Apache drifting wildly

[WA2ROC]

Last night the vfo on my Apache started drifting all over the band after working several stations on 20 meter AM net.  As I pressed the SPOT button, the signal heard in the receiver started to "warble" and drift +/- 20 khz.

I switched to a dummy load and tried 80 meters which worked perfectly.  40, 20, 15 and probably 10 meters were all "warbly" when the spot button was pressed.  The vfo operates on 1.75 to 2.0 MHZ for 80, and 3.5 to 3.8 MHZ or so on all the other bands.

I suspect either dirty switch contacts inside the vfo box or maybe bad silver mica caps in the 40-20-15-10  meter oscillator circuit. 

Before I wrestle it off the operating table and onto the workbench to open it up, how hard will I be working to get inside the vfo box and is there a better way to fix the problem?

[Jim, W5JO]

Maybe dirty bandswitch contacts.  Work it through the different bands several times then see what happens.  If that doesn't do it you may have to open it to clean those contacts.  I would not suggest replacing caps wholesale until you have looked at all other possibilities.

[WA2ROC]

That's pretty much what made it come back to life last night, rotating the band switch a number of times.

Guess I'll pull it out of the case and see how to get inside the VFO box.

Where's that Ben-Gay?

[Pete, WA2CWA]

There's no easy way to work on the VFO while its mounted in the Apache. If it's never been done in the life of the product, and since you have the VFO opened up and on the bench, I would replace all the fixed capacitors and check/replace any resistors that are out of tolerance. If you find one flaky cap and replace it, the odds are more will pop up as soon as you button up the VFO and reinstall it in the Apache.

[Jim, W5JO]

What Pete said, but remember that at least one of the caps on each coil is a low value temperature compensating type.  They don't do well with a lot of heat a soldering iron can put on the leads, so heat sink between were they are soldered and the cap before you try to remove them.  You will want to keep them if they are good. 

You can check the value with a good cap checker but can't check the temp part so have faith and put them back with your fingers crossed.  Also do everything possible to clean the contacts with a pencil eraser rather than use chemicals.  If not possible to use the eraser, then be very sparing with the chemicals.

[Steve - K4HX]

I'm not meaning to single out anyone, since this has been happening a lot lately. This topic was moved because it belongs in the Technical forum. This is where technical questions should be posted.

The object of the Handbook section (where the question was originally posted) is to have links and articles that will help us to build and experiment with our hobby through sharing (one of the hallmarks of amateur radio).

Each and everyone of you have input to this and there is already consideration to putting the material on CD as it grows. 

Please direct comments, questions, and other technical discussion to the Technical Forum . The Handbook Forum is for sharing complete, ready to use information, modifications, building projects, etc. If in doubt, post it in the Technical Forum. The AM Forum Administrators will gladly move posts from the Technical Forum into the Handbook Forum, if deemed appropriate.

So please post your helpful hints, Mods, links and projects with good documentation so all care share the AM/Vintage radio experience.

Please remember when using links be sure you have permission to do so from the site you are providing the link to. It is wise to get the persmisson in an email to have in case of issues.

Thanks,

The AM Fone Team

[WD5JKO]

  Dick,

   I was listening to you last night as the VFO misbehaved. You drifted up 10 or 20 Khz in erratic jumps over about a 30 second period. I never missed a word!

   You have been given some sound advice so I will not repeat it. But while in there look for signs off poor solder connections, or anything that finds a ground by pressure, metal to metal. This could include the ground wiper on the tuning capacitor if so equipped, or a ground lug that might be tight, but not making good contact due to corrosion.

   I once had a similar issue with the VFO in a BC-458 Command transmitter. The VFO coil in the metal box is a Hartley design, and the bottom of the coil is grounded to the aluminum floor with screws securing the coil form. This "pressure contact" from a tinned terminal to the aluminum had deteriorated due to galvanic corrosion. All I had to do was remove the screws, and slide some sand paper under there, grit side up, and then grit side down. The VFO sounded like RTTY before, and was very stable after that simple fix.

Good Luck,

Jim
Wd5JKO

[K4RT]

Before I wrestle it off the operating table and onto the workbench to open it up, how hard will I be working to get inside the vfo box and is there a better way to fix the problem?

I can't add anything to the technical advice already given, but having had the VFO enclosure open in my Apache recently I suggest keeping close track of the enclosure screws.  I dropped one and searched for it very carefully with a mirror and lamp, finally concluded that it dropped to the floor and rolled, figuring it would eventually turn up.  When I fired up the transmitter, the HV shorted to ground -that missing screw was inside the chassis nestled in some wires, causing the short. I was lucky that there is no serious damage. Finally getting time now to work on it again.

Brad

[WA2ROC]

Thanks to all of you who have suggested several very worthwhile places to start.

Brad: since you said you had the "vfo enclosure opened", I take that to mean that the screws holding the rear plate on the enclosure can be removed (after taking out the tubes in the way) and maybe the contacts on the bandswitch can be cleaned. 

I turned the Apache back on yesterday and the vfo ran just fine after warming up an hour or two.  If any of the caps were out of tolerance, I believe the vfo would have been way off frequency to begin with.  I'm hoping that it is a contact problem, but I have the caps on order too.

To the AM Fone Team:  Sorry for posting in the wrong place,  I felt, at the time, that the post was aimed at people that had experience with this particular Transmitter, and that's why I put it in the Transmitter section.

[K4RT]

Brad: since you said you had the "vfo enclosure opened", I take that to mean that the screws holding the rear plate on the enclosure can be removed (after taking out the tubes in the way) and maybe the contacts on the bandswitch can be cleaned. 

Dick - Correct - I removed the tubes first, then removed the screws and rear plate. I have attached a couple of photos I snapped after I removed that plate.

73,

Brad

[Steve - K4HX]

Not a problem Dick. I just wanted to point out that the Transmitter section, as well as the Receiver, Antennas, Restoration, etc sections are part of the Online Handbook. So, one should go there for answers rather than to post questions. So, when you fix the VFO in your Apache, the Transmitter section would be a good place to post details of the problem and how you fixed it.

Good luck with the Apache.

[WA2ROC]

Thanks, Brad, for the photos.  The components I may need to change are the ones that are closest to the camera at the top of the second photo and it looks like the bandswitch can be cleaned just as easy with this corner of the vfo case removed.

And Yes, I will make sure to account for all the screws!

[WD5JKO]

  Those pictures Brad attached are fantastic! I magnified one, and there seems to be at least one marginal solder connection. I know this is not Dick's VFO, but I am mentioning it just the same. Also that terminal lug under the choke might not have a good bite, and be touching the aluminum well. This relates back to my post where I mention galvanic corrosion.

Jim
WD5JKO

[K4RT]

 Those pictures Brad attached are fantastic! I magnified one, and there seems to be at least one marginal solder connection. I know this is not Dick's VFO, but I am mentioning it just the same. Also that terminal lug under the choke might not have a good bite, and be touching the aluminum well. This relates back to my post where I mention galvanic corrosion.

Jim
WD5JKO

Jim,

Nice work! I did not notice that solder connection.  Earlier, I found one like that where the wire had been pushed through a lug for a tube socket but not soldered - the builder apparently overlooked a few in what otherwise appears to be a very nice construction job. I'll check that terminal lug screw/nut, too.

Many thanks.

73,
Brad

[W2PFY]

The first thing I would do is change out the tubes in the OSC & buffer and see if there is a change? Just because they test good on a tester may not be an indicator of a good tube in the circuit.........

[WA2ROC]

Just ordered the tubes and it is easy to swap them without wrestling the chassis out of the case to check operation.. 

I still plan to open the VFO box and clean the switch contacts and maybe change the caps. 

Over the last 2 days it seems to be working fine but you have to remember this old saying:

"Problems that go away by themselves always return with several of their friends".

[K4RT]

Jim,

I opened the VFO enclosure to inspect. The lead to the lug was soldered, but not well. I heated it and added a small amount of solder for a solid connection. Also, the screw holding the lug was a bit loose so I tightened it up.  Thanks again.

73,
Brad