admiral 24c15 with 20b1 chassis attempt at restore

[fumplet]

Oh is it easier just to take out the tumbler? and get to the contacts, or I can just keep on spraying and wiping.
Sound and focus solved: Dang audio output tube socket was dirty or tube wasnt in all the way, and I decided to check it because I thought I saw a focus/sound issue in troubleshooting chart that said check audio output tubes..Well if a tube goes cold, then whatever its doing fades out and when elememnt cooled it starts to work again. Bingo clean 6v6 socket with tuner spray, lube and yep. Here I thought I had screwed it up installing the sound area and b+ resistors. Now the sound just might stay level and not fade in and out at all.

[bandersen]

Ni, it is not easy to remove. Spray cleaner onto cloth and wipe contacts. You're probably only going to ever use channels 3 and 4 so I'd concentrate on those.

[fumplet]

Ok no idea why audio output tube filament was not staying on. Its not doing it now. Same as the 6bg6 tube where the filament was shutting off. Mess with it long enough and they quit doing it. anyway 2 things noticed were a 14000 ohm resistor that actually says 14000 aerovox on it which I think is supposed to be 15000 and measures 19000, plus theres 2 coils that look like they are corroding and should I be worried about the resistor value being over 25 percent off and the coils looking greenish? I see no reference to this resistor even in the changes being 14000 ohms and I have chassis run 18. Apparently sound keeps going from having it fully cranked and medium volume to blasting volume and having to turn it down. Was normal volume response with loud at full knob and I wanted to test the focus knob rheostat voltage center pin and the second I touched the center pin with meter it made popping noise and volume went to fully cranked medium volume. Wild, now focus is sorta near center of adjustent after using 2 of the 12k ohm 20 watt resistors in parallel for the 6 k can ohm.

[bandersen]

Where is the resistor connected? Which coils?

[fumplet]

the resistor is connected to the 1st audio I.F. T201 and the 6k ohm can ohm resistor riveted to the side of the chassis. The coils are connected in series and go to pin 8 of the 6ac7 tube. The coils look like they are each wound around a resistor. Coils are about the size of a dime and about thick as 2 dimes stacked in the photo

[bandersen]

Yes, 15K. It's probably and old repair. I doubt it will make much difference if you replace it.
Those are peaking coils. The green is copper corrosion. Maybe the set was in a humid environment.
If they go open, the video quality will suffer. I don't know of any sure-fire treatment.

[fumplet]

replaceable coils or just hope they dont go open? L304 33k ohm resistor and L305 1 meg resistor on parts list. I wonder if you could get a 250 microhenry coil and just take one end off of the resistors lead and put the appropriate resistor in place.
Side note: The focus is now at one end again of knob for adjustment. Amazing!
Weird: when I crank up the volume quickly......when its capable of full volume.....the picture tweaks a bit then goes back to normal.
250 uH coils are on evilbay, I bet if I took turns off I could get down to 150 uH for the 2nd coil.

[Electronic M]

You can probably find more correct peaking coils on mouser.

[bandersen]

Has anyone ever tried replacing peaking coils? I haven't so can't offer any advice.
Not all inductors are created equal. There are other factors like "Q" and the winding pattern.
It's my understanding they wind the coils to dampen the "Q". It also means if the coil goes open, some signal still gets through.

[fumplet]

Originally Posted by Electronic M

You can probably find more correct peaking coils on mouser.

What qualifications do i need other than 250 and 150 uH. The schematic doesnt list the Q properties.

[bandersen]

We don't know - that's the problem. It also should be wound around a carbon comp resistor.
Back in the day, you would just get one from Admiral or one of the other parts vendors (Merit, JW Miller, etc). Those days are long gone.

You go with what you have, scavenge from a parts chassis or experiment.

[fumplet]

Ok so the fun begins with the oscilloscope and parts...i did order 250 and 155 and 160 coils off evilbay we will see. Makes me wonder if this is the vertacal jumpy issues that is intermittent.
So thats very interesting has to be wound around a carbon comp resistor so the coil must be like a choke.

[bandersen]

Not, not at all. A "choke" block Rf from getting into a circuit. These coils help it get through by peaking the response at certain frequency ranges. The ultimate goal is to get around 4MHz bandwidth out of the video amp.

[fumplet]

Just got my sencore ringer yoke and flyback tester in the mail, more fun. Hmmm 4 mhz bandwidth, interesting. so really the only way I would screw it up is if I used one with 2 low a resistor value in it for a peaking coil?

[bandersen]

No, using a coil with the wrong characteristics could affect the bandwidth. I suppose you'll have to attach the resistor in parallel to the coil rather than have the coil wrapped around it. Not sure what effect that will have.

Seriously though. This is a really subtle thing and I really doubt you're going to notice much of anything either way without test equipment. Try shorting on of the coils out with a jumper wire and see if you notice any difference.

None of this has anything to do with your bigger issues. The focus control should not be at one extreme and you image is off center. I would not be surprised if the two are related. Namely, the focus coil is not mounted in the correct positon.