VFO 122 60 cycle hum

[kd1nw]

Hi All,

I got a report of 60 cycle hum using the VFO122 on a Viking II. So, I looked into it with just LV on, I can hear it modulating the VFO output. If I switch to crystal it's pure with no hum. The hum is on all the vfo freqs 160, 80, 40 etc (switch positions 2 and 3). I've cleaned the VFO bandswitch. I've tried swapping muliple 6AU6, 6AH6, 0A2, 0B2 with no difference. I tried swapping the 5V4 in the V2 and I paralleled a 33uF across C10 with no difference. I also tried switching my crystal from position 1 in the V2 to position 5 thinking there might be some coupling or interaction going on but no effect. I don't have a scope available to me, it's not working. Does anyone have advice on where to look next?

Thanks 73
Kevin KD1NW

[WD5JKO]

  Kevin,

   If the VFO induced hum is AM, then the VII buffer, driver, and PA stages being class C stages should be an effective limiter getting rid of any AM on the RF drive signal. My bet is that the hum is FM. While pondering this, ask yourself if this issue always existed, or if it is new development? The first place to look would be the ground between the VFO and the Viking II. Is it "as designed", or maybe a wire broke somewhere?
  
   A quick thing to try is to add a braid from the back of the VFO to the Viking II, chassis to chassis.

Some good reference material from Janis AB2RA:
http://wireless-girl.com/Projects/AMTransmitters/JohnsonVFOkeyingMod.pdf


Jim
Wd5JKO

[kd1nw]

Thanks Jim, Good question but, I don't know if this is a pre-existing issue. I bought the rig and vfo at nearfest this fall. I replaced electrolytics and papers but i have no prior experience with this setup. I agree it seems like FMing the vfo but I have no real way telling I guess. Both the V2 and vfo are as designed with no significant mods. That is, 33uF ecaps instead of 15uf. I tried to stay within the not lower and not higher than 100% increase in capacitance. Both the V2 and the vfo are in very good shape with no evidence of previous work or mods. I'll try connecting braid from vfo to V2 as you have suggested, that's certainly easy enough. I'll also check out Janis' site again, I have looked before but its possible i missed something.

Thanks!!

[KA0HCP]

Check the filament leads on the VFO 12AU7.

It has two grounds and center fed +.  This balanced feed is supposed to eliminated 60hz hum. Perhaps one of the ground leads is broken.

[W2JBL]

    You are likely getting RF back into the VFO. First (not related to the RF problem but easy to do) try a new tube in the VFO- this could be caused by filament to cathode leakage. Make sure all the VFO cabinet screws are installed using star washers and are tight. Go up one screw size (#4 to #6) if any holes are stripped. Johnson put them there for good reason. Change the control cable to the VFO to shielded cable. Put .01 MF bypass caps across the filament, B+ and keying leads to ground where they enter the VFO. Make sure you have a well shielded high impedance coax on the output, like RG-62 ( 92 ohm car radio antenna cable). I had the problem on 15 and 10 meters and these steps (which should really be done anyway- it's good engineering practice) cured the problem.

[flintstone mop]

    You are likely getting RF back into the VFO. First (not related to the RF problem but easy to do) try a new tube in the VFO- this could be caused by filament to cathode leakage. Make sure all the VFO cabinet screws are installed using star washers and are tight. Go up one screw size (#4 to #6) if any holes are stripped. Johnson put them there for good reason. Change the control cable to the VFO to shielded cable. Put .01 MF bypass caps across the filament, B+ and keying leads to ground where they enter the VFO. Make sure you have a well shielded high impedance coax on the output, like RG-62 ( 92 ohm car radio antenna cable). I had the problem on 15 and 10 meters and these steps (which should really be done anyway- it's good engineering practice) cured the problem.

Good suggestions. I dealt with that too and it seemed to be RFI related. RF getting back into the Exciter circuits. Shielded cable from the VFO to the transmitter and grounding. You'll have to experiment with the connection of the ground.

Fred

[DMOD]

You might also check to make sure there is grounding "carry-through" on the octal accessory plug pins 1 and 2.

Phil - AC0OB

[kd1nw]

Thanks everyone. I tried a .022 across the filaments of the 6AU6 and the hum was gone. So I figured if I removed it the hum would come back. It didn't come back. The only other thing that I did was, I moved the braid of the octal control cable to inspect it to see if it was loose. So it looks like a grounding issue with the cable. I can touch up that connection later but the hum has gone from an obvious level to a very low level (hard to hear).

Thanks 73 - Kevin KD1NW

[DMOD]

Good deal Kevin and it sounds as if you had a ground loop, but I would keep a cap across the filament at the tube socket to kill any RF and buzz.

Phil - AC0OB

[kd1nw]

Thanks Phil, I found some .01 ceramics that are much smaller than the .02 i was messing with. I'll put one in for good measure

[VE3LYX]

I had a similar problem on a DX60B so I will tell how I cured it and perhaps that will help U. I made my VFO (12ax7) in a stainless kleenex box. Immediately it had a bad hum but like yours only when the VFO was switched in. On crystal it was perfect. So it was as it is with you obvious the VFO addition was the culprit. I bypassed and filtered and farted around for weeks to no avail. finally in desperation I took some scrap litz wire (only because I had it. I am not recommending it because it is litz wire) and I wound a homemade sheild the entire legth of my vfo to rig cable . End of problem and I have been using it ever since talking even to some of you SO the "shield it" recommendation could in fact be bang on correct. I am no hum guru, in fact I am trying to get it out of my ARC5 PSU (ARRL handbook Economy Power Supply) right now but this worked for me with the VFO problem. mm I wonder.......................?
don

[kd1nw]

Thanks Don, I agree, it appears to have been a problem with the shield on the control cable. I'll go back and make sure the connection is good. 73 Kevin KD1NW

[kd1nw]

Hi,

I thought the hum was gone but it isn't. I found the issue but I'm not sure of the answer and wanted to check first. I think the problem is leaky bypass cap or caps C41, C42 and C53 in the Viking II. They are line bypass caps that are part of the line RF filter assembly. I converted to a 3 wire cord and connected black to fuse (L13A), neutral to L13B and the green to chassis. With the radio off, cord plugged in, I find continuity to chassis (appox 100 ohms) with my tester from either side of all 3 RF chokes, it gives a raspy tone (60hz?) not sine. I disconnect the green wire and there is no continuity. I also ran the radio with the vfo without the green wire connected and there was no hum at all on the signal. So, with the green wire connected I must be getting AC leaking to the chassis? Can I just clip the bypass caps C41, C42 and C53 out for the time being and leave the RF chokes there? If that works I guess I could get proper line bypass caps or just remove the whole RFC assembly and free up some space in that area.

Thanks and 73
Kevin - KD1NW 

[KA0HCP]

Line bypass caps should be replaced with modern safety caps.

[kd1nw]

Okay thanks Bill