My First Tube Radio Restoration Log

[G7PSK]

You really need to megger the windings of the transformer. Old style varnish insulation absorbs moisture which will lead to breakdown of the windings, either put the transformer in a warm oven (not hot) to dry out or short out the secondaries and apply about six to twelve volts to the primary winding in order to warm the transformer safely, Keep the transformer just warm to the touch for eight to ten hours this will drive out the moisture.

[ElectroIrradiator]

Apply the same treatment to the audio output transformer, now you are at it.

[EpicIntelGamer]

You really need to megger the windings of the transformer. Old style varnish insulation absorbs moisture which will lead to breakdown of the windings, either put the transformer in a warm oven (not hot) to dry out or short out the secondaries and apply about six to twelve volts to the primary winding in order to warm the transformer safely, Keep the transformer just warm to the touch for eight to ten hours this will drive out the moisture.

I appreciate the suggestion but unfortunately most of the outside of the transformer is covered in liquid electrical tape and is quite sealed up (due to the repairs of the wires I had to do). So I'm not going to be doing this to the transformer, I'll cut my losses and just continue onward.

Apply the same treatment to the audio output transformer, now you are at it.

I don't think there is an audio output transformer on this radio....

Thanks anyway guys!

And a bit of an update, I took apart one of the IF transformers because the wire that was going in the top was in need of replacement. A lot of the coating on the windings have broken off but I patched it up with liquid electrical tape and hopefully everything will be good to go.

[ElectroIrradiator]

I don't think there is an audio output transformer on this radio....

There has to be one. Philips didn't start to experiment with direct drive speakers in domestic tube radios until the late fifties.

Schematic claims the output transformer is not on the main chassis, but may be attached directly to the speaker. Check whatever is connected between '50' (the speaker connector) and '46' (the actual speaker).

[EpicIntelGamer]

I don't think there is an audio output transformer on this radio....

There has to be one. Philips didn't start to experiment with direct drive speakers in domestic tube radios until the late fifties.

Schematic claims the output transformer is not on the main chassis, but may be attached directly to the speaker. Check whatever is connected between '50' (the speaker connector) and '46' (the actual speaker).

Well now that you mention it, there is a transformer on the speaker itself.

Like I said though I really cannot be bothered to do any of that elaborate stuff to these transformers.

Thanks anyway.

[Chryseus]

You don't need to generally do anything with the transformers, I've never had a transformer that has a good resistance check fry itself.

[EpicIntelGamer]

You don't need to generally do anything with the transformers, I've never had a transformer that has a good resistance check fry itself.

Good then!

Also, more good news/updates. I figured out how to wire up and test the speaker!

I applied 15VDC through a 330ohm resistor (just to make sure it doesn't pull too much current but really I found out its not needed) into the field windings and I managed to clip on some crocadile clips to the output of the audio transformer (directly to the voice coil) and I powered it using a very small few watt amplifier that I had and it seems to work just fine!

I'm very happy to know that the speaker is working and doesn't sound too bad either!

I also managed to re-install and completely connect the main transformer after replacing all but 2 wires coming off of the transformer and consulting with the schematic for about an hour.

I'm also ordering all of the capacitors tonight so I'll post a list here shortly that I hope someone can look at and see whether or not the ones I've chosen for replacement will work.

Thanks.

[EpicIntelGamer]

So can I use these (in no particular order) to replace the caps in the picture below? (the top three in the picture are all in the big electrolytic can)

Electrolytics to replace the big can (top three in the list in the picture)
mylar and ceramic for the rest

Will this work? Also, what are those mmf caps? I didn't try to find replacements for those.

[fpliuzzi]

using the nomenclature shown in your example capacitor list Mmf means micromicrofarad, which is the same as picofarad.

1 Mmf = 1pF
1450 Mmf = 1450pF = 1.45nF
 
  and so on...

Frank

PS, On old schematics I've seen micromicrofarads written as mmfd, MMFD, MMF, Mmf, PF, pF (etc).

[EpicIntelGamer]

using the nomenclature shown in your example capacitor list Mmf means micromicrofarad, which is the same as picofarad.

1 Mmf = 1pF
1450 Mmf = 1450pF = 1.45nF
 
  and so on...

Frank

PS, On old schematics I've seen micromicrofarads written as mmfd, MMFD, MMF, Mmf, PF, pF (etc).

Thanks for the info! I do believe though that they actually are not normal capacitors, but rather variable caps and possibly part of the tuning thing so I guess I didn't need to know anyways, but its better to know it for later anyway!

I just ordered all of the caps and the tube I broke and surprisingly it came out to be almost exactly $50 on ebay! That's on top of the $20 I've already spent on heatshrink and wires!

This is getting quite expensive but hopefully this will be all I need to buy.

Thanks.

[fpliuzzi]

Thanks for the info! I do believe though that they actually are not normal capacitors, but rather variable caps and possibly part of the tuning thing so I guess I didn't need to know anyways, but its better to know it for later anyway!

If I'm looking at the correct schematic, it looks like items 11, 12, and 23 (the 4.72nF, 1.45nF, and 1pF caps) are not depicted as variable capacitors. All three are shown on figure 3 (bottom view) of the pdf I downloaded as though they're fixed value capacitors.

The 1pF cap (item  23), shown in figure 3 between the two wafers of the front panel center switch, may be a mica type capacitor. If it is you will not need to replace that one, unless it is cracked.

Depending on the construction of items 11 and 12, these two capacitors may have to be replaced, along with the electrolytics and any paper type capacitors. From the bottom side photo that you supplied, the silver colored capacitors (items 11 and 12 with the dots painted on them) may be flat paper capacitors in plastic shells. If they are, they'll be very unreliable after all these years.

Anyway, that's my take on those three particular capacitors using the info I have on hand. I now defer to the forum members who work on vacuum tube antique radios on a regular basis (possibly for decades).

Regards,
Frank

[Chryseus]

Any square shape mica caps are usually good, as for a 1pF capacitor you can just twist two insulated wires together and that will work fine.
If you're unsure if 11 and 12 are mica (I can't tell either) just leave it in place, you'll notice poor performance on the medium and short wave bands if they're bad.

[EpicIntelGamer]

Good news guys!

I think I have all of the parts now so it should just be a few hours to solder in the rest of the parts and then I should be able to power this up for the first time later today!

I'm really excited!

[EpicIntelGamer]

Well it looks like I actually don't have two of the caps, must still be in the mail.

They are tiny value caps that are replacing some paper caps.

I'm thinking I might just go ahead with the rest of the replacement and then just leave those originals intact. I'm going to run this on a tiny ballast, and with a variac so what's the worst that can happen?

Let me know what you think everyone.

[Chryseus]

Give it a go, I usually give a radio a run after replacing the main electrolytic, assuming none of the paper caps are completely gone you should get something out of it.
My method of repairing a radio is as follows:

Replace main electrolytic can and any leaking or exploded capacitors.
Using a variac and / or isolation transformer with a ballast (I.E 40W lightbulb):
Disconnect the HT output so you only have the rectifier tube and main filter capacitor, check that the high voltage comes up to the correct range (usually will be high due to little load) maybe 260V depending on various factors, check the filaments all light up.
Insert an ammeter in series or just monitor the HT voltage, connect the HT to the rest of the set and power it up, all being well the voltage and current draw should remain at a reasonable level and you should get sound out of the speaker if all is well.
If the current draw is reasonable but the HT is low then you probably need a bigger ballast or you can go without it which I usually do at this point.
If you get excessive current draw then you most likely have a bad capacitor or in some cases a shorted resistor (although I've never actually seen a shorted resistor).
Once it's working leave it on for a couple of hours to make sure nothing fails, particularly if you've only replaced some components, after that you can tweak it and perform an alignment as needed.

[EpicIntelGamer]

Give it a go, I usually give a radio a run after replacing the main electrolytic, assuming none of the paper caps are completely gone you should get something out of it.
My method of repairing a radio is as follows:

Replace main electrolytic can and any leaking or exploded capacitors.
Using a variac and / or isolation transformer with a ballast (I.E 40W lightbulb):
Disconnect the HT output so you only have the rectifier tube and main filter capacitor, check that the high voltage comes up to the correct range (usually will be high due to little load) maybe 260V depending on various factors, check the filaments all light up.
Insert an ammeter in series or just monitor the HT voltage, connect the HT to the rest of the set and power it up, all being well the voltage and current draw should remain at a reasonable level and you should get sound out of the speaker if all is well.
If the current draw is reasonable but the HT is low then you probably need a bigger ballast or you can go without it which I usually do at this point.
If you get excessive current draw then you most likely have a bad capacitor or in some cases a shorted resistor (although I've never actually seen a shorted resistor).
Once it's working leave it on for a couple of hours to make sure nothing fails, particularly if you've only replaced some components, after that you can tweak it and perform an alignment as needed.

I did try it out, first slowly, only going to about 80V then shutting it off to check the caps and what not. I brought it up to about 115 a few times and I notice that when I turn it on it initially draws a bit of current, then draws less for about 4 seconds, then the current returns to its initial range. I'm guessing that initial current is the filaments warming, then it settles when they warm and then it raises when the tubes begin to function?

Unfortunately all I get is a hum out of the speaker. The rectifier and output tubes get warm, but the other 4 tubes are not getting nearly as warm and are taking extremely long to warm up.

I'm pretty sure the speaker is wired on correctly but I could be wrong.

Help?

[EpicIntelGamer]

Oh and there was another knob on the front of this, but its contacts were horribly broken and I removed it.

The only two connections to the knob were from the rectifier's one leg to the one side of a capacitor. I don't know what this knob did and if maybe that is whats stopping this from working.

EDIT: after looking at the schematic, I found it is the tone control knob.

It looks like it either connects the cap to the one pin on the output tube or the other pin. Or maybe just one. The schematic isnt too clear on switches.

[EpicIntelGamer]

Any help? I don't know about tuning knobs, does anyone else?

[EpicIntelGamer]

Actually I think that judging from the schematics as well as the hum that I don't have the main filter caps hooked up correctly.

The schematic isn't very clear as to the polarity of the electrolytic filter caps 10x 10y and 10z, can anyone help.

[G7PSK]

My experience with those old radios is that they always hum without a signal, it sound like your output stage is working have yo tried turning the volume control the hum loudness should alter as you do, if it does you need to work backwards checking the if stages and ending at the input. This is where some form of signal injector comes in handy, there are plenty of designs around using just one transistor. It could well be that replacing the capacitors has altered the tuning of the if transformers so they will need adjusting but don't touch them yet.

[Chryseus]

The tone control switch isn't critical and should not stop it working, if you want to replace it just get a single pole switch and connect it between capacitor 22Y and the screen grid of the 42 (output tube).
Capacitors 10X and 10Y have their negative leads connected to the chassis, 10Z is connected between the positive side of 10Y with the negative side of 10Z going to the center tap of the transformer high voltage winding.

[G7PSK]

One of the oldest ways to get rid of hum is to switch the low frequency transformer leads over.

[Chryseus]

One of the oldest ways to get rid of hum is to switch the low frequency transformer leads over.

I can't really see how that works..

[EpicIntelGamer]

There's something not working correctly in the radio part of this radio. Me and someone else online have been troubleshooting it and I have got audio out of it but only by connecting in my own audio.

[mamalala]

One of the oldest ways to get rid of hum is to switch the low frequency transformer leads over.

I can't really see how that works..

Old tube radios (and also tube amplifiers) usually have AC heating for the tubes. Then, the rectification is usually not perfect. There is the "trick" to inject some AC (or "rectified" DC) at a 180° phase shift, which in turn will cancel out some of that. I have seen a lot of professional tube amps (for example the 30 watts amp in the Klein+Hummel OX studion monitors) that had a pot especially for that purpose: to adjust the amount of "directly injected hum" that cancels it out.

Or simplified: -1 + +1 = 0....

Greetings,

Chris