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Restored 42-321 - Printable Version

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Restored 42-321 - andrekp - 08-19-2009

I'm new here, so hi everyone!

Longish post, and covers more than just electronics, but I didn't want to break it up:

I am really a tube guitar amp guy, so I know very little about tube radios, except what I can figure out on the fly, or transfer from my knowledge of working on tube guitar amp circuits, or my long-ago Marine Corps avionics days. But I recently became fascinated by the idea of getting an old tube radio in a wooden cabinet to tinker with.

I found a 42-321 at an "antique mall" for a decent price with a tag that said it worked. I figured what the heck.

It was in pretty good shape overall. The cabinet had apparently been stripped down to bare wood and veneer, as it was bare and the trim was no longer painted. One small patch of veneer toward the back had chipped off, but nothing major. Still had both its knobs. Grill cloth was adequate (but more on that later). Looking in the back, arial was intact, chassis was just dusty as all getup, and from what I could see the speaker was intact. Someone had put a new cord on it at some point (brown, non-polarized, lamp cord). The one cap visible was clearly old, if not original.

Operating under the (I know, foolish) assumption that SOMEONE had turned it on already, in order to decide it was working, I very carefully did the same. Worked fine with a slight hum and a little unsteadiness (as compared to its newly restored state now).

Pulling the chassis revealed that the caps were all old/original. I suspect that someone, probably a long time ago, might have done some work on this. Obviously, the cord was newer, though maybe not too new as it was non-polarized. All the caps were the old dingy yellow, wax covered monstrosities, and were all marked Philco. Wiring was obviously old, but was a mixed bag in terms of insulation. In the end, there was only one length that was the old rubber type with the insulation crumbling off that needed replacing. The rest was either cloth, or some other old but intact type.

So I found the schematic and layout at Nostalgia Air and started studying the thing. Ordered a new set of caps from Antique Audio and replaced the old ones. Used the same values for all. Basically, I used the technique of cutting out the old ones, but leaving their leads to form a loop to solder the new, much smaller replacements in. I figured this would be minimally invasive to the rest of the componants/wiring. Replaced the dual filter cap with a pair of 22mf electrolytics I had on hand. Then I added a fuse and put in a new polarized cord with the hot wire running directly to the fuse first, then to the one/off switch, as I should be. Other than replacing one bit of crmbling wire, I left the rest of the electronics as is.

On to the cabinet: I wanted to keep it simple, and as the wood on this thing had really nice grain, I just wanted to emphasize it a bit. So I put a couple coats of natural (tint-wise) Danish oil on it. It looks really nice and was exactly the effect I was looking for. For the grill - well the old one was odd. Somebody had taken a hard painting canvas, cut it to size, pasted the grill cloth onto the front, then pasted that into position within the cabinet. It did the job of keeping the grill cloth taught and in place, but it also (obviously) blocked the speaker! (the only sound would come out the back side of the cabinet) Clearly a cosmetic fix. So I had ordered some Philco diamond pattern grill cloth (the wrong year for this, but I liked it and am not the Smithsonian). Using that same hard frame, I cut out out in the middle so the speaker would not be blocked, and mounted the new cloth on it. Worked well. I was going to get some
dark brown paint at re-paint the trim, but I couldn't find a brown dark enough by itself and got bored with the idea for now. Cabinet looks great either way - so maybe eventually.

So I vacuumed all the dust off of everything, sealed it all back up, and fired it up. It plays beautifully. (sounds a lot better than the solid state radio that I've been using for AM). Nice warm, slightly compressed, tube sound. Love it. Looks killer too. Alignment seems pretty good. A station I know to be at 610 comes up around 625-30 on the dial. Reception is good all up and down the range. I see no reason to screw around with that any (especially since I don't have a sig gen and don't really know how anyway!).

As I said, I think it migt have been worked on in the past. Evidence: the apparently newer cord, the lack of consistent rubber wiring, the obvious cosmetic alterations, 100% intact speaker. But it also was either old or original, as evidenced by the old Philco labelled caps. However, I find it hard to believe that caps from 1942 (or possibly earlier) were intact with no sign of leakage and still operated within the circuit. But what do I know about these things? An equally likely possibility is that someone found it original and tried to fix it up minimally for sale.

A couple of questions came up during the rebuild:

1). Inside the chassis, right in the middle, there is what appears to be a big cap, that has a wire wrapped around it for a few turns. The layout calls it a condenser and choke assembly. No values for either the cap or coil are marked on the schematic. (On the schematic, it's a cap, leading to a coil, leading to ground) I presume that the cap part is marked on its case. I assume it's just what it appears to be - a cap wrpped by wire. I didn't touch it for now, but maybe since it's probably an old cap I should. Could I just slide out the cap, install a new one in its place with the same value, maybe just setting it inside the coils (since the new cap will be smaller)... Or maybe get a new cap, wrap a few turns of wire around it, and use that... Or maybe just leave it alone, since it's all working fine for now. Any thoughts?

2). On the top of the chassis there is another mystery (to me) cap. .0015mf 600v. On the schematic, it runs from one end of the arial transformer to ground. Physically, it runs from one lug of what must be that transformer to a tab on the tuning cap, that is effectively the chassis. I have a .001 630v on hand. I'm not sure if this cap is performing some sort of tuning function, or just blocking something, so I wonder if .001 is close enough for these purposes, or if I should hunt up a .0015?

3). The old filter cap was marked as a 20-20 on the schematic. The one I removed was a much bigger 50-30, or something like that. I knoew I had a pair of 22's sitting around, so I just planned on using those and didn't order anything different (didn't see the values of the original until I pulled it). I used the 22's and there is no hum. So I assume that's adequate filtering and the bigger cap was either a later over-rated replacement, or whatever Philco had on hand when it was built. Sounds like I'm good to go with the 22's right?

4). I have a set of Philco decals from Antique Audio. They look to be a water slide type like for model airplanes. (true?) Does anyone know if I can simply apply these over the danish oil finished wood, or if I need to do something further if I want to use them. I know water slides can be picky about surfaces. I can do an experiment with an unneeded decal if it comes to it, but if anyone knows offhand from experience...

5). Did these things have an open back, or some sort of harboard covering? Mine has no back, and the few pics I've seen of others, when they show the back, have no back. Just curious.

Anyway, a great experience with a gret result. Love the radio. I put on a ballgame and it was like a sound from my distant youth. Not sure how often they go this easy...

Maybe when I figure out how, I'll get a pic or two up here.

Andre

Re: Restored 42-321 - codefox1 - 08-19-2009

Well the first 3 are easy enough

1. You can wire in a .25 mfd 630 volt capacitor and replicate the windings as on the original. Actually the set will probably play OK without the windings. Rather that than have a possibly dangerous leaky capacitor there.

2. The .001 you have on hand is probably OK. But you can get .0015 caps easily enough.

3. The 22 mfds you have are OK to use. A larger value would cut down some on the hum, but I wouldn't go above 40 mfd for fear of shortening the life of the rectifier.

The remaining questions I think will be picked up by the more experienced.

If the alignment has not been performed, you should do so with a signal generator. Then you can get the stations to come in where they are supposed to be. Whenever you do a wholesale replacement of capacitors, an alignment is in order.

Best of luck on your venture. Quite a few of us on this forum also build and fix guitar amps.

Re: Restored 42-321 - andrekp - 08-20-2009

Thanks codefox1.

Yeah, tube guitar amps are a gateway drug for sure...

1). In my digging around here I saw a post that seems to be talking about the same part. If I understood correctly, this was a tuned circuit (to IF) that is not entirely necessary now. Replacement with just a cap is OK. Not sure I fully understand why, but maybe someone can elaborate.

2). Yeah, I just don't have a .0015 on hand, and no immediate need to place an order which could include it. I was just thinking that given its location and small size, it could be just doing a blocking job of some sort and its value was less critical.

3). Like I said, no hum right now, so I'll just assume the pair of 22's is adequate filtering for the moment. The old caps were probably just what was on hand in this less value-important area.

I'm not particularly concerned with the alignment for now, which seems pretty close. Don't really have a sig gen anyway. Unless someone knows of a quick and dirty for minor adjustment using a known radio station.

Is there any real benefit to putting in "new" tubes if the ones in there now are working? I ask because in some applications (like guitar amps) the old ones can still work but start getting dull after a time, but before they fail outright. But maybe radios don't operate in such a way that it makes a difference. (A full set for this thing looks to be <$30)

Re: Restored 42-321 - Ron Ramirez - 08-21-2009

Hi Andre, and welcome!

andrekp Wrote:Is there any real benefit to putting in "new" tubes if the ones in there now are working?

Generally, no. If the old tubes still work fine, there is no real benefit to replacing them.

Since you already have answers to three of your initial questions (thanks, Codefox! Icon_smile ), here are my thoughts on your other two:

4. The decals are lacquer decals designed to be placed in water to be removed from their backing and placed on the radio. Since they are designed to be used with lacquer finishes, I do not know how they would react to Danish Oil.

5. As far as I know, all AC/DC Philco sets had backs. I would think the 42-321 probably did as well, although I could be wrong. Note that this is NOT true of prewar AC-only sets, which generally did not have a back.

Re: Restored 42-321 - andrekp - 08-25-2009

OK, thanks everyone.

Maybe I'll take one of the decals I'm not using and try it one some Danish-oiled scrap and see what happens. I'd rather not do any further finishing if I don't have to - as it looks great with the Danish oil and my goal was a nice piece for my own enjoyment - not a period perfect restore.

I suppose I could make a typical looking back out of some hardboard with some vent holes cut. It'd mostly just be to keep nosey critters from sticking their noses in.

So if I attempt to replicate that cap & coil combo, what is the critical feature of the coil? The number of turns, the size of the turns, the fact that they wrap the cap, or....? I ask because, obviously, a new cap is not going to be thumb-sized like the old one, so if I reuse that same coil wire, the cap will be swimming in it! Does that matter? (I was thinking just slide the old cap out and put the new one in its place.)

Andre

Re: Restored 42-321 - codefox1 - 08-26-2009

So far as the capacitor with the winding, you can either leave the existing coil there, and put the new cap inside the cardboard tube that remains after you gut out the old capacitor, or make a new coil with a few turns of wire in the same fashion over the new smaller component. In practice it doesn't make a lot of difference. I know that some people just replace the cap, forget about the winding, and the radio plays just fine.

Re: Restored 42-321 - Ron Ramirez - 08-26-2009

You may want to read Chuck's info on these LC wavetraps designed to be resonant at the set's IF frequency:

http://www.philcorepairbench.com/tips/svctip37.htm

While it mainly describes the Philco "special" capacitor which was used after WWII without the wire wound around its body, the info is still applicable to your radio.

Re: Restored 42-321 - andrekp - 09-09-2009

Back to the .0015 cap. Surprisingly, I have had difficulty locating one. (.001 & .0012, but no .0015)

However, the more I look at it, the more I wonder if I even need it. The cap is part of the antenna transformer cicuit. The secondary of that circuit has one side connected to the internal antenna, and the other connected to the cicuit. However, the .0015 cap is on the primary side of that trannie. On the primary side, one side CAN BE connected to an external antenna, while the other side is connected to ground via that .0015 cap.

Which leads me to two conclusions:
1). It should only be needed if I am using an external antenna (which I am not).
2). It is likely just a cap used to shunt high frequencies to ground, or to somehow help develop the frequencies coming into the antenna. In either case, it shouldn't really matter the exact size.

So I figure for now, since I'm not using an external antenna, I'll just leave the original cap in for its looks, as it is not harming anything, nor can it, even if it is shot.

Make sense?

(I did order a .22 cap to replace the one in that coil/cap combo I noted above. I figure I'll just slide out the old cap and put the new one in its place, using the original coil.)

Re: Restored 42-321 - Raleigh - 09-09-2009

JustRadios.com offers .0015uf, 630 and 1600 volt, caps in several different styles and materials. However, your analysis of the need for the cap when you are not using an external antenna makes sense, so maybe you don't need it after all.

The only problem with buying from Just Radios is the $15 minimum order amount. I order from them a good bit, but I bundle my orders to meet the minimum amount. Dave Cantelon, the owner, gives good service. Just Radios is the only place I know to buy caps in the prewar values. I don't know how much it matters, but I like to use the original values.

Re: Restored 42-321 - exray - 09-09-2009

That cap is simply for decoupling and to isolate an external antenna from the chassis. Most any similar value would do. Your .001 would be fine. Do use a 600 volt cap though.

Re: Restored 42-321 - andrekp - 09-10-2009

Thanks, I'll add justradios onto my list.

I'll probably just throw the .001 in there when I get to doing that internal one. Right now it does nothing anyway, as the circuit is: Chassis => Antenna coil => .0015mf cap => Chassis.

Re: Restored 42-321 - andrekp - 09-14-2009

I slid the .2 cap out of the coil wrapped around it (it meaures .28mf) and put some zip ties on the coil to keep it together. Then I attached a new .22 cap in place of the old one. Everything works exactly as before.

A replaced the .0015 cap with the .001 I had on hand. No change in anything, but I wouldn't expect one.

The radio has some hiss, probably from the CC resistors it uses. I probably won't worry about changing anything else, unless I reach a point where I want to rewire the entire circuit. A project not for now.

I wonder if there is a way, without using a sig gen, to tweak the two IF transformers slightly to optimize their tuning? i.e. work from the end of the circuit to the front, and adjust each on in turn until the station tuned sounds the loudest...or something like that?

Re: Restored 42-321 - exray - 09-15-2009

Its not highly recommended but you can simply peak them by ear if the radio hasn't been severely monkeyed with. If you find the adjustment WAY off you might want to delay the procedure until you can get a sig gen. Normally a fresh tweaking will just be a fraction of a turn on the trimmers.

Re: Restored 42-321 - andrekp - 09-15-2009

Well the reception is nice and clear all up and down the dial. It's just that everything is about 30khz high on the dial. I've been looking over the schematic, and thinking about how it works, and I think I could probably do a very light tweak on the trimmer to the oscillator circuit and move the entire band down a small bit. Shouldn't have any reason to screw around with anything else at this point, since otherwise reception is pretty ideal. Should really even need a sig gen for that, as I'm not going far, the radio will be in tune with itself, and for all I know - I'm putting it right back where it's supposed to be.

I'll mark it first. It'll either work or it won't.

Re: Restored 42-321 - andrekp - 09-15-2009

OK so I tweaked the trimmer on the oscillator and moved the stations down a little on the dial.

The dial is now pretty accurate all the way up to about 1200, at which point stations are low (i.e. 1540 is at about 1450). It's almost like the very top of the dial is forshortened. I get all the expected stations, as compared to a digital radio that works, but the top ones a squeezed down on the dial. (this is the first time I've done a full spectrum comparison - All the ones I've previously spot checked ahve been fine, but I've never noticed the squeezing at the very top.)

My suspician is that this must be because the antenna tank trimmer needs tuning. I arrive at this conclusion through the idea that the oscillator should only adjust WHERE the stations are on the dial, but the antenna tank would control the spread of the band. I'm thinking that maybe the band pass of the antenna squeezes on the ends and it's currently adjusted so that it is above the middle so most of the dial is fine, but the top end is starting to get into the squeezed end.

Does this make sense?

Will adjusting the tank trimmer spread the band out properly (then allowing me to move it into proper position with the oscillator trimmer again)?