[morzh]

Eric

A stupid question: IF alignment usually requires applying the signal to either the plate of the RF amp (with the tube out) or to the grid of the tube. This sometimes calls for pulling off the grid caps.

if you took of anything: have you put it back?

[Eric Adams]

No Morzh, Nothing removed. No grid caps on on this model.

[morzh]

You need to check if your oscillator in fact oscillates.
If you have a handheld radio, you could try to move it near your radio and see if you hear the beatings.
I inever tried it on tube radios burt suppposedly that can be used.

Guys, what do you think?

I mean, the scope is the best way of course but short of having one.....

[Eric Adams]

Morzh, could you explain just how to do that? BTW I do have a rather basic scope but haven't learned how to use it yet.

[BrendaAnnD]

On your hand held or other radio, tune to a blank spot on the dial near the top of the dial. Tune the radio under test around between 1000-1200 KHz and listen for a carrier on the hand held radio. This will manifest as a quieting and a low hiss.

[Eric Adams]

Thanks. I tried it with a good radio to see what you meant. The radio in question doesn't do a thing. Any ideas on how to find out what the problem is with the oscillator?

[Sean]

Have you cleaned, I mean really cleaned, the pins on the oscillator tube? I use 400 grit sandpaper on every pin on those Loctal tubes.

[morzh]

Eric

Using scope is really a simple thing. You pretty much connect two grounds together (the scope lead's GND and the Chassis) and then put the probe to the point of interest.

Of course in case of tube radios you have to observe the basic safety, like not touching the two grounds with both hands etc, using an isolation transformer if the integrity of the isolation is in question (or even if it is not) etc.
Use proper V/div setting to not overload your scope input, in your case at least 10V/div and use AC coupling and not DC in this case as you do not want to adjust hundreds of volts every time you move the probe and you are only interested in the signal itself.

This is as much as there's to it.

Again, be safe.


This said,

1) connect the two chassis together (probe's GND to chassis). Better even do it when both scope and radio are unplugged from the Mains.
2) Set your scope to AC coupling (a mech. switch on old scopes).
3) Set to 10V/Div
4) touch the probe to whatever the output of your oscillator is. If it is the plate load (the transformer etc) - touch the plate. Proceed from there.


Have you put the sch link here yet?

[Ron Ramirez]

Sean raises a great point. You have to be sure the pins of each loctal tube are clean. You should also clean each loctal socket pin as well. DeoxIT sprayed into each little pin hole of each loctal socket, followed up with one of those small brushes such as designed to clean between teeth, works well.

I had mentioned mica caps earlier. Yes, they can and do go bad, and if all else fails, they may still require replacement.

[Eric Adams]

Thanks guys but no good. It looks like I'll be taking a close look at the mica caps.

Eric

[jerryhawthorne]

Eric, how about also posting the voltages you find on the oscillator tube, that would certainly help us in seeing what is going on.
Jerry

[Arran]

Very likely that a bad mica is the culprit, either as a coupling cap between the oscillator tank circuit and the tube or a cap in the tank circuit itself. Calling the oscillator tube and oscillator is rather a misnomer, the tube does not oscillate the tank circuit is what oscillates, the tube just provides a kick of voltage at the right time to keep the tank circuit oscillating. If the oscillator coil has continuity then one of the micas has to be the problem, sometimes more then one if they ended up with a bad batch. You can use a silver mica capacitor or a precision ceramic one as a replacement, make sure that you use the correct value and observe where the original leads are routed, not just connected, before you take it out.
Regards
Arran

[Eric Adams]

OK Arran, I have to admit that we're getting in a little past my understanding and experience. I'm not sure that I know where to look next. Here is the schematic: http://www.nostalgiaair.org/Resources/637/M0013637.htm
If you could look it over and give me some kind of idea which mica caps I should be looking at I would appreciate it. I haven't had any luck testing caps so I have a cap tester on its way.

Thanks,
Eric

[jerryhawthorne]

Eric, after looking at the schematic you posted, that is a VERY complex radio featuring AM and FM and a lot of switching. Scares me! Correct nomenclature for the "Oscillator" tube is a converter or first detector. The tank circuit is as I recall 505 on the schematic and is labeled BC Osc. Not much there. With all the switching going on with the band switch, I would start there. Some real good cleaning. So many contacts to be made. Do you get anything on the FM? You will need an antenna.
Jerry

[BrendaAnnD]

I gotta say that is the ugliest schematic I have ever seen!