Airline 14WG-625A oscillator dead/dying/intermittent

[electra225]

I have an Airline radio that is a mystery. Well-Gardner chassis, traditionally has been a hot performer. Six tuber with RF amp, but not a tuned RF stage. I was going to pull it apart to paint the cabinet and to replace the volume control which refuses to be cleaned. The volume hops up and down, if you manipulate the volume control, the volume returns. It finally got to the place where the lower half of the volume control was dead. The volume control there now is not the original one. The shaft is about 1/4" too long. Anyways.....

I fired the radio up to confirm my diagnosis on the volume control. The set was essentially dead, as in no audio from the speaker. It was dead all across the dial. I pulled the chassis and started checking voltages. I found when I put the DC voltage probe from the VTVM on pin 4, the mixer grid of the 12SA7 tube, the radio would play. Sensitivity seemed normal, so I connected another VTVM on DC negative volts to the grid of the oscillator. With the probe of the first VTVM on pin 4, the oscillator would run. Remove the probe and the oscillator voltage would slowly drop until the radio was dead. The chassis has been recapped, but years ago. Something is killing the oscillator, but what? I need to study the drawing to see what is going on.

I've done some more tinkering and have found that when I put the probe of the VTVM really on any point in the signal path up to and including the hot end of the volume control, the radio will play. It seems to me like the probe of the VTVM is providing a ground (or something). Do you reckon it's possible that the volume control is the cause of all this nonsense? The radio has always played strong. I played it maybe a month ago, in its spot in the rotation. I played it all day and it worked well outside of the volume hopping up and down, maybe every hour or so. The element in the volume control is cracked so the radio only works with the volume advanced all the way. The audio is somewhat distorted, not horrible, but I attributed that to the volume control. I believe, before I get excited, I'll at least sub in another volume control, then see how it works. I have no theory nor explanation how a defect in the volume control could kill the oscillator......

[Motorola minion]

After looking at the factory schematic BUT never having serviced a W-G set myself, this could happen with any radio having AVC line.

Automatic Volume Control is a side job for a detector diode. A negative DC is taken from the audio line via high-impedance loop to control the gain of the RF and IF stages using negative bias to reduce gain on strong signals. This also how sets in early 30s and before without AVC controlled volume, restricting signal coming IN at the front end.

Whereby the audio signal is taken off the other side of the IF transformer secondary thru which the diode will not pass a positive voltage. It is filtered, so that only low frequency audio remains. The LEVEL of this detected audio signal determines how negative the AVC line goes.

First, the .5 meg volume control also functions as part of a voltage divider (exact circuit varies by mfr) and if it were open, the setting wouldn't matter if the "X" side per the schematic, would no longer have its ground reference. This would allow the AVC line (X) to go way negative and cut off (via neg bias) the mixer-oscillator and the IF and/or RF amp tubes as well. This "loop" was a popular and simple method to implement a very useful feature.

Old radio questions are always inspiring to solve another set's issues, I have recapped a Capehart AA4 battery set with 1R5-1U4-1R5-3V4, etc. All voltages check, but the oscillator is quiet I know its not just "a tube" LOL

[electra225]

You already know this, Dave, the first thing I check when I suspect a dead oscillator is the oscillator grid voltage. Regardless of the model radio you are working with, every superhet chassis needs negative voltage of some description on the grid of the oscillator, pin 5 on a 12SA7 pentagrid converter tube. That is a universal rule of a superheterodyne circuit, radio and TV. I have challenges working with oscillator circuits, but try to always remember to do that first. If I have negative voltage on the oscillator grid, I need to look somewhere else for the trouble. I have a little Arvin 444-A four tube wonder that has an oscillator issue I never have figured out for sure. It also has a "sometimes" dying oscillator. I am suspecting the coil, but have never seen an intermittent oscillator coil. They either work or they don't for me.

I like Well-Gardner chassis and actually seek them out. Typically high quality much like Stromberg, Capehart, the big boys. Typically easy to service, not finicky about adjustments. I say "typically". This one may be the exception. Thanks for the input on the volume control. I think that will be my next step, then go from there.

[electra225]

If somebody posted on a forum that a wonky volume control could kill an oscillator, I would have been skeptical, to say the least. I am skeptical no longer. I replaced the volume control with one I dug out of a parts chassis. I'm not even sure it will work with the cabinet. The shaft is a tad shorter than the one that was there, but it was too long. The radio works as well as it ever did, maybe even a bit better. So, I reckon it's fair to say that if you are working on a radio with a dead oscillator or with an intermittent oscillator, check to see if the volume control seems to be working normally. If in doubt, test it. I would not have believe this in a million years if I hadn't experienced it myself.....

[electra225]

Repainting the cabinet on this set will be my next project. I haven't decided on color(s) yet. Lavendar and black? Burgundy and Vintage Blush? Sea green and black? The feet are hard rubber, riveted on. I am in the process of digging out the rivets. I will replace these with stick-on felt feet. This set has a genuine glass dial cover. Other than being ugly and having chipping around the knobs, the paint on this cabinet is in decent shape. A quick sand and primer should be all it needs.

[electra225]

This chassis has a really hot front end for not having a tuned RF stage. Typical of a Wells-Gardner chassis, this thing is a dandy performer.

[electra225]

I have the cabinet painted. I will save pictures until I get it back together. IT turned out really nice. I learned a couple of tips that I'll share, hoping it might help someone.

First, use fresh primer, the same brand as the paint you'll use. Next, sand plastic or bakelite, especially on the top and sides of the cabinet, with 150 grit dry sandpaper. Next, wash the cabinet in hot water and dishwashing soap. This gets all the "greasy grubbies" off the cabinet. Rinse and let it air dry. If you have to help it dry, use a new roll of paper towels, not those left on the kitchen counter or where they could be contaminated. Best to use compressed air or let it "sun test" to get it good and dry. After well dry, I spray primer, two coats. I let it dry to the touch, 30 minutes max, (go by the directions on the can, but this works for Rustoleum), then shoot the color. A light coat first, followed by a medium coat. Let this tack, then shoot a heavy coat. The problem I had with the Truetone cabinet was that my primer was old and reacted with the paint. Different substrate materials and different paint colors will react differently to the paint. I painted this Airline cabinet with solid "Cranberry" Rustoleum paint. I painted the knobs "Vintage Blush". I think it will be just fine........

[electra225]

It's done! I wanted to two-tone it, but the paint turned out so nice, I didn't want to press my luck.

[Hi-Fi-Mogul]

Not a cliche' this time, but Just WOW !!!

Congrats on another superb effort.

[electra225]

Thanks, Mr. Mogul! This radio has record player parts in it. I used the switch/volume control out of a junker record player chassis in this radio chassis.

This set didn't have the original knobs anymore and the ones there wouldn't work with the shorter volume control. The knobs I used are for a TV chassis. One knob says "horizontal" the other says "contrast". I filled the script best I could. Thank God for glazing putty.......

Another little tidbit of information on this set is that the backwards, weirdly-shaped dial is probably as accurate as any I own. The station it is on is 960 Khz, which is really close to where the pointer is.

[William]

Very sharp, nice job, Greg.

Bill

[electra225]

Thanks, Bill !