Johnson Challenger Audio

[WD4DMZ]

This past week I acquired a Challenger on ebay. It is really clean, the tubes are good and it has the push to talk mod. And it works.

Other than the PTT mod all of the components appear original so the first thing I did was check out the circuitry. Then replaced the power supply electrolytic caps, bypass electrolytics and any wax capacitor.

Output is very good with 30+ watts on AM. It appears to put out almost 90watts on CW. Does that seem high?

The AM audio is clear but very high pitched. No low frequency at all. Since I have never heard a Challenger on the air this might be normal.

Anyone have any ideas on how to improve the audio?

Thanks, Rich

[Pete, WA2CWA]

see specs
Consider roughly ~ 65% efficiency - and probably higher line voltage

[WD4DMZ]

Thanks for the reply and the ad. Nice to have for the file.

This must be a slightly different version than mine as it has a 6AU6 for the osc. Mine has a 6DS5, which is similar to a 6AQ5.

Regards, Rich

[K1JJ]

Hi Rich,

Congrats on a nice find!

The info above shows a pair of 6DQ6 finals which are good for 18 watts diss each X2 = 36 watts. That's about the equivalent of a single 6146B.   It is screen modulated which is good for maybe 25% efficiency at best. So with reasonable headroom for the audio peaks, maybe  backed off to 17 watts of AM carrier.     (70 watts input X 25%)   Johnson advertises 70 watts AM input to hide this PissWeak fact.  That is about 53 watts of diss on the finals -   maybe 55 watts input is more reasonable for a pair of 6DQ6s = 14 watts output.

There are some good methods out there to modify the audio to make it BC hi-fi.  Check the DX-60 mods QIX did a few years back. I'll bet the general circuit could be easily adapted to the Challenger.  Otherwise, I think to modify the exisiting circuit to sound clean and broad-banded would be just as much work. I can't tell from the fuzzy schematic, but if it is controlled carrier, that wud have to go.

I have always liked the Challenger and Navigator Johnson rigs. I've often thought of using a Navigator with its sexy VFO -  using the QIX AM screen mod circuit.  Beautiful looking rig, like a mini Ranger.

T

Broadcash Audio for your DX-60:

http://amfone.net/Amforum/index.php?topic=26603.0

[Pete, WA2CWA]

Thanks for the reply and the ad. Nice to have for the file.

This must be a slightly different version than mine as it has a 6AU6 for the osc. Mine has a 6DS5, which is similar to a 6AQ5.

Regards, Rich

EF Johnson made two versions of the Challenger. As you point out, one had a 6AU6 oscillator and one had the 6DS% Oscillator. There are circuit and component parts list differences between the two versions. Voltage chart obviously had differences. They issued two different manuals.

[K9EID]

Many of the AM transmitters of that era had small coupling capacitors of .01, .05 which produced higher pitched, lots of mid range audio as the microphones of the day were crystal or ceramic.  Over the years audio has become more pleasant, balanced audio with a bit of low frequency energy.  Replacing some the couple caps between stages with .1, .47,.68  will larger values will produce more pleasant audio.  Bob Heil, K9EID

[WD4DMZ]

Good info all. Thanks.

The input and coupling caps were all stock .001 MFD. I added .005  to .01 to each section and the audio range was significantly broadened and lowered but sounded nasty. I cut back to .002 and found a reasonable compromise. Acceptable for now but could still use some tweaking. Might try an amplified D104. It worked wonders for my Globe Scout but that uses Heising (SP?) modulation and has a mic gain pot.

Regards, Rich

[DMOD]

Good info all. Thanks.

The input and coupling caps were all stock .001 MFD. I added .005  to .01 to each section and the audio range was significantly broadened and lowered but sounded nasty. I cut back to .002 and found a reasonable compromise. Acceptable for now but could still use some tweaking. Might try an amplified D104. It worked wonders for my Globe Scout but that uses Heising (SP?) modulation and has a mic gain pot.

Regards, Rich

I am not surprised the audio is subpar because of the coupling capacitor values and the screwy impedances in the audio circuits.

I usually use this circuit when a Challenger is on the operating table.

Phil  

[WD4DMZ]

Excellent. Thanks, Rich

[DMOD]

Excellent. Thanks, Rich

I don't know what mic you're using but if you find you need some pre-emphasis to cut through the "crud" and increase articulation, you can add a 0.47 100V film type cap across the Rnew 560 ohm cathode resistor in the first stage of the speech amp.

If you find you need more mic gain, then you can reapply a 10uF cap across the 1.5k, R17*.


Phil - AC0OB

[WD4DMZ]

Again, thanks for the great inputs. I had it on the air today on 7295 KHZ for a regular forum with others who know how I usually sound and the reports were surprisingly good. I will leave it alone for now. Often I keep fixing things that are good enough until I break them!

The mic I am using is a D104 carcass with a Kobitone 25,000 ohm impedance replacement element.

One issue is that I do not have a crystal for 7295 to use on it. Today I used a Heathkit VF1 which drives it well but wanders about +/- 150 khz. It is on a separate Heathkit PS23A which has plenty of power so the drift is in the VF1. That amount of drift may actually be pretty good for a VH1.

May have to just break down and purchase a crystal or perhaps a Viking 122.

Rich

[Pete, WA2CWA]

Again, thanks for the great inputs. I had it on the air today on 7295 KHZ for a regular forum with others who know how I usually sound and the reports were surprisingly good. I will leave it alone for now. Often I keep fixing things that are good enough until I break them!

The mic I am using is a D104 carcass with a Kobitone 25,000 ohm impedance replacement element.

One issue is that I do not have a crystal for 7295 to use on it. Today I used a Heathkit VF1 which drives it well but wanders about +/- 150 khz. It is on a separate Heathkit PS23A which has plenty of power so the drift is in the VF1. That amount of drift may actually be pretty good for a VH1.

May have to just break down and purchase a crystal or perhaps a Viking 122.

Rich

The first several posts should give you some great info to cure the stability problem:
https://forums.qrz.com/index.php?threads/some-thoughts-on-stabilizing-the-heathkit-vf-1-vfo.614936/

[WD4DMZ]

More great info. In my stash of tubes I found a new 6AH6 and a used 0B2. I will try them now!

Thanks, Rich

[WD4DMZ]

Made the tube swap and there is improvement in the drift issue.

I am using a small digital freq meter to dial it in. The tuning mechanism is pretty crude but usable.

Thanks again for the tips.

Rich