Johnson Ranger question

[W6MQI]

I have two caps coming off of SW3B pin 6 both 150 pf in series the schematic shows them going to ground. The caps in my unit go to a ground tab on the bottom of L10 (Under side of chassis) which is insulated from ground with an insulated washer. So which is it grounded or not grounded?

 Thanks Dave

[wa3dsp]

Dave,

  See the links below. I made this W8JI mod when I completely overhauled my Ranger. Yes it is a floating ground but making the changes eliminates that. It is hard to say why Johnson did it that way but I have had no problems after making the change. I also had no apparent problems before the change but it made sense so I did it.

The mod -
http://www.w8ji.com/johnson_vfo_chirp_jump.htm

My restoration -
http://www.crompton.com/hamradio/JohnsonRanger/RangerRestoration.html

[AB2EZ]

Dave

In a stock Ranger, the bottom of L10 is directly connected to ground with a 0.002uF = 2000pF capacitor... and via a sequence of TVI chokes, additional capacitors to ground. Therefore, in a stock Ranger, it won't matter whether the pair of 150pf (net capacitance 75pF) capacitors is connected directly to ground or to the bottom of L10.

There are at least two reasons why you might want these two capacitors in series connected directly to ground.... but (as discussed next) they don't make a significant difference,.

1. In a stock Ranger, there is (in my opinion) too much bypass capacitance across the B+... and this limits the high frequency modulation response of the Ranger. Some of the bypass capacitors should be removed and others reduced to 0.001uF... so that the total capacitance across the modulated B+ (i.e. the sum of: C47, C48, C83, C66, C69, and 0.25 x C67) is around 0.002uF (instead of the stock total of around 0.0105uF = 10,500 pF). This means that the total capacitance from the bottom of L10 to ground will be significantly less than it is now (although still large compared to 75pF). For example, if you remove C48, C83, C66 and C67... leaving only C47 (0.002uF = 2000pF) directly RF- bypassing the bottom of L10 to ground, then it still won't make a significant difference whether the pair of 150pF capacitors in series goes directly to ground or to the bottom of L10.

2. The voltage at the bottom of L10 is the modulated B+... and the pair of 150pF capacitors will let some significant percentage of the modulation of the B+ go to the output of the transmitter... unless there is a DC/audio frequency path to ground across the output of the transmitter. However, the output DC blocking capacitor (C37=0.002uF) will let a much larger percentage of the modulation of the B+ through to the output of the transmitter... unless there is a DC/audio frequency path to ground across the output of the transmitter. Therefore, again, it won't make a significant difference whether the pair of 150pF capacitors in series goes directly to ground or to the bottom of L10.

Stu