[Arran]

Ron;
  Did you power the set up with the shorted 6X5 installed? Was that 6X5 in the set when you got it or was it a replacement you already had? Since this was a high hour set, and obviously the victim of a lightning strike on the antenna coil primary, the power transformer may have already been damaged. I would conduct a resistance check on the two legs of the H.V secondary winding, and a resistance check on the primary, shorted turns on either would cause it to heat up and smoke. This is something I usually do before I even begin work on an old radio, along with all of the other transformers and coils, let's just say I didn't used to and got burned.
 I found a note in my 1937 Sylvania manual that recommends fusing one leg of the transformer primary in any set that uses a rectifier with an indirectly heated cathode, specifically a 5Z4 or a type #84 but it would also apply to the #84's octal cousin as well. Most of the transformer problems I've encountered were in sets from the 1940-42, and they were in sets that used 5Y4Gs, not 6X5s of any sort, the last one being a CGE KL-500. I think maybe it's just a pitfall of immediate pre war radios, maybe they were trying to cut back on materials, less insulation, thinner gauge wire to save on copper, who knows? I have noticed that most companies reduced the size of their power transformers in that era, then went in the reverse direction after the war. We all know about the failure prone output transformers in 1939-42 Philcos which may or may not be related. 
Regards
Arran

[Ron Ramirez]

From my workbench:

Well, Arran,

I hate to admit it, but yes - I failed to follow my normal protocol which is to test tubes before applying power. The 45 to 90 seconds of operation with a shorted rectifier certainly did not help the situation, although I think the damage was probably already done.

The shorted Brand Z 6X5 was in the radio when I acquired it.

I just ran a resistance check of the power transformer windings. There are no resistances on the schematic, so I have nothing to compare the following readings to, but I think the following speaks for itself:

AC primary, two primaries connected in parallel for 115 volt operation - 7 ohms
6.3V winding - 0.5 ohms
One half of HV secondary - 188 ohms
Other half of HV secondary - 122 ohms

While the two halves would not have exactly the same resistance, I think the discrepancy is great enough to indicate shorted turns in the half reading 122 ohms.

I also measured from the main B+ line to ground. I get a measurement of 58K ohms. There are two resistors in series between B+ and ground - a 22K and a 33K which adds up to 55K. The 22K resistor measures around 23K and the 33K measures 35K which accounts for the 58K between B+ and ground. If there was a partial short or leakage between B+ and ground, I would expect the 58K measured resistance to be lower. So I feel confident in my diagnosis of a bad transformer.

Hammond sells a power transformer that looks like it would be a good replacement. The 270DAX has the same mounting footprint, and has a HV secondary of 520 VCT @ 104 mA and a 6.3V winding @ 3.5A. The original transformer is rated at 570 VCT @ 70 mA and 6.3V @ 2.5 A. Slightly lower B+ would not be a bad thing.

If I go the new route, though, it will have to wait a while. I might take a look at some other Philco sets of this time period and see if there might be another set that uses a similar transformer, and then look for a junk set using such a transformer.

I think that after I replace the power transformer, I will install silicon rectifiers and a dropping resistor in place of the 6X5 before I apply power again.

In other news...The flocking fibers arrived today. Perhaps next week I'll try that and see how it comes out.

[Ron Ramirez]

The junk set idea is out. Similar six-tube Philcos with vertical mounting power transformers all have high voltage secondaries that are only capable of 45 to 52 mA; the 42-720 transformer is rated at 70 mA.

So it looks like it's the Hammond replacement or nothing.

[Arran]

Ron;
  Have a look at what Edcor has to offer, they seem to have a larger selection of vertical mounted transformers, and their pricing is better then Hammond. I would also not overlook the option of using a transformer with a 5 vac rectifier winding, I think that would be a better idea then using silicon diodes, which, in addition to being instant on, can also short out. Keep your eyes open for other makes and models of radio chassis for a donor transformer, such as a junker post war radio-phono combo unit or tube type cabinet stereos, I've used those as organ donors before.
 I don't think that 45 to 90 seconds was enough to have damaged the HV winding on the transformer, it must have been damaged to start with. It was obviously a high hour set, and the victim of a lightning strike, it's also possible that some foolish person tried plugging the set in and powering it up before you got it, the brand Z 6X5GT may have been defective, or it may have gone to short thanks to a shorted filter or paper cap. That being said a 70 ma current rating is right at the upper limit for a 6X5 of any sort, though at least they did not completely ignore it like brand Z did with their 1940 line.
 I can't say that the 7 ohm primary resistance is wrong, given that yours was likely designed for 115-230 volts at 50-60 cps. The RCA Victor service manual I have near me now seems to quote about 9 ohms as a resistance for a typical power transformer primary, this is for a 105 vac -125 vac transformer at 50-60 cps, 13.5 ohms for a 25-60 cps transformer.
Regards
Arran

[sam]

hey ron
would the 41-240 help you
sam

[Ron Ramirez]

Arran

Thanks. I just visited Edcor's site. Unfortunately, I did not see anything that would work well without some modifications. The smallest Edcor transformers I found that I thought might work have a larger footprint than the Hammond 270DAX and, therefore, might not physically fit on the 720 chassis. The Hammond 270DAX has the same footprint as the original transformer.

I will, however, keep an eye out for a junk set of some other brand.

It looks like what makes this set draw more B+ current compared to six-tube Philco domestic sets are the two 7B5 tubes connected in push-pull. Six-tube Philco table models intended for sale in the USA such as the 41-240, 42-345, 40-140, and others have a single ended output, and their power transformers are only rated at 45 to 55 mA HV DC output.

Sam - check your PM. I could use a 41-240 for other parts.

[Steve Davis]

Hey Ron, here you go, maybe you could do something like this.   http://www.ebay.com/itm/121468391940?_tr...EBIDX%3AIT

Steve

[Ron Ramirez]

Well, the 42-720 chassis isn't nearly as large as a 118 chassis...

[Arran]

Ron;
  I was thinking that a transformer from a 40-180 would work, but the B+ is only around 180-200 volts in those models, that was how they got away with using a type #84 tube and how the later versions got away with a 6X5. Your set seems to use about 250, which with the current draw of the 7B5s, was likely running the 6X5 pretty hard. If you could convert the set over to using a transformer with a 5 v rectifier winding, that would be the best solution for the long haul.
  If you could fit any r one of these on the chassis it would be winners, a 570 volt HV winding is sort of odd, though I did find one with that:

https://www.edcorusa.com/xpwr008

Or this one which is even closer to the original, add an extra $1.54 for 240 volt line operation.

https://www.edcorusa.com/xpwr014

This seems to be the closest with voltages, since it has a a 570 v HV winding, though more expensive then the second one, and probably overkill for amperage, you could use a 5U4 with it though and kiss the wimpy 6X5s and silicon diodes goodbye:

https://www.edcorusa.com/xpwr049

Regards
Arran

[Ron Ramirez]

This morning, I removed the power transformer and performed a limited autopsy (limited in that I did not open up the windings).

[Image: http://www.philcoradio.com/images/phorum...e00020.jpg]

It doesn't look bad in the photo, does it? The only tell-tale signs of burning are some of the wires exiting the windings, and the charred appearance on the inside of the bell on the left. But the smell of roasted transformer is very strong.

Note the handwriting on the winding:

[Image: http://www.philcoradio.com/images/phorum...e00021.jpg]

'Tis a shame to lose this transformer, but if this radio was indeed a preproduction sample (and it gives many indications that it is), someone took it home and used it to death.

Arran, I really appreciate the effort you made to find suitable Edcor replacements. There is only one problem - the mounting width of the holes on either side of the bell is 2-1/2 inches. The original transformer only has 2-1/8" on either side of the bell, and as you will see in the photo below, there isn't much room to go much more than this.

[Image: http://www.philcoradio.com/images/phorum...e00022.jpg]

In addition, whether I purchase Edcor or Hammond, another hole will have to be drilled where the melted wax from the transformer formed a round indentation on the chassis - both new transformers have wires protruding from both bells while the original had all wires coming out one end only (hence, only one large hole in the chassis).

Arran, you've just about convinced me to rewire the rectifier socket, use a new transformer with a 5V winding, and use a 5Y3GT in place of the 6X5. But there is still the issue of the mounting holes. The Hammond 270DX has a footprint of 2" across the bell x 1.938" between bells. Now, there's the 270EX, more current capacity, with a footprint of 2" x 2.688".

270DX - 550 VCT @ 104 mA; 5V @ 2A; 6.3V @ 3A
270EX - 550 VCT @ 144 mA; 5V @ 3A: 6.3V @ 4A

Both exceed the 42-720's requirements (570 VCT @ 70 mA; 6.3V @ 2.5A) and are both small enough to fit. As you notice in the photo just above, there are holes drilled already to allow for up to a 2.75" mounting length (between bells).

The 270DX is $59.07 at Mouser; the 270EX is $64.76.

So I need to raise a little $$. Anyone need some Philco coils rewound? I'll place an ad in the For Sale or Trade section since ads are not allowed here.

[Arran]

Ron;
  It looks like Philco punched a hole in the chassis where wires would normally come out the other side of the transformer, but then they decided to block the hole with something. With regard to mounting, that is a tricky issue, the first one I linked to is the Edcore XPWR008, the width at the bell ends is 3.22'', the space between mounting holes on the ears of each bell is 2.5'', the maximum length from the outside of one bell to the next is 4.189''. The distance between the mounting holes from one bell to the next is 2.955'', I'm not sure whether that is between centers or the maximum or minimum as they look elongated or slotted, I think it may be minimum as the holes for the leads are exactly 3'' from center to center. 
  For the Edcor XPWR014, the end bells are the same as the XPWR008, with the mounting holes at 2.5'' on center. The maximum length of the transformer is 3.435'' so more then 1/2'' shorter, and the slots are marked as 2.201'', which I think is a minimum measurement given that the holes for the wires are 2.246'' on center. For the Edcor XPWR049, the one with the 6 amp rectifier winding, the dimensions are the same as the XPWR008. It looks like any way you go there is going to be some drilling and tapping involved. 
   Since most of the transformers looked at have a H.V winding that exceeds the original in current capacity, except that the net high voltage will likely be higher then the ratings posted, except for the Edcor XPW014 which has it's rated at 75 ma. What I would like to see are more reproduction power transformers with a horizontal mounting, that style was popular with many radio manufacturers.
Regards
Arran

  

[Ron Ramirez]

Arran

That isn't a hole in the chassis - re my previous post:

In addition, whether I purchase Edcor or Hammond, another hole will have to be drilled where the melted wax from the transformer formed a round indentation on the chassis - both new transformers have wires protruding from both bells while the original had all wires coming out one end only (hence, only one large hole in the chassis).

What you see there is a spot neatly made by the wax that melted and came out of the old transformer. There is a hole cover in the other bell of the original transformer, and much of the wax collected there when it melted out and left a bowl-like indentation in the chassis. No hole in the chassis there. I will have to drill a hole, and enlarge it with my Ryobi "Dremel" tool and a metal cutting bit.

Anyway, I'm going to have to go with Hammond because an Edcor would be rubbing against the rectifier tube and one of the output tubes on one side, and against the volume control on the other side - that is, if it would even go in that limited space.

[Arran]

Ron;
  I guess I missed that part about the spot of wax, at least it gives you a reference of where to put the hole. I wonder if the production 42-720s, if it ever made it that far, used a different chassis and or power transformer? It looks almost like they used the same iron cores for that transformer as what they used for the 42-380 style chassis, but vertically mounted, since the wires all come out the same side. It's curious that Edcore can't make a narrower transformer, maybe they are concerned with core saturation more so then Hammond? But from what I heard Hammond tends to underrate the current and primary voltage ratings of their transformers, what I can't stand are those cheesy plastic tags they glue on to their transformers now, the old ones had a nice metal plate riveted on. 
Too bad that you are in Indiana, I could lone you a Greenlee punch to make that extra hole rather then you having to flog one out with a file or rotary tool. I have an incomplete set of those punches, though I have most of the important sizes, sometimes they have sets on fleabay for good prices, but it only makes sense to buy if you do a lot of this sort of thing.
Regards
Arran

[Ron Ramirez]

I did a bit of research on this subject. I read that Hammonds tend to run warm to semi-hot, so the ratings are probably a bit under-rated as you say. This is why I am considering a 270EX instead of a 270DX - my radio will never draw as much current as a 270EX is rated for unless it has a major problem!

The 42-720 is not listed in Philco's 1942 RMS Year Book. Thankfully the 42-724 chassis is (nearly) identical. Maybe the 720 had a very short run? It did come out in January 1942, after we had entered the war, and Philco would shut its civilian radio production down by Spring '42 to convert 100% to war work for the gubmint.

Michael Feldt has a similar prototype 43-720. You can see it here. Either his cabinet has been refinished, or else Philco did not apply the fake burl photofinish to the upper third of that cabinet. His cabinet does not appear to have the decals around the knobs, and the knobs are 1937-1940 Philco rather than the slightly smaller 1942 knobs. His has the proper escutcheon. Oddly enough, his set has five tubes while mine has six. I have a prototype 46-421 in a very different cabinet than a production 46-421, that has the same type of typewritten label inside as his 43-720. But I digress.

I should write to Michael and see if he has a larger scan of that dial scale as I will need to have one made for mine. I could scan and recreate mine but it might be quite the chore since it is broken in three places.

Philco also issued a new farm radio in January 1942, the 42-131.

http://www.philcoradio.com/gallery/1942b.htm#a

The cabinet is identical to the 42-720 but only has two controls instead of four. I have seen one or two of them on eBay in the past, never at a swap meet. 10,550 were made but where are they? This may be one of those models with few survivors. But I digress again.

[Ron Ramirez]

Having finished the Canadian 39, before I start on another set I decided to revisit this 42-720.

That dial backing plate really looks bad...

[Image: http://www.philcoradio.com/images/phorum...e00023.jpg]

Here's how it looks in back:

[Image: http://www.philcoradio.com/images/phorum...e00024.jpg]

I removed this metal piece, and using a combination of scraping and sanding, managed to remove all of the old, worn, blackened flocking.

[Image: http://www.philcoradio.com/images/phorum...e00025.jpg]

I found that the backing plate of my 41-246 is identical, so I used it as a pattern as to where to mask it off. I then proceeded to paint it brown.

[Image: http://www.philcoradio.com/images/phorum...e00026.jpg]

The next step is to apply the new flocking, once the paint dries. I will use the same method used by Greg (NostalgiaRadioTime) as outlined in this thread. Wish me luck...I shall need it...