[Dan Walker]

I just bought this Philco  model 20.   Everything seems to be there.
  Is there anything particular I should know about this radio, before I get into replacing the caps?
 Dan in Calgary.

[gregb]

Nice find Dan, don't see to many Canadian models for sale. Don't use electrolytic's in the filter can, use 630 volt poly caps.

Gregb

[Steve Davis]

Don't use electrolytic's in the filter can, use 630 volt poly caps.

I have read this many times but, I don't understand why. When I look at the 20 schematic, I see the power supply is virtually identical to the one used in a 70 or a 90 or just about any of the other Philco models. So why are electrolytic capacitors OK in a 70 or a 90 but not in a 20?


Steve

[Radioroslyn]

Hey Dan,
Just push in your 71A and you'll be good to go[Image: http://philcoradio.com/phorum/images/smi...on_lol.gif]
On model 20's pull the spkr plug and measure resistance against each other. All pin should have a max of about 7,000 ohm or less.
Check the primary and secondary of the rf coils and audio driver transformer. RF coil less than 10 ohms audio trans less than 2,000 ohms.

[Radioroslyn]

Don't use electrolytic's in the filter can, use 630 volt poly caps.

I have read this many times but, I don't understand why. When I look at the 20 schematic, I see the power supply is virtually identical to the one used in a 70 or a 90 or just about any of the other Philco models. So why are electrolytic capacitors OK in a 70 or a 90 but not in a 20?


Steve

There is one big difference if you look at the values of the caps the 20 uses much smaller capacitance. Electrolytic caps in these small values don't hold up well. I replaced the caps in my 87 which uses these small values w/ 4.7mf@ 450v (two in series to give me about 2.3mf @900v).  Lasted about 4 month and the input cap shorted! Since then I've been using polys and haven't had a problem. Methinks it has something to do with something called ripple current.

[morzh]

Steve

I think it was that the el. caps were not really available during the 20-s time. Mershons were used in 70 and that waslater, and they were expensive.

Now if you mean, why not use them today....no electrical reasons why, but:

1. Some folks want to preserve authenticity: in 20 the caps are paper.
2. The caps in 20 are 2uF and in 70/90 are, I believe, 6uF or so. 2uF electrolytic cap is a flimsy creature (well, 6uF is too) and will not take the ripple current. To make a proper 2uF cap that would one would need to serialize bunch of hi-ripple higher capacitance caps. It is easier to do for 6uF, especially considering that most times it is replaced with 10uF value which is easily achieved: I serialize 22uF with high enough ripple to do it.


This said, in a 20 can one easily has the space to place a large film 2uF cap. Makes it easier.

[Steve Davis]

OK, so it is the LOW value electrolytics  that are not happy with the high ripple. I understand.

Steve

[morzh]

Steve

They could be any value, but due to today's construction of 2uF electrolytic being you know how "hyoooge" (about 4mm diameter and 10mm hight) it won't be happy.
In the olden days they were...well, you saw the Mershons - these could take an amp or two of ripple without any ill effects.

I wonder if today one could find a small value lytic of large dimensions; I haven't seen it, at least not in catalogues.

[Mondial]

Really don't think small value high voltage electrolytics in larger physical dimensions are made any more. In fact some of the larger dimensioned axial electrolytics sold today are actually fake.

Sprague used to make the "Atom" series replacement electrolytics with reasonably large dimensions. If you buy one of these today and take it apart you will find that they actually stuff a tiny cap inside the larger can, just to make it look substantial like the older production. And for this deception they charge a ridiculous price.

[gregb]

Sorry Steve I didn't answer sooner, but what Morzh said, these new manufacture low value caps won't stand up due to the high ripple current.

Gregb

[Arran]

Lets see, in addition to using film or a motor run cap rather then electrolytics in the power supply, the resistors often drift up in value substantially, the audio interstage transformer may be open (a common failure in any late 20s early 30s set with push-pull output) and one of the windings in the RF coils can go open, the primary I think. Sometimes the field coil will need to be repaired or rewound, I think that Mike (Morzh) had this issue with one of his model 20s, but the pole piece unbolts from the rear bracket so it's fairly easy to get the coil out for servicing. So no, a recap is only part of what needs to be done to make a model 20, or many other late 20s to mid 30s radios, perform well again.
Regards
Arran

[morzh]

Arran

You are scaring the guy

yes the problems are typical but not necessarily encountered. The speaker to be non-working is 50/50, with the field coil being the worst that can happen.
In my first 20 I had to re-cone the speaker and put a new transformer in it, but the field was OK.
In my other 20 Delux.....I think the field coil was bad.
I am yet to look into the plain 20 which Tom gifted me.

Haven't encountered open coils.

To me the nastiest part of restoring the 20 is emptying that capacitor array can. It is truly messy operation. Somehow a similar can in 16 emptied just fine, a can in 111 was a walk in the park, but this one.......nah-stee.

Other than that.......

[462ron]

With all this info on electrolytics vs poly caps, what is considered the cutoff point as far as when it's safe to use an electrolytic? Under how many mfds should you switch from electrolytic to poly?

[Arran]

Arran

You are scaring the guy

yes the problems are typical but not necessarily encountered. The speaker to be non-working is 50/50, with the field coil being the worst that can happen.
In my first 20 I had to re-cone the speaker and put a new transformer in it, but the field was OK.
In my other 20 Delux.....I think the field coil was bad.
I am yet to look into the plain 20 which Tom gifted me.

Haven't encountered open coils.

To me the nastiest part of restoring the 20 is emptying that capacitor array can. It is truly messy operation. Somehow a similar can in 16 emptied just fine, a can in 111 was a walk in the park, but this one.......nah-stee.

Other than that.......

Mike;
  The open RF coil problem seems to be more common on some models then others, but it doesn't mean that this set has that problem, it does however have the obligatory broken speaker grille. I was trying to remember which issue you had with the speaker in one of your sets, I thought it was either a field coil or an output transformer, you actually encountered both but in different sets. It's not difficult to test either the field coil or the output transformer, and it isn't always an issue. I would think that the output transformer would be a more common point of failure then a field coil, but you never know if or when someone might have tried plugging the set in and BBQing the speaker field so it's worth knowing before you start.
   As for the condenser can, this is where a second hand toaster oven comes in handy, the model 16 and 111 must have had their cans lined with fish paper if they pulled out easily, I cleaned out a can in a Westinghouse and had to resort to driving a large screw into one of the paper caps to pull the guts out as it wasn't lined.
Regards
Arran

[morzh]

I usually look at the ripple rating and roughly choose it to be equal or better than the B+ current draw.
At around 10uF some high-ripple (not GP) rated caps become adequate, I usually serialze 22uF caps to get to the target for 8/10 uF values.
Again I am talking only about the first rectifier cap; the second after the filter choke (or field coil used as such) could be a GP and its ripple rating is not as important anymore.