[morzh]

Hi Terry,

The big one definitely! It's a mess to take apart and clean the tar. After that it's a piece of cake. Keep the value of the input cap the same the others aren't as critical.

Meaning the can assembly? Ugh....dread at the thought Those are 1uF and 0.13uF.
Are the 1.0uF really electrolytics? They show "+" sign on the sch.

There are a few bakelite block caps and a metal can with a few .25 or .5 in it if memory serves me correct.

They are all .05uF (50nF) except one that is 0.01uF (10nF), also one fat box combining a 250 Ohm resistor and the cap.

I measured two of those yesterday (I disconnected one terminal to isolate the cap from the rest of the sch): one gave me 100nF i/o 50nF (not sure what to attribute it to), and another one was direction-dependent and kept giving me 75nF in one direction and 350nF (0.35uF) in another, the measurement sometimes for no reason would get down to 120nF (0.12uF).

>Actually, it was my intent today to measure the capacitance of those by de-soldering the surrounding wires. I wonder myself how they have survived these 80 years

It's the leakage that is problematic.

This may be leakage indeed.
But, considering 80 years, and I almost don't hear any 60Hz buzz, and the music is decent quality, the distortion clearly being the Amplifier type (one of the tubes I think is a bit off) - they survived remarkably well, still being somewhat capacitors, and not just bad resistors!

If you think of it, today's electrolytics at full rating (ripple curr. / temp ) are guaranteed for 2000 hrs - it is a pain to pick a good one today for a power application.

Q: The Tube 24A, #3 (the detector tube) has the Plate Voltage in the table equal to 35V. In fact it is 110V-120V.
The rest of the tubes have correct plate voltages, plus-minus 5% or so.
The plate voltage is acquired through a T-filter of two resistors (100K and 500K) and one capacitor (0.25uF) from the "J" voltage point (lower voltage Field Coil contact). This one being 136V also provides the Plate voltage for the 1-st Audio (tube 27) and it is correct (120V or so). The only explanation I have is that the tube is not conducting proper (large enough) current and so presents to little a load, and so does not drop the voltage enough.

This to me may be the result of the lower than needed screen grid voltage on the 1-st audio 24 tube, but it is not listed anywhere - in the table it is vacant.
The voltage is obtained from the "F" node which measures at 12.3V and then filters through 250K resistor and 0.25uF cap, becoming 9.2V at the grid.
I wonder if the cap is leaky and lowers the voltage more than it needs to be.
Well, seems like I explained it to myself and now am gonna go and measure the voltage without the cap.

Any thoughts?

[morzh]

Well, I de-soldered this capacitor, both ends (it's a dual one) and measured it: the leakage is nothing to speak of (on the order of 10 MegaOhms) and the capacitances are 0.35uF and 0.45uF, the latter being the one I was worried about.
The sch. shows 0.25uF and I don't think it matters here, if it is alrger - it is a filter; the more the better (to a point, of course) .

Then I measured the plate voltage of the detector tube while removing and touching the cap wire to the screen grid. No effect whatsoever. It stays 109V with or without the cap.

PS> I did try the tube swap (there are three 24 tubes here). Also no difference.
PPS> While measuring plate voltage (RF screen installed - no good reception) I touched the plate of the 1-st RF 24 tube with the meter, and the reception just rocked me! The sound was loud, clear, well-regulated by the Volume regulator - really great, no noise, almost no distortions. I just wonder if my Detector's plate voltage actually is OK. While in this good reception, I measured it's plate voltage - same 109V, not even close to 30V, like the chart says.
PPPS> The Cathode voltage at the detector is 1.4V (supposed to be 8V). At 8V the cathode current would be 8V/50K=0.16mA, so the drop on 600K of the plate load would be 0.16mA*600K=96V, so 136V-96V=40V which is way close to 30V than 109V is.

[Radioroslyn]

A few thoughts. If all the resistor are good then I would check the ground rivet. Sometimes they can develop a high resistance. The other thing is I've seen on some of these diagrams/voltage measurements the - lead is connected to the cathode rather than chassis ground.
You mentioned how the set sounds, try hooking up a modern 8 ohm speaker to it. It really sounds great! The spider and paper on the original are pretty stiff thus inhibiting the frequency response.
By the by are you from Wallace and Gromett land?
Terry

[morzh]

A few thoughts. If all the resistor are good then I would check the ground rivet. Sometimes they can develop a high resistance.

Where I saw the GND connections with rivets, I checked them - seem to be OK.

The other thing is I've seen on some of these diagrams/voltage measurements the - lead is connected to the cathode rather than chassis ground.

Even if so, the voltage will vary by the aforementioned 1.4V, which will make it 107.6 i/o 109V. Not much closer to 30V. Alas!

You mentioned how the set sounds, try hooking up a modern 8 ohm speaker to it. It really sounds great! The spider and paper on the original are pretty stiff thus inhibiting the frequency response.

I believe that. A tube amp coupled with modern speaker is a win-win combination.
Thing is - I no longer have the spider. The speaker did have the cone fully destroyed, but the spider was intact; the speaker shop confirmed that and re-coned it using original spider; somehow when it came to me, the spider was broken in half.
The Speaker shop felt responsible, so they took the speaker back and just put all modern stuff in there; the spider is no longer a part of this speaker.
So the speaker is still dynamic (with the field coil) vs. perm. magnet one. But the cone is modern.

By the by are you from Wallace and Gromett land?
Terry

Hmm, let me brush up on my trivia (not big on it at all, nor ever was)... By this do you mean the GB, our archenemy? Who now are conquering us back using TV reality shows?
If so, no, I am a Jersey man. Though originally from the ole Soviet Union (my Ellis Island was a very long time ago). I grew up with all tube equipment around me. Even in 80-s/beg. 90-s the TV sets were tube ones, and those modern ones with thyristor flyback section or with voltage multipliers were extremely unreliable. So I had my share of being exposed to the enchanting warm glow (back then we did not appreciate it much).

[Radioroslyn]

Let me scratch my head a bit more. I've got a working 20 I'll take a look at it and see what the voltages are on it. Just for laughs you could ground the cathode and see what the plate voltage is.
The caps in the big can are paper/non polarize. Once you get the can hot (in the oven @ 275 for 30 mins. You'll need something like a cork screw to pull out the paper caps, there in there pretty tight.
Yes GB. You type with a bit of an ascent. I get over to Jersey a lot for work. Tomorrow I'm headed up to Basking Ridge.
Terry

[morzh]

Thanks Terry, I appreciate the help.

Actually,, these voltages are measured relative to....God knows what.

Look! The table says for the Cathode of the 27 tube (1st Audio) that it is 8V.
Well, that's just dandy, considering the Cathode of the 27 is....tied to the Chassis GND.

So, relative to where is the voltage (and the rest of them) taken?

Yes GB. You type with a bit of an ascent.

First time anyone took me for an Englishman. It's probably the typing. When I speak, it's just about anything. One time it was, someone thought I was an Irishman. I don't look like no Irishman. Especially not then. Now I am all grey, there's at least something......

I get over to Jersey a lot for work. Tomorrow I'm headed up to Basking Ridge. Terry

Almost 60 miles up from where I am. I am in Jackson, where the Great Adventure is.

[Arran]

I don't think that the Philco 20 used electrolytic filter caps, I think they were like the earlier sets like the 511, 76, 86, and my Philco 96 where they used large value paper caps, the same is true of most AC radios from the 1928-30 period. The electrolytic filter caps that came out around that time were primarily wet electrolytics not the so called dry electrolytics, the 1931 Philcos used wet electrolytics originally made by "Mershon" in copper cans. You can replace these paper filter caps by using polyester film types of the same value and voltage, then you won't have to worry about them drying outlike you would with an electrolytic once they are inside the can.
When it comes to the capacitors you are really making more work for yourself trying to troubleshoot which ones are more leaky then others, unless this is just an educational exercise, they are 80 years old and if they haven't failed by now then it's just a matter of time. I would not want to take a gamble on burning out a speaker field coil or a power transformer just because an 80 year old cap might pass the test on an ESR meter, a cap might read fine on the ESR but fail shortly after you start putting voltage across it in service. Capacitors are cheap, maybe between $.25 to $1, transformers can cost $50 on up for a new one.
Regards
Arran

[morzh]

I am not sure they used lytics either, and from what I read, Mershons, when they appeared, were used on hi-end radios. Wide use of them came in 40-s, not in beg 30-s.
But why the heck do they show "+" signs on 3 out of 4 filter caps? Both in the sch and in the can layout?

BTW what are the voltage ratings of the filter caps / bakelite potted caps? Or should I start making educated guesses? I think for the filter 600V is a good number for the two in the LC filter. The one across the choke can be 50V (I think 20-30V is the dropout) but I would use a 100V, and the one across the multi-tap resistor needs to be at least 300V (150V dropout), so might as well make it the same 600V as the other two.

Yes, I will replace them. I just want to get the reception working properly, and not the way it is now - loud and clear when I touch the plates or remove the screen.(I think one tube is bad and hence my reception problems, and I even know which one is bad, by swapping them around).
---------------------------
So, who knows what is the reference point of the voltage measurement? I would assume GND (chassis) but then some things I cannot explain or understand (like the 8V at a cathode tied to the Chassis etc).

[Ron Ramirez]

Yup, the original capacitors in Model 20 filter cans are all paper.

If you're referring to the schematics on my website, I added those + and - symbols in case someone wanted to use electrolytics to replace the caps inside those cans. The symbols are also there to help beginners see which side has positive voltage and which side is negative.

Arran is right, it is better to use yellow mylar (polyester) to replace these caps - there is plenty of room inside the can, and they shouldn't go bad nearly as fast as a replacement electrolytic will.

I have to disagree with you regarding Mershons. Philco's first radio to use a Mershon was model 21 - it used one as the input filter. The other filters are paper. This was in early 1931. Then came the 70 and 90, which each used two Mershons. The 70 and 90 went into production in the spring of 1931, and were introduced to the public in June of that year. Model 112 was the last Philco to use all-paper filters (1931). The 112 was a slight revision of the 111 that came out in January 1931 as Philco's first superheterodyne radio. The 112, with its push-pull 45 output tubes, continued to use a large can filled with paper caps until its circuitry was slightly changed in the latter part of 1931 to use two 47 output tubes; at this time, two Mershons were also added to it.

Most of Philco's 1932 and early 1933 line continued to use copper Mershon electrolytics, although a few soon appeared that used aluminum cased electrolytics, such as the 53C cigar box pee-wee, and models 89 and 19.

By 1933-34 Philco had switched completely to aluminum cased electrolytics and no longer used the copper-cased Mershon electrolytics.

The copper Mershons are now highly collectible. When polished up and installed in a clean chassis, they look great in a 70 or a 90, or any Philco of the period that used them, for that matter.

[morzh]

The copper Mershons are now highly collectible. When polished up and installed in a clean chassis, they look great in a 70 or a 90, or any Philco of the period that used them, for that matter.

I take it, they are used for their exterior only, housing a new electrolytic cap inside? Or do they survive to be actually used by itself?

[Ron Ramirez]

Some folks stick new electrolytics inside the old copper cans - although opening them without damaging them, from what I have read, can be a challenge; others wire new electrolytics under the chassis and leave the Mershons electrically disconnected but in place on the chassis, "for show."

I am not aware of any Mershon that is still any good. At any rate, I've never owned a Philco that still had a working Mershon. On the other hand, I have come across a few 1937-era Philco aluminum electrolytic caps that were still good after 60-70 years. I don't keep using them, however; they get emptied and rebuilt with new electrolytics.

[codefox1]

If they don't leak today, they will next month or year, or whatever. Replace 'em and the radio will outlive you. Guess that's te best we can do.

[morzh]

Great news!

The radio works now fully.

The tube I suspected of wrongdoing happened to be bad indeed. Probably lost emission.
Once I replaced it, my antenna circuit started working and I no longer receive only one station, but the whole gamut of them - bunch of news, music, and - yes - commercials
Small grief compared to the joy of listening to the radio .

All in all, two tubes were bad: one is the Push-pull 71A, and another one - 24A, one of the three. The rest seems to be fine - I replaced All 24A to see if there's a change in reception - there's none. (And I do not have tester, so at this point this is the way for me to test them).

Time to try to change them capacitors

[Radioroslyn]

Great news!!! If you would like I might be able to find my way over to your place and we could test some toobs.
Terry

[morzh]

Great news!!! If you would like I might be able to find my way over to your place and we could test some toobs.
Terry

Sure Terry.
Write me an e-mail to
"morzhblya at yahoo dot com".

I will respond and we can make an arrangement.

Mike.