I just got a Stewart-Warner 305 TRF set in very good cosmetic condition. I checked all the tubes, good, and powered it up with an ARBE-III and 25-30 foot outside antenna.

Powering it up revealed that the second audio transformer is open, so I used headphones on the first audio stage. After playing the radio for a while I was struck by two things. 1) This radio really is touchy about going into oscillation. I was constantly adjusting the Volume Control potentiometer to try to keep the radio from going into regeneration. 2) It wasn't as sensitive radio as other 3 dialers I've used with the same antenna.

Any ideas, comments or suggestions where to troubleshoot?

Thanks,
Chris

Chris,

The oscillation problem is common to many TRF regenerative battery sets and does require a bit of tinkering with the dials to get the best signal and volume - which is usually with the volume and battery control knobs set to a point just before oscillation. I'd give it a thorough going over for loose connections, dirty contacts in the base of the tube sockets, and open windings in the volume/battery rheostats. The wipers on the rheostats need to be clean too. Lastly, how did you check the tubes? A good filament does not mean good emission. I have found it useful in these old TRF sets to swap the tubes around a bit - and have often been surprised with the results. An O1A that works 'okay' as an audio amplifier may not perform well at all as a detector and vice versa.

The coils in the Stewart-Warner sets (amongst many others) seem to be particularly hygroscopic. I've noticed that the pperformance of these sets may be greatly improved by baking the chassis in a (preferrably convection) oven for a few hours.

Take all of the screw terminals and both loosen and tighten them several times. A small amount of added resistance in an RF circuit can cause a great deal of mischief. I hav found that reflowing the solder at all of the soldered connections can often improve a recalcitrant set's performance, too.

A 25 foot aerial really doesn't cut it with these sets. The forst stage is designed for a bit more pickup. Try connecting the aerial post of your set to an unused conductor in your home telephone wiring. You will be suprised by the improved performance, U think, and will not need to operate the set with the gain turned up, in that area of instability.

Check the grid leak and its condenser, an off value can promote instability.

The "Volume Control", in this instance, is a potentiometer across the filament circuit, whic, in effect, varies the bias of the RF tubes. A faulty potentiometer, which has poor contact, can cause no end of trouble here.

When operating this model set, it is better to use the filament rheostat, and the detector tuning knob for volume control, and use the potentiometer as a sensitivity control.

This set will operate best if the two UX-201A tubes in the RF amplifier stages are pretty closley matched, for then, with practice one can learn to operate the set with both RF stages regenerative, just below the point of oscillation. The regenerative RF stages have much greater selectivity, in addition to increased sensitivity.

Whilst the adjustment of a set operating under such conditions can be quite critical, once ine gets the hang of it, your DX will be great.

The gridleak tests at about 1.5 megaohms. I'll clean up the contacts and renew solder joints this summer. Out of curiousity, how do hydroscopic RF coils affect performance?

Thanks a lot the advice!

If the coils have moisture in them, it lowers their Q greatly and it will cause the set to loose some sensitivity. But more important, it will cause them not to tune where they are supposed to tune and they will drift as the set dries out. This may or may not be a problem with a TRF set, but a superhet, it can be a major problem.
Curt

Chris,

Check your PM - have sent some hints and suggestions for your 305...

Thanks for all the suggestions so far. I had the radio going for over an hour the other night. I logged my stations, and I didn't see any drift to speak of.

This summer, I'll go through the set and see what I can improve on.

Thanks!

Followup -- I pulled out the set and wired it up last night. It was making all the usual oscillations and performance issues. In frustration, I tuned it to one station I could always get on it and started substituting tubes. Then I played with the rheostat and potentiometer.

Eureka!

This radio likes to work with the filament rheostat about 3/4 turn clockwise on and the potientometer somewhere between 1/2 and 3/4 to the right. So I guess some tubes (and sets) run better with low A voltage. Whuda thought?

Chris,

Do you have the operating instruction placard tacked into the lid of your radio? It gives some useful hints on adjusting the controls and initial settings. If you had both (pot & rheo) wide-open, I can see why you were having problems - the radio was in oscillation and you're just not going to get any sort of performance in that instance.

The purpose of the potentiometer is to alter the effective grid bias on the tubes in the RF stage.

Your Stewart Warner is not neutralised, and careful adjustment of this bias, along with the filament voltage of the RF tubes is the only way to keep it from breaking into uncontrollable oscillation.

The set sounds as if it is operating as designed.

Whilst the adjustment and tuning of tis sort of set is rather critical, with a little practice you should be able to get excellent performance from it.

Chris,

Here are some photos of the placard that was originally attached to the lid....





I'm sorry about the quality - that's the best I can do this early in the AM! Hopefully you can mag them up and get a little useful info. I hesitate to scan it - I'm afraid I'd destroy it trying to take it off the lid.

Thanks Jackson!

No problem Chris....