Frank -KB3AHE) on an EF Johnson Invader and after the 24-hour burn in period - I find the actual frequency about 400 KHZ higher than the slide rule dial. I’ve tried to pull the freq with L8 and C52 but have ran out of adjustment. I used a high quality Teflon coax cable for the VFO mods and I kept the two runs as short as possible but I am wondering about the total capacitance involved with the cables. Has anyone run into this one while performing the mod and found a workable solution?

 73's
 Jim

[N3DRB The Derb]

just sub a wider range varaible for c52? may not be that simple. Frank knows evyhing here is to know about hat radio. 

[WD4KCM]

More specifically - the VFO is 400 to 500 KHZ low in frequency - making the resultant transmit frequency 400 to 500 KHZ higher than the slide rule dial. I need to get the VFO back in the 4.9 to 5.5 MHZ range for proper operation.  Any suggestions are welcomed. 

73's

Jim

[Steve - WB3HUZ]

Is there an alignment procedure in the manual? Would this pull it back in? Just a guess....

[w4bfs]

was that teflon coax very small stuff, if so it may have more pF/ ft capacity than a larger type ... this would tend to lower resonant freq.... 73 ...John

[WD4KCM]

 I had a chance to dig into this deeper and I found that L8 was almost at the bottom of the coil form. A re-adjustment of L8 has the VFO back in the proper frequency range. If I pull the freq dead on at one end - I am still off at the opposite end of the band. L8 adjustments works agains the results of C52 adjustment.  I am not having a lot of success with getting the slide rule dial to track with the actual freq. I am 20 KHz off at the band edged – Interesting high on the high end – and 18 KHz low on the low end of the band. Very close (within 1 KHz) at the middle of the band. Is this "normal" operation for this vintage of equipment or should I keep trying to get it closer?

73's

Jim 

[WQ9E]

Jim,

If I am reading your post correctly you are off around 20 khz low at one end and 18 khz high at the other, is that correct?  None of my Johnson gear could be confused with S line or Drake dial tracking but it really should do better than this amount of calibration error.  The worst case errors on any of my Johnson gear is within 5 khz after calibration although over time it can certainly get worse.

As to whether this is good enough that is entirely up to you the operator.  Since you will probably be using a well calibrated receiver for your frequency readout the calibration error on the Invader could be considered just a nuisance.  But if you still want to try to improve the calibration here are a couple of hints:

1.  Any chance that the VFO capacitor rotation is not correctly set with respect to the dial indicator?  The Invader is one of the few pieces of Johnson gear I have never used but check your manual to see if it gives the correct dial setting for full mesh (i.e. fully meshed at the top indicated frequency or fully meshed at the dial stop, etc.)  If this setup is off then it will be impossible to get the intended calibration accuracy across the range.

2.  It sounds like your unit was already highly adjusted by a previous owner since the coil setting was so far off.  As a new starting point, set C-52 at the middle of its range and then adjust L-8 for zero beat at 4.9 mhz. (I am assuming that the Invader uses a 4.9 to 5.5 megahertz VFO).  Then iterate from there to see if the new solution provides better linearity.  Since L-8 and C-52 do both impact frequency if the starting point for one or the other is too far off you will not be able to converge to the proper setting.

3.  Any chance someone has bent any of the plates on the VFO tuning cap?  Since the adjustment of L-8 was so far off it is possible someone who didn't understand the VFO alignment created some issues with their lack of understanding.  The tuning caps I am familiar with in the various Johnson VFO's have solid plates that are not meant to be bent but it doesn't guarantee someone didn't try this anyway.

Good luck getting it to your satisfaction.

[WD4KCM]

Thanks for the comments - You are correct on the frequency error. I'll confirm the dial to VFO cap position and go back thru the alignment process to see if I can get it closer to the slide rule dial. All the VFO caps are in good shape - no bending - so I should have a chance to make the unit track better.  The VFO is 4.9 to 5.5 MHz -with the higher VFO frequencies resulting in the lower operation frequency.  Any thoughts as to what function C55 plays in the alignment procedure.

[WQ9E]

Jim,

C-53,54, and 55 form the temperature compensation network for the VFO to reduce the amount of frequency change during warm-up.  As such they are not used to set the basic tracking of the VFO.

At a "normal" ambient temperature changing the setting of C-55 (which is a differential capacitor) should have almost no effect on frequency as both C-53 and 54 would have the same capacitance but as the temperature goes up C-54 (an N-750 temperature coefficient unit) has its capacitance affected by temperature so properly adjusted the change in C-54 offsets the frequency change due to heating of other components.  Typically you would start with the differential capacitor at its middle setting and then adjust as necessary for proper temperature compensation and if you haven't changed the setting I would leave it alone because optimizing this setting can be tedious (especially if you tend towards being a perfectionist!)

73, Rodger WQ9E