[Phlogiston]

If this set were built by the same guy that built mine, intermittent contacts would be expected.

[462ron]

Hi Ed, FWIW, I am on the opposite side of the country from you being in PA. On most of my SW sets, there isn’t much at all from 8-18 MC either other than some random static and beeps most of the time. Since you can hear your signal generator, you may be looking for something that isn’t there. Do you have access to another SW radio to compare yours to? Just throwing this out there!

Ron

[EdHolland]

Ron, Thank you

"Do you have access to another SW radio to compare yours to?..."

Oh boy do I, an embarrassment of them actually, vintage and modern.

Yesterday I poked around more, and attended to some suspect looking solder joints. After this, I was seeing occasional oscillator failure with the original 6A8 tube. I also noticed that there was a blue glow in the tube when not oscillating, suggesting that it is gassy. With a replacement 6A8, oscillator was fine, but even on the temporary antenna, still a bit deaf.

Today, I moved the radio to my radio desk, and connected it to the longwire. Tonight, both SW bands are teaming with stations, and everything seems to be operating just as it should, with good reception. This set just needs the right aerial. What I think was happening is that the temporary antenna was just cancelling out the unintended antenna action from connection to the AC line. On a big antenna, with a good ground, all is fine.

Anyway, now it is on to fixing the last caps, and gentle restoration/preservation of the cabinet finish.

Cheers

Ed

[462ron]

OK then, sounds like you have it squared away. Since I relocated my repair bench, I no longer have access to the phone line I was using as my long wire antenna. I’m using a 20’ length of wire now, I will have to try and get a better setup. 

Ron

[EdHolland]

I tested on the bench, with "A few feet of wire" many times, but I think those radios had different front ends, possibly an RF amp. This one just behaved very strangely with this set-up.

It makes me wonder what the listener of 1938 was to expect from their SW radio, and how many would have gone to the lengths (pun intended) of putting up a big wire. That brings me back to an interest in "all wave" aerials that became popular about this time, as sets with SW bands became available, and how many of these would have been purchased and set up.

Anyway, now it is on to the cabinet

[EdHolland]

Hmm! Since the radio is on my bench still, I turn it on for a bit of a play whenever I'm in the garage. Today I turned on, and a second or two later I heard "POP!" and saw a spark between the first IF can and the chassis.

Aha!, here might lie the source of the odd instability I have occasionally run into, where it sounds like the IF is oscillating/regenerating. More later.


Oh, and I tested the temporary antenna I had been using on a modern Grundig portable. No wonder I was confused. It made signals worse, just as it had with the 1938 radio.

Cheers,

Ed

[EdHolland]

More investigation yesterday. Carefully (and with some difficulty) I was able to remove the can from around the first IF transformer. Before this, I tested its continuity to the chassis again, and grounding did seem precarious. Inside, there was no obvious damage from the short, but the trimmers were very dusty. Wiring and wax dipped coils all looked good. On the inside of the can, there is one spot that looks to be where the flash-over occurred, at a position near the trimmers, and coincident with the high voltage locations. No sign of what caused the trouble.

This will give an opportunity to replace the slightly shabby wire to the grid of the #78 IF valve, and perhaps the other wires running under the chassis. I may get it running again this afternoon.

The transformer is interesting. Two coils, spaced quite far apart - perhaps ~3/4" - wound on a wooden dowel and dipped in beeswax. The compression trimmers are carried on a square ceramic support attached to the dowel.

Cheers

Ed

[Ron Ramirez]

The IF transformer construction you mention, Ed, is typical of 1930s-1940s home radio IF transformers. Most 1939-1942 Philco IF transformers, with the exception of most Tropic models, are made in this fashion. Other manufacturers made IF transformers like this also.

[EdHolland]

Thanks Ron, I just thought it was interesting - especially to see wood used as an electronic material!

I've seen the insides of the IF's of my 16X, this radio, our '41 RCA and '47 Zenith. The late thirtiesor seems to usher in permeability tuning with iron/ferrite cores, and sizes gradually reduce. To me, they are interesting components.

Ed

[EdHolland]

Here is a view with the can removed. Sorry the picture is a little blurry, I was a bit hurried, and working in low light.

The can was put back together after cleaning and adding a new top wire, and the radio played nicely again.

   

[Ron Ramirez]

Yup...it looks just like a 1939-1942 Philco IF transformer.

[EdHolland]

Still fiddling with this radio... A good job impatient customers are not waiting!

There was still a weird instability, which may now be cured. Symptoms were that the set's gain would jump suddenly high, but also the IF bandwidth was obviously narrower. Occasionally it would break into oscillation, and this could be triggered while trying to adjust the IF transformers. Sometimes it would be somewhere inbetween these conditions, but audio would sound dull, if loud enough. I Found and did the following:

A closer inspection unearthed (pun intended) a blob of solder that had run from a tag strip associated with the #75 valve. This could be provoked into shorting out the detector diode circuit. I also removed the can from the second IF transformer, and cleaned up the trimmers. Lots of dust and debris on these, just like there had been on the first one shown above. It seems better than ever, nice clear audio on broadcast band, and fair enough sensitivity on SW. Selectivity is another matter on SW, - the antenna tuning circuits do not track at all well, and can't seem to be adjusted better with the single trimmer provided for each. Perhaps this is just how things were?

[EdHolland]

Change of track today, to remove the temporary power supply setup and complete this area of the chassis. I restuffed each of the electrolytic caps, trying a new technique. Since I have some aluminium solder on hand for repairing the voice wires of Magnepan speakers, I decided to try soldering the caps within the can, rather than drilling holes. Of course, the can must still be opened - careful hacksawing along a line drawn carefully about 1/2 in up from the base where the part would sit on the chassis. The replacement caps were soldered in place, and the tops replaced and secured using a band of aluminium foil tape. This looks reasonable to me, although I did not really like having to cut the cans.

With this all back together, and wiring returned to something like factory, the radio plays well, and the top side of the chassis is complete. Credit to the previous person who worked on this, wiring changes were logical enough that returning to original was straightforward, and nothing had been cut short.

Back to the cabinet tomorrow

[EdHolland]

Today I refitted the push buttons and coil set for the MW presets. This was a bit fiddly, as it was necessary to replace the 12 rather heavy gauge wires that link the two assemblies. The other connecting wires were returned to the appropriate tabs. I was very confused when powering up to test - all bands were receiving a steady level of noise, with no tunable signals at all. The IF was working but nothing getting through from the mixer. Oscillator working just fine, voltages checked.

Eventuially I found the trouble, just before I had resigned myself to removing the preset components again. The frame for the pushbuttons was just touching the 2 meg resistor that feeds AVC voltage to the signal grid of the 6A8. All the incoming signals were being shorted out! That was easy to fix, and the chassis is electrically complete again. All the presets tuned up to stations and work well.

Cheers

Ed

[EdHolland]

Now I understand why two of the #2 screws that secure the tuning dial surround were missing. The wood there is really thin, and the holes were drilled right at the edge of the cutout. I made some thin pieces to make "back up" material for new screws, and build up the area a bit - there is plenty of clearance for this. I bought new screws, but of course they are bright brass. After testing a few ideas, I found that heating them to orange heat in the upper part of the propane torch flame did a great job of creating an "antiqued" finish that matches the others, and the escutcheon, rather well.

I was hoping to post some pictures today, but since the cabinet needed extra attention, the chassis never made it into position.