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 Post subject: Ranger I VFO instability
PostPosted: May Wed 04, 2022 10:56 pm 
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Location: West Concord, MA
Back with a new wrinkle for the old Ranger. It transmits exceptionally well using a crystal, but the normally stable VFO has been acting up lately. It will run fine for a few minutes to a half hour, then suddenly jump 100-200-300 kcs up or down frequency and transmit there. Tapping on the VFO box will sometimes bring it back. Sometimes not.

I have replaced the 6AU6 and the OA2. I have replaced the 18K dropping resistor (tested good but looked stressed) and the 470 ohm B+ dropping resistor (tested good but looked way undersized). I resoldered marginal or cold solder joints found on the terminal strip and ground bus to chassis. I have carefully cleaned the CRYSTAL/VFO/ZERO rotary switch with DeOxit. Going back into the VFO box is not fun, especially working on it from the removable side panel, but I'll do it. I just need some ideas on what culprits to zero in on. Reflowing any questionable solder joint is obvious. Any other ideas? Thanks again!


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 Post subject: Re: Ranger I VFO instability
PostPosted: May Thu 05, 2022 12:45 am 
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The fact that tapping it can restore the correct frequency almost guarantees that there is an intermittent connection somewhere. Once you rule out all the usual suspects - solder joints, ground lugs, tube sockets, and switch contacts, then the remaining possibilities boil down to parts that are partially failing. Using a long, thin insulated wand, try tapping individual components. Or try spraying with "compressed air" which will temporarily cool the component.

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 Post subject: Re: Ranger I VFO instability
PostPosted: May Thu 05, 2022 7:50 am 
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Location: Lincoln City, OR 97367
Greetings to W1GFH and the Forum:

The Ranger VFO operates on three different frequencies. These are then multiplied to get output on the desired band. 160 and 80 are covered by one tank circuit, 40 through 10 by a second, and 11 all by itself with a third. The third for 11 meters is actually a fixed capacitor added across the 40 meter tank.

So, the first question to ask is: Do these frequency jumps occur on all bands, or just 40 through 10 or 160 and 80?

Noting the above may help to narrow down the problem. Note that there is a cam driven band switch in the bottom of the VFO housing. This may also require careful cleaning with De-Ox-It. Do be careful to wash it out with an evaporating cleaner afterwards or to be very careful not to get the stuff on the switch insulator. (Note; the photo I have of the transmitter shows that band switch as being ceramic; such a switch is more resistant to uptake of contact cleaner, but exercise due caution.)

There are four fixed mica "postage stamp" capacitors in the unit; these are normally OK, but we are now starting to see them fail after these many years. (I have had three fail fairly recently, one in the BFO of my HQ-129-X receiver, one in the BFO of a BC-348 receiver and one in an HP 410B VTVM.) To be on the safe side, you might wish to replace these with modern dipped silver mica caps.

There are also trimmer and padder capacitors that have mechanical joints that may require cleaning and lastly, C1, which may also require cleaning and lubrication of its rotor bearings and contact surfaces.

Hopefully, you will be able to find something that is easy to reach and that will be the only thing that you need to deal with. Just in case, the above are some potential trouble spots that I can think of.

Good Luck,

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Jim T.
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 Post subject: Re: Ranger I VFO instability
PostPosted: May Thu 05, 2022 3:40 pm 
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Location: West Concord, MA
Did the suggested test: transmitted for an hour QSO on 75 meters, monitoring sig with BFO on another receiver. VFO rock solid (or as solid as a Ranger VFO can be). So the problem is particular to 40 meters.

I will check the 40 HI and 40 LO trimmer caps for loose grounds. I will check and clean the contacts on the rotary band switch that are accessible from the bottom of the chassis. But getting to the rotary band switch contacts in the bottom of the VFO may not be possible without a major teardown. : ( Not sure if I should risk just squirting DeOxit in there, I'll have to think on that one.


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 Post subject: Re: Ranger I VFO instability
PostPosted: May Fri 06, 2022 12:37 am 
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If that's the white wafer that's visible on the bottom, you'd probably be okay spraying some contact cleaner in there. (if the brand you are using is Plastic Safe).

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 Post subject: Re: Ranger I VFO instability
PostPosted: May Fri 06, 2022 1:39 am 
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Location: West Concord, MA
Thoroughly cleaned the bandswitch. There is some black crud on the contact surfaces, but a few applications of DeOxit made some progress. By working the bandswitch, I was able to get the VFO to stay on frequency on 40M. Poked around with a stick while the VFO was in ZERO (spotting) mode and being monitored on a receiver with the BFO on.

Could not reproduce the fault by tapping on the postage stamp caps or anything in the VFO compartment. Tapping on the 40 LO trimmer only induced a few KC of drift. Tube V4 the 6CL6 buffer/multiplier is very touchy, it feels somewhat loose in the socket, and rocking or moving it induces a few KC of drift, but nothing like the 200 KC jumps I was having before.

Tightened a lot of ground points. Removed the fiber insulators to ground the plate tuning cap per https://www.w8ji.com/johnson_vfo_chirp_jump.htm - but now the frequency in ZERO (spotting) is a few KC off from the transmit frequency. Adjusting the 40 LO or HI trimmers don't remedy that. I'll try the contact cleaner on the wafer switch in the VFO, the DeOxit I have is plastic safe.

But I'm kind of playing whack-a-mole now. I noticed the 15uf C59B electrolytic at the driver transformer center tap is bulging and near to bursting. The schematic shows C59B with positive to ground. However upon investigation, it appears to have been installed backwards (negative to ground). I had an 8uf Iaying around, so I tried using that in the correct orientation, and it worked, but mod current went down to nothing. So I'll pick up some new 15uf caps tomorrow and readjust the mod current resistor and hope that problem will be solved so I can get back to VFO troubleshooting.


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 Post subject: Re: Ranger I VFO instability
PostPosted: May Fri 06, 2022 8:09 pm 
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Location: West Concord, MA
C59B fixed. On to the VFO. Got some DeOxit into the rotary switch and blew it out with compressed air. But it definitely appears to be the 6CL6 buffer (V4) that's causing this instability. Moving it around in the socket, or even lightly tapping the table induces a small frequency shift. I can also hear intermittent scratchy noise on the ZERO (spot) signal, like tube noise or a loose connection.

I will try swapping the 6CL6s, tightening the socket contacts, cleaning the tube pins, and a few other things.

** UPDATE: the metal tab for pin 6 of the 6CL6 socket was very loose in the phenolic base. I sacrificed one of my tech syringes by filling it with E6000 glue and squirting a tiny amount on the socket tab to stabilize pin 6. Hopefully when it cures, the issue will be cured. **

*** UPDATE: that was it. It is CURED. (fingers crossed). :lol: And the ZERO/spot vs VFO frequency disparity was also cured when I reinstalled the VFO compartment shielding.


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 Post subject: Re: Ranger I VFO instability
PostPosted: May Sat 07, 2022 9:47 am 
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Location: Lincoln City, OR 97367
Greetings to W1GFH and the Forum:

Glad you found it. Enjoy your Ranger!

73,

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Jim T.
KB6GM


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 Post subject: Re: Ranger I VFO instability
PostPosted: May Fri 13, 2022 7:09 pm 
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Posts: 373
Location: West Concord, MA
Just a follow up...

I thought I had it cured, but the problem soon returned with a vengeance. Tapping on the rig, flicking rotary switches, poking tubes, even the T/R relay going into transmit - any vibration at all - would induce wild frequency shifts and general instability. While looking at the capacitors in the VFO I noticed that the long screws that oscillator coil L1 is mounted on were completely loose, so there was about a half inch of freeplay on each side. Any movement or vibration of the coil also caused the 47pf N150 temperature compensating capacitor directly attached to the 40 meter side of the coil to move around. I spent a few minutes adjusting nuts and tightening them to firmly secure the screws to the coil. That made a BIG difference. The VFO is now stable on 40 meters after an hour of testing.

Tracking down what's causing VFO problems in these older rigs can be tricky. I hope this helps somebody down the line.


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 Post subject: Re: Ranger I VFO instability
PostPosted: May Sat 14, 2022 10:38 am 
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Joined: Oct Thu 01, 2020 10:48 am
Posts: 139
"Tracking down what's causing VFO problems in these older rigs can be tricky."

You got that right!

"I hope this helps somebody down the line."

Big help! Here is the takeaway:

There are 3 areas of VFO problems:
1. Bad parts.
2. Poor continuity (dirty wipers, cold solder joints...)
3. Mechanical instability.

I typically go after 1, and overlook 2 and 3.


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